Tibetan Plateau and Climate Change
A study of the role of the Tibetan Plateau′s thermal forcing in modulating rainband and moisture transport in eastern China
Acta Meteorologica Sinica,2015,Vol 73,No. 01
【Abstract】 This paper reviews the recent progress in the studies of effects of the Tibetan Plateau′s thermal forcing on the rainband and moisture transport pattern and their interdecadal variability in eastern China in the four respects:(1)variations in the thermodynamic process of the three-ladder terrain in China and seasonal advance of the monsoon rainband, (2)thermal forcing of the TP′s land-air process and monsoon vapour transport pattern,(3)cooling effect of the TP snow cover on the distribution of moisture transport and rainband in eastern China,and(4)the relations of changes in the TP′s apparent heat source and decadal variation in monsoon rainband as well as the possible modulation.The major research results are concluded as:(1)The special three-ladder terrain in western China intensifies the thermal contrast between the land and the ocean.In particular,the cascade of western China′s elevated lands with the seasonal variations in thermal forcing extending toward the northeast seems to act as a "dynamic attractor" driving the monsoon rainband to move to the northwest. (2)The decreases/increases in the TP heat source could lead to the anomalies in the pathway and strength of vapour transport from the low-latitude oceans to the continents,regulating changes in East Asian monsoons and the spatial-temporal evolvement of rainfall in China.During the TP strong and weak heating years,the monsoon precipitation patterns in China tend to be characterized as " North floods-South droughts" and " North droughts-South floods",respectively.As a strong precursor signal,the anomalies of heating over the TP result in distinctive patterns of moisture transport in the East and South Asian regions and summer precipitation in China. (3)The mid-lower reaches of the Yangtze River are a distinctive confluent area of southward and northward air flows,where the differences in summertime column vapour fluxes between winter high and low snow cover years of the TP-key area were in a similar pattern of correlation vector between the summer TP heat source and vapour transport fluxes.The winter TP snow cover condition might significantly affect the structure of summer water vapour transport for the Meiyu rainfall in the reaches of the Yangtze River. (4)The patterns of the interdecadal precipitation variability in China are generally identified with the pattern of "southern flooding and northern drought" as well as trends of turning wet in the west over the last decades.In association with the trends in the TP spring apparent heat source rising again after a fall in recent 10 years,a turning point could be expected in the eastern China′s precipitation patterns.
Chinese Journal of Atmospheric Sciences,2015,Vol 39,No. 02
【Abstract】 Climate change for the 21st century over the Tibetan Plateau(TP) is projected using multiple climate models within the phase five of the Coupled Model Intercomparison Project under the Representative Concentration Pathway 4.5 (RCP4.5) scenario. These models have a demonstrated ability to simulate modern climatology. The results show an annual warming trend of 0.26 °C per decade, which correlates positively with the topographical height in 2006-2100.With respect to the reference period 1986-2005, the TP annual temperature increases 2.7 °C in the 2090 s, which is stronger than the warming in the early and middle 21st century. In the early, middle, and end periods, annual warming is 0.8–1.3 °C, 1.6–2.5 °C, and 2.1–3.1 °C, respectively. Temperature increases are seen in all seasons, with the strongest warming occurring in winter. On the contrary, overall annual precipitation increases slightly on the TP, with a trend of 1.15% per decade during 2006–2100 and an increase of 10.4% in the 2090 s relative to the reference period. Annual precipitation ranges from –1.8% to 15.2% in the early period, from –0.9% to 17.8% in the middle period, and from 1.4% to 21.3% in the end period. Precipitation generally increases in all seasons; the summer increase is larger compared with other seasons, particularly for the end of the 21st century. The annual precipitation increase occurs mainly in summer. It is noted that the above results differ somewhat among models, which indicates a relatively large level of uncertainty and a relatively high (low) reliability of temperature(precipitation) projection.
Quasi 3-year period response of the rainy season precipitation over the eastern parts of Northwest China to the spring sensible heat flux over the eastern part of the Tibetan Plateau
Acta Meteorologica Sinica,2015,Vol 73,No. 04
【Abstract】 By using the precipitation data at 39 stations in the eastern parts of Northwest China during 1960–2010 and the NCEP/NCAR reanalysis data, the quasi-periodic cycles of precipitation during the rainy season over the eastern parts of Northwest China, spring sensible heat flux over the eastern part of the Tibetan Plateau and their synergistic coupling are analyzed through the Multi Taper Method-Singular Value Decomposition (MTM-SVD). The results show that both the sensible heat flux and precipitation have a significant quasi 3-year period, and their coupling field also shows a significant 3-year period. When the spring sensible heat flux becomes stronger (weaker) over the eastern part of the Tibetan Plateau, there is more (less) precipitation over the eastern parts of Northwest China during the rainy season. The most significant period of such quasi 3-year period synergistic relationship appears during 1960–1982, and there is an adjustment stage in 1983–1990, with the quasi 3-year period appeared again after 1990s. The continuous heating process of the former can affect the latter, which are mainly embodied in August.
Chinese Journal of Atmospheric Sciences,2015,Vol 39,No. 06
【Abstract】 The spatial distribution of the vertical variation in cloud over the eastern Tibetan Plateau during summer (June to August) of 2006–2010 was studied using CloudSat data. The results showed that: (1) The cloud over the eastern Tibetan Plateau could develop in the stratosphere in summer, and existed as water cloud below 5 km, mixed with liquid and ice crystals from 5 to 10 km, and was ice cloud above 10 km. However, due to the problem of the inversion algorithm of the cloud phase from CloudS at data, the cloud top of the water cloud and mixed cloud may have been lower, and the cloud base of the ice clouds higher, than was actually the case. (2) There were some spatial differences between the spatial distribution of the summer mean water vapor flux and the cloud water path. The latitudinal distribution of cloud water indicated that the cloud water was sufficient from 26.5°N to 30.5°N. The longitudinal distribution of cloud water showed that the cloud water to the east of 95°E was more abundant than to the west. (3) The cloud in the study area was mainly single-layer, especially over the eastern Tibet Plateau. The average thickness of the single-layer cloud was 4182 m, and the cloud top and cloud thickness descended in a fluctuating fashion from south to north, limited by the water vapor amount. The occurrence frequency of multilayer cloud was reduced obviously to the north of 27°N, which illustrated that powerful convection was more conducive to the development of multilayer cloud.
Reversed Phase Change of the Temperature in the Upper and Lower Troposphere over the Tibetan Plateau in Summer and Its Relationships to Precipitation and Atmospheric Circulation under the Background of Global Warming
Chinese Journal of Atmospheric Sciences,2015,Vol 39,No. 06
【Abstract】 Using linear-trend estimates, empirical orthogonal function (EOF) analysis, correlation and composite analysis, the vertical variation of temperature in the troposphere over the Tibetan Plateau in summer and its relationships to precipitation and atmospheric circulation are examined, based on monthly NCEP/NCAR (National Centers for Environmental Prediction/National Center for Atmospheric Research) reanalysis data and precipitation data from 596 stations in China. The characteristics of the vertical variation of temperature reveal that the temperature in the lower to middle-upper troposphere over the Tibetan Plateau in summer shows a significant warming trend since 1971, while the temperature in the high troposphere shows a significant cooling trend. In terms of interannual and interdecadal the temperature in the lower to middle-upper troposphere and the high troposphere are negatively correlated, and all have the cycles of 2–4 and 8–13 years. The first EOF mode of the vertical temperature anomaly averaged along 27.5°N–40°N over the Tibetan Plateau in summer shows a reversed phase change of an increase in the lower to middle-upper troposphere and a decrease in the high troposphere. Its time coefficient shows a long-term positive trend, and there is abrupt change in 1978 and 1994. The relationships between the interannual and interdecadal reversed phase change of temperature, i.e., an increase in the lower to middle-upper troposphere and a decrease in the high troposphere over the Tibetan Plateau in summer, and precipitation in China in summer, are explored. When the temperature over the Tibetan Plateau increases in the lower to middle-upper troposphere and decreases in the high troposphere, summer rainfall in China shows a southern type pattern, in which more precipitation in the regions south of the Yangtze River and southern China and less precipitation in the northeast of China are the main distributional characteristics. In addition, less precipitation occurs in the local areas of the Yangtze River Basin and some areas of northwestern China, while more precipitation occurs in the local areas of the eastern part of North China, the central and eastern regions of the Tibetan Plateau, and northwest Xinjiang. The abnormal interdecadal distribution of precipitation is more significant than the interannual distribution. Analysis of the circulation reveals that when the temperature over the Tibetan Plateau increases in the lower to middle-upper troposphere and decreases in the high troposphere, there is abnormally high pressure in middle and high latitude regions of East Asia, while in middle and low latitude areas there is abnormally low pressure. Meanwhile, a relatively significant relationship is found between circulation and precipitation.
Variation of the Tibetan Plateau summer monsoon under the background of global warming and its impact on the climate in southwestern China
Acta Meteorologica Sinica,2015,Vol 73,No. 05
【Abstract】 The new Tibetan Plateau (TP) summer monsoon indices were defined according to the vorticity characteristics at 600 hPa over the TP, and their impacts on the southwestern China climate had been analyzed by using the NCEP/NCAR reanalysis data from1951 to 2012 and the surface meteorological data at 116 stations in southwestern China from1960 to 2012 under the background of global warming. The intensification of the TP summer monsoon has reached the peak in the late 1990s, consistent with the global warming trend, but the locations are different from each other. There are two indices, the intensity and the position of the central longitude of the TP summer monsoon, which have significant effects on the southwestern China climate change. A stronger TP summer monsoon is favorable for more moisture convergence, more cloudy and rainy days, less sunshine, weaker evaporation and smaller diurnal temperature range in southwestern China. Moreover, the meteorological elements vary more significantly in the Sichuan-Chongqing region with the deep updraft development than in the Yunnan-Guizhou region with the ascending flow limited to below 600 hPa. When the central longitude moves to the east, the whole southwestern China is controlled by the abnormal sinking motion, which contributes to a higher temperature. There appears abnormal strong moisture divergence in the middle-western part of Sichuan, Guizhou, and Guangxi, which result in higher temperature, lower humidity and less rainfall over the above regions. Since the 21 st century, the shrinking intensity and eastward-moving of the TP summer monsoon conspire to cause the drying trend of southwestern China. The new TP summer monsoon indices can not only reflect the monsoon circulation on the TP, but also have a good relationship with the climate change in southwestern China. This research provides a theoretical foundation for the drought-flood prediction of southwestern China.
Acta Meteorologica Sinica,2015,Vol 73,No. 06
【Abstract】 The accuracies of the GLASS (Global LAnd Surface Satellites) albedo products in 1 km resolution are assessed using the ground measurements of year 2003 at three stations over the Tibetan Plateau. The results show that the overall change trends of the GLASS black-sky and white-sky albedos are consistent with the ground-observed albedos, which could indicate the state of surface fair well; both the small fraction of snow cover and cloud cover have great effect on the accuracy of the GLASS albedo products and cloud cover might result in higher values of GLASS albedo products than ground measurements; the RMSEs (root-mean-square errors) between the GLASS black-sky albedos, GLASS white-sky albedos and ground measurements are evidently decreased when eliminating the effect of snow and cloud cover, which are 0.0155 and 0.019, respectively.
The interannual variation of wind speed in the Tibetan Plateau in spring and its response to global warming during 1971–2012
Acta Meteorologica Sinica,2016,Vol 74,No. 01
【Abstract】 Based on the daily observed data of 73 stations in the Tibetan Plateau and the JRA-55 reanalysis data, by using year-to-year differences and the equation of kinetic energy budget, the variation of wind speed in the Tibetan Plateau in spring during the period of 1971–2012 and its response to global warming are investigated. It is indicated that there is a statistically significant decrease trend of spring wind speed in the Tibetan Plateau as the climate is warming, while the variation tends to be steady as a hiatus of warming occurs at the beginning of the 21st century. Although the climatic trend of wind speed is contrary to surface temperature, they exhibit an in-phase variation on an inter-annual scale, in which case the spring wind speed in the Tibetan Plateau increases when the surface temperature increases in the Tibetan Plateau, Indo-China Peninsula and India Peninsula but decreases in the north and east of Asia. A decline trend of spring wind speed in the Plateau at the end of the 20th century due to more rapid warming in the North Asia than in the South Asia, and an increasing trend of wind speed in the years followed is accompanied by warming in central Plateau and South Asia while cooling in North and East Asia. The asymmetrical thermal conditions of both the Tibetan Plateau and its purlieus make an impact on atmospheric baroclinicity by which the variation of wind speed in the Plateau can be influenced in two ways: one is changing directly the pressure gradient force near surface, and another is affecting the transportation of kinetic energy from the upper levels to the middle levels. Meanwhile, the applicability of the three different reanalysis wind data to the Tibetan Plateau is investigated, respectively.
Characteristics of Precipitation Based on Cloud Brightness Temperatures and Storm Tops in Summer Tibetan Plateau
Chinese Journal of Atmospheric Sciences,2016,Vol 40,No. 01
【Abstract】 Characteristics of precipitation types are investigated in summer over the Tibetan Plateau based on the merged 2A25 and 1B01 datasets issued by TRMM (Tropical Rainfall Measuring Mission). The statistics show that many more stratiform precipitation samples are included in 2A25, version 7, in summer over the Tibetan Plateau, and their ratio reaches 85%. In the definition of precipitation types based upon the cloud top brightness temperature (i.e., cloud phases) observed by the thermal infrared channel of VIRS (Visible and Infrared Scanner), results indicate about 43% and 56% of precipitation with ice, mixed ice and water in the top precipitating clouds, respectively, in summer over the Tibetan Plateau. This compares to versus about 77%, 22% and 1% for deep weak convective precipitation, shallow precipitation, and deep strong convective precipitation, respectively, according to storm top classification. The spatial distribution indicates that the frequency and rain intensity of precipitation with ice or mixed ice and water in the top of precipitating clouds increase from the western plateau towards the eastern and southeastern plateau. However, the storm top altitudes for these precipitating clouds decrease from the western and central plateau towards the eastern plateau. In the category of precipitation types defined by storm top altitudes, their spatial distributions show that the frequency of the deep strong convective and shallow precipitation increases from the western plateau towards the eastern plateau, while the frequency of the deep weak convective precipitation shows that it is less in the western and northern plateau, opposite to that in the southern plateau where it is usually one time higher than that in the northern plateau. The spatial distribution of rainfall intensity for both deep weak convective precipitation and shallow precipitation also displays an increasing trend from the western and central plateau towards the eastern plateau, with the opposite variation of storm top altitudes for both precipitation types. In conclusion, the variation in the frequency and intensity of precipitation in summer over the Tibetan Plateau increases from the western to the eastern plateau, contrary to the altitude variation of cloud top and storm top.
Characteristics of Land–Atmosphere Energy Exchanges over Complex Terrain Area of Southeastern Tibetan Plateau under Different Synoptic Conditions
Chinese Journal of Atmospheric Sciences,2016,Vol 40,No. 04
【Abstract】 The geographical environment of the Tibetan Plateau is complex. Previous studies of land surface–boundary layer processes mainly focused on different underlying surfaces without complex topography. The characteristics of land–atmosphere exchanges of energy on different underlying surfaces over complex terrain area of the Tibetan Plateau under different synoptic conditions are analyzed in the present study based on observations collected at four ground sites during May 20th to July 9th 2013 over Linzhi. Results indicate that in the case when the downward shortwave radiation is roughly the same at all the sites, sensible heat flux at the northern slope site, which is covered by broad-leaved forest and characterized by steep terrain, is greater than that at the other three sites; latent heat flux at the southern site, which is covered by wheat and thus with large vegetation fraction, is the largest among the four sites. Energy fluxes show obvious diurnal variation at each individual site. In sunny days, sensible heat flux and net radiation are significantly greater than that in rainy days. However, latent heat flux has little changes under different synoptic conditions. The influence of complex terrain of the Tibetan Plateau on sensible heat flux is more significant than that of different synoptic conditions. Different topographies have significant impacts on latent heat flux on rainy days. When the warm moist southwesterly flow in front of the South Asia monsoon trough affects Linzhi, the weather will be dominated by cloudy and rainy days. Otherwise sunny days are prevalent in this region. Apparently the variation of monthly land–atmosphere fluxes over Linzhi area is modulated by the South Asia monsoon activities.
The simulation analysis of the precipitation over the southern slopes of the Tibetan Plateau based on WRF model
Acta Meteorologica Sinica,2016,Vol 74,No. 05
【Abstract】 Impacts of cumulus parameterization schemes, grid nesting approach and model resolution on the simulation of precipitation in July 2006 over the southern slope of the Tibetan Plateau have been studied by using WRF model. The result shows that the simulated precipitation is very sensitive to cumulus parameterization scheme. Different schemes have different results, and the result simulated by using Grell-Devenyi mass flux scheme is better than results using other schemes. The comparison of results of five sensitivity experiments indicates that, the use of cumulus parameterization scheme, the increased resolution, and the nesting approach can improve the simulated intensity and spatial distribution of precipitation. The simulation is better when these three approaches are used jointly. These approaches can improve the wind field simulation, which is beneficial for the simulation of water vapor transport and its convergence, and vertical heating and convection, and the distribution of vertical velocity. The simulated atmospheric humidity decreases without using a cumulus parameterization scheme, while model resolution and the nesting approach have little impacts on the humidity simulation.
A Climatology of the Surface Heat Source on the Tibetan Plateau in Summer and Its Impacts on the Formation of the Tibetan Plateau Vortex
Chinese Journal of Atmospheric Sciences,2016,Vol 40,No. 01
【Abstract】 Based on NCEP/DOE (National Centers for Environmental Prediction/Department of Energy) reanalysis data of surface sensible heat and latent heat fluxes on the Tibetan Plateau (TP) and datasets of the Tibetan Plateau Vortex (TPV) recognized from MICAPS (Meteorological Information Comprehensive Analysis and Process System) weather maps, this paper studies the near 30-year (1981–2010) climatological characteristics of surface heating and generating frequency of TPVs over the TP in summer, and analyzes the temporal correlation between the TP surface heating and the TPV statistics and its physical cause. The following results are obtained. The climatological mean of TP surface sensible heat fluxes in summer is 58 W m−2, showing an overall weak decreasing trend in the near 30-year period. An increasing trend is apparent in the early 1980s and most of the first decade of the 21st century, but a fluctuating decline between them. The surface sensible heat shows a quasi-3-year periodic oscillation, and an abrupt climate change starts around 1996. The climatological mean of surface latent heat fluxes in summer is 62 W m−2, showing fluctuating changes accompanied by an increasing trend over the near 30-year period. The surface latent heat shows a quasi-4-year periodic oscillation, with an abrupt increase beginning around 2004. The climatological mean of the surface heat source in summer is 120 W m−2; sensible heat and latent heat on the ground contributes the same to the surface heat source over the TP in summer. The surface heat source shows a modest weakening trend overall, with a strong phase between the 1980s and 1990s, an obvious weak phase in the first six years of the 21st century, and then becomes strong again. The surface heat source shows a quasi-3-year periodic oscillation and an abrupt change from strong to weak around 1997. Based on identification using the MICAPS weather maps, the linear frequency of summer TPVs over the near 30-year period showed a certain degree of decline, with a higher frequency mainly concentrated in the 1980s to 1990s. The generating frequency of TPVs shows a quasi-7-year periodic oscillation, and features an abrupt change around 1998. The generating frequency of TPVs over the same period is highly positively correlated to sensible heat but weakly negatively correlated to latent heat, but compared with the surface heat source over the TP, is still a significant positive correlation. On the climate scale, therefore, stronger periods of TP surface heat, especially surface sensible heat, correspond to the favorable formation of TPVs. From the perspective of the temporal correlation of climate statistics, this study reveals important impacts of the TP surface heating on promoting TPVs and convective activity.
Features of land surface process over wetland at Tibetan Plateau during soil freezing and thawing periods
Acta Meteorologica Sinica,2017,Vol 75,No. 03
【Abstract】 Using observed data at Longbao wetland, Yushu from July 2015 to July 2016, features of soil temperature, soil moisture and surface energy budget were analyzed. The results show that the frozen period of soils lasts from preceding December to subsequent April. The soils at deeper depths freeze later than that near the surface. The thawing process is faster than the freezing process. The soil at 5–40 cm depth completely freezes within 51 d and thaws within 19 d. The magnitude of annual variability of soil water content is up to 0.6 m3/m3. The soil moisture at 5–40 cm decreases during the freezing period and soil moisture at 5–10 cm increases during the thawing period. When the soil is frozen, sensible heat flux increases while latent heat flux decreases during the daytime, net radiation and soil heat flux both decrease, and the diurnal variation of soil heat flux becomes larger. After thawing, latent heat flux, net radiation and soil heat flux all increase during the daytime. Surface albedo, the Bowen ratio, and the thermal conductivity and diffusivity of soil increase after freezing and decrease after thawing. The soil thermal capacity decreases after freezing and increases after thawing.
Chinese Journal of Atmospheric Sciences,2016,Vol 40,No. 01
【Abstract】 The activities of the Asian summer monsoon are modulated by large-scale land–sea thermal contrast and the thermal forcing of the Iranian Plateau–Tibetan Plateau. The vast quantity of latent heat released by Asian monsoon precipitation forms a kind of feedback to the atmospheric circulation. This feedback is so complicated that it is very important to reveal the physical processes involved. Doing so will help us to understand the formation and variation of the climate pattern, as well as improve the accuracy of weather predictions and climate forecasts. The main subtropical climate system existing in the upper troposphere in boreal summer is characterized by the huge South Asia High (SAH) and the Upper-Tropospheric Temperature Maximum (UTTM). This paper introduced the development of the theory of the temperature–vertical heating gradient (T–QZ), and used it to explain the formation mechanism of the SAH and UTTM. It iwas proven that the latent heating over the subtropical eastern Eurasian continent, the sensible heating over the central continent, and the longwave cooling in the upper troposphere were the causes of the development of the SAH and UTTM in the upper troposphere in South Asia. This paper also documented the limitation of the Gill model in studying upper-troposphere dynamics and provided a relevant solution.
Spatial and temporal distribution characteristics of surface heat fluxes over both Tibetan Plateau and Iranian Plateau in boreal spring and summer and their relationships
Acta Meteorologica Sinica,2017,Vol 75,No. 02
【Abstract】 Elevated heat sources over the Tibetan Plateau (TP) and the Iranian Plateau (IP) have significant impacts on East Asian climate. Based on monthly mean surface heat fluxes extracted from the ERA-interim reanalysis data for the period of 1979 to 2011, surface thermal characteristics during boreal spring and summer over both TP and IP and their relationships were analyzed. The results show that the basic spatial and temporal characteristics of surface heat fluxes over TP and IP are different in the spring and summer, and surface heat fluxes in specific regions of these two plateaus exhibit different characteristics on interannual and interdecadal time scales. Over TP, surface sensible heat flux (SH) in the western part is stronger than that in the eastern part during the spring and summer, whereas the spatial distribution of surface latent heat flux (LH) is opposite with larger values in the eastern part. SH peaks in the spring and exceeds LH before the summer, but LH is larger than SH in the summer. On the interannual time scale, SH is negatively correlated with LH in western TP during the spring and summer, and SH anomalies over western TP can persist from spring to summer. On the interdecadal time scale, there is a significant difference in surface heat fluxes between eastern and western parts of TP. Springtime (summertime) SH over eastern TP exhibits a significant decreasing trend and experienced an interdecadal change in1998 (2001), switching from positive to negative anomaly. However, springtime SH over western TP exhibits a significant increasing trend and experienced a negative-to-positive interdecadal change in 2003. The LH over eastern TP displays a significant decreasing trend only in the spring and exhibited a positive-to-negative interdecadal change in 2003. The LH over western TP exhibits a significant decreasing trend in both the spring and summer and experienced an interdecadal change at the beginning of the 21st century, switching from positive to negative anomaly. Over IP, both SH and LH are uniformly distributed during the spring and summer. SH peaks in the summer, LH is strong in the spring but weak in the summer, and it is smaller than SH all year round. SH over IP is stronger than SH over TP in each season. On the interannual time scale, spring and summer SH (LH) anomalies over the entire IP are uniformly positive (negative). There is a significant negative correlation between SH and LH over IP. SH and LH anomalies over IP can persist for quite a while. A significant difference is found in the interdecadal variability of surface heat flux between northern and southern parts of IP. The spring and summer SH (LH) over northern IP exhibited a significant increasing (decreasing) trend and experienced an interdecadal shift in the end of the 20 century, when the anomaly of SH (LH) switched from negative (positive) to positive (negative). There is no significant trend in SH and LH over southern IP during the spring and summer. However, SH over southern IP in the spring and summer experienced a negative-to-positive interdecadal change, while springtime LH experienced a positive-to-negative interdecadal change at the end of the 20th century. The relationship between surface heat fluxes over these two plateaus is as follows: SH over IP is positively correlated with SH over western TP and is negatively correlated with SH over eastern TP in the spring; LH over IP is positively correlated with LH over eastern TP in the spring; SH over IP in the spring is negatively correlated with SH over eastern TP in the subsequent summer.
Acta Meteorologica Sinica,2017,Vol 75,No. 02
【Abstract】 The gravity wave drag triggered by the Qinghai-Tibet Plateau remains unclear at present. To address the problem, a parameterization scheme for subgrid-scale orographic gravity wave drag was introduced into the GRAPES_Meso and a suite of numerical experiments were conducted. Several conclusions from the results are as follows. (1) According to the vertical distribution of orographic gravity wave drag along 30°N, the blocking drag mainly exists in the lower levels (from level 1 to 5) while the gravity wave drag mainly exists between level 5 and level 10. According to the horizontal distribution, the blocking drag, which is dominant on level 3, mainly exists in the flanks of the Qinghai-Tibet Plateau. Large values of blocking drag are located from the eastern Qinghai-Tibet Plateau to northern Yunnan-Guizhou Plateau. (2) Analysis of the Froude number and the altitude of circumfluent flow show a large gradability, and thus the highest altitude of circumfluent flow is located at the area of Himalayas and eastern flank of the Qinghai-Tibet Plateau. The larger the Froude number is over a specific area, the higher the altitude of circumfluent flow is in the area. (3) With the adoption of the subgrid-scale orographic gravity wave drag parameterization scheme, the model results reflect more accurate representation of the breaking of orographic gravity waves in lower and higher levels, as well as its upward transport. (4) Moreover, both the single case study and the batch experiments show positive impacts on the simulation of wind field and precipitation, which leads to the improvement of model prediction accuracy.
The Comparison of Cloud Base Observations with Ka-Band Solid-State Transmitter-Based Millimeter Wave Cloud Radar and Ceilometer in Summer over Tibetan Plateau
Chinese Journal of Atmospheric Sciences,2017,Vol 41,No. 04
【Abstract】 Understanding the observation abilities and advantages of ceilometer and cloud radar will be helpful for their operational applications in cloud observation. During the summer of 2014, the third Tibetan Plateau Atmospheric Science Experiment was carried out and clouds were observed using Ka-band solid-state transmitter-based millimeter wave cloud radar, Lidar and ceilometer. Based on the cloud measurements in this experiment, the algorithm to determine cloud base using cloud radar observations was developed. The biases of cloud base and data acquisition ratios by ceilometer and cloud radar for low clouds, medium clouds and altocumulus were analyzed. Reasons for the overestimation of cloud base by ceilometer were explained. The improved observation mode was introduced and the observation skill was simulated. The results indicate that (1) the cloud radar observation ability for altocumulus is better than that of ceilometer, the cloud radar echoes below the ceilometer-derived cloud bases are possibly clutter, and its observation for low clouds should be improved. Blockages of low clouds affect the observations of medium clouds and altocumulus by ceilometer. Compared with ceilometer, the cloud radar missed some shallow altocumulus. (2) Most of cloud bases observed by ceilometer are located within the cloud echoes observed by cloud radar. The temporal and spatial correspondences of cloud base observed by cloud radar and ceilometer are poor. (3) Increasing the transmitter power of ceilometer and improving the operational mode of cloud radar can improve the cloud observation abilities. The cloud radar and ceilometer data merging would enhance cloud observation abilities. The research results provide references for usages of cloud radar and ceilometer data and evaluation of observation abilities of cloud radar and ceilometer. They are also helpful in designing operational modes for cloud radar, and promoting the development and application of advanced cloud observation technology.
Low Frequency Oscillation of Precipitation and Diurnal Variation Characteristic of Land–Air Process at Shiquanhe Station and Linzhi Station in Tibetan Plateau in the Summer of 2014
Chinese Journal of Atmospheric Sciences,2017,Vol 41,No. 04
【Abstract】 Using the original data of 3 m eddy correlation system and 10 Hz turbulence data at Shiquanhe station and Linzhi station, which are respectively located at the southwestern and southeastern Tibetan Plateau, obtained during “the 3rd Tibetan Plateau Atmospheric Scientific Experiment”, station observation data provided by China Meteorological Administration, JRA-55, and GPCP (Global Precipitation Climatology Project) daily reanalysis data in July and August of 2014, the diurnal variations of surface layer meteorological elements and turbulent changes under the background of 10–20-day low frequency oscillation in two stations are studied respectively. The results are as follows. The 10–20-day low frequency component of circulation fields in the upper and lower troposphere and water vapor fields and heat sources at the two stations are opposite during their dry and wet phases. The variations of low frequency surface sensible and latent heat fluxes have influences on precipitation, and these influences are different in the western and eastern Plateau. The low frequency oscillation at Shiquanhe station propagates from west to east, whereas the oscillation at Linzhi station propagates from east to west. Thus, there are two types of low frequency oscillations from different origins. Surface layer meteorological elements and turbulent flux have obvious diurnal variations in the dry and wet phases. The highest temperature usually appears at 1400 BJT (Beijing time) but at 2000 BJT in the dry and wet phases at Shiquanhe station. Based on Bowen ratio, it can be found that the latent heat is dominant throughout the day at Shiquanhe station in the wet phase, while latent heat is dominant before 0600 BJT and sensible heat is dominant after 0600 BJT during the dry phase. At Linzhi station in both wet and dry phases, sensible heat is always dominant before 0800 BJT and latent heat is dominant after 0800 BJT. The average kinetic energy of turbulence and average wind speed are positively correlated at the two stations, and the vertical momentum is transported downward while heat and water vapor are transported upward.
Estimating planetary boundary layer height over the Tibetan Plateau using COSMIC radio occultation data
Acta Meteorologica Sinica,2018,Vol 76,No. 01
【Abstract】 Many previous studies on the planetary boundary layer (PBL) over the Tibetan Plateau (TP) were based on sparse conventional observations, which led to limitations on overall understanding of the PBL over the TP. Global Positioning System Radio Occultation (GPS RO) measurements contained useful information about the PBL due to its high accuracy and high vertical resolution. Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) RO measurements from 2007 to 2013 were used to derive the PBL height over the TP. The top of the PBL was defined as the height at which the vertical gradient of the refractivity was the minimum. The PBL height derived from COSMIC RO was highly correlated with collocated radiosonde data with a correlation coefficient of 0.786, a mean PBL height difference of approximately 0.049 km and a root-mean-square difference near 0.363 km. The PBL height over the TP decreased from west to east with the amplitude of annual mean PBL height ranging within 1.8 to 2.3 km over the central-western TP and 1.4 to 1.8 km over the eastern TP. The maximum PBL height was found over southwestern TP. In addition, the PBL height over the TP had a distinct seasonal variation. The PBL height was more than 2.0 km during winter over most TP regions. Following the approach of spring, the PBL height started to shrink over most TP regions except southern TP, where the PBL height increased to 3.0 km due to the influence of the Indian monsoon. The PBL over the TP strengthened during summer with the PBL height more than 1.8 km. The PBL height reduced again during the autumn. There were two maxima over Taklimakan Desert and the monsoon region, which is located at the north and south of the TP, respectively. The maxima of PBL height occurred at summer over the desert and April before the monsoon onset over the monsoon region. The convergence of horizontal winds and extensive ascending motions over the central and western TP provided the impetus for the development of the PBL, and the large-scale descending motions restrained the development of PBL over the eastern TP. The seasonal distributions of PBL height were consistent with that of sensible heat flux over the TP. The PBL height derived from COSMI RO had similar spatial and temporal patterns with that from ERA-Int (European Centre for Medium-Range Weather Forecasts Reanalysis Interim), but the PBL height derived from ERA-Int was underestimated almost everywhere over the TP. The GPS RO can capture the structure of PBL over TP very well. However, the presence of strong inversion layer, or cloud with large liquid or ice water content, may cause uncertainties in the derived PBL height.
Structure analysis of heavy precipitation over the eastern slope of the Tibetan Plateau based on TRMM data
Acta Meteorologica Sinica,2017,Vol 75,No. 06
【Abstract】 The horizontal and vertical structure of a heavy precipitation system occurred over the eastern slope of the Tibetan Plateau on 21 July 2013 has been studied by using TRMM data, NCEP-FNL operational global analysis data and ERA-Interim analysis data in combination with Doppler radar and other surface observational data. The result indicated that the heavy precipitation system, which was triggered by unstable stratification combined with 700 hPa shear line and surface convergence line, consisted of a main stratiform precipitation cloud cluster and several scattered convective precipitation clouds characterized by high precipitation rate. Although the total amount of convective rainfall was less than that of the stratiform rainfall, averaged convective precipitation rate was 4.7 times larger than the stratiform precipitation rate and convective rainfall accounted for 25.6% of the total rainfall. The horizontal scale was about 20–50 km for a reference precipitation rate of 10 mm/h. The spectral distribution of convective precipitation rate indicated that it was largely concentrated within the range of 1–50 mm/h, and the rainfall with precipitation rate of 20–30 mm/h made the greatest contribution to total rainfall. Meanwhile, almost 90% of the stratiform rainfall was below 10 mm/h. Vertically, the top of the heavy precipitation system could reach up to 12 km above the ground with a low core structure (below 6 km). Most precipitation occurred in the levels from ground to near 7 km and the distribution of precipitation rate in vertical was inhomogeneous. In general, precipitation decreased with increasing height, but within a certain range of height, convective precipitation increased with height. As a verification dataset and supplement for TRMM PR, Doppler radar observations also showed the characteristics of low precipitation center in this rainfall event.
A spatial-temporal distribution characteristics of the atmospheric methane in troposphere on Qinghai–Tibetan Plateau using AIRS data
China Environmental Science,2017,Vol 37,No. 08
【Abstract】 A contrastive study was conducted on the AIRS retrieval results and the observational data of methane concentration at Waliguan atmospheric background station. The distribution variation characteristics of atmospheric methane concentration over the Qinghai–Tibetan Plateau from 2003 to 2015 were analyzed. It was shown that the AIRS retrieval data displayed the same monthly, annual and seasonal variation trend as well as segmental variation characteristics with those of Waliguan station. The methane concentration was featured high in the southeast and low in the northwest along the geographical line of the east edge of Qiangtang Plateau-the northwestern Three-River-Source region, and decreased significantly as the altitude rose, with the highest and the most sensitive variability in the south central region of the Qinghai–Tibetan Plateau. From 2003 to 2015, the methane concentration in the Qinghai–Tibetan Plateau continued to rise, with the fastest growth in autumn and the slowest in winter, at an annual growth rate of 5.2 nmol/(mol·a), while the growth rate from 2013 to 2015 was lower than the global average. The seasonal variation showed a unimodal curve, with the highest value in summer and the lowest value in spring, and with the altitude rising, the seasonal variation was more significant.
A statistical forecast model for summer precipitation in eastern China based on spring sensible heat anomaly in the Tibetan Plateau
Acta Meteorologica Sinica,2017,Vol 75,No. 06
【Abstract】 Based on station observations and high resolution gridded precipitation data, the relationship between spring sensible heat in the Tibetan Plateau (STPSH) and summer precipitation in eastern China (SPEC) is investigated in terms of decadal change and interannual variability by using the maximum covariance analysis. Further attempt has been made to establish a statistical model for forecasting the SPEC using the timescale decomposed regression approach. Results indicate that for the decadal component, a significant correlation exists between the STPSH and SPEC in most part of eastern China in June, July and August with the explained variance fractions of 75.6%, 99.9% and 79.7%, respectively. For the interannual component, however, the significantly correlated regions are distributed in southern China, the coastal area of northern China, and the Yangtze–Huai River valley in June; in July, the correlated areas are located over the southwestern part of South China, the Yangtze River valley, the southeastern part of Northeast China and the middle-lower reaches of Yellow River; high correlation is found over Northeast China and the western part of South China in August. The explained variance fractions are 42.7%, 43.4% and 32.0%, respectively. The explained variance analysis and the hindcast examination suggest that the best prediction skill of this model occurs in most part of eastern China in July. The areas with high predictability are the southern region of the Yangtze River in June, and northeastern China and western part of South China in August. The model can reasonably describe the relation between the STPSH and SPEC and quantitatively forecast local precipitation in June, July and August. Therefore this model might be used for short-term operational climate prediction.
Numerical experiments and dynamic diagnosis of the upper-level jet stream impact on the Tibetan Plateau shear line
Acta Meteorologica Sinica,2018,Vol 76,No. 03
【Abstract】 Using the NCEP 1° × 1° FNL analysis data and the mesoscale model WRF, numerical experiments have been conducted to simulate a Tibetan Plateau shear line process. The study focused on the impact of the upper-level jet stream intensity on the Tibetan Plateau shear line. In combination with the ω equation, factors affecting the ascending motion in the Tibetan Plateau shear line were also analyzed. The results showed that the intensity of the upper-level jet stream had an important influence on low-level wind fields. The increases in jet stream would intensify the wind shear along the Tibetan Plateau shear line, and the shear line would become longer. Meanwhile, the enhancement of the upper-level jet stream intensity was also favorable for the convergence of water vapor along the Tibetan Plateau shear line. The upper-level jet stream can affect positive vorticity, convergence and ascending motion along the Tibetan Plateau shear line by influencing the vertical configuration of lower-level convergence and upper-level divergence. The diagnostic analysis of the ω equation showed that the Laplace term of temperature advection played a dominant role in the ascending motion along the Tibetan Plateau shear line, and the low-level warm advection was favorable for the development of ascending motion along the shear line. The influence of the variation of upper-level jet stream intensity on the differential term of the vorticity advection was greater than that of the Laplace term of temperature advection, and the enhancement of the upper-level jet stream intensity would enlarge the positive contributions of the differential vorticity advection term and the temperature advection term, which is more favorable for the development of ascending motion and the maintenance of the Tibetan Plateau shear line.
Review of the Impact of the Tibetan Plateau Sensible Heat Driven Air-Pump on the Asian Summer Monsoon
Chinese Journal of Atmospheric Sciences,2018,Vol 42,No. 03
【Abstract】 The study during the past twenty years on the Tibetan Plateau sensible heat driven air-pump (TP-SHAP) and its impacts on the Asian summer monsoon is reviewed. The impact mechanisms are summarized from the perspective of energy ( θ), potential vorticity–diabatic heating (PV– Q), and angular momentum conservation (AMC). It is demonstrated that the surface sensible heat on the plateau’s slopes plays a significant role in changing the energy of the air parcel moving towards the plateau and generating its vertical pumping. The PV forcing induced by the TP-SHAP generates a strong and vast cyclonic circulation surrounding the plateau near surface, which transports abundant water vapor from the sea to the land to feed the monsoon convective precipitation. Through changing the temperature and pressure fields structure over the TP, the plateau surface sensible heat also produces minimum absolute vorticity and PV in the near tropopause, and induces a monsoon meridional circulation which is opposite to the Hadley circulation under the constraint of angular momentum conservation. It provides a favorable large-scale ascending background for the monsoon development over the Asian summer monsoon area. A brief comment on the recent discussion of the TP impact on Asian monsoon mechanism is provided as well, and an outlook on future study is given at the end.
Review of Current Investigations of Cloud, Radiation and Rainfall over the Tibetan Plateau with the CloudSat/CALIPSO Dataset
Chinese Journal of Atmospheric Sciences,2018,Vol 42,No. 04
【Abstract】 Clouds over the Tibetan Plateau (TP) and associated precipitation and radiation affect the structure of the TP diabatic heating. The Cloud Sat/CALIPSO launched in 2006 has provided quantitative information on three-dimensional distribution of clouds. This paper reviews the studies on the macrophysical and microphysical properties, the connection with precipitation, the cloud radiative forcing and the problem in numerical simulations of clouds over the TP based on the CloudSat/CALIPSO data. It is pointed out that there is less water vapor above the TP, which limits the vertical height of clouds on the plateau and significantly affects the cloud thickness and the number of cloud layers. The relative contribution of single layer cloud on the cloud fraction and its seasonal variation is greater than that in other Asian monsoon regions. The convective cloud in summer is relatively shallow, the cumulus cloud occurrence frequency is the highest, and the drop spectrum in the cloud is wider over the TP. Cirrus and cumulus clouds are major cloud types that have precipitation. The cloud contribution to total precipitation decreases with the increase in the number of cloud layers. The number concentration of ice particle tends to be denser at higher altitudes when precipitation enhances. In the summer, net radiative cooling effect occurs over a layer of only 1 km thick at the height of 8 km over the TP, where the cooling is very strong. Below the cooling layer is a strong radiative heating layer located between 4 and 7 km. Finally, the future studies are proposed.
Study on the Applicability of Simulating the Atmospheric Boundary Layer in Nagqu, Tibetan Plateau Using WRF Model
Acta Meteorologica Sinica,2018,Vol 76,No. 06
【Abstract】 The WRF (Weather Research and Forecasting) Model was employed to simulate the atmospheric boundary layer characteristics over Nagqu area in the Tibetan Plateau. By comparing simulations with results from the comprehensive campaign “the Third Tibetan Plateau Scientific Experiment”, four boundary layer schemes were evaluated for their applicability over complicated underlying surface in Nagqu area. The results indicate that the YSU, MYJ, ACM2 and BouLac boundary layer parameterization schemes have cold biases in the simulation of 2 m air temperature and ground surface temperature. The BouLac scheme yields the minimum cold bias of surface temperature. By analyzing components of the energy balance, it was found that the cold bias of temperature mainly came from lower downward long wave radiation and unrealistically strong exchange of the sensible heat flux and latent heat flux. For the simulation of vertical structure of the atmospheric boundary layer, local schemes show better results of wind and relative humidity than that of nonlocal schemes. In Nagqu area, the BouLac scheme has a good effect on simulating surface temperature and vertical distribution of potential temperature and relative humidity in the boundary layer.
The climatic characteristics of summer convection over the Tibetan Plateau revealed by geostationary satellite
Acta Meteorologica Sinica,2018,Vol 76,No. 06
【Abstract】 Based on the infrared TBB of the geostationary meteorological satellite FY-2 E from 2010 to 2014, the climatic characteristics of summer convection over the Tibetan Plateau and its surrounding areas are analyzed. The analysis shows that in May, the main convection occurs in the eastern edge of the Tibetan Plateau. In June, following the onset of the Asian summer monsoon, the strongest convection (severe convection) occurs in the southeastern Tibetan Plateau. In late summer, strong southwesterly winds transport abundant moisture to the eastern and central areas of the Tibetan Plateau, leading to the formation of an active convection belt in the southeastern Tibetan Plateau. In the western plateau, the area with convection frequency greater than 6% reaches the southern plateau at about the 37th pentad, and gradually moves northward until the end of July and the beginning of August. In the central plateau, convection (severe convection) becomes active since early (mid) June, and maintains over the entire late summer with three major northward movements until reaching 34° N. Convection in the eastern Tibetan Plateau is relatively active since the beginning of May and its northward stretching time is slightly later than that over the central plateau. Two high intra-seasonal variability centers are located in the middle branch of the Brahmaputra and the southeastern plateau. Summer convective activities are unevenly distributed in these regions, which are prone to drought and flood disasters. The first leading mode of the convection frequency is the reverse mode of that in the Indian monsoon region and the southeastern plateau while the second leading mode reflects the tripolar variation pattern over the western plateau, the India continent west of 80° E and the South Asian continent east of 80° E.
Characteristic of Adiabatic and Diabatic Water Vapor Transport from the Troposphere to the Stratosphere over the Tibetan Plateau and its Comparison with the Rocky Mountains in the Summer
Chinese Journal of Atmospheric Sciences,2019,Vol 43,No. 01
【Abstract】 The Asian summer monsoon regions are mainly atmospheric composition transport pathways from the troposphere to the stratosphere, and have large contribution to the variation of the stratospheric water vapor. Previous research shows that the Tibetan Plateau (TP) and its surround regions contribute most water vapor transport from the troposphere to the stratosphere over Asian summer monsoon regions. The multi-year average Aura Microwave Limb Sounder (MLS) satellite observations, and the ERAi and MERRA reanalysis datasets are used to diagnose the water vapor maintenance and quantify the water vapor transport from the troposphere to the stratosphere over the TP and the Rocky Mountains (RM) in July and August. The three-dimensional structure of isentropic surfaces and the tropopause is favorable for adiabatic water vapor transport from the troposphere to the stratosphere over the TP. According to the quantified result from ERAi, there is a significant zonal adiabatic water vapor transport pathway from the northeast of the South Asian high to the central Pacific at 340–360 K, and the averaged water vapor mass flux is nearly 7 × 10 3 kg s −1 during July and August. Strong diabatic water vapor transport pathway is found in the southern flank of the TP at 370–380 K, which is controlled by deep convection and large-scale ascending motion, and the averaged flux is about 0.4 × 10 3 kg s −1 during July and August. Besides, at 350–360 K, there is a weak meridional adiabatic water vapor transport pathway from the Iranian Plateau to western flank of the TP, and a weak zonal adiabatic water vapor transport pathway is also found from the eastern flank of the RM to the western Atlantic, where the water vapor mass flux is about 2.5 × 10 3 kg s −1.
Chinese Journal of Atmospheric Sciences,2019,Vol 43,No. 01
【Abstract】 The Tibetan Plateau snow cover is an important land surface factor, whose time scale of change is longer than that of the atmosphere and shorter than that of the ocean. This study analyzes the influence of the Tibetan Plateau snow depth anomaly on extended-range prediction skill over extratropical regions. The reforecast data from DERF2.0 (Dynamic Extended Range Forecast 2.0) model provided by the National Climate Center of China and the daily snow depth data inversion calculated by scanning multichannel microwave radiometer (SMMR) and special sensor microwave imager (SSM/I) from 1983 to 2014 are used. The results show that the skill in extended prediction of DERF2.0 is much higher in abnormal years than in normal years, especially over regions significantly affected by snow cover in the Tibetan Plateau like the Tibetan Plateau region, Lake Baikal region and the North Pacific region. With the extension of the forecast leading time, the skill in extended prediction attenuates the slowest in more-snow years and attenuates the fastest in normal snow years. The above results show that the predictable time is longer in abnormal years of the Tibetan Plateau snow. The skill in extended prediction is improved, which can be seen from the first pentad in the Tibetan Plateau snow abnormal years, especially in more-snow years. The influence of the snow cover is obviously earlier than that of the ocean. The Tibetan Plateau snow cover has an important contribution to the skill in extended prediction, suggesting that the Tibetan Plateau snow anomaly is a potential source of prediction for extended-range prediction in East Asia.
Acta Meteorologica Sinica,2018,Vol 76,No. 06
【Abstract】 Convective systems originated from Tibetan Plateau are classified and their horizontal and vertical dynamic and thermodynamic structures are examined using the ISCCP data of tracks of convective systems, the NCEP/NCAR reanalysis data and the TRMM 3 B42 from 1998 to 2004 during northern hemisphere summer (June–August). The results reveal that convective systems mainly form over the southeastern Tibetan Plateau. Convective systems are classified into three categories: staying over the Plateau and southward and eastward moving out of the Plateau. The movement and development of convective systems are determined by surrounding environmental meteorological conditions. The convective systems with strong upward motion and plenty of water vapor may move out of the Plateau, whereas those with weak upward motion and small amount of water vapor stay and dissipate over the Plateau. The southward and eastward movements of convective systems are steered by surrounding northerly and westerly winds.
CLM4.5 Model Simulation of Soil Moisture over the Qinghai-Xizang Plateau and its Performance Evaluation
Chinese Journal of Atmospheric Sciences,2019,Vol 43,No. 03
【Abstract】 The Community Land Model version 4.5 (CLM4.5) driven by atmospheric forcing data (CRUNCEP, with a spatial resolution of 0.5° × 0.5°) is used to simulate the spatial and temporal variations of soil moisture in the Qinghai-Xizang Plateau for the period 1981–2016. After comparing and validating the simulation with the station data, two reanalysis products (i. e., the ERA-Interim and GLDAS-CLM) and the microwave remote sensing FY-3 B/MWRI data, we find that CLM4.5 can well reproduce the spatiotemporal variations and the long-term trends of soil moisture in the Qinghai-Xizang Plateau. The land surface data with a high spatial resolution (0.1° × 0.1°), which is derived from various satellite remote sensing in the Qinghai-Xizang Plateau, provides a more detailed description of the spatial features of soil moisture. The results of the comparison show that the spatial and temporal distributions of simulated soil moisture are consistent with station observations. The simulation is significantly correlated with observations in all soil layers but with positive systematic deviations. And the consistency between simulation and observations gradually decreases from the surface downward. The spatial distributions of the simulated and reanalyzed soil moisture in the Qinghai-Xizang Plateau are consistent, which are characterized by a gradual increase from northwest to southeast. The high-value areas are found in the Sanjiangyuan Wetland and the southeastern Plateau, while the dry regions are located in the Qaidam Basin and the Tarim Basin. The soil moisture generally increases from surface downward. The temporal variation of soil moisture presents a band-like alternate dry and wet pattern. There is little difference in the variation trend of soil moisture in different layers. The simulation also reasonably reproduces the monthly variations in soil moisture in summer. The soil moisture in the southwestern region increases in a wide range. The soil moisture in the southern periphery of the plateau obviously increases in summer. The dry area in the northern Qaidam Basin also markedly shrinks to the north. The simulated soil moisture by CLM4.5 provides a more detailed description of spatial distribution and monthly variations of soil moisture in the summer compared with the reanalysis data and microwave remote sensing data.
Acta Meteorologica Sinica,2018,Vol 76,No. 06
【Abstract】 In order to investigate the reliability of the observation of precipitable water (PW) in the Tibetan Plateau, in this article, we compare and analyze five different observations collected at Naqu, Gaize and Shenzha from June to September 2015. Observations include the GNSS PW, the radiosonde PW, the FY-3 visible infrared scanning radiometer sensing data (FY-3 PW), the MODIS remote sensing data and the NCEP gridded reanalysis data (NCEP PW). The result shows that the radiosonde PW and the GNSS PW are closed with the differences less than 2.5 mm. FY-3 PW is significantly drier than the other observations, and the deviations are larger than 6 mm. The degree of dispersion for PW observed by the automatic sounding systems at Gaize and Shenzha stations is obviously larger than that by the L-band electron radiosonde at Naqu Station, and the root-mean-square error (RMSE) is larger than 4 mm.
Impact of the spring sensible heat flux over the Tibetan Plateau on summer rainfall over East China and its role in rainfall prediction
Acta Meteorologica Sinica,2018,Vol 76,No. 06
【Abstract】 By using observational data from 71 stations over the central and eastern Tibetan Plateau, monthly rainfall data from 756 stations throughout China, HadISST v1.1 and ERA-Interim reanalysis data, we investigated the impacts of the spring sensible heat flux over the Tibetan Plateau along with the global SST on summer rainfall over East China. A prediction equation was established and the effect of spring sensible heat flux over the Tibetan Plateau on the prediction of rainfall in China. The present study aims to improve the prediction skill of summer rainfall over China by using the sensible heat observations over the Tibetan Plateau in the preceding spring. Results show that there is a close relationship between the spring sensible heat flux over the Tibetan Plateau and rainfall over East China. When the sensible heat anomaly over the Tibetan Plateau increases in the spring, rainfall increases over the middle and lower reaches of the Yangtze River Basin in the spring and throughout the Yangtze River Basin in the subsequent summer, whereas rainfall over Southeast China decreases. The increase of the sensible heat flux over the Tibetan Plateau is closely related to a Rossby wave train along the middle-high latitudes of the northern hemisphere. The anticyclone over the North Pacific caused by the disturbance extends southwestward to the western North Pacific and transports abundant moisture to the Yangtze River Basin, which is conducive to the development of rainfall. In summer, along with the increase in sensible heat in the Tibetan Plateau, the South Asia high pressure is located to the east, the Northwest Pacific subtropical high (WPSH) is located to the south. In the north of the West Pacific subtropical high is the cyclonic circulation anomaly. Under the control of the western Pacific subtropical high, rainfall over Southeast China is inhibited. However, rainfall over the Yangtze River Basin increases as abundant moisture is transported to this region by southerly flow to the west of the western Pacific subtropical high. The southerly flow converges with the northerly flow to west of the cyclonic anomaly over the Yangtze River Basin. The sensible heat anomaly over the Tibetan Plateau in the spring could be a key precursor signal of summer rainfall over Southeast China and the Yangtze River Basin. With its impact considered, correlations between the rainfall predictions in South China and the Yangtze River Basin and observations are increased, and the explained variance of the prediction equation for regional precipitation increases by about 15%.
Acta Meteorologica Sinica,2018,Vol 76,No. 06
【Abstract】 Based on observations of a mobile C-band dual polarimetric radar (C-POL) and the Chinese new generation weather radar deployed in Naqu (CINRAD/CD) from 30 July to 5 August 2014, two hailstorm events and their associated dynamic, thermal-dynamic and microphysical characteristics were demonstrated using wind fields retrieved from observations of the two Doppler radars and identification technique of dual polarimetric radar hydrometeors. The convective cells mostly appeared in the afternoon in the Tibetan Plateau. Although the horizontal and vertical scales of the convective cells were small, they occurred frequently and evolved rapidly, and generally lasted for tens of minutes. In the RHI (range height indicator) images of ZH, ZDR and Class, the dynamic and microphysical processes can be seen clearly. Hydrometeor particles reached higher levels following the “0 line” and grew quickly, accompanied by increases in the echo intensity, and eventually formed a hail wall dropping down on the other side of main updraft. From the consecutive three RHI scans, it can be seen that the particles changed from wet snow to hailstorm during the evolutional process in one convective cell. The height of the echo was lower and its intensity was very weak when the convective cell was initially triggered. However, large amounts of wet snow appeared above the melting level, indicating that the updraft was strong enough to transport wet snow back to higher levels before the snow completely melted below the melting level. Through physical processes such as condensation, rime and attachment, the wet snow could rapidly grow into hailstones in just over 10 min. During the re-condensation of wet snow, the unstable structure further developed while the ascending and descending motions strengthened due to the latent heat release. Therefore, the occurrence of wet snow in a newly generated weak echo region above the melting level usually indicates strong updraft and convective cell would develop rapidly.
Acta Meteorologica Sinica,2018,Vol 76,No. 06
【Abstract】 Based on reanalysis data, multiple sets of satellite retrieval data and the WRF model simulation data, a gravity wave process on 10 January 2005 was identified over the Tibetan Plateau. The impact of the gravity wave on snowfall over the western Tibetan Plateau was also studied. It is found that the gravity wave was located to the left of jet stream exit, and covered most part of the Plateau from southwest to northeast. The wave-like temperature advection pattern and the strengthening of non-equilibrium flow provided environmental condition favorable for the development of gravity wave. Wavelet cross spectrum analysis showed that vertical vorticity and horizontal divergence in the middle troposphere were consistent with the polarization state of gravity waves. Snowfall was detected over the updraft region in the western Tibetan Plateau, which can be well simulated by WRF. Simulation can also reproduce the mesoscale gravity wave that can hardly be identified in reanalysis data. Numerical simulation showed that the strong diabatic heating near the surface resulted in static instability. Convections were thus easily triggered over the warmer region, which led to latent heat release by condensation over high altitudes. The melting-induced cooling near the ground associated with convection forced the gravity wave development. The formation of solid hydrometeors took place over updraft regions, and the subsequent arrival of cold air provided favorable condition for water vapor transport. Weak large-scale ascending motion later replaced the strong updrafts that entered the snow region intermittently. Solid hydrometeors then fell onto the ground and precipitation eventually occurred over the western Tibetan Plateau.