Because of the importance of the impact of the Tibetan Plateau on atmospheric general circulations and climate across China, Asia, and even the world, Chinese and Japanese scientists jointly constructed an integrated atmospheric observing system, especially for the water vapor observation, across the Tibetan Plateau and its adjacent areas during the period of 2005-2009 under the JICA (Japan International Co-operation Agency) project (JICA/Tibet Project). The JICA/Tibet Project aims at understanding processes of the land-atmosphere interaction over the Tibetan Plateau and their impacts on the severe weather and climate over the Tibetan Plateau and the area to its east in the East Asian region. The project is designed in an attempt to alleviate impacts of meteorological disasters in these areas through improving the prediction skill. The implementation of the project has enhanced the capability of monitoring the Plateau atmosphere. The numerical forecast techniques are developed through assimilating observed data into the numerical model. Based on the investigation of observed surface energy balance, the land surface model is improved. It is found that the diurnal variation of precipitation over the Plateau is closely related with water vapor, and the latent heat release is a main factor affecting the Plateau vortex. By analyzing observed seasonal features of the tropopause, the evidence of strong stratosphere and troposphere exchange over the Tibetan Plateau is provided. It reveals that the interannual variability of summer rainfall in East China corresponds to that of vegetation index over the Plateau. The crossing hemispheric circulations driven by the thermal and mechanical forcings of the Plateau play an important role in water vapor transports not only over East Asia, but also in the global scale.
An intensive field observation was implemented for the Tibetan Plateau from early spring to the mature season of the Asian summer monsoon in 2008. Atmospheric conditions in the early spring and the pre-monsoon season are investigated in detail using radiosonde observation data. In early spring, atmospheric profiles show unstable stratification, which can result in strong dry convection. In the mixing layer developed by convection, the potential temperature (PT) increases significantly during the day near the surface. On the other hand, there is no clear tendency in the variation in PT above the mixing layer. In the pre-monsoon season, there is a clear increasing trend of PT at each level from the surface to the upper troposphere. The largest heating amount and trend are observed in the upper troposphere. Since cumulus activities become less frequent in the pre-monsoon season, the upper tropospheric heating in this season cannot be caused by latent heat release, as noted in past studies. On the other hand, the profiles of PT show quite stable stratification and the mixing layer is shallow in this period. Under such a condition, the remarkable heating in the upper layers cannot be due to the development of the mixing layer. In past studies, a dry convection induced by strong surface heating is thought to be a possible cause of atmospheric heating over the Tibetan Plateau in the pre-monsoon season. However, the observation results reveal that such a locally induced convection is not enough for the significant heating. In addition, the temporal variations of PT at the eastern and western part of the plateau show some similarity. This similarity and the characteristics in dry and wet convections denote some synoptic scale processes may have important roles in the heating over the Tibetan Plateau in the pre-monsoon season.
The Sichuan Basin (SCB), located east of the Tibetan Plateau, is surrounded by mountains. This study examines the influence of terrain and weather events on the diurnal cycle of the planetary boundary layer (PBL) and the vertical structure of the atmosphere over the SCB, by utilizing field data obtained mainly from Wenjiang Station (western SCB) for the period 20 June-19 July 2008. The results show inversion layers of specific humidity and prevailing wind directions in the lower troposphere from the southwest and northeast. Migration of the 500 hPa trough to the western SCB results in the formation of low-level jets and cyclonic circulation, induced by terrain obstruction of northward airflow entering the basin from its southeast corner. Migration of the 500 hPa trough to the west (east) of Wenjiang Station results in an abruptly decreased (increased) surface downward short-wave radiation flux, which is associated with an increased (decreased) cloud fraction. In turn, this results in decreased (increased) upward sensible and latent heat fluxes, and a decrease (increase) in the elevation of the mixing layer (ML). Rapid increase in the ML height depends not only on the intensity of nocturnal temperature inversions, but also on the stability of the lower troposphere. At Wenjiang Station, the period prior to rainfall events is marked by increasing water vapor, instability and wind speed, and the wind direction changes from westerly to easterly. Subsequent to rainfall events, the PBL is rapidly restored to its normal state. The PBL is an important source of water vapor for rainfall. An increase in water vapor in the lower troposphere during night-time contributes to nocturnal rainfall. Basin terrain may play an important role in the formation of the southwest China vortex.
This paper presents the characteristics of air-lake-land interactions, which arise from the different thermal properties of lake and land surfaces, as observed at Erhai Lake in Yunnan Province, China. The heat balance over the lake was estimated by using data on the lake surface obtained from the Erhai automatic weather station and solving the surface heat balance equation (HBE) for a mixed layer. Meteorological station data were also used to estimate the long-term heat balance. The surface temperature from in situ observations was compared with that from satellite observations, and the two results were found to be in reasonably good agreement. The satellite data were used to validate the heat balance calculation. The mixed layer depth of Erhai Lake was 6 m in 2008 and 8 m from 2003 to 2008. Lake heat storage showed large annual variation of 50 W m-2 in amplitude, with a maximum from March to April and a minimum in November. The phases of the latent heat, the sensible heat, and the difference between surface and air temperatures tended to be delayed on the lake as compared with the land. The phase lag in the seasonal variation of evaporation was found to clearly depend on lake depth. The daily change in convection over the lake-land system was greatest in the daytime over the mountainous area and in the nighttime over the lake area. These features arose from differences in the thermal properties of the lake and land surfaces. Non-dimensional evaporation increased with increasing annual mean air temperature, and there was no systematic difference between regions. Nearly 92% of solar radiation was absorbed by the lake within a depth of 2 m. From this finding, the daily range of the lake’s surface temperature was quantitatively determined to be about 5°C.
The middle reaches of Yarlung Zangbo River (YR) and its two tributaries is a region in central Tibet Autonomous Region where the industrial and agricultural economy is most concentrative and developed. Remote sensing of land surface parameters has significant meaning for crop growth monitoring, crop yield assessment and disaster monitoring. It also has great theoretical importance for understanding energy and water cycle in local area and even for surrounding regions. In this paper, four scenes data in 2003 under nearly clear weather conditions are selected as the spring, summer, autumn and winter cases. Different algorithms for AVHRR and MODIS data will be applied to retrieve land surface temperature (LST), albedo, Normalized Difference Vegetation Index (NDVI), vegetation coverage, emissivity, and water vapour content, respectively. The derived LST, albedo are validated with field measurements and MODIS products. The estimations are in good agreement with in-situ measurements, with MODIS retrievals being slightly better than those of AVHRR. This means that the adopted remote sensing retrieval algorithms are not only applicable but also simple for the study area. As called the “water vapour inflow corridor”, the valleys of YR and its two tributaries are proved to have higher LST and larger atmospheric water vapour content. Spatiotemporal variations of land surface parameters in the middle reaches of YR and its two tributaries can also be clearly identified.
The Atmospheric Infrared Sounder (AIRS) on board the Aqua satellite provides the estimate of precipitable water vapor (PWV ), which can be assimilated into numerical prediction models to improve precipitation forecasts. In this study, the AIRS retrieval of PWV is evaluated against ground-based GPS measurements at 24 stations over the Tibetan Plateau (TP) and its surroundings. First, the PWV estimates from the GPS delay signals at these stations are improved by applying a new scheme for computing the water-vapor-weighted mean atmosphere temperature, a key parameter in the GPS PWV retrieval. Compared with a traditional retrieval, this revision can improve the PWD estimate by up to 6% in some cases. Second, the newly retrieved PWV data are used to evaluate the AIRS product. Prior to the evaluation, an elevation correction is made to these GPS PWV data to account for the elevation difference between the GPS stations and the corresponding AIRS footprints. This correction effectively removed false negative biases in the AIRS product. Nevertheless, an average negative bias of 2 mm still exists in the AIRS product. A further analysis suggests that the negative bias may be attributed to the cloudclearing algorithm in the AIRS retrieval scheme.
The influences of biomass, snow grain size and frozen ground covered with snow are problems that should be solved in the estimation of snow quantity when using a snow retrieval algorithm. This paper focuses on the latter problem in that algorithm: the influence of frozen ground covered with snow. The algorithm is developed for the ground station BJ, located in a region of seasonally frozen ground at low altitude on the Tibetan Plateau. An improved radiative transfer model (Improved RTM) with a frozen soil layer is developed for the station BJ on the seasonally frozen ground at low altitudes on Tibetan Plateau. On the other hand, the detection method of the apparent freezing signal of soil and the signal of apparent snow based on the microwave were developed because two problems, which are the estimation error caused by the incorrect detection of snow in the period without snow cover and the estimation error caused by the apparent snow cover being undetectable, were found out. These errors were discovered upon comparative verification of estimated snow depth, based on the normal snow retrieval algorithm, and in situ (measured) snow depth. An enhanced retrieval algorithm for BJ station is then created, by introducing the improved RTM and the detection method into the normal retrieval algorithm. Output from this improved algorithm for BJ was then compared with in situ snow depth. It was found that the latter depth was much larger than estimated snow depth. The former was too large because of snowdrift; it was therefore corrected to a more realistic depth, using a decrement proportion based on the in situ observation. A comparison of estimated snow depth from the improved snow retrieval algorithm and the more realistic in situ snow depth was then done, thereby evaluating the effectiveness of the algorithm. This verification showed good agreement.
“Cold source” forcing effect of winter Tibetan Plateau snow cover (TPSC) on the subsequent summer atmospheric circulation anomalies over East Asia is a hot issue concerned by atmospheric scientists. Based on a statistical analysis on the mean and anomalies of snow cover (SC) depth observations at 113 stations across Tibetan Plateau, 51 stations in the eastern Tibetan Plateau (TP) were identified as the “strong signal” key stations of TPSC and the area wherein the key stations are distributed as the key area of TPSC. It was found from a comparative analysis with the winter SC areas and days of 1997-2010 from NOAA satellite products that the overall changes in winter SC at the 51 stations could not only possess a “strong signal” character but also represent the variation of whole TPSC. The distinctive differences between key area high and low SC years were found in winter (DJF)/spring (MAM) TP Q1 (apparent heat source) and Q2 (apparent water vapor sink) as well as the subsequent summer (JJA) full column water vapor flux and satellite remote sensed equivalent blackbody temperature (TBB). This paper also investigated the correlation between precursory “strong signal” character of winter SC condition in the key area and the subsequent summer water vapor transport structure in East Asia including China. Significant negative differences of winter Q1 between high and low SC years over the key area of TPSC on the height-longitude section along 32.50 N clearly show that the larger winter TPSC forcing the local atmosphere built a “cold source column” structure. This study also reveals that the summer water vapor flow confluence over the mid-lower reaches of the Yangtze River moved southwards (northwards) following the high (low) winter SC of the key area. The winter TPSC conditions were significantly correlated with the structure of subsequent summer moisture flow, i.e., the winter TPSC conditions might result in the south-north shift of the subsequent summer Meiyu system and the Western Pacific subtropical high system from their normal positions. The winter TPSC depth was closely related with the summer precipitation not only in eastern China, but also over the areas of Japan, South Korea, and Russian Far East. The above conclusions also suggest that the variations of TPSC condition and its cold source intensity might lead to the anomalous changes of East Asian climate.
The accuracy of 0-36 h real-time flood forecasting is largely determined by the quantitative precipitation forecasts (QPFs), but convective weather remains a significant challenge for numerical weather prediction systems. Therefore, it is crucial to improve the QPFs’ accuracies to predict and prevent flash flood disasters. A coupled atmosphere-hydrology system with the WRF model (together with a three-dimensional variational data assimilation system, 3DVAR) and a distributed biosphere hydrological model (WEB-DHM) is described. This system was then applied to the flood forecasting of the Nanpan River Basin (Yunnan province, China) for 1 July 2008. Based on the available observations (230 surface meteorological sites, 10 conventional Radiosonde sites, and 8 ground-based GPS stations), a series of experiments were conducted with the WRF-3DVAR to investigate the contributions of different observations to QPF accuracy and flood forecasting. Forced with the observations or the WRF model outputs, WEB-DHM predicts stream-flow at the basin outlet. The overall better performance by the assimilation experiments over the no assimilation case has been clearly demonstrated in the predictions of the 0-36 h heavy rainfall (magnitude and spatial pattern) and flash flood occurrence ( peak value and time). The WRF-3DVAR only assimilating GPS data performs poorly, showing the necessity to improve both the assimilation technique and the spatial resolution for the operational numerical weather forecasts. To our knowledge, this work is the first to utilize comprehensive observations around the Tibetan Plateau with a coupled atmosphere-hydrology system to improve short-term flood predictions.
The one-dimensional structure of a daytime planetary boundary layer (PBL) with shallow cloud development was captured by intensive sonde observation during the late winter season at the central Tibetan Plateau (TP), and its relationship to patchy snow cover conditions was revealed. The diurnal change of potential temperature was evident in the atmosphere up to 1 km above ground, indicating PBL development, and frequent cloud formation in the afternoon and night over the PBL was confirmed by the increase of relative humidity in the sonde data profile and abrupt decrease of brightness temperature in the satellite images. Day-to-day changes of PBL and nighttime stable layer developments were dependent on the speed of the sub-tropical jet stream prevailing 5 km above the surface and the changing of snow cover conditions after snowfall events. Numerical simulations confirmed that the increase of land-surface albedo, imitating the continuous snow covers, could suppress the PBL and cloud development. The positive feedback of land-atmosphere interactions through the PBL development on patchy snow cover and re-distribution process of shallow and dry snow cover were discussed.
The Tibetan field observatories, which are constructed in a project of the Japan International Co-operation Agency in 2007, provide an opportunity to establish four observation sites equipped with same sensors on and around Tibetan plateau for better understanding the monsoon climate features over the Tibetan plateau. By using same analyzing technique, the present paper examines the seasonal and diurnal variations in (1) radiation components, (2) turbulent momentum, heat, water vapor, and CO2 fluxes, and (3) soil temperature, moisture, and heat flux of the four homogenous sites (BJ at Bu-Jong (Naqu), DL at Da-Li, LZ at Lin-Zhi, and WJ at Wen-Jiang) on and around the Tibetan Plateau, in 2008. Seasonal variation in the sensible heat flux at the BJ site indicate that the sensible heat flux was stronger thereat compared with other sites in spring (March to May). After the monsoon had set, sensible heat flux decreased in all four sites. On the other hand, the seasonal variations in latent heat flux suggest that the latent heat flux increases rapidly with the water content in the soil that rises from March to June at the LZ, DL, and WJ sites. An annual cyclic pattern of CO2 flux at sites DL and WJ was obvious. Finally, At the sparsely vegetated BJ site the seasonal variation in CO2 flux was small due to the limited CO2 absorption in summer and its subsequent limited release in other seasons; and at the very grassy LZ site, the CO2 absorption responds to seasonal march of monsoon. Energy partitioning dramatically responds to the onset of monsoon at the BJ site.
Surface energy-balance closure in a typical gully region of the Loess Plateau on the eastern edge of Qinghai-Xizang Plateau were studied based on the turbulence and radiation data observed at the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL) in a period of 6 consecutive sunny days for each month from June to August 2008. The results show that the sensible heat flux considering the vertical advection item of energy transport to the energy budget equation (Hec + Hmv) in the gully region on summer sunny days accounted for 39.4% of net radiation, and the surface soil heat flux considering the heat storage term from the surface to the flux-plate (G + ΔG) for 29.6%, and the latent heat flux (LE) account for 26.5%, after all corrections. The closure rate of surface energy for the typical environment of the Loess Plateau reached 93.2% and was higher during the day than at night. Consideration of soil heat storage (ΔG) and the vertical advection effect (Hmv) significantly improved the surface energy closure rate. It is seen that in the typical semi-arid Loess Plateau region, even in the rainy season, sensible heat is still dominant over various components of surface energy expenditure.
Turbulence measurements at 3 m performed at the base camp of Mt. Everest during the spring of 2005 were used to study the atmospheric turbulent characteristics under conditions of down-slope ambient wind (katabatic wind). The cases where large-scale forcing resulted in a down-slope ambient wind were considered. Firstly, the normalized standard deviations of wind speed (u and v components) were larger than those reported in the literature. Then the (co)spectral characteristics of turbulence under near neutral stratification were described depending on the prevailing wind; case A−southerly with wind speed larger than 6 m s-1 during daytime, case B−southerly with wind speed less than 6 m s-1 and case C−northerly wind. The analysis of the averaged spectra and co-spectra revealed that low frequency perturbations had a large influence on the variance of u and w wind components, and also altered the co-spectra of momentum and sensible heat flux under near neutral stratification. The features of spectral shape and power were different from the previous ones under ideal flat and homogeneous conditions. The possible cause of these features is due to strong down-valley flow induced by (normal) glacier wind.
Land surface pattern and the quality of turbulent data exert important influences on calculations of surface momentum and heat fluxes. This article assesses the excellence of quality-controlled data, which are at Wenjiang station in April, July, and September 2007 and January 2008, and uses high-quality data to analyze the seasonal changes of flux footprint and calculate the aerodynamic roughness length z0. We use total flux data to analyze the seasonal changes of fluxes for different underlying surfaces. Footprint analyses show that the distribution of flux contribution in source regions is related to not only wind directions but also underlying surfaces. An estimate of z0 is near 0.9, but the value of z0 is close to 0.05 when wind direction is near NNW. There are obvious seasonal changes in heat fluxes, but not in momentum flux.
Integrating the clear-sky NOAA/AVHRR satellite data with station meteorological observations, we conduct a remote sensing estimation of surface energy fluxes of the Sichuan Basin based on surface energy balance. The correlation between the estimation of satellite pixel and the ground-based observed value of Wenjiang Atmospheric Boundary Layer Observation Station is analyzed to verify the satellite remotely-sensed surface energy flux. Result indicates that the distributions of satellite remotely-sensed surface energy fluxes are consistent with those of the underlying surface. There exist remarkable positive correlations among the satellite-estimated net radiation flux, soil heat flux, sensible heat flux, and the ground-based observed values of key components. All correlation coefficients significantly exceed 0.9. Hence, the remotely-sensed estimations of net radiation flux, soil heat flux, and sensible heat flux are reliable to a certain extent. Since the surface energy balance at Wenjiang test site is usually unclosed, the correlation coefficient between the satellite pixel latent heat flux estimated by surface energy balance method and the value measured by eddy covariance method of the key components is only 0.717. If the unclosed value of surface energy balance is taken into consideration, the correlation coefficient increases to 0.945, meaning that the deviation between the two decreases obviously. It is thus unadvisable to use the residual of surface energy balance equation to estimate the latent heat flux in the Sichuan Basin, because the unclosed energy can cause large errors. Closure of surface energy is required when the residual method is applied.
Using wind and temperature data with high vertical resolution obtained from Global Positioning System (GPS) radiosondes which were launched four times a day at Gerze (32.2°N, 84.4°E) and Litang (30.0°N, 100.3°E) stations during the People’s Republic of China (PRC)-Japan cooperative JICA project in 2008, the characteristics of gravity waves such as vertical energy propagation directions, intrinsic frequencies, vertical wave-lengths, and horizontal propagation directions over the Tibetan Plateau are investigated by hodograph method. The statistic characteristics of the gravity waves show that the activities of the gravity waves over the western and eastern Tibetan Plateau have significant spatial variations except for horizontal propagation directions, and that the gravity waves over the eastern Tibetan Plateau are easier to be triggered. The energy propagates upward for almost half of the observed gravity waves, and the energy propagation is downward for the other half of the gravity waves, indicating that the wave sources in lower and upper troposphere are equally important over the Tibetan Plateau. The dominant gravity wave frequencies lie in the frequency range of 2-2.5 times of Coriolis parameter f at Gerze station, while the dominant gravity wave frequencies are in the range of 1.5-2 times of f at Litang station. The dominant vertical wavelengths are 1.5-2.5 km over Gerze and Litang. For Gerze station, the dominant gravity waves propagate along the west-southwestward directions, while the dominant gravity wave propagating directions are north-northwestward for Litang station.