Search

Boreal summer precipitation over the Tibetan Plateau (TP) is difficult to predict in current climate models and has become a challenging issue. To address this issue, a new analog-based correction method has been developed. Our analysis reveals a substantial correlation between the prediction errors of TP summer precipitation (TPSP) and previous February anomalies of sea surface temperature (SST) in the key regions of tropical oceans. Consequently, these SST anomalies can be selected as effective predictors for correcting prediction errors. With remote-sensing-based and observational datasets employed as benchmarks, the new method was validated using the rolling-independent validation method for the period 1992–2018. The results clearly demonstrate that the new SST analog-based correction method of dynamical models can evidently improve prediction skills of summer precipitation in most TP regions. In comparison to the original model predictions, the method exhibits higher skills in terms of temporal and spatial skill scores. This study offers a valuable tool for effectively improving the TPSP prediction in dynamical models. [ABSTRACT FROM AUTHOR]

The Tibetan Plateau (TP) impacts local and remote atmospheric circulations, wherein it mechanically and thermally affects air masses or airflows. Moreover, the TP provides a key channel for substance transport between the troposphere and the stratosphere. This study reviews recent advances in research regarding land–atmosphere coupling processes over the TP. The TP experiences climate warming and wetting. Climate warming has caused glacier retreat, permafrost degradation, and a general increase in vegetation density, while climate wetting has led to a significant increase in the number of major lakes, primarily through increased precipitation. Local and regional climates are affected by interactions between the land and the atmosphere. Namely, the TP drives surface pollutants to the upper troposphere in an Asian summer monsoon (ASM) anticyclone circulation, before spreading to the lower stratosphere. Further, the thermal forcing of the TP plays an essential role in the ASM. TP forcing can modulate hemispheric‐scale atmospheric circulations across all seasons. The TP interacts with remote oceans through a forced atmospheric response and is substantially affected by the evolution of the Earth's climate via promoting Atlantic meridional overturning circulation and eliminating Pacific meridional overturning circulation. The extensive influence of the TP is facilitated by its coupling with the ASM in the summer; whereas its winter influence on climate mainly occurs through Rossby waves. The observed increasing trends of temperature and precipitation over the TP are projected to continue throughout the 21st century. Plain Language Summary: The impact of the Tibetan Plateau on atmospheric circulation and the climate has been of great interest to the scientific community. Here, we review the literature on the mechanisms of its climate effects, to provide an overview of recent progress in the field and directives for future research. Key Points: We summarize recent advances in climate change research and observations of the land–atmosphere coupling processes over the Tibetan Plateau (TP)We highlight the impact of the TP on the global climate, including atmospheric species transport, circulation, and air‐sea interactionsWe conclude projected future climate changes over the TP and discuss future research directives for assessing the TP's global climate impact [ABSTRACT FROM AUTHOR]

Extreme precipitation events frequently occur at the southeastern edge of the Tibetan Plateau (SETP), causing severe disasters. In this study, we selected the top 100 regional extreme precipitation events over the SETP region during the period of 2001–2020, and analyzed their evolutionary characteristics of large-scale thermodynamic anomalies prior to the extreme precipitation events occurring, with the aim of exploring their precursor signals. The results show that, accompanying the wave train propagating across the Eurasian continent and reaching East Asia, the extreme events over SETP during the summer season are dominated by the background large-scale atmospheric circulations characterized by the strengthened Southern Asia high (SAH), the westward-extended Western Pacific subtropical high (WPSH), and an intensified eastern Asia trough. Additionally, an analogue of low-level vortex embedded in the background large-scale circulations is developed at least 4 days prior to the occurrence of extreme events. Under the combined effects of these anomalies, the warm and cold air converge in the SETP area. Further analysis also suggests that the upper-troposphere divergence aloft combined with lower pressures at surface level lead to the upward vertical motion of circulations, along with the enhanced water-vapor transport conveyed both by the East Asian summer monsoon and the Indian summer monsoon. All anomalies mentioned above provide the favorable environment for the occurrence of precipitation extremes in the SETP region. [ABSTRACT FROM AUTHOR]

Variations of precipitation over the Tibetan Plateau (TP) have great hydrometeorological impacts on local and surrounding areas. Focusing on the TP precipitation in the boreal summer, this study investigates the seasonal‐interannual predictions of eight dynamical models from the North‐American Multi‐Model Ensemble (NMME) using different observational data sets for verification. Results show dynamical prediction skill of the TP precipitation is regionally dependent with significantly high skills in the southwestern TP up to about three‐season ahead. Prediction skill of precipitation anomalies over the southwestern TP is significantly reliant on the models' ability in reproducing the observed relationship between the anomalous southwest‐TP precipitation and Pacific Sea surface temperature (SST). Using the different gridded observational data sets as verifications can lead to variations in prediction skills, and the multi‐observation mean tends to obtain the highest skills over the southwestern TP. These results provide a deeper understanding of dynamical seasonal‐interannual predictability of summer precipitation in TP. Plain Language Summary: The Tibetan Plateau (TP) is characterized as a strong heat source in boreal summer, largely contributed by enormous latent heat release from precipitation, influencing regional and even global climate. Reliable prediction of the summer precipitation over TP is one of the most important and challenging tasks. However, dynamical predictability of TP precipitation in models largely remains unexplored. In this study, based on different observational data sets and the eight dynamical models from the North‐American Multi‐Model Ensemble (NMME), the seasonal‐interannual predictions of summer precipitation over TP are examined, and results show obvious regional differences, with considerably higher skills in the southwestern TP. It is found that the prediction skills in the southwestern TP are closely dependent on the performance of models in catching the relationship between the Pacific SST and TP precipitation, and also on the verification data sets. These findings would deepen our understanding of dynamical seasonal‐interannual predictability of the TP summer precipitation. Key Points: Dynamical prediction skills of the summer precipitation over Tibetan Plateau (TP) are regionally dependent in North‐American Multi‐Model Ensemble, with higher skill in the southwestern TPAbility of the models in reproducing the Pacific Sea surface temperature‐precipitation relation is highly responsible for the southwest‐TP prediction skillsPrediction skills vary with the observational data sets and their mean tends to provide the highest skill scores over the southwestern TP [ABSTRACT FROM AUTHOR]

In this study, we analyze the relationships between summer afternoon low cloud cover and environmental conditions over the Tibetan Plateau (TP). Using in situ measurements, satellite data, and reanalysis, and based on theoretical analysis, we find that there is stronger thermal turbulence, lower temperature, and higher frequency of low cloud formation for the same surface relative humidity over the eastern and central TP compared with eastern China. With the same sensible heat flux, decreased air density enhances buoyancy flux, which increases the planetary boundary layer height and moisture vertical transport. At the same time, with the same near‐surface relative humidity, lower temperature over the TP decreases the lifting condensation level, which increases the probability of the air parcel reaching this level. Compared to the low‐elevation region in eastern China, these two mechanisms enhance low cloud occurrence in the afternoon over the TP. Plain Language Summary: Previous studies suggested that planetary boundary layer processes play an important role in the formation of more low clouds in the afternoon over the Tibetan Plateau (TP). However, detailed mechanisms for the differences in low cloud occurrence in the afternoon over the TP versus eastern China (low‐elevation region) remain unclear. Using in situ measurements, satellite data, and reanalysis, and based on theoretical analysis, we first quantify the more frequent low cloud occurrence over the TP, and then propose two mechanisms: (i) compared to eastern China, in the afternoon over the TP, the lower near‐surface temperature results in a lower height where condensation occurs (given the same relative humidity), and (ii) the lower air density (given the same surface sensible heat flux) results in a larger buoyancy flux, leading to a deeper boundary layer. These two mechanisms work together to increase cloud formation in the afternoon over the TP. Key Points: There are more summer afternoon low clouds over the Tibetan Plateau (TP) than over eastern China as a result of two mechanismsCompared to eastern China, with the same relative humidity, lower temperature over the TP results in a lower lifting condensation levelWith the same surface sensible heat flux, lower air density over the TP results in a larger buoyancy flux and a deeper boundary layer [ABSTRACT FROM AUTHOR]

Correct understanding of the land-surface processes and cloud-precipitation processes in the Tibetan Plateau (TP) is an important prerequisite for the study and forecast of the downstream activities of weather systems and one of the key points for understanding the global atmospheric movement. In order to show the achievements that have been made, this paper reviews the progress on the observations for the atmospheric boundary layer, land-surface heat fluxes, cloud-precipitation distributions and vertical structures by using ground- and space-based multiplatform, multisensor instruments and the effect of the cloud system in the TP on the downstream weather. The results show that the form drag related to the topography, land–atmosphere momentum and scalar fluxes is an important part of the parameterization process. The sensible heat flux decreased especially in the central and northern TP caused by the decrease in wind speeds and the differences in the ground-air temperatures. Observations show that the cloud and precipitation over the TP have a strong diurnal variation. Studies also show the compressed-air column in the troposphere by the higher-altitude terrain of the TP makes particles inside clouds vary at a shorter distance in the vertical direction than those in the non-plateau area so that precipitation intensity over the TP is usually small with short duration, and the vertical structure of the convective precipitation over the TP is obviously different from that in other regions. In addition, the influence of the TP on severe weather downstream is preliminarily understood from the mechanism. It is necessary to use model simulations and observation techniques to reveal the difference between cloud precipitation in the TP and non-plateau areas in order to understand the cloud microphysical parameters over the TP and the processes of the land boundary layer affecting cloud, precipitation and weather in the downstream regions. [ABSTRACT FROM AUTHOR]

The effects of the Tibetan Plateau (TP) and its second staircase terrain (i.e., the extension of the TP) on summer rainstorms over North China (NC) from the perspective of large‐scale water vapour transport were investigated based on the frequency of summer rainstorms from 70 observational stations in NC and National Centers for Environmental Prediction reanalysis data from 1961 to 2010. Seven rainstorm cases over NC during 1981–2016 acquired from ERA‐Interim reanalysis data and observational hourly precipitation data were selected. Two water vapour transport channels provided favourable backgrounds for rainstorms over NC. One channel flowed along the northern edge of the TP via westerlies. The other channel flowed along the eastern edges of the TP and its second staircase terrain accompanied with southerly monsoon airflow, with water vapour sources from the Bay of Bengal, South China Sea, and western Pacific. The abundant water vapour that was transported along the two channels not only offered a favourable moisture background for rainstorms over NC but also resulted in the development of water vapour flux vortexes that supplied convergence conditions for rainstorms in situ. At low levels, water vapour transport mainly flowed along the eastern edge of the TP second staircase terrain. The results from the Weather and Research Forecasting model simulation of a randomly selected rainstorm case originating from the TP to NC confirmed the vital effect of the TP second staircase terrain on rainstorms over NC accompanied with water vapour transport. [ABSTRACT FROM AUTHOR]

Abstract This study investigated the effect of convection over the Naqu region of the central Tibetan Plateau (TP) on rainstorms in the downstream areas of the Yangtze River Basin (YRB) accompanied by water vapor transport during August 15–19, 2014. The hourly maximum C-band frequency-modulated continuous-wave (C-FMCW) radar echo intensity at Naqu obtained from the Third Tibetan Plateau Atmospheric Scientific Experiment (TIPEX-III) was applied to represent local convective motion during this rainstorm process. Results based on hourly rain-gauge station data, National Centers for Environmental Prediction (NCEP) Final (FNL) Operational Global Analysis data, and Weather Research and Forecasting (WRF) simulations revealed that convection at Naqu was a strong signal over the TP. Convection over the Naqu region could impact rainstorms in the middle and lower reaches of the YRB via a three-dimensional water vapor flux vortex (WVFV) structure with high-level divergence and low-level convergence. The eastward propagation of the WVFV structure would enhance convection and thereby develop rainstorms downstream of the YRB. A FLEXible PARTicle (FLEXPART) dispersion model tracked the trajectory of air masses originating from the TP toward the middle–lower reaches of the YRB, which supported the robustness of the diagnostic results. Highlights • C-band radar echo intensity was employed to measure convection. • Convection over the Tibetan Plateau had an eastward propagation. • This convection was sustained and developed by moisture flux vortex. • The eastward movement of convection facilitated rainstorm over the Yangtze River. [ABSTRACT FROM AUTHOR]

It is well documented that the East Asian summer monsoon has been experiencing a steady weakening trend in recent decades. Because the Asian summer monsoon (including both East Asian monsoon and South Asian monsoon) is the largest and most pronounced monsoonal system in the world, its change in strength may exert a profound impact on global weather and climate systems, especially on the rainfall pattern in South and East Asia. On the other hand, as a vast elevated landmass, the Tibetan Plateau forms a huge heat source protruding into the free atmosphere. Setting against the backdrop of global climate change, whether or not does the change of this heating affect the change of Asian summer monsoon and thus rainfall distribution? Here we show that the apparent heat source over the Tibetan Plateau is closely correlated with the East Asian summer monsoonal circulation, and that the weakening of the East Asian summer monsoon is closely associated with the decreasing trend of the Tibetan Plateau apparent heat source. Further analysis indicates that the change of rainfall pattern in China in recent decades is consistent with the decreasing of the East Asian summer monsoon. [ABSTRACT FROM AUTHOR]

The gross primary production (GPP) at individual CO 2 eddy covariance flux tower sites (GPP Tower ) in Dali (DL), Wenjiang (WJ) and Linzhi (LZ) around the southeastern Tibetan Plateau were determined by the net ecosystem exchange of CO 2 (NEE) and ecosystem respiration ( R e ). The satellite remote sensing-VPM model estimates of GPP values (GPP MODIS ) used the satellite-derived 8-day surface reflectance product (MOD09A1), including satellite-derived enhanced vegetation index (EVI) and land surface water index (LSWI). In this paper, we assembled a subset of flux tower data at these three sites to calibrate and test satellite-VPM model estimated GPP MODIS , and introduced the satellite data and site-level environmental factors to develop four new assimilation models. The new assimilation models’ estimates of GPP values were compared with GPP MODIS and GPP Tower , and the final optimum model among the four assimilation models was determined and used to calibrate GPP MODIS . The results showed that GPP MODIS had similar temporal variations to the GPP Tower , but GPP MODlS were commonly higher in absolute magnitude than GPP Tower with relative error (RE) about 58.85%. While, the assimilation models’ estimates of GPP values (GPP MODEL ) were much more closer to GPP Tower with RE approximately 6.98%, indicating that the capacity of the simulation in the new assimilation model was greatly improved, the R 2 and root mean square error (RMSE) of the new assimilation model were 0.57–4.90% higher and 0.74–2.47 g C m −2 s −1 lower than those of the GPP MODIS , respectively. The assimilation model was used to predicted GPP dynamics around the Tibetan Plateau and showed a reliable result compared with other researches. This study demonstrated the potential of the new assimilation model for estimating GPP around the Tibetan Plateau and the performances of site-level biophysical parameters in related to satellite-VPM model GPP. [ABSTRACT FROM AUTHOR]

Abstract: The land surface soil moisture is a crucial variable in weather and climate models. This study presents a land data assimilation system (LDAS) that aims to improve the simulation of the land surface soil moisture and energy fluxes by merging the microwave remote sensing data and the general circulation model (GCM) output into a land surface model (LSM). This system was applied over the Tibetan Plateau, using the National Centers for Environmental Prediction (NCEP) reanalysis data as forcing data and the Advanced Microwave Scanning Radiometers for EOS (AMSR-E) brightness temperatures as an observation. The performance of our four data sources, which were NCEP, AMSR-E, LDAS and simulations of Simple Biosphere Model 2 (SiB2), was assessed against 5 months of in situ measurements that were performed at two stations: Gaize and Naqu. For the surface soil moisture, the LDAS simulations were superior to both NCEP and SiB2, and there was more than a one-third reduction in the root mean squared errors (RMSE) for both of the stations. Compared with the AMSR-E soil moisture retrievals, the LDAS simulations were comparable at the Gaize station, and they were superior at the Naqu station. For the whole domain inter-comparison, the results showed that the LDAS simulation of the soil moisture field was more realistic than the NCEP and SiB2 simulations and that the LDAS could estimate land surface states properly even in the regions where AMSR-E failed to cover and/or during the periods that the satellite did not overpass. For the surface energy fluxes, the LDAS estimated the latent heat flux with an acceptable accuracy (RMSE less than 35W/m2), with a one-third reduction in the RMSE from the SiB2. For the 5-month whole plateau simulation, the LDAS produced a much more reasonable Bowen Ratio than the NCEP, and it also generated a clear contrast of the land surface status over the plateau, which was wet in the southeast and dry in the northwest, during the monsoon and post-monsoon seasons. Because the LDAS only uses globally available data sets, this study reveals the potential of the LDAS to improving the land surface energy and water flux simulations in ungauged and/or poorly gauged regions. [Copyright &y& Elsevier]

Arctic region (AR) and Tibetan plateau (TP) are the most sensitive regions to climate change and have synchronously experienced accelerated warming over recent decades. Based on the multi-year observational data of PM 2.5 and meteorology, we investigated the modulation of AR and TP's anomalous warming on the variations of PM 2.5 pollution and regional transport over central and eastern China (CEC), including major PM 2.5 pollution regions in northern China (NC), the Yangtze river delta (YRD), central China (CC), and southern China (SC). Results show that the thermal effects of TP and AR modulated PM 2.5 pollution in different ways respectively with prominently influencing the interannual changes of PM 2.5 concentrations and heavy PM 2.5 pollution occurrences. The impacts of anomalous warming in TP and AR on PM 2.5 pollution consistently enhanced regional PM 2.5 transport from NC to YRD and SC, while the warming anomalies over AR and TP respectively enhanced and inhibited the southward transport of PM 2.5 from NC. In association with the synergistic impact of warming anomalies in AR and TP, the regional PM 2.5 transport over CEC presented the inverse patterns of the northward anomalies between NC and CC and the southward anomalies from the eastern part of NC, YRD to SC, forming the regional distribution with negative PM 2.5 anomalies in NC and CC and positive PM 2.5 anomalies in YRD and SC. Our results highlight the effect of warming anomalies in AR and TP on the PM 2.5 pollution over China with the implication of source–receptor relationship of regional transport of air pollutants. • The climate changes of AR and TP prominently alter the PM 2.5 concentrations and heavy pollution occurrences in CEC. • The warming anomalies over AR and TP respectively enhance and inhibit the southward transport of PM 2.5 in the CEC. • The synergistic effect of anomalous warming in AR and TP induces an inverse anomaly of PM 2.5 transport pattern over CEC. [ABSTRACT FROM AUTHOR]

The eastern periphery of the Tibetan Plateau (EPTP) is prone to frequent and severe Persistent Extreme precipitation (PEP) events in summer. Given the complexity of weather systems and the intricate nature of terrain over this region, the generation and development mechanisms of the PEP in the EPTP remain to be determined. In this study, the formation and persistence mechanisms are further explored from the perspective of the general features of large-scale circulations, moisture source diagnosis and the influence of steep topography by using a thermal-dynamical diagnosis method, a mesoscale numerical simulation and a Lagrangian identification of the main moisture sources. The results show that during PEP events, the corresponding atmospheric circulation systems are characterized by an anomalous Rossby wave train at middle levels, an intensified westerly jet, an eastward extension of the South Asian high and a westward extension of the western Pacific subtropical high, all of which are possible to facilitate the development of potential instability and anticyclonic convergence of water vapor transport. The evaporative moisture sources from the Southeastern Asia (SEA), Bay of Bengal (BOB), and Southern China Sea (SCS) are remarkably enhanced during PEP events, with a contribution of 56.44%, which is 28.5% higher than the climatic mean. The further sensitive experiment indicates that the steep terrain could enhance the continuous transport of positive potential vorticity and the downward propagation of upper-level isentropic potential vorticity disturbances, thereby playing an essential role in the triggering and development of PEP events. • The synoptic circulation patterns are identified for PEP events over the EPTP. • The SEA, BOB, and SCS acts as the main enhanced moisture source contributors. • The steep terrain facilitates the transport of potential vorticity disturbances. [ABSTRACT FROM AUTHOR]

Containing elevated topography, the Tibetan Plateau (TP) has significant thermodynamic effects for regional environment and climate change, where understanding energy and water exchange processes (EWEP) is an important prerequisite. However, estimation of the exact spatiotemporal variability of the land-atmosphere energy and water exchange over heterogeneous landscape of the TP remains a big challenge for scientific community. Focused on the above scientific question, a series of atmospheric scientific experiments and research programs have been conducted since the 1960s, quantitatively evaluating both the spatial distribution and the multi-timescale variation of EWEP via observation, remote sensing, and numerical simulation. Based on the three main approaches, the major advances on EWEP over the past 25 years are systematically summarized in this work. Observations reveal distinct characteristics of the energy balance components and micrometeorological parameters. The roughness length for momentum is generally one order of magnitude higher than that for heat, and a distinct diurnal cycle of the excess resistance for heat transfer (kB −1) is captured. These progresses via observations further contributed to the improvement of remote sensing parameterization and numerical simulation of EWEP, e.g., the daily sensible heat flux can be overestimated by approximately 50% using a fixed kB −1, while this overestimation can be mitigated with the observation-captured diurnal variation in kB −1 taken into consideration. Moreover, multisource (multispectral, thermal, and microwave) satellite data have been successfully used to retrieve key land–atmosphere properties, which offers a feasible way to monitor EWEP at different spatiotemporal scales: A decreasing trend of sensible heat flux and an increasing trend of latent heat flux over the TP from 2001 to 2012 were reported. Hourly data of land surface heat fluxes over the entire TP were first obtained, with root mean square errors of 76.6 W m−2 (net radiation flux), 60.3 W m−2 (sensible heat flux), 71.0 W m−2 (latent heat flux) and 37.5 W m−2 (soil heat flux), superior to the previous flux products. The total annual evaporation is approximately 51.7 ± 2.1 km3 year−1 for high-elevation lakes with ice sublimation component accounting for around 10–25%. In addition, different numerical models have been evaluated and improved to study EWEP over heterogeneous land surfaces. The simulation accuracy of land surface temperature and surface energy balance in arid and semiarid areas was improved via an improved heat roughness parameterization scheme in Noah. The sensible heat flux was also effectively improved in the CoLM model by adopting an independent method to determine aerodynamic roughness length. All these results advanced the understanding of different aspects of EWEP over the TP by using in situ measurements, multisource satellite data and numerical modeling. Future studies are recommended to focus on the optimization of the current three-dimensional comprehensive observation system, the development of applicable parameterization schemes and the investigation of EWEP on weather and climate changes over the TP and surrounding regions. [ABSTRACT FROM AUTHOR]