| 1. |
- Chen, Aifang, 1990, et al.
(författare)
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Rising future tropical cyclone-induced extreme winds in the Mekong River Basin
- 2020
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Ingår i: Science Bulletin. - : Elsevier BV. - 2095-9273 .- 2095-9281. ; 65:5, s. 419-424
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Tidskriftsartikel (refereegranskat)abstract
- © 2019 Science China Press The societal impact of extreme winds induced by tropical cyclones (TCs) is a major concern in the Mekong River Basin (MRB). Though no clear trend of landfalling TC intensity along the Vietnam coastline has been observed since the 1970s, climate models project an increasing TC intensity in the 21st century over the Western North Pacific, which is the primary TC source region influencing the MRB. Yet, how future TC activities will affect extreme winds quantitatively in the MRB remains unclear. By employing a novel dynamical downscaling technique using a specialized, coupled ocean-atmospheric model, shorter return periods of maximum wind speed in the MRB for 2081–2100 compared with 1981–2000 are projected based on five global climate models under the RCP8.5 scenario, suggesting increases in the future tropical cyclone intensity. The results point to consistently elevated future TC-related risks that may jeopardize sustainable development, disrupt food supply, and exacerbate conflicts in the region and beyond.
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| 2. |
- Kukulies, Julia, et al.
(författare)
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Mesoscale convective systems in the third pole region: Characteristics, mechanisms and impact on precipitation
- 2023
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Ingår i: Frontiers in Earth Science. - 2296-6463. ; 11
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Forskningsöversikt (refereegranskat)abstract
- The climate system of the Third Pole region, including the (TP) and its surroundings, is highly sensitive to global warming. Mesoscale convective systems (MCSs) are understood to be a vital component of this climate system. Driven by the monsoon circulation, surface heating, and large-scale and local moisture supply, they frequently occur during summer and mostly over the central and eastern TP as well as in the downstream regions. Further, MCSs have been highlighted as important contributors to total precipitation as they are efficient rain producers affecting water availability (seasonal precipitation) and potential flood risk (extreme precipitation) in the densely populated downstream regions. The availability of multi-decadal satellite observations and high-resolution climate model datasets has made it possible to study the role of MCSs in the under-observed TP water balance. However, the usage of different methods for MCS identification and the different focuses on specific subregions currently hamper a systematic and consistent assessment of the role played by MCSs and their impact on precipitation over the TP headwaters and its downstream regions. Here, we review observational and model studies of MCSs in the TP region within a common framework to elucidate their main characteristics, underlying mechanisms, and impact on seasonal and extreme precipitation. We also identify major knowledge gaps and provide suggestions on how these can be addressed using recently published high-resolution model datasets. Three important identified knowledge gaps are 1) the feedback of MCSs to other components of the TP climate system, 2) the impact of the changing climate on future MCS characteristics, and 3) the basin-scale assessment of flood and drought risks associated with changes in MCS frequency and intensity. A particularly promising tool to address these knowledge gaps are convection-permitting climate simulations. Therefore, the systematic evaluation of existing historical convection-permitting climate simulations over the TP is an urgent requirement for reliable future climate change assessments.
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| 3. |
- Lin, Changgui, 1985, et al.
(författare)
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A new perspective on solar dimming over the Tibetan Plateau
- 2019
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Ingår i: International Journal of Climatology. - : Wiley. - 0899-8418 .- 1097-0088. ; 39:1, s. 302-316
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Tidskriftsartikel (refereegranskat)abstract
- © 2018 Royal Meteorological Society Solar radiation changes (dimming/brightening) have recently received growing attention within the research community, although there is currently no generally accepted explanation. This article aims to provide a new perspective for identifying the reasons behind solar dimming/brightening by using long-term measurements of direct and diffuse solar radiation, unlike previous studies which have focused on global solar radiation. We postulate that extinction processes can be more readily revealed by direct and diffuse radiation measurements with the help of a modelling tool that treats the two components separately. An example is presented for Golmud and Lhasa in the northern and southern Tibetan Plateau (TP) respectively, over the period 1957–2013. The following is found: (a) ground-based observed cloud cover alone hardly explains the observed solar dimming at the two sites; (b) both the cloud-free direct radiation transmittivity (τdir) and diffuse radiation proportion (pdif) declined; (c) variations in pdifare overwhelmingly dominated by variations in aerosols, while those in τdirare related to variations in both aerosol and water vapour; (d) on top of the suggested reduced aerosol concentrations, decreased snow cover can partly explain declined diffuse radiation via lowered surface albedo; (e) the decline in τdircan be partly attributed to the wetting atmosphere via strengthened absorption; and (f) the impact of volcanic eruptions was also identified from such radiative parameters, lasting into the following summer and especially strong in the southern TP.
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| 4. |
- Lin, Changgui, 1985, et al.
(författare)
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Impact of model resolution on simulating the water vapor transport through the central Himalayas: implication for models’ wet bias over the Tibetan Plateau
- 2018
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Ingår i: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 51:9-10, s. 3195-3207
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Tidskriftsartikel (refereegranskat)abstract
- © 2018 The Author(s) Current climate models commonly overestimate precipitation over the Tibetan Plateau (TP), which limits our understanding of past and future water balance in the region. Identifying sources of such models’ wet bias is therefore crucial. The Himalayas is considered a major pathway of water vapor transport (WVT) towards the TP. Their steep terrain, together with associated small-scale processes, cannot be resolved by coarse-resolution models, which may result in excessive WVT towards the TP. This paper, therefore, investigated the resolution dependency of simulated WVT through the central Himalayas and its further impact on precipitation bias over the TP. According to a summer monsoon season of simulations conducted using the weather research forecasting (WRF) model with resolutions of 30, 10, and 2 km, the study found that finer resolutions (especially 2 km) diminish the positive precipitation bias over the TP. The higher-resolution simulations produce more precipitation over the southern Himalayan slopes and weaker WVT towards the TP, explaining the reduced wet bias. The decreased WVT is reflected mostly in the weakened wind speed, which is due to the fact that the high resolution can improve resolving orographic drag over a complex terrain and other processes associated with heterogeneous surface forcing. A significant difference was particularly found when the model resolution is changed from 30 to 10 km, suggesting that a resolution of approximately 10 km represents a good compromise between a more spatially detailed simulation of WVT and computational cost for a domain covering the whole TP.
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| 5. |
- Lin, Changgui, 1985, et al.
(författare)
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Present and future European heat wave magnitudes: climatologies, trends, and their associated uncertainties in GCM-RCM model chains
- 2022
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Ingår i: Earth System Dynamics. - : Copernicus GmbH. - 2190-4979 .- 2190-4987. ; 13:3, s. 1197-1214
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Tidskriftsartikel (refereegranskat)abstract
- This study investigates present and future European heat wave magnitudes, represented by the Heat Wave Magnitude Index-daily (HWMId), for regional climate models (RCMs) and the driving global climate models (GCMs) over Europe. A subset of the large EURO-CORDEX ensemble is employed to study sources of uncertainties related to the choice of GCMs, RCMs, and their combinations. We initially compare the evaluation runs of the RCMs driven by ERA-interim reanalysis to E-OBS (observation-based estimates), finding that the RCMs can capture most of the observed spatial and temporal features of HWMId. With their higher resolution compared to GCMs, RCMs can reveal spatial features of HWMId associated with small-scale processes (e.g., orographic effects); moreover, RCMs represent large-scale features of HWMId satisfactorily (e.g., by reproducing the general pattern revealed by E-OBS with high values at western coastal regions and low values at the eastern part). Our results indicate a clear added value of the RCMs compared to the driving GCMs. Forced with the emission scenario RCP8.5, all the GCM and RCM simulations consistently project a rise in HWMId at an exponential rate. However, the climate change signals projected by the GCMs are generally attenuated when downscaled by the RCMs, with the spatial pattern also altered. The uncertainty in a simulated future change of heat wave magnitudes following global warming can be attributed almost equally to the difference in model physics (as represented by different RCMs) and to the driving data associated with different GCMs. Regarding the uncertainty associated with RCM choice, a major factor is the different representation of the orographic effects. No consistent spatial pattern in the ensemble spread associated with different GCMs is observed between the RCMs, suggesting GCM uncertainties are transformed by RCMs in a complex manner due to the nonlinear nature of model dynamics and physics. In summary, our results support the use of dynamical downscaling for deriving regional climate realization regarding heat wave magnitudes. © Author(s) 2022.
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| 6. |
- Lin, Changgui, 1985, et al.
(författare)
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Summer afternoon precipitation associated with wind convergence near the Himalayan glacier fronts
- 2021
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Ingår i: Atmospheric Research. - : Elsevier BV. - 0169-8095. ; 259
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Tidskriftsartikel (refereegranskat)abstract
- Little is known about the effects of glacier-air interactions on the Himalayan glacier mass balance. Until this knowledge gap is filled, a reliable projection of the future changes in the Himalayan glaciers is hardly possible. Here, we describe the drying effect of the katabatic winds on the up-valley summer monsoon flows by creating favorable conditions for local convergence-induced precipitation to occur near the glacier fronts. We postulate that this retarding effect on the up-valley monsoon flows results in a negative feedback mechanism mediated by glacier-air interactions, in which glacial retreat pushes precipitation upwards as the down-valley katabatic winds weaken, resulting in greater local precipitation and enhanced snow accumulation across the upper parts of the Himalayan glaciers. Our analyses are based on the exclusive data recorded in the Khumbu valley and the Langtang valley in the Nepalese Himalayas. These data revealed higher afternoon precipitation in summer associated with surface wind convergence near the glacier fronts and a sharp decrease in the temperature lapse rate over the glacier surfaces. The principle of the observed phenomena was proven by our high-resolution modeling sensitive experiment, which involved two simulations, one with the present glaciers and the other without. This numerical experiment also supports the proposed negative feedback. Furthermore, we report a low deuterium excess near the glacier fronts, indicating below-cloud re-evaporation facilitated by the local convergence induced by the dry katabatic winds. Our study suggests that current models may overestimate the retreat of Himalayan glaciers because they have completely ignored the glacier-air interactions. © 2021 The Author(s)
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| 7. |
- Ou, Tinghai, et al.
(författare)
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Simulation of summer precipitation diurnal cycles over the Tibetan Plateau at the gray-zone grid spacing for cumulus parameterization
- 2020
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Ingår i: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 54, s. 3525-3539
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Tidskriftsartikel (refereegranskat)abstract
- The Tibetan Plateau (TP) is often referred to as the “water tower of Asia” or the “Third Pole”. It remains a challenge for most global and regional models to realistically simulate precipitation, especially its diurnal cycles, over the TP. This study focuses on evaluating the summer (June–August) precipitation diurnal cycles over the TP simulated by the Weather Research and Forecasting (WRF) model. The horizontal resolution used in this study is 9 km, which is within the gray-zone grid spacing that a cumulus parameterization scheme (CU) may or may not be used. We conducted WRF simulations with different cumulus schemes (CU experiments) and a simulation without CU (No_CU experiment). The selected CUs include the Grell-3D Ensemble (Grell), New Simplified Arakawa-Schubert (NSAS), and Multiscale Kain-Fritsch (MSKF). These simulations are compared with both the in-situ observations and satellite products. Results show that the scale-aware MSKF outperforms the other CUs in simulating precipitation in terms of both the mean intensity and diurnal cycles. In addition, the peak time of precipitation intensity is better captured by all the CU experiments than by the No_CU experiment. However, all the CU experiments tend to overestimate the mean precipitation and simulate an earlier peak of precipitation frequency when compared to observations. The frequencies and initiation timings for short-duration (1–3 h) and long-duration (> 6 h) precipitation events are well captured by the No_CU experiment, while these features are poorly reproduced by the CU experiments. The results demonstrate simulation without a CU outperforms those with a CU at the gray-zone spatial resolution in regard to the precipitation diurnal cycles.
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| 8. |
- Ou, Tinghai, et al.
(författare)
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Wet bias of summer precipitation in the northwestern Tibetan Plateau in ERA5 is linked to overestimated lower-level southerly wind over the plateau
- 2023
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Ingår i: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 61:5-6, s. 2139-53
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Tidskriftsartikel (refereegranskat)abstract
- The Tibetan Plateau (TP), also called the Third Pole, is considered to be “the world water tower”. The northwestern TP (NWTP), which has an average elevation higher than 4800m, is an arid region where the summer precipitation is largely overestimated by the ERA5 global reanalysis product. We hypothesize that this wet bias is mainly caused by unrealistic lower-level winds that trigger strong convection over the region; it can be reduced by using a high-resolution regional climate model with a large domain that allows realistically representing interactions between the Westerlies and Asian summer monsoons. Here, downscaling using the Weather Research and Forecasting (WRF) model driven by ERA5 was conducted with a large domain (8°‒50° N, 65°‒125° E) at 9km for the period 1979‒2019 (WRF9km). Precipitation values from WRF9km and ERA5 were evaluated against satellite observations; compared with ERA5, WRF9km captured the climatological summer precipitation over the NWTP with a much-reduced wet bias. The ERA5 overestimation is mainly caused by excessive convective precipitation, likely linked to strong vertical motions over the NWTP induced by an overestimated lower-level southerly wind.
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| 9. |
- Tang, Wenjun, et al.
(författare)
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A revisit to decadal change of aerosol optical depth and its impact on global radiation over China
- 2017
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Ingår i: Atmospheric Environment. - : Elsevier BV. - 1352-2310 .- 1873-2844. ; 150, s. 106-115
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Tidskriftsartikel (refereegranskat)abstract
- Global radiation over China decreased between the 1960s and 1990, since when it has remained stable. As the total cloud cover has continued to decrease since the 1960s, variations in aerosols were suggested in previous studies to be the primary cause for variations in global radiation over China. However, the effect of aerosols on global radiation on a decadal scale has not been physically quantified over China. In this study, aerosol optical depth (AOD) data since 1980 are estimated by combining horizontal visibility data at stations in China and AOD observed by the moderate resolution imaging spectroradiometer (MODIS). It is found that the AOD exhibits decadal changes, with two decreasing periods (before the end of 1980s and after 2006) and one increasing period (from 1990 to 2006). With the derived AOD, a clear-sky model is then applied to quantify the role of aerosols in the variations in global radiation over China. The results show that aerosol direct effect cannot fully explain the decadal variations in the global radiation over China between 1980 and 2010, though it has a considerable effect on global radiation climatology. There are significant differences between the trends of clear-sky global radiation impacted by aerosols and those of all-sky global radiation impacted by aerosols and clouds, and the correlation coefficient for the comparison is very low. Therefore, the variations in all-sky global radiation over China are likely to be due to changes in cloud properties and to interactions between clouds and aerosols.
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| 10. |
- Wang, Yan, et al.
(författare)
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Evaluation of precipitable water vapor from four satellite products and four reanalysis datasets against GPS measurements on the Southern Tibetan Plateau
- 2017
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Ingår i: Journal of Climate. - 0894-8755. ; 30, s. 5699-5713
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Tidskriftsartikel (refereegranskat)abstract
- © 2017 American Meteorological Society. The southern Tibetan Plateau (STP) is the region in which water vapor passes from South Asia into the Tibetan Plateau (TP). The accuracy of precipitable water vapor (PWV) modeling for this region depends strongly on the quality of the available estimates of water vapor advection and the parameterization of land evaporation models. While climate simulation is frequently improved by assimilating relevant satellite and reanalysis products, this requires an understanding of the accuracy of these products. In this study, PWV data from MODIS infrared and near-infrared measurements, AIRS Level-2 and Level-3, MERRA, ERA-Interim, JRA-55, and NCEP final reanalysis (NCEP-Final) are evaluated against ground-based GPS measurements at nine stations over the STP, which covers the summer monsoon season from 2007 to 2013. The MODIS infrared product is shown to underestimate water vapor levels by more than 20% (1.84 mm), while the MODIS near-infrared product overestimates them by over 40% (3.52 mm). The AIRS PWV product appears to be most useful for constructing high-resolution and high-quality PWV datasets over the TP; particularly the AIRS Level-2 product has a relatively low bias (0.48 mm) and RMSE (1.83 mm) and correlates strongly with the GPS measurements (R = 0.90). The four reanalysis datasets exhibit similar performance in terms of their correlation coefficients (R = 0.87-0.90), bias (0.72-1.49 mm), and RMSE (2.19-2.35 mm). The key finding is that all the reanalyses have positive biases along the PWV seasonal cycle, which is linked to the well-known wet bias over the TP of current climate models.
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| 11. |
- Wang, Yan, et al.
(författare)
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The Formation of a Dry-Belt in the North Side of Central Himalaya Mountains
- 2019
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 46, s. 2993-3000
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Tidskriftsartikel (refereegranskat)abstract
- ©2019. American Geophysical Union. All Rights Reserved. South Asian monsoon crosses the Himalayan Mountains (HMs) and brings moisture for precipitations in the South Tibetan Plateau. A distinct dry-belt was found in the north of the central HM region, where there are the highest and steepest mountains in the world. Through in situ and remote-sensing observations and convection-permitting numerical experiments, the current study demonstrates that the formation of the dry-belt is mainly due to the depletion of water vapor when the monsoonal flow climbs the steep south slope of the HMs. The foehn phenomenon is notable over the north slope of the HMs, but the hot and dry downslope flow does not significantly reduce the amount of the precipitation; instead, it can delay the peak of the diurnal precipitation in the north side of the HMs.
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| 12. |
- Zhou, X., et al.
(författare)
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Dynamical impact of parameterized turbulent orographic form drag on the simulation of winter precipitation over the western Tibetan Plateau
- 2019
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Ingår i: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 53:1-2, s. 707-720
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Tidskriftsartikel (refereegranskat)abstract
- Sub-grid orographic drag directly acts on wind and impacts the regional water cycle through control of atmospheric water vapor (AWV) transport. The effect of turbulent orographic form drag (TOFD) on wind and precipitation is investigated in this study using the WRF model for a winter month over the western Tibetan Plateau (TP), where solid precipitation supplies large amounts of water resources. The diurnal cycle of wind components and atmospheric circulation simulated with TOFD are consistent with observations and ERA-Interim data, whereas stronger westerlies exist in the simulation without the TOFD scheme. The latter results in more zonal AWV transport from the west and more precipitation over the western TP and surroundings. The implementation of the TOFD scheme leads to reduced biases, when evaluated with two observation-based precipitation products. It is therefore concluded that this scheme has a clear dynamical control on the regional atmospheric water recharge and thus the parameterization of the small-scale orographic drag in the model helps to improve the prediction of wintertime precipitation in the western TP region.
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| 13. |
- Zhou, X., et al.
(författare)
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Evaluation of WRF Simulations With Different Selections of Subgrid Orographic Drag Over the Tibetan Plateau
- 2017
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Ingår i: Journal of Geophysical Research: Atmospheres. - 2169-897X .- 2169-8996. ; 122:18, s. 9759-9772
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Tidskriftsartikel (refereegranskat)abstract
- ©2017. American Geophysical Union. All Rights Reserved. Weather Research and Forecasting (WRF) simulations with different selections of subgrid orographic drag over the Tibetan Plateau have been evaluated with observation and ERA-Interim reanalysis. Results show that the subgrid orographic drag schemes, especially the turbulent orographic form drag (TOFD) scheme, efficiently reduce the 10m wind speed bias and RMS error with respect to station measurements. With the combination of gravity wave, flow blocking and TOFD schemes, wind speed is simulated more realistically than with the individual schemes only. Improvements are also seen in the 2m air temperature and surface pressure. The gravity wave drag, flow blocking drag, and TOFD schemes combined have the smallest station mean bias (−2.05°C in 2m air temperature and 1.27hPa in surface pressure) and RMS error (3.59°C in 2m air temperature and 2.37hPa in surface pressure). Meanwhile, the TOFD scheme contributes more to the improvements than the gravity wave drag and flow blocking schemes. The improvements are more pronounced at low levels of the atmosphere than at high levels due to the stronger drag enhancement on the low-level flow. The reduced near-surface cold bias and high-pressure bias over the Tibetan Plateau are the result of changes in the low-level wind components associated with the geostrophic balance. The enhanced drag directly leads to weakened westerlies but also enhances the a-geostrophic flow in this case reducing (enhancing) the northerlies (southerlies), which bring more warm air across the Himalaya Mountain ranges from South Asia (bring less cold air from the north) to the interior Tibetan Plateau.
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