Convection-permitting Model Reveals New Features of Atmospheric Water Cycle over the Asian Water Tower


Future changes in precipitation on the Tibetan Plateau (TP) may have a profound influence on the ecosystem and environment of high-altitude East Asia. Despite this, the climate modeling community faces challenges in accurately simulating precipitation and the water cycle on the TP. 
One possible route to overcoming these challenges may be to use convection-permitting models (CPMs) to simulate the atmosphere over the Tibetan Plateau. Such models explicitly resolved deep convection and may improve simulate precipitation over the TP, according to a new paper published in Journal of Geophysical Research: Atmospheres and conducted by researchers from the Institute of Atmospheric Physics at the Chinese Academy of Sciences (IAP, CAS), Chinese Academy of Meteorological Sciences (CAMS) and the UK Met Office.
"Climate models have wet bias in simulating the TP precipitation, which is generally attributed to the overestimation of moisture transport through the southern edges of the TP", explains Tianjun ZHOU, the corresponding author of the study and a senior scientist at IAP.  "However, from the process-oriented perspective of atmospheric water cycle, we found that the added value of the CPM is dominated by the realistic depiction of the precipitation process and its upscale effect, rather than the reduced northward transport of water vapor".
Using a CPM based on the Met Office’s Unified Model, the research group conducted a traditional ("convection-parameterized") simulation (LSM) with the model resolution of 13.2km, and a CPM simulation with the resolution of 4.4km, both focusing on the precipitation over the TP during the summer of 2009. 
Their results show the wet bias in the simulated TP precipitation has been obviously reduced from 61% in the LSM to 14% in the CPM. They found that this improvement occurs because of the CPM converted approximately 25-percent less of the moisture into precipitation than the LSM.
Figure The schematic diagrams of the water cycle over the TP in a. LSM and b. CPM. Units: 107 kg s-1. (Image by ZHAO Yin)
This improvement in precipitation, in turn improves the simulation of atmospheric circulation, explains ZHAO Yin, the papers first author: "because the CPM treats rainfall processes in a completely different way, changes in latent heating give a more realistic simulation of the large-scale circulation over the TP in summer, which plays a key role in the moisture transport over the TP."  
"The improved circulation improves the transport of moisture into and out of the TP", added ZHAO. "Overall, the amount of moisture convergence is reduced, and this is key to the smaller wet bias in the CPM. This sheds light on what cause the precipitation biases that we see in the current generation of global and regional models: they are upscale effects of the latent-heat released by convective storms on the TP. Improving the simulation of these storm is therefore important for obtaining better prediction of future climate change in high-altitude East Asia".
This work is jointly supported by the Second Tibetan Plateau Scientific Expedition and Research (STEP) program (grant No. 2019QZKK0102), National Natural Science Foundation of China (No. 41988101, 42005039, 41875132), K.C.Wong Education Foundation and the UK‐China Research & Innovation Partnership Fund through the Met Office Climate Science for Service Partnership (CSSP) China as part of the Newton Fund.
Published article:
Zhao Y, Zhou TJ, Li PX, Furtado K, Zou LW. (2021) Added value of a Convection Permitting Model in simulating Atmospheric Water cycle over the Asian Water Tower. Journal of Geophysical Research: Atmospheres, doi:
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