Modeling that allows for convection improves simulated precipitation on the Tibetan Plateau

The Tibetan Plateau (TP) is the highest and largest plateau in the world and is widely known as “the roof of the world”, “the water tower of the world” and “the third pole”. The thermal and mechanical forces of the TP play an essential role in influencing the global climate and precipitation is one of its most important components of the water cycle.

However, accurate simulation of TP precipitation is a long-standing global challenge. Current state-of-the-art climate models tend to overestimate rainfall during the TP. The wet bias on the TP in current numerical models could be a combined result of the dynamic core of the model, inadequate physical parameterizations of the model, and relative coarse resolution of the model. Deep convection parameterization was considered to be the largest source of model uncertainty in precipitation simulation.

Due to the rapid development of high-performance computing resources, models that allow for convection (CPM), which with a horizontal grid spacing of less than 5km, are built to partially solve (rather than parameterize) convective heat transport and path to fundamental advances in our understanding of the factors that influence clouds and precipitation, have become important tools for climate research.

Recently, as part of the Climate Science for Service Partnership China (CSSP China; https: //www.metoffice.gov.UK /Research/approach/cooperation/Newton /cssp-china /index) and Convection-Permitting Third Pole (CPTP, approved by WCRP-CORDEX as a flagship pilot study; http: // rcg.gvc.GU.I know/cordex_fps_cptp /), researchers from the Chinese Academy of Sciences Institute of Atmospheric Physics, the Chinese Academy of Meteorological Sciences at the China Meteorological Administration and the UK Met Office, have jointly investigated the added value of a CPM in simulating characteristics of TP precipitation and explained the possible reasons for excessive TP precipitation in mesoscale convection parameterized models.

Their results show that two mesoscale models (MSM) have notable wet polarizations on the TP and can overestimate summer rainfall by more than 4.0mm per day in parts of the central and eastern TP. Additionally, both MSMs have more frequent light precipitation, increasing the horizontal resolution of the MSMs alone does not reduce excessive precipitation. Further investigation reveals that MSMs have a spurious precipitation spike in the early afternoon, which may be linked to a strong dependence on available convective potential energy (CAPE) that dominates wet biases.

“Here, we highlight that the sensitivity of the CAPE to surface temperatures can cause MSMs to have a spurious hydrological response to surface warming. Users of climate projections should be aware of this potential model uncertainty when investigating future hydrological changes on the TP. “said Dr. Puxi Li, the lead author of the article, a researcher from the Chinese Academy of Meteorological Sciences.

By comparison, CPM removes afternoon spurious rains and thus significantly reduces the wet bias simulated by MSMs. “The CPM also better represents the frequency and intensity of rainfall, and is therefore a promising tool for dynamic downscaling on the TP,” added Dr Kalli Furtado, the study’s second author.

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This work was recently published in Quarterly journal of the Royal Meteorological Society (https: //doi.org /10.1002 /qj.3921).