Convection-permitting modelling improves simulated precipitation over the Tibetan Plateau

November 04, 2020

The Tibetan Plateau (TP) is the highest and most extensive highland in the world, and is widely known as "the Roof of the World", "the World Water Tower" 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 water-cycle components.

However, accurately simulating precipitation over the TP is a long-standing worldwide challenge. Current state-of-the-art climate models tend to overestimate precipitation over the TP. The wet bias over the TP in current numerical models could be a combined outcome of the model's dynamical core, inadequate model physical parameterizations and relative coarse model resolution. The deep convection parameterization has been regarded has the largest source of model uncertainty in simulating precipitation.

Due to the rapid development of high performance computing resources, convection-permitting models (CPMs), which with horizontal-grid spacing of less than 5 km are constructed to partially resolve (rather than parameterize) convective heat and moisture transport, and thereby offer a path towards fundamental advances in our understanding of factors influencing clouds and precipitation, have become important tools for climate research.

Recently, under the Climate Science for Service Partnership China (CSSP China; https://www.metoffice.gov.uk/research/approach/collaboration/newton/cssp-china/index) and Convection-Permitting Third Pole (CPTP, which was endorsed by WCRP-CORDEX as a Flagship Pilot Study; http://rcg.gvc.gu.se/cordex_fps_cptp/), researchers from the Institute of Atmospheric Physics at Chinese Academy of Sciences, the Chinese Academy of Meteorological Sciences at China Meteorological Administration and the UK Met Office, have jointly investigated the added value of a CPM in simulating precipitation characteristics over the TP, and explained the possible reasons for excessive precipitation over the TP in the mesoscale convection-parameterized models.

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

"Herein, we highlight that the sensitivity of CAPE to surface temperatures may cause the 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 over the TP", said Dr. Puxi Li, the paper's lead author, a researcher from the Chinese Academy of Meteorological Sciences.

By comparison, the CPM removes the spurious afternoon rainfall and thus significantly reduces the wet bias simulated by the MSMs. "The CPM also better depicts the precipitation frequency and intensity, and is therefore a promising tool for dynamic downscaling over the TP", Dr. Kalli Furtado, the second author of the study, added.
-end-
This work was recently published in the Quarterly Journal of the Royal Meteorological Society (https://doi.org/10.1002/qj.3921).

Institute of Atmospheric Physics, Chinese Academy of Sciences

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