The diurnal cycle of precipitation is a vital aspect of our weather and climate and is an important metric for understanding the effects of climate change (Figure. 1). The current generation of climate models is known to struggle in terms of reproducing the diurnal cycle over East Asia. This has severe adverse consequences for the simulation of clouds, precipitation, and regional circulations. These problems are believed to arise from the fact that East Asia’s diurnal cycle is governed by a complex set of interactions between land-surface, topography and large-scale air motions, which are difficult for climate models to capture.
Figure 1. Late-afternoon convection and precipitation over land. (Image by LI Puxi)
One route to improved simulation of the diurnal cycle is to use convection-permitting models (CPMs), which can resolve small-scale processes associated with deep convective cloud better than low-resolution climate models. This approach removes many of the uncertainties associated with lower-resolution models and has been shown to improve predictions of the diurnal cycle elsewhere in the world. However, to date, there have been no season-long CPM simulation focused on EASM due to their high computational cost.
Recently, under the Climate Science for Service Partnership (CSSP China) (http://www.lasg.ac.cn/cssp/index.html), supported by the UK-China Research and innovation Partnership Fund, LI Puxi, a PhD candidate from the Institute of Atmospheric Physics, Chinese Academy of Sciences, along with his mentor Prof. ZHOU Tianjun, have collaborated with Prof. LI Jian in the Chinese Academy of Meteorological Sciences and Dr. Kalli FURTADO from the UK Met Office, to analyze a regional convection-permitting simulation, which encompasses an entire warm-season of the East Asian monsoon.
Their results show that the CPM significantly improves the simulation of precipitation. In fact, many climatologically important aspects of East Asian rainfall only emerge when simulated with the CPM. For example, the late afternoon precipitation over the Mei-yu region and southern China (Figure. 2), as well as the diurnal cycle of low-level winds over those two regions are better simulated by the CPM. The study also highlights some areas where further development of convective-scale model is also needed. For example, the CPM is shown to overestimate rainfall over central eastern China, a typical model bias of CPM which the authors show is related to excessive sensible heating flux from the surface.
Figure 2. Normalized mean diurnal cycle of summer mean precipitation amount (first column), frequency (second column) averaged over the four sub-regions from rain gauge data (OBS; black line), convection-permitting model (CPM-4p4; red line) and large-scale convection-parameterized model (LSM 13p2; blue line). The unit of x axis is local solar time in hours. (Image by LI Puxi)
"In contrast to previous real-time precipitation forecast using CPM over EASM, this is the first time in literature to investigate the ability of a season-long regional CPM continuous simulation to reproduce the diurnal cycle of precipitation, as well as the associated atmospheric circulation over EASM”, as reported by the first author of the paper Mr. LI Puxi.
Researchers plan to follow up with this study. “We are systemically investigating the added value of CPM over EASM and hope to use the CPM to deepen our understanding of physical process of some unique features of EASM”, Prof. ZHOU Tianjun, the corresponding author of the study, added.
This work is recently published in Climate Dynamics.
The UK China Research and Innovation Partnership Fund (Newton Fund)
The UK-China Research and Innovation Partnership Fund is a bilateral partnership between the UK and China. It is funded in China by a range of partners and in the UK by the Newton Fund.
The Newton Fund builds research and innovation partnerships with 17 active partner countries to support economic development and social welfare, and to develop research and innovation capacity for long-term sustainable growth. It has a total UK Government investment of ￡735 million up until 2021, with matched resources from the partner countries. The Newton Fund is managed by the UK Department for Business, Energy and Industrial Strategy (BEIS), and delivered through 15 UK delivery partners, which include the Research Councils, the UK Academies, the British Council, Innovate UK and the Met Office. For further information visit the Newton Fund website (www.newtonfund.ac.uk) and follow via Twitter: @NewtonFund.
Puxi Li, Kalli Furtado, Tianjun Zhou*, Haoming Chen, Jian Li, Zhun Guo and Chan Xiao (2018). The diurnal cycle of East Asian summer monsoon precipitation simulated by the Met Office Unified Model at convection-permitting scales. Climate Dynamics, 1-21. doi: 10.1007/s00382-018-4368-z
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