Abstract
Dynamical downscaling generally performs poorly on the Tibetan Plateau (TP), due to the region’s complex topography and several aspects of model physics, especially convection and land surface processes. This study investigated the effects of the cumulus parameterization scheme (CPS) and land-surface hydrology scheme (LSHS) on TP climate simulation during the wet season using the RegCM4 regional climate model. To address these issues and seek an optimal simulation, we conducted four experiments at a 20 km resolution using various combinations of two CPSs (Grell and MIT-Emanuel), two LSHSs (the default TOPMODEL [TOP], and Variable Infiltration Capacity [VIC]). The simulations in terms of 2-m air temperature, precipitation (including large-scale precipitation [LSP] and convective precipitation [CP]), surface energy-water balance, as well as atmospheric moisture flux transport and vertical motion were compared with surface and satellite-based observations as well as the ERA5 reanalysis dataset for the period 2006–2016. The results revealed that the model using the Grell and TOP schemes better reproduced air temperature but with a warm bias, part of which could be significantly decreased by the MIT scheme. All schemes simulated a reasonable spatial distribution of precipitation, with the best performance in the experiment using the MIT and VIC schemes. Excessive precipitation was produced by the Grell scheme, mainly due to overestimated LSP, while the MIT scheme largely reduced the overestimation, and the simulated contribution of CP to total precipitation was in close agreement with the ERA5 data. The RegCM4 model satisfactorily captured diurnal cycles of precipitation amount and frequency, although there remained some differences in phase and magnitude, which were mainly caused by the CPSs. Relative to the Grell scheme, the MIT scheme yielded a weaker surface heating by reducing net radiation fluxes and the Bowen ratio. Consequently, anomalous moisture flux transport was substantially reduced over the southeastern TP, leading to a decrease in precipitation. The VIC scheme could also help decrease the wet bias by reducing surface heating. Further analysis indicated that the high CP in the MIT simulations could be attributed to destabilization in the low and mid-troposphere, while the VIC scheme tended to inhibit shallow convection, thereby decreasing CP. This study’s results also suggest that CPS interacts with LSHS to affect the simulated climate over the TP.
Highlights
The Tibetan Plateau (TP), known by scientists as the “Third Pole”, is well recognized as exerting significant influence on regional and even global weather and climate systems through its thermodynamic and mechanical forcing (Duan et al 2012; Wu et al 2015)
The physical parameterizations in the model contain the radiation package of the National Center for Atmospheric Research (NCAR) community climate system model version 3 (CCSM3) (Kiehl et al 1996); the Rapid Radiation Transfer Model (RRTM) (Mlawer et al 1997); the planetary boundary layer (PBL) scheme developed by Holtslag and Boville (1993), which allows for non-local transport in the convective boundary layer; the large-scale cloud and precipitation scheme (Pal et al 2000), known as the SUBgrid EXplicit moisture scheme (SUBEX), which accounts for sub-grid variability in clouds; and several optional cumulus parameterization scheme (CPS), such as the Kuo (Anthes 1977), Kain–Fritsch (Kain and Fritsch 1993), Grell (Grell 1993), MIT-Emanuel (Emanuel 1991), and Tiedtke (Tiedtke 1989)
Different combinations of CPSs and land-surface hydrology scheme (LSHS) were used for the following reasons: (1) When comparing the GTP and MIT and CLM4.5 with TOP (MTP) or Grell and CLM4.5 with VIC (GVC) and MIT and CLM4.5 with VIC (MVC) simulations, in which the same LSHS was used, the climate effects caused by different CPSs could be detected; (2) Comparing the GTP and GVC or MTP and MVC simulations, in which the same CPS was used, could detect the climate effects caused by different LSHSs; (3) By further comparing the two pairs of simulations, the interactions between CPSs and LSHSs could be revealed
Summary
The Tibetan Plateau (TP), known by scientists as the “Third Pole”, is well recognized as exerting significant influence on regional and even global weather and climate systems through its thermodynamic and mechanical forcing (Duan et al 2012; Wu et al 2015). The soil waterheat physics associated with soil freezing–thawing processes in the CLM has a significant effect on surface energy flux, the overlying atmosphere, and the TP climate, and the precipitation overestimation by RCMs is appreciably alleviated by revising the soil water-heat physics (Wang et al 2016). These studies highlight the importance of the LSM in accurate RCM simulations. The objective of this study was to investigate the roles CPS and LSHS play in the simulation of regional climate and to explore how key physical processes represented by these parameterization schemes act on climate.
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