Sensitivity of high-temperature weather to initial soil moisture: a case study with the WRF model
Authors: Zeng X.-M. , Wang B. , Zhang Y. , Song S. , Huang X. , Zheng Y. , Chen C. , Wang G. 
Published: May 2014

Abstract. Using the Weather Research and Forecasting model (WRF), we investigate the sensitivity of simulated short-range high-temperature weather to initial soil moisture for the East China extremely hot event in late July 2003 via a succession of 24 h simulations. The initial soil moisture (SMOIS) in the Noah land surface scheme is prescribed for five groups of designed simulations, i.e., relative to the control run (CTL), SMOIS is changed by −25, −50, +25 and +50% in the DRY25, DRY50, WET25 and WET50 groups, respectively, with ten 24 h-long integrations performed in each group. We focus on above-35 °C (standard of so-called "high-temperature" event in China) 2 m surface air temperature (SAT) at 06:00 UTC (roughly 12:00 LT in the study domain) to analyze the occurrence of the high-temperature event. Ten-day mean results show that the 06:00 UTC SAT (SAT06) is sensitive to the SMOIS change, i.e., SAT06 exhibits an apparent rising with the SMOIS decrease (e.g., compared with CTL, DRY25 results in a 1 °C SAT06 rising in general over land surface of East China), areas with above-35 °C SAT06 are most affected, and the simulations are found to be more sensitive to the SMOIS decrease than to the SMOIS increase, suggesting that hot weather can be amplified under low soil moisture conditions. With regard to the mechanism of influencing the extreme high SAT06, sensible heat flux shows to directly heat the lower atmosphere, latent heat flux is found to be more sensitive to the SMOIS change and results in the overall increase of surface net radiation due to the increased greenhouse effect (e.g., with the SMOIS increase of 25% from DRY25 to CTL, the ten-day mean net radiation is increased by 5 W m−2), and a negative (positive) feedback is found between regional atmospheric circulation and air temperature in the lower atmosphere (mid-troposphere) due to the unique dynamic nature of the western Pacific subtropical high. Using a method based on an analogous temperature relationship, a detailed analysis of physical processes shows that for the SAT change, the diabatic processes (e.g., surface fluxes) are affected more strongly by the SMOIS change than the adiabatic process (i.e., downward airflow, or convection) in the western Pacific subtropical high in the five groups of simulations. Very interestingly, although the diabatic processes dominate over the convection process during the daytime and nighttime, respectively, they do not show to necessarily dominate during the 24 h-long periods (e.g., they are primary in the WET and CTL simulations only). It is also found that as the SMOIS decreased, the SAT06 is increased, which is largely because of the reduced cooling effect of the diabatic processes, rather than the temperature-rising effect of convection. Unlike previous studies of heatwave events at climate time scales, this paper presents a sensitivity of simulated short-range hot weather to initial soil moisture, and emphasizes the importance of appropriate initial soil moisture in simulating the hot weather.