Simulation of the Microphysical Processes and Effect of Latent Heat on a Heavy Rainfall Event in Beijing

1. Introduction

Microphysical processes play a very important role in cloud formation and development, thermal and dynamical processes，precipitation forms, and the large-scale circulation around(Zhu et al.,2006). Research on microphysical processes in precipitation has began since the 1960s and 1970s (Mason, 1971;Gu，1980). Cold and warm cloud processes both play very important role in precipitation in some study (Colle et al.，2004, Wang et al., 2007；Huang et al., 2014). Latent heat produced in microphysical processes provides the cloud and precipitation with a significant heat source, and also affects the thermal and dynamical structure of cloud and precipitation distribution(Guo et al.,1999; McGee and and van den Heever ,2014).

Beijing and its surrounding areas experienced a heavy rain from 21 to 22 July, 2012. A total of 79 people were killed and 1.9 million people across the capital city were hit by this deluge. During this process, the mean rainfall amount was 170 mm, and the maximum rainfall amount reached up to 460 mm. The rainbelt had a southwest-northeast trend and the precipitation process could be divided into warm area phase and cold front phase. There has been much research on this rainstorm in large and meso-scale (Sun et al., 2012; Sun et al., 2013), but little attention has been paid to the microphysical processes and latent heat effect in the two precipitation phases. So in this paper the contents mentioned above are talked about based on WRF simulation results.

2.Data and Method

This paper employs the meso-scale numerical model WRF with cumulus parameterization scheme BMJ and microphysical scheme WDM6 (Lim and Hong, 2010). It centered on Beijing (39.9°N, 116.38°E) and the simulation time is from 2000 BST 20 to 2000 BST 22 July, 2012. The initial and boundary field data are all from NCEP 1°´1° data by 6 hours. In order to have a better understanding of the microphysical processes in this rainstorm, we improved WRF and added the output of conversion amount in microphysical processes.

In order to have a clear idea of diabatic effect in the precipitation process, microphysical processes can be divided into two categories, with and without phase changes. The processes with phase changes include latent heat release processes (warming processes) which warm the air, and latent heat absorption processes (cooling processes) which cool the air. Warming processes include condensation, accretion, freezing, desublimation and nucleation. Cooling processes include evaporation, melting and sublimation. According to model results, the diabatic warming, cooling and net heating rate (Rw,Rc,Rt) can be calculated using the method described in Hjelmfelt et al.(1989) and Guo et al.(1999) like this:

Rw= (Lv/Cp)*(pcond(+)+prevp(+))+(Lf/Cp)*(pihmf+pihtf+pgfrz+pgacr+psacr+piacr+pgacw

+psacw)+(Ls/Cp)*(pgdep(+)+psdep(+)+pidep(+)+pigen)

Rc= (Lv/Cp)*(pcond(-)+prevp(-)+pgevp+psevp)+(Lf/Cp)*(psmlt+pgmlt+pimlt+pseml

+pgeml)+(Ls/Cp)*(pgdep(-)+psdep(-)+pidep(-))

Rt= Rw+Rc

The variables Lv, Lf, and Ls represent latent heat constant of evaporation, melting, and sublimation, respectively. Cp represents wet air specific heat at constant pressure. Pxxxx is the conversion rate of microphysical processes with the unit of kg kg^-1×s^-1.

3. Conclusion (1) The main microphysical processes in the two phases were all cold cloud processes, while warm cloud processes supplemented. The accumulated conversion amount and conversion rate in warm area phase were much larger than those in cold front phase.

(2)In the whole precipitation process, 72.4% of the total rainwater came from warm area phase and only 27.6% from cold front phase. Rainwater mainly came from cold cloud processes and little contribution was made by warm cloud processes. The main source of rainwater was melting of graupel(gmlt),73% of the total rainwater source amount. accretion of cloud water by rain (racw) and melting of snow(smlt) ranked second, accounting for 13% respectively.

(3)The warming and cooling rate in two phases had the similar changing trend, with two net heating centers at about 3km and 6km and minimum net warming rate at about melting layer for melting of ice particles. The vertical structure of latent heat was favor for the maintenance of updraft and development of cloud. Latent heat effect in warm area phase was more obvious than that in cold front phase.

(4)In two phases, the major contributor to latent heat release was all warm cloud processes. In warm area phase, warm cloud processes contributed 77.35% to the total accumulated latent heat release amount and that in cold phase was 66.56%.The increase of latent heat release in cold cloud processes(from 22.65% to 33.44%) indicated that the importance of thermal development of cloud contributed by cold cloud was improved.

(5) Cold cloud processes were the main processes of latent heat absorption, with melting of graupel and snow (gmlt,smlt) the most obvious in two phases. The latent heat absorption contribution of warm cloud processes was very little and mainly came from evaporation of rainwater. But the latent heat effect of evaporation of rainwater (revp) was significantly reduced in cold front phase than that in warm area phase.

References:

Colle, B. A., and Y. Zeng, 2004: Bulk Microphysical Sensitivities within the MM5 for Orographic Precipitation. Part I: The Sierra 1986 Event, Mon. Wea. Rev., 132, 2780–2801.

Gu, Z. C., 1980: Bases of Clouds, Fogs and Precipitation Physics (in Chinese), Science Press, 173-179.

Guo, X., H. Niino, and R. Kimura, 1999: Numerical modeling on a hazardous microburst-producing hailstorm. Towards Digital Earth- Proc. of the International Symposium on Digital Earth, Science Press, Beijing,1, 383~398.

Hjelmfelt, M. R., R. D. Roberts, H. D. Orville, et al., 1989: Observational and numerical study of a microburst line-producing Storm, J. Atmos. Sci., 46: 2731-2744.

Sun, J., N. He, G. Wang, et al., 2012: Preliminary analysis on synoptic configuration evolvement and mechanism of a torrential rain occurring in Beijing on 21 July 2012, Torrential Rain and Disasters (in Chinese), 31: 218-225.

Sun, J., S. Zhao, S. Fu, et al., 2013: Multi-scale characteristics of record heavy rainfall over Beijing area on July 21, 2012, Chinese J. Atmos. Sci. (in Chinese), 37: 705-718.

Wang, J., X. Li, and L. D. Carey, 2007: Evolution, Structure, Cloud Microphysical, and Surface Rainfall Processes of Monsoon Convection during the South China Sea Monsoon Experiment, J. Atmos. Sci., 64, 360-380.