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Radiative Effects on Precipitation: a South China Storm Case Study

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Radiative Effects on Precipitation: a South China Storm Case Study Journal of the Meteorological Society of Japan, Vol. 84, No. 4, pp. 691--704, 2006 691 Radiative Effects on Precipitation: A South China Storm Case Study Guanqiang ZHOU Shanghai Typhoon Institute/Chinese Meteorological Administration, Shanghai, P.R. China Department of Atmospheric Science, School of Physics, Peking University, Beijing, P.R. China Chunsheng ZHAO Department of Atmospheric Science, School of Physics, Peking University, Beijing, P.R. China Ying DUAN Hebei Meteorological Bureau, Hebei, P.R. China and Yu QIN Department of Atmospheric Science, School of Physics, Peking University, Beijing, P.R. China (Manuscript received 1 July 2005, in final form 27 April 2006) Abstract In this paper the effects of radiation on a meso-scale precipitating system is investigated during a severe storm in South China on June 8, 1998. This was done by using the Pennsylvania State University (PSU)/National Center for Atmospheric Research (NCAR)/Meso-scale Model 5 Version 3 (MM5_V3) after the introduction of radiative transfer schemes that are able to treat water clouds, ice crystals, snow, and groupel. The results suggest that the rainfall patterns do not differ too much for the various radiation schemes used in the numerical calculations, but rather influence the rainfall intensity in the central areas. The radiative effects on precipitation are more significant during daytime, as compared to night- time. The diurnal variation of rainfall is enhanced by the radiative processes. Computed precipitation intensities, and radiative cooling/heating rates, are dependent on the specific radiative transfer scheme used. The results suggest that model improvement of daytime cloud radiative processes is crucial for a better representation of such effects on meso-scale precipitating system. 1. Introduction key component. Previous research indicates that radiation and cloud microphysics are in- The radiative transfer process is one of the terdependent (Ramaswamy and Detwiler 1986; most important physical processes in the atmo- Pradelle and Cautenet 2002). On the one hand, sphere, and cloud-radiation interaction is the radiative transfer processes change the atmo- spheric thermal state through scattering, ab- Corresponding author: Chunsheng Zhao, Depart- sorption, emission by gases and particles, which ment of Atmospheric Science, School of Physics, then modifies the dynamical structures and Peking University, Beijing, 100871, China. E-mail: [email protected] microphysical processes (coagulation, gelation, ( 2006, Meteorological Society of Japan collision and coalescence etc.) in clouds. For ex- 692 Journal of the Meteorological Society of Japan Vol. 84, No. 4 Table 1. Summary of the past modeling results, the percentages of increase or decrease in precipita- tion due to the longwave (LW) effects are against with the run without radiation, while those due to the shortwave and longwave (LW and SW) are against with the result due to LW except for marked by * (against with no radiation run) and # (ratio of no radiation to radiation). NA is Not Available and no is no such study in that paper. Integrated LW LW and Region and model Study time only SW Dimension Midlatitudes, cloud Chen and Cotton (1988) 4 h 0% no 2d resolving Tripoli and Cotton (1989) 16 h NA NA 2d Chin (1994) 8 h 11% À7% 2d Tao et al. (1996) 12 h 8% À6% 2d Tropics, cloud resolving Chin et al. (1995) 10 h 15% À18% 2d Fu et al. (1995) 12 h 5% À10% 2d Xu and Randall (1995) 15 d NA NA 2d Tao et al. (1991) 8 h 20% no 2d Tao et al. (1996) 12 h 36% À7% 2d Dharssi et al. (1997) 16 h 30% no 2d Tropics, regional Dudhia (1989) 18 h no 36%# 2d Churchill and Houze (1991) steady state 0% 0% 2d Miller and Frank (1993) 24 h no 18–21%* 2d ample, radiative cooling increases the relative et al. (1995) examined all of these and one of humidity in the atmosphere, and benefits the their conclusions confirmed the destabilization production of liquid and solid water substances. of the tropical environment by IR cooling Furthermore, these radiative processes affect (1st mechanism). Tao et al. (1996) performed a the radiant flux down to the surface, changing comprehensive study of cloud-radiation mecha- the surface temperature and hence the convec- nisms in the tropics and midlatitudes, by using tion, especially near the ground. As a result, the Goddard Cumulus Ensemble (GCE) model. the vertical development of clouds is modified. They emphasized that large-scale radiative This is the indirect effect of radiation on cloud. cooling is the dominant process for surface pre- Also, the changed cloud microphysical proper- cipitation enhancement; the cloud-top cooling ties make the cloud radiative properties differ- and cloud-bottom warming mechanism effect is ent. Therefore, radiation processes and cloud slight, and the differential cooling between the processes interact and produce the variation of cloudy and clear regions has little effect on pre- other atmospheric processes and weather situa- cipitation enhancement. But other research, tions. The study of the interaction between such as Xu and Randall (1995), indicated that clouds and radiation is necessary for the prob- the 2nd mechanism is the dominant factor that lems of weather forecasting and climate change. affects precipitation. Generally, the 1st mecha- Previous research results show that radiative nism enhances precipitation by increasing the transfer processes play an important role in relative humidity, the 2nd mechanism modifies surface precipitation (Tao et al. 1996; Fu et al. precipitation by changing convection and the 1995; Dudhia 1989 and etc., see in Table 1). effect of 3rd mechanism is much weaker. Three mechanisms are suggested in the former One can also find in Table 1 that the modified studies based on modeling: (1) large scale long- ratios of the precipitation in different studies, wave cooling (Dudhia 1989), (2) IR cloud-top which employ different models, case study cooling and cloud-bottom warming (Chen and date, geographic position selection, etc, are Cotton 1988; Ackerman et al. 1988; Lilly 1988) significantly different. The comparative study and solar radiation cloud-top heating and (3) by Kay et al. (2001) suggests that there is the secondary circulation caused by horizontal significant disagreement in accuracy among ra- differential radiative heating between cloudy diative transfer schemes concerning the radia- and clear regions (Gray and Jacobson 1977). Fu tive transfer properties. Another point, which August 2006 G. ZHOU et al. 693 should be focused on, is that most of the studies cooling/heating of atmosphere is ignored, that are two-dimensional and based on cloud resolv- is ðqT/qtÞrad ¼ 0. A simple radiative cooling ing models. These methods of model structure rate (KdÀ1) algorithm is provided for ‘Simple’, and application are advantageous for research so that ðqT/qtÞrad ¼1:8 À 0:017ðT À 273:16Þ, of thermal dynamic processes, but not good where T is the atmospheric temperature in enough for real-time simulation or prediction. unit K. Because of the shortage of downward This paper is focused on the effects of radiative longwave and shortwave surface radiant fluxes processes on three-dimensional meso-scale pre- for the boundary layer energy budget in option cipitation, and the differences derived from ra- ‘None’ and ‘Simple’, a surface radiative scheme, diative transfer schemes. which is based on atmospheric column inte- The South China severe storm case on June grated vapor, and the low/middle/high cloud 8th, 1998, is selected for this study. The obser- fraction derived from the relative humidity, is vational data (shown in Fig. 1f ) indicate that employed to supply the diurnal cycled down- the precipitation has a northeast-southwest ward shortwave and longwave radiant fluxes pattern and two strong rainfall centers, Wu- to the surface. The general introduction for the zhou center (C1) near (23.5N, 111E) with a Cloud, CCM2 and RRTM radiative transfer 24-hour rainfall amount of 120 mm, the Pearl schemes follow (also summarized in Table 2). River Delta center (C2) near (22N, 114.5E), more than 175 mm, and a relatively weak 2.1 Cloud radiative transfer scheme center (C3) near (26.5N, 118E), about 60 mm For shortwave radiation, water vapor is a within 24 hours. The synoptic analysis shows unique absorber and the absorption calculated that a quasi-stationary frontal system con- as a function of its path; allowing for solar ze- trolled the weather over South China, and the nith angle changes (Lacis and Hansen 1974). drainage area of the Yangtze River. C1, located Clear-air and cloud scattering are both in- within the front and its precipitation, was pro- cluded. Clear-air scattering is taken to be uni- duced by a low pressure vortex and is convec- form and proportional to the air mass path tive, C2 located in the warm sector and rainfall, length, again allowing for variable solar zenith was produced by a low level jet. More informa- angle, with a constant scattering of 10% for the tion about detailed description could be found whole atmosphere. All cloud and precipitation in Sun (2002). are treated as one type of cloud, and the cloud In this paper, the model and radiative fraction is either 1 or 0 in each grid box; the schemes are described in section 2. Section 3 cloud back-scattering or albedo and absorption describes the design of an idealized study and are bilinearly interpolated from tabulated func- the analyses of the results. Section 4 provides tions of m and lnðw/mÞ (where m and w are the a discussion on a variety of different runs, and cosine of the solar zenith angle and the inte- the conclusions are given in section 5. grated liquid water path, respectively) derived from theoretical values in Stephens’ (1978). 2. Model and radiative schemes Stephens (1984) broad band temperature- The non-hydrostatic PSU/NCAR (Pennsylva- dependent emissivity function, which is based nia State University/National Center for Atmo- on Rodgers (1967) upward and downward emis- spheric Research) Meso-scale Model (MM5V3), sivity, is employed for the clear-air longwave with a new cloud microphysical scheme (China vapor absorption calculation.
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