Journal of the Meteorological Society of Japan, Vol. 84, No. 4, pp. 691--704, 2006 691

Radiative Effects on : 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% 7% 2d Tao et al. (1996) 12 h 8% 6% 2d Tropics, cloud resolving Chin et al. (1995) 10 h 15% 18% 2d Fu et al. (1995) 12 h 5% 10% 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% 7% 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 . 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 (Kd1) algorithm is provided for ‘Simple’, and application are advantageous for research so that ðqT/qtÞrad ¼1:8 0:017ðT 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 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 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. It is not accurate Academy of Meteorological Science [CAMS] for conditions below a cloud ceiling; the error is scheme) coupled with Lou et al. (2003) is se- up to 20 Wm2. Stephens (1978) scheme is used lected as the dynamical model for this paper. for the liquid cloud longwave absorption. In The CAMS scheme is a two-parameter explicit this scheme, the cloud water is assumed to moisture scheme, in which, five types of water have a constant absorption coefficient, which is substances (11 prediction variables) and 31 slightly different for upward and downward microphysical processes are calculated. There radiation. Ice, and precipitating particles are are five radiative transfer scheme choices similarly treated, but with smaller absorption in MM5V3: None, Simple, Cloud, CCM2 and coefficient used for the larger size. The overlap RRTM. In the option of ‘None’, the radiative technique by Stephens (1984) is used to com- 694 Journal of the Meteorological Society of Japan Vol. 84, No. 4

Fig. 1. 24 h surface precipitation pattern. a, b, c, d, e and f are the results of NoRad, SimpleRad, CLDRad, CCM2Rad, NewRad and observation, respectively. The unit is mm. August 2006 G. ZHOU et al. 695

Table 2. Summary of the radiative schemes; LW for longwave and SW for shortwave; * RRTM em- ploys cloud scheme SW algorithm. Cloud scheme CCM2 scheme RRTM scheme* New scheme model — d-Eddington (2 stream) — d-4 stream gas LW CO2,H2OH2O, O3,CO2 H2O, O3,CO2 H2O, O3,CO2, CH4,N2O SW H2OH2O, O3,CO2 —H2O, O3,CO2 gas LW Stephens (1984) Band absorption for O3 Correlated k Correlated k algorithm overlap and CO2; Ramanathan distribution distribution and technique et al. (1986) method and the the continuous for H2O continuous H2O absorption absorption of vapor SW Lacis et al. (1974) Band model, O3 in — Correlated k vapor path upper; H2O and CO2 distribution function method in the lower method water LW Stephens (1978) High/middle/low cloud Similar to Cloud Water, ice, snow, substance water cloud calculated by RH; scheme rain and graupel method; similar emission and 5 kinds, 8 types method for ice, absorption by effective of water cloud, rain droplet and cloud fraction re for water, ice snow and snow; but different SW Stephens (1978), Water and ice cloud, re — algorithm for and all as one and partial and LW and SW kind of cloud overlap of cloud ice optics technique

bine the effect of non-gray carbon dioxide and and other gas absorptivity and emissivity cal- cloud. See Dudhia (1989) for more detail about culations. For cloud, the emissivity is calcu- this scheme. lated by using a broad-band emissivity tech- nique (a negative exponential function of the 2.2 CCM2 radiative transfer scheme liquid water path), and refreshed by the param- The absorption by ozone, water vapor and eter of effective cloud fraction in each model carbon dioxide are calculated separately in each layer. shortwave radiation band; the atmosphere The delta-Eddington solution (Joseph et al. is divided into two layers vertically: ozone 1976) is employed for the radiative transfer in the upper and the lower for all other mass flux calculation, in each layer for both longwave absorption and scattering. The optical proper- and shortwave radiation, and consequently this ties (optical depth, single scattering albedo is a delta-2 stream scheme. and asymmetry factor) of two types of clouds, namely water cloud and ice cloud, are calcu- 2.3 RRTM scheme lated separately and their values are deter- RRTM is a rapid radiative transfer model mined by liquid/ice water path and effective ra- for gas radiation calculations in the longwave dii. The effect of partial cloud and cloud overlap region, with high precision and efficiency. H2O, are also treated, in which the optical depth is CO2,O3,N2O, CH4, CCl4, CFC-11, CFC-12, adjusted by timing a factor of 3/2, the exponent CFC-22 and the continuous water vapor ab- of cloud fraction. sorption band are calculated in RRTM (Mlawer A band emissivity technique is used for car- 1997). The correlated-k distribution method is bon dioxide and ozone in clear sky for longwave employed for the detailed absorptive spectrum radiation; and the method in Ramanathan and calculation. Three gases (H2O, CO2 and O3) Downey (1986), is employed for the water vapor are calculated and a scheme similar to Dudhia 696 Journal of the Meteorological Society of Japan Vol. 84, No. 4

(1989) is used for longwave water substances gases is calculated. 3) A better method for cal- when it is coupled to MM5. And the Dudhia culation of non-grey gaseous absorption (121 in- (1989) shortwave scheme is employed for the tervals) is used. Fu et al. (1995) examined the shortwave radiation. Thus, the RRTM in MM5 accuracy by using the line by line integration is very similar to the Dudhia cloud radiative method and the results show that the differ- scheme, and just advanced in calculation of ence of heating rate is less than 0.05 Kd1, and clear sky gases radiative properties. the radiant flux difference is less than 1 Wm2. 4) Detailed parameterizations for the radiative 2.4 New coupled radiative transfer scheme properties of water substances are employed, A radiative transfer scheme is coupled to the which are advantageous for study of the cloud MM5V3 (including the CAMS explicit moisture radiative properties, and discussion of its scheme) model by the authors. The new coupled dependency. scheme is based on the delta-4 stream approxi- For this study, the snow radiative parame- mation radiative transfer parameterization by terization is added. The effective size ðDesÞ is Liou et al. (1988), the correlated-k distribution employed to represent the snow radiative prop- method for calculation of non-grey gases radia- erties. Small snow particles (Des < 300 um) are tive properties, by Fu and Liou (1992) and pa- treated as ice crystal, and the parameteriza- rameterization of ice crystal optical properties tions of ice crystal are employed to calculate (Fu and Liou 1993). In this scheme, the optical its radiative properties. The graupel parame- spectrum are divided into 18 bands (6 for solar terizations are used to build the large snow and 12 for ), and the radiative fluxes crystals (Des > 300 um) radiative calculation. are calculated individually in each band and Stephens (1984) suggested approximations for z 1 vertical atmospheric model layer. Five kinds of particle optical depth (t re ), and single scat- gases, including H2O (g), CO2,O3,N2O and tering albedo (1 $ z re). So the snow optical CH4, and four types of water substances, in- depth and single scattering albedo are ts ¼ cluding cloud water, ice crystal, rain water and tgqsDeg/ðqgDesÞ and $s ¼ 1 ð1 $gÞDes/Deg, graupel, are calculated. An effective radius respectively. The subscript s and g are for parameterization is employed for the calcula- snow and graupel, and De and q stand for effec- tion of cloud water and ice crystal radiative tive size and water mixing ratio. properties. Particularly, the cloud water is 3. Simulation and results divided into 8 types based on effective radius, to describe the different properties. While 3.1 Simulation design the scheme coupled to MM5V3, CO2,N2O and The initial data came from the Regional Me- CH4 are assumed to be uniform, and their vol- teorological Center in Guangzhou, China. It is ume ratios are set to 330 ppm, 1.6 ppm and grid data with 1 1 degree of horizontal resolu- 0.28 ppm respectively, O3 is uniform horizon- tion, 11 vertical layers (1000, 925, 850, 700, tally and the vertical profile in the MM5 is 500, 400, 300, 250, 200, 150, 100 hpa) and tem- used. An extra layer, which is above the top poral 6-hour interval resolution, assimilated by model layer and whose optical properties are the TLAPS (Tropical Limited area Assimilation same to the first top layer, is set up to prevent and Prediction System). The observational excessive heating of the top layer. results of HUAMEX (Huanan/South China There is some development in the new Meso-scale Experiment), SCSMEX (South coupled scheme from others: 1) a higher China Sea Monsoon Experiment), and the data precision approximation parameterization is from Hong Kong, Macau, and Taiwan are all in- employed. For example, Kay et al. (2001) sug- cluded in this assimilation. gested that the delta-4 stream approximation In this single domain simulation, the configu- is more accurate on calculating radiant flux rations are set to be 60 seconds for the dynami- than the two stream approximation and broad cal time step length, 20 minutes for the radia- band method. And the difference between the tive step length and 23 sigma layers vertically. delta-4 stream approximation, and DISORT The simulation period is from June 8th at 12:00 (Discrete Ordinate Radiative Transfer Method) UTC (@20:00 local time on the 8th) to the 9th 32 stream is negligible. 2) Absorption by more at 12:00, 1998, 24 hours totally. The grid size August 2006 G. ZHOU et al. 697 distance is 30 kilometers, with 75 grid points in and two relatively weaker centers, C3 and C5 latitude and 81 in longitude. The 24-category near (27N, 117.5E). The locations of C1@C3 USGS (the United States Geological Survey) are the same as that in section 1. All simulation data is used for land use, vegetation, vegetation results are close to the observational results fraction, etc. The physical schemes are selected for C1. The results of CCM2Rad and NewRad as follows: CAMS (Lou et al. 2003) explicit correspond to the observed value of 120 mm; moisture scheme, Betts/Miller (Betts and CLDRad is somewhat larger for the 150 mm Miller 1986) cumulus scheme, MRF (medium- area; but those of NoRad and SimpleRad are range forecast model) planetary boundary layer much smaller and the differences are about scheme (Hong and Pan 1996, vertical mixing 30 mm. At C2, the simulated rainfall is 150, moist adiabatic in clouds), and multilayer soil 150, 175, 150 and 175 mm for run 1@5, respec- temperature model, in which a slab scheme is tively. The rainfall of C3 is equivalent for each used to calculate the surface temperature ten- simulation. However large differences exist for dency according to the residual of the surface C4. The maximum rainfall is 120, 90, 150, 90 energy budget (Blackadar 1978). Relaxation/ and 150 mm, respectively and the maximum inflow-outflow lateral boundary conditions, up- discrepancy is up to 60 mm. This is equivalent per radiative boundary condition, and other to the observed value. C5 has the strongest pre- MM5 recommended options are employed. cipitation for CLDRad, but the weakest for In order to estimate the radiative effects on SimpleRad. There is also some difference in the precipitation, and the differences derived the central positions at C2 and C3, e.g., New- in the radiative transfer schemes, five runs are Rad C2 is displaced southwestward and NoRad designed as follows: C3 is displaced southward; but the difference is not very distinct. (1) No radiative heating (NoRad); The surface precipitation indicates that the (2) Simple cooling radiative scheme (Sim- effect of radiation, and the difference derived pleRad); from various schemes have a slight influence (3) Dudhia cloud radiative scheme (CLDRad); on the pattern, while they have a significant (4) CCM2 radiative scheme (CCM2Rad) and; influence on the precipitation intensity, espe- (5) New coupled radiative scheme (NewRad). cially at the centers. These results are in good There is no run for the RRTM scheme be- agreement with the previous conclusions of the cause it is too similar to the CLDRad scheme. cloud-radiation interaction study. They suggest Their differences are only in the gaseous radia- that the radiation does not change the overall tive calculation. squall line structure, e.g., in Chin et al. (1995), Tao et al. (1996), but enhances the cloud anvil, 3.2 Surface precipitation pattern e.g., in Chen and Cotton (1988), Tripoli and The simulated 24-hour surface precipita- Cotton (1989), Churchill and Houze (1991) and tion patterns of the different runs are depicted Chin et al. (1995). in Fig. 1. All runs produce a very similar overall precipitation pattern, i.e., a northeast- 3.3 Domain averaged precipitation southwest structure as seen in the observations Domain averaged integrated rainfall (DAIR), (Fig. 1f ). The major characteristics of the ob- and domain averaged rain rate (DARR) are cal- servational data can also be found in the simu- culated for the averaged temporal trend discus- lations, including the and posi- sion. The DAIRs show a similar general tempo- tions of the three centers (C1@C3). Therefore, ral changing trend (Fig. 2a). But their relative the simulation results are reliable and can be changes, which are normalized by the NoRad used for the work in this paper. The disagree- results, are much different in time (Fig. 2b). ment among the simulations exists only in a They show positive anomalies at nighttime (be- few places, such as in the area around (26N, fore simulated 10:00) and negative anomalies 118E), and the rainfall centers. Thus radiation in the daytime, and the maximum and mini- influences the precipitation pattern slightly. mum of different radiative scheme runs is For the centers, all simulations obtain three much different (see in Table 3). It also indicates strong centers, C1, C2 and C4 (25N, 115.5E), that a different radiation scheme results in 698 Journal of the Meteorological Society of Japan Vol. 84, No. 4

Fig. 2. Domain averaged integrated rainfall and rain rate. Domain averaged integrated rainfall, dif- ference to NoRad run, rain rate and rain rate difference to NoRad shows from a to d, respectively.

Table 3. The maximum ratios of difference in radiation run from NoRad run. SimpleRad (%) CLDRad (%) CCM2Rad (%) NewRad (%) Integrated rainfall þ3.65 þ4.27 þ2.80 þ4.37 6.00 4.12 2.63 5.64 Rain rate þ5.56 þ14.49 þ5.60 þ14.02 19.62 17.46 6.87 24.17

different effects on the meso-scale precipitation. by the radiative impact has an obvious varia- The variations among the radiative runs are tion except for SimpleRad, which is suggested larger during the daytime than those at night- by Tao et al. (1996), Sui et al. (1998), etc. time. The radiation forced rainfall variation in The DARR temporal trend (in Figs. 2c–2d) the runs is comparable to those of LW and SW shows larger differences and better diurnal in Table 1. Another notable result is that the variation. Comparing to the NoRad result, the relative precipitation change (Fig. 2b) caused radiative rain rate changes have temporal August 2006 G. ZHOU et al. 699

Fig. 3. (a) domain averaged downward longwave and shortwave radiant fluxes at the surface and (b) the differences from NoRad run in units of Wm2.

trend. The diurnal variation of rain rate is obvi- computes three variables for the model, includ- ous except for the SimpleRad run in the first ing the radiative cooling/heating rate, down- 3-hour period. A larger rain rate is found at ward longwave (LW), and shortwave (SW) night and smaller in the daytime, with an radiant flux at the surface. So the possible ex- inverse temporal trend of u (cosine of the solar planation may be found from them. zenith angle). But obvious differences exist Figure 3 depicts the domain averaged down- among the runs (Table 3) and the relative ward longwave and shortwave radiant fluxes change ratios (Fig. 2d) are much larger than at the surface, and the differences from the those of DAIR (Fig. 2b). The conclusions show NoRad run. One can find in Fig. 3a that there that radiation enhances the surface precipita- is not much difference in LW radiant flux tion at nighttime, and reduces it during the except for CLDRad, whose value is about daytime. Thus the solar radiation decreases 70 Wm2 more than NoRad. The result of Sim- the meso-scale precipitation while the IR radia- pleRad is almost the same as NoRad because tion does the inverse. The consequences of the the same algorithm is used; and CCM2Rad simulated radiation’s effect on precipitation are and NewRad are about 20 Wm2 more. How- varied: they are similar in SimpleRad, CLDRad ever, the SW flux is much more varied. The and NewRad runs, but agree less in the maximum difference of CLDRad and NewRad CCM2Rad run (about 1/3 of the three formers’). is about 300 Wm2 at 16:00 (local noon); the re- The results show that SimpleRad is not proper sult of CCM2Rad is a bit less, the maximum is for simulating the radiation effects of diurnal less than 100 Wm2. As a result, the total radi- variations. ant flux from the atmosphere to the surface, which is important for the surface energy bud- 4. Discussion on the precipitation get, is greatly modified by the different radia- variation tive scheme (Fig. 3b). When comparing NoRad It is found in section 3.2 and 3.3 that the to the other methods, CLDRad and NewRad radiative process has a significant effect on the calculated more radiant flux to the surface, intensity of precipitation instead of the overall CCM2Rad produced less, and SimpleRad pro- pattern, as well as the differences among duced a negligibly negative value. the results of the different radiative transfer As is introduced in section 2, the radiative schemes. But how are these discrepancies pro- cooling rate is 0 for ‘NoRad’. The domain aver- duced? Since the effect on the overall pattern aged radiative cooling/heating rate (in unit of is slight, the domain averaged results will be Kd1) of SimpleRad, CLDRad, CCM2Rad and discussed. As is known, the radiative scheme NewRad is shown in Fig. 4. The radiative cool- 700 Journal of the Meteorological Society of Japan Vol. 84, No. 4

Fig. 4. Domain averaged radiative cooling/heating rate of (a) SimpleRad, (b) CLDRad, (c) CCM2Rad and (d) NewRad. Units: Kd1. ing rate of SimpleRad is linearly increased in the upper atmosphere, while CLDRad and from about 2.2 at the surface to 1.0 at the NewRad have a larger heating rate in the lower top, but has no obvious diurnal variation for atmosphere in the daytime. The differences in the small temperature change at the same the daytime among the schemes are much height. The cooling rate in the nighttime is larger than those at night. about 2 Kd1 except in a few regions, e.g. in the The difference in downward longwave and low atmosphere for CLDRad and near the top shortwave radiant flux and radiative cooling/ for CCM2Rad. In the daytime, the results are heating from the radiative schemes results very different: the heating rate of CCM2Rad is in vertical velocity differences. Comparing to up to 5 Kd1 in the upper atmosphere, while NoRad, the strong lower and weak upper cool- CLDRad is 1 and NewRad is 0.3; in the lower ing structure of SimpleRad increases the stabil- atmosphere, the heating rate of CLDRad and ity and hence suppresses convection. In the NewRad is up to 0.5 and the heating region of other three simulations, the effects of radiation NewRad is somewhat larger, and the value of are different in stages: the cooling becomes CCM2Rad is less than 0 (cooling). Generally, weaker step by step after sunrise, transforms the nighttime radiative cooling of CLDRad, to heating before and after noon, and followed CCM2Rad and NewRad is equivalent and has by enhanced cooling. So the convection has a stronger cooling than SimpleRad in the upper a corresponding variational trend of weaker, atmosphere. CCM2Rad has a stronger heating stronger and weaker. Furthermore, the down- August 2006 G. ZHOU et al. 701

Fig. 5. Domain averaged vertical velocity of (a) NoRad and the difference of (b) SimpleRad, (c) CLDRad, (d) CCM2Rad and (e) NewRad from NoRad. Units: 102 ms1.

ward radiant flux influences the convection, es- indirect effect of radiation on convection. As pecially in the lower atmosphere. The greater a result, the atmospheric vertical velocity is radiant flux produces a higher surface tempera- modified by radiative processes and varies due ture, which increases the ascent. This is the to different radiative schemes (shown in Fig. 5). 702 Journal of the Meteorological Society of Japan Vol. 84, No. 4

comes progressively weaker, because the Sim- pleRad run has an enhanced negative vertical velocity change and an almost constant radia- tive cooling; for the other three schemes, the relative vertical velocity and their rainfall in- tensity, have a more significant diurnal varia- tion due to the radiative effects than that of the NoRad. Now, the question is what produces the difference among CLDRad, CCM2Rad and NewRad. One can find that the lower and mid- dle radiative heating of CLDRad and NewRad is stronger (or cooling weaker) than that of CCM2Rad during about 12–20 hours into the Fig. 6. The sensitivity of saturated vapor simulation, so their precipitation intensity is pressure to temperature. relatively reduced during this period. After about 16 hours, their convection in the low level atmosphere is stronger and therefore the pre- It is well known that higher relative humid- cipitation of CLDRad and NewRad is larger ity benefits precipitation. So radiative cooling than that of CCM2Rad a few hours later due and higher vertical velocity enhance precipita- to the delay of convective forcing. tion by increasing the relative humidity. Fur- The different radiative cooling/heating and thermore, the saturation vapor pressure ðEsÞ the vertical velocity change ascribed to the ra- is more sensitive to temperature ðTÞ in higher diative cooling/heating and downward long- temperature areas (Fig. 6). Tetens experiential wave and shortwave radiant flux, result in the formula is as following: (Murray 1967) Es ¼ variation of precipitation intensity from differ- 6:017 expðaðT 273:16Þ/ðT bÞÞ, where a ¼ ent radiative schemes. Also, these discussions 17:27 and b ¼ 35:86. One can get the sensitivity can be employed for the different precipitation of Es to T from qEs/qT ¼ Es að273:16 bÞ/ accompanying specific locations, such as pre- ðT bÞ2. The mean T profile of NoRad is used cipitation centers. Although the discussion is to calculated the sensitivity shown in Fig. 6. So focused on a South China storm case, the anal- the cooling/heating, and updraft in the lower ysis can be employed for other cases. Further- and middle atmosphere, produce more conver- more, the combination of direct and indirect sion of water from gas to liquid or solid and impacts will provide a potential solution for hence the dominant precipitation variation. quantitatively estimating the effect of the radi- This result is similar to the conclusion in Tao ative transfer process on precipitation. et al. (1996) that the effect of radiation on pre- 5. Conclusions cipitation is larger in the tropics than in midla- titudes for the higher temperature and water A radiative transfer scheme, which has vapor abundance. higher precision of radiant flux, gaseous ab- It is found in section 3.3 that the DARR dif- sorption calculation, and relatively detailed ra- ference among the radiative schemes is small diative property description of water substan- at night and quite variable in the daytime. Be- ces, is coupled to the MM5V3. The study of the cause the radiative cooling and vertical velocity radiative effects on meso-scale precipitation, change in the four radiative runs (SimpleRad, and their dependency on the radiative schemes, CLDRad, CCM2Rad and NewRad) is similar in is carried out using a South China severe storm the first 12 hours of simulation, the precipita- case on June 8th, 1998. tion variation from NoRad is almost the same, The simulation results indicate that radia- except from 3 to 5 hours in CCM2Rad, where tive transfer processes influence surface pre- the radiative cooling is somewhat weaker dur- cipitation significantly. The precipitation in- ing this time. For the remaining duration of tensity, especially at the rainfall centers, is the simulation, the precipitation intensity be- strongly modified by radiation. But their effect August 2006 G. ZHOU et al. 703 on the rainfall pattern is slight. Radiation en- J. Atmos. Sci., 45, 1606–1623. hances the precipitation in night time, but Betts, A.K. and M.J. Miller, 1986: A new convective reduces it in the daytime and increases the di- adjustment scheme. Part I: Observational and urnal cycle. Concerning intensity, the effects theoretical basis. Quart. J. Roy. Meteor. Soc., during the daytime are much larger than that 112, 677–692. ——— and ———, 1986: A new convective adjust- at nighttime. ment scheme. Part II: Single column tests us- There are obvious differences among the var- ing GATE wave, BOMEX, ATEX, and Arctic ious radiative scheme simulations. Concerning air-mass data sets. Quart. J. Roy. Meteor. Soc., the precipitation patterns, their differences are 112, 693–709. small in overall structure, but large on the cen- Blackadar, A.K., 1978: Modeling pollutant transfer ters’ locations and amounts. The domain aver- during daytime convection. Preprints Fourth aged rainfall is very uncertain due to the radia- Symposium on Atmospheric Turbulence, Diffu- tive scheme. Furthermore, the uncertainty is sion and Air Quality. Reno. Amer. Met. Soc., much larger in the daytime than at night. 443–447. So the effects of radiation on mesoscale precipi- Chen, S. and W.R. Cotton, 1988: The sensitivity of a tation are dependent on the using of the radia- simulated extratropical meso-scale convective system to longwave radiation and ice-phase tive transfer scheme, videlicet the accuracy of microphysics. J. Atmos. Sci., 45, 3897–3910. radiative transfer processes description is im- Chin, H.-N.S., 1994: The impact of the ice phase and portant for meso-scale precipitation, especially radiation on a midlatitude squall line system. that during the daytime. The discussions show J. Atmos. Sci., 51, 3320–3343. that the different radiative cooling/heating ———, Q. Fu, M.M. Bradley, and C.R. Molenkamp, rate, and downward radiant flux to the surface, 1995: Modeling of a tropical squall line in two which are more different in the daytime, lead to dimensions and its sensitivity to environment the various cooling/heating patterns and con- winds and radiation. J. Atmos. Sci., 52, 3172– vections, especially in the lower and middle 3193. atmosphere, and then result in the varying Churchill, D.D. and R.A. Houze Jr., 1991: Effect of precipitation. radiation and turbulence on the diabatic heat- ing and water budget of the stratiform region These conclusions suggest that for the study of a tropical cloud cluster. J. Atmos. Sci., 48, of radiative effects on meso-scale precipitation, 903–922. the accuracy of radiative transfer calculation Dharssi, I., R. Kershaw, and W.-K. Tao, 1997: Sensi- during daytime is more important, and a poten- tivity of a simulated tropical squall line to tial key area of study for improvement. longwave radiation. Quart. J. Roy. Meteor. Soc., 123, 187–206. Acknowledgements Dudhia, J., 1989: Numerical study of convection ob- The authors thank Dr. Xiaofeng Lou for her served during the winter monsoon experiment warm-hearted help with the model coupling using a two-dimensional model. J. Atmos. Sci., and providing the model MM5V3 with the new 46, 3077–3107. cloud microphysical scheme (CAMS scheme) Fu, Q. and K.-N. Liou, 1992: On the correlated k- distribution method for radiative transfer in developed. We are also grateful to Zhenglong nonhomogeneous atmospheres. J. Atmos. Sci., Li, Brent C. Maddux and Prof. M. Kaplan for 49, 2139–2156. the linguistic correction. We thank the two ——— and ———, 1993: Parameterization for the anonymous reviewers for their kind and valu- radiative properties of cirrus cloud. J. Atmos. able comments to improve the writing. This Sci., 50, 2008–2025. research was supported by National Natural ———, S.K. Krueger, and K.-N. 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