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Aerosol Effects on the Development of a Supercell Storm in a Double JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116, D02204, doi:10.1029/2010JD014128, 2011 Aerosol effects on the development of a supercell storm in a double‐moment bulk‐cloud microphysics scheme Kyo‐Sun Sunny Lim,1 Song‐You Hong,1 Seong Soo Yum,1 Jimy Dudhia,2 and Joseph B. Klemp2 Received 1 March 2010; revised 14 September 2010; accepted 14 October 2010; published 21 January 2011. [1] This study investigates the aerosol effects on the development of an idealized three‐dimensional supercell storm, focusing on storm morphology and precipitation during a quasi steady state of a storm. The impact of the aerosol concentration on the simulated storm is evaluated by varying the initial cloud condensation nuclei (CCN) number concentration in the Weather Research and Forecasting Double‐Moment Six‐Class microphysics scheme. A right‐moving, quasi‐steady supercell with two diverging echo masses was reproduced, compared with the previous modeling study. In the experiment with a high CCN number concentration, storm intensity was weakened, and surface precipitation was reduced. On the other hand, the simulation that excluded the graupel substance produced a weaker low‐level downdraft, thus less near‐surface vorticity, compared with the simulation that included graupel. The CCN number concentrations did not affect the storm structures in the absence of graupel. In addition, the aerosol effects on the surface precipitation with respect to the initial CCN value were diametrically opposed. The major reason for the different responses to aerosol can be attributed to the exaggerated snow mass loading across the convective core when the graupel species is excluded. The results indicate that graupel species and related microphysics are crucial to the realistic representation of the aerosol‐precipitation interactions within a supercell storm. Citation: Lim, K.‐S. S., S.‐Y. Hong, S. S. Yum, J. Dudhia, and J. B. Klemp (2011), Aerosol effects on the development of a supercell storm in a double‐moment bulk‐cloud microphysics scheme, J. Geophys. Res., 116, D02204, doi:10.1029/2010JD014128. 1. Introduction [3] Studies of aerosol effects on large‐scale stratiform clouds via several general circulation models [Lohmann [2] The increase in aerosol particles in the atmosphere et al., 1999; Ghan et al., 2001; Rotstayn, 1999, 2000; caused by industrialization is known to change cloud Menon et al., 2002; Rotstayn and Lohmann, 2002; Lohmann microphysics and precipitation processes. Increased aerosol and Feichter, 2005; Takemura et al., 2005] suggest that the concentrations raise the number concentrations of cloud suppression of precipitation with increased aerosols, con- condensation nuclei (CCN) and cloud droplets, thus reduc- sistent with the second indirect effect of aerosols, can alter ing droplet size and increasing cloud reflectance [Twomey, the radiative fluxes due to changes in cloud lifetime or liquid 1974, 1977, 1991]. This is generally considered the first water path. Meanwhile, it is not certain whether the second indirect effect of atmospheric aerosol. Previous studies indirect effect of aerosol also applies to deep convective pointed out that a decrease in droplet size is also likely to clouds. Recently, efforts to investigate the effects of aerosol impact precipitation [Gunn and Phillips, 1957; Warner, on deep convective clouds have been carried out through 1968; Albrecht, 1989]. The decrease in droplet size can several observational and modeling studies [Rosenfeld, 1999, suppress the conversion of the droplet to drizzle or rain, 2000; Wang, 2005; Tao et al., 2007; Fan et al. 2007; Lerach thereby inhibiting rainfall and prolonging cloud lifetime. et al. 2008]. This precipitation suppression is commonly considered to be [4] Lerach et al. [2008] simulated a supercell storm using the second indirect effect of atmospheric aerosol and may a bin‐emulating, double‐moment bulk microphysics scheme substantially alter the water mass budget of clouds, altering including hail under an environment with a convective their persistence and albedo, and, possibly, the climate. −1 available potential energy (CAPE) of 3130 J kg and veering winds from the surface to 2 km above ground level 1 and showed a decrease in precipitation with increasing Department of Atmospheric Sciences and Global Environment aerosols. Wang [2005] investigated the response of tropical Laboratory, Yonsei University, Seoul, South Korea. 2Mesoscale and Microscale Meteorology Division, National Center for deep convection to the increase of CCN concentration with a −3 Atmospheric Research, Boulder, Colorado, USA. set of 30 initial CCN profiles ranging from 50 to 6000 cm and found that increasing CCN concentration causes a Copyright 2011 by the American Geophysical Union. stronger convection, leading to the increase in precipitation 0148‐0227/11/2010JD014128 D02204 1of16 D02204 LIM ET AL.: AEROSOL EFFECTS ON A SUPERCELL STORM D02204 as well as the expansion of the cloud coverage. However, ment of the supercell storm will be investigated, recognizing when the initial CCN concentration exceeds a certain level, the importance of the dense ice particles in simulating the many of the above effects become insignificant, implying supercell storm [Gilmore et al., 2004]. that a more substantial aerosol effect on deep convective [6] Section 2 outlines the numerical experiments conducted cloud be seen over a clean rather than polluted regions. With in this study, and their results are discussed in section 3. a weak wind shear and a lower CAPE ( = 960 J kg−1), Fan Concluding remarks appear in section 4. et al. [2007] showed a nonmonotonic precipitation response to increasing aerosols. The precipitation increased with increasing aerosols, and then decreased in the extremely 2. Numerical Experimental Setup −3 high aerosol cases (over 5000 cm ) owing to suppression of [7] The model used in this study is the Advanced convection from depleted water vapor and inefficient coa- Research WRF version 3.1 [Skamarock et al., 2008], which lescence. Tao et al. [2007] showed that three different deep was released in April 2009. The WRF model is a state‐of‐ convective cloud systems result in differing responses of the‐art mesoscale numerical weather prediction system surface precipitation with increasing aerosol concentrations serving both operational forecasting and atmospheric during the mature stage of the simulations. They suggested research needs. The WDM6 scheme [Lim and Hong, 2010] that evaporative cooling in the lower troposphere is a key is a double‐moment bulk‐cloud microphysics scheme based process in determining whether high CCN reduces or on the WSM6 microphysics scheme. In addition to the enhances precipitation. Lee et al. [2008] also investigated prediction for the mixing ratios of six water species (water aerosol effects on precipitation with five different sets of vapor, cloud droplets, cloud ice, snow, rain, and graupel), initial soundings and concluded that increasing aerosol can the number concentrations of cloud droplets and raindrops either decrease or increase precipitation for an imposed are also predicted in the WDM6 scheme, together with the large‐scale environment supporting cloud development. prognostic variable of cloud condensation nuclei (CCN) Aerosol effects on cloud microphysics and precipitation number concentration. The ice‐phase microphysics [Hong could be nonmonotonic under the different meteorological et al., 2004] is identical for both the WDM6 and WSM6 and aerosol conditions because of the complicated coupling schemes. The WRF Double‐Moment Five‐Class (WDM5) between cloud microphysics and storm dynamics [Seifert scheme, which excludes the graupel substance, has also been and Beheng, 2006; Fan et al., 2007; Tao et al., 2007; developed [Lim and Hong, 2010]. Both the WDM6 and Yang and Yum, 2007; van den Heever and Cotton, 2007; WDM5 schemes were implemented in WRF version 3.1. Lee et al., 2008; G. Li et al., 2008]. Khain [2009] and Khain [8] The formulation of warm‐rain processes such as auto- et al. [2008] analyzed aerosol effects on precipitation using conversion and accretion in the WDM6 scheme is based on the mass and heat budgets and concluded that in the case the studies of Cohard and Pinty [2000]. Autoconversion when the condensation loss increases more than the con- parameterization is based on the numerical simulation of densation generation, a decrease in precipitation takes place. the stochastic collection equation suggested by Berry and They also concluded that many discrepancies between the Reinhardt [1974], which is built on the observation. In results reported in different observational and numerical addition, the accretion process is obtained by an analytical studies for aerosol effects on surface precipitation can be integration of stochastic collection equation. The mass‐ attributed to the different atmospheric conditions and cloud weighted mean terminal velocity, which is responsible for the types analyzed. sedimentation of each hydrometeor mass, can be obtained by [5] The purpose of this study is to investigate aerosol integrating the terminal velocity of hydrometeor. Sedimen- effects on the development of a supercell storm, focusing on tation fluxes for both the number concentration and mixing storm morphology and precipitation during a quasi steady ratio of rain are computed in the WDM6 scheme. Thus, dif- state of a storm. A supercell thunderstorm, characterized by ferential settling between drops can be simulated. The number‐ a deep, persistent, rotating updraft [Doswell
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