Abstract
Abstract. Three configurations of a bulk microphysics scheme in conjunction with a detailed bin scheme are implemented in the Weather Research and Forecasting (WRF) model to specifically address the role of the saturation adjustment assumption (i.e., condensing/evaporating the surplus/deficit water vapor relative to saturation in one time step) on aerosol-induced invigoration of deep convective clouds. The bulk model configurations are designed to treat cloud droplet condensation/evaporation using either saturation adjustment, as employed in most bulk models, or an explicit representation of supersaturation over a time step, as used in bin models. Results demonstrate that the use of saturation adjustment artificially enhances condensation and latent heating at low levels and limits the potential for an increase in aerosol concentration to increase buoyancy at mid to upper levels. This leads to a small weakening of the time- and domain-averaged convective mass flux (~-3%) in polluted compared to clean conditions. In contrast, the bin model and bulk scheme with explicit prediction of supersaturation simulate an increase in latent heating aloft and the convective updraft mass flux is weakly invigorated (~5%). The bin model also produces a large increase in domain-mean cumulative surface precipitation in polluted conditions (~18%), while all of the bulk model configurations simulate little change in precipitation. Finally, it is shown that the cold pool weakens substantially with increased aerosol loading when saturation adjustment is applied, which acts to reduce the low-level convergence and weaken the convective dynamics. With an explicit treatment of supersaturation in the bulk and bin models there is little change in cold pool strength, so that the convective response to polluted conditions is influenced more by changes in latent heating aloft. It is concluded that the use of saturation adjustment can explain differences in the response of cold pool evolution and convective dynamics with aerosol loading simulated by the bulk and bin models, but cannot explain large differences in the response of surface precipitation between these models.
Highlights
All but the bulk-cond model configuration predict an increase in the domain-averaged precipitation at 120 min in polluted compared to clean conditions, the magnitude of the increase is much larger using the bin model compared to any of the bulk configurations
Previous studies of aerosol effects on convective development and cumulative precipitation have hypothesized that results using bin and bulk microphysics models do not agree because of the use of a saturation adjustment scheme, which is commonly employed in bulk microphysics models (e.g., Khain and Lynn, 2009; Lebo and Seinfeld, 2011; Fan et al, 2012)
To quantitatively address this issue, we employ a high-resolution cloud resolving model (CRM) to study the effects of increased aerosol loading on a supercell storm using four microphysics model configurations: 1. Bin Model – All hydrometeors are represented using binned distributions following Lebo and Seinfeld (2011)
Summary
Recent studies have investigated the effects of increased aerosol loading on the behavior and response of deep convective clouds (e.g., Khain et al, 2004; Khain and Pokrovsky, 2004; Khain et al, 2005; Wang, 2005; Koren et al, 2005; Grabowski, 2006; Seifert and Beheng, 2006; Teller and Levin, 2006; Van den Heever et al, 2006; Fan et al, 2007; Tao et al, 2007; Van den Heever and Cotton, 2007; Khain et al, 2008; Lee et al, 2008a,b; Rosenfeld et al, 2008; Fan et al, 2009; Khain and Lynn, 2009; Koren et al, 2010; Noppel et al, 2010; Ekman et al, 2011; Lee, 2011; Lebo and Seinfeld, 2011; Grabowski and Morrison, 2011; Seifert et al, 2012; Morrison, 2012; Tao et al, 2012). Changes in cloud properties resulting from aerosol loading can have potentially significant effects on the radiative forcing, precipitation patterns and amounts, and storm severity. Lebo et al.: Dependency of Aerosol-Cloud Interactions on Saturation Adjustment is highly challenging. This is especially true in a mixed-phase convective environment. The initial effect of an increase in aerosol loading may be to suppress the collisioncoalescence process and mitigate the formation of precipitation within a warm cloud or the warm region of a mixed-phase cloud (i.e., Gunn and Phillips, 1956; Squires, 1958; Albrecht, 1989), feedbacks with the environment or changes in the microphysical process rates in other regions of the cloud may result in a negligible change in precipitation, or even potentially an increase
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