Abstract
Abstract. The potential effects of increased aerosol loading on the development of deep convective clouds and resulting precipitation amounts are studied by employing the Weather Research and Forecasting (WRF) model as a detailed high-resolution cloud resolving model (CRM) with both detailed bulk and bin microphysics schemes. Both models include a physically-based activation scheme that incorporates a size-resolved aerosol population. We demonstrate that the aerosol-induced effect is controlled by the balance between latent heating and the increase in condensed water aloft, each having opposing effects on buoyancy. It is also shown that under polluted conditions, increases in the CCN number concentration reduce the cumulative precipitation due to the competition between the sedimentation and evaporation/sublimation timescales. The effect of an increase in the IN number concentration on the dynamics of deep convective clouds is small and the resulting decrease in domain-averaged cumulative precipitation is shown not to be statistically significant, but may act to suppress precipitation. It is also shown that even in the presence of a decrease in the domain-averaged cumulative precipitation, an increase in the precipitation variance, or in other words, andincrease in rainfall intensity, may be expected in more polluted environments, especially in moist environments. A significant difference exists between the predictions based on the bin and bulk microphysics schemes of precipitation and the influence of aerosol perturbations on updraft velocity within the convective core. The bulk microphysics scheme shows little change in the latent heating rates due to an increase in the CCN number concentration, while the bin microphysics scheme demonstrates significant increases in the latent heating aloft with increasing CCN number concentration. This suggests that even a detailed two-bulk microphysics scheme, coupled to a detailed activation scheme, may not be sufficient to predict small changes that result from perturbations in aerosol loading.
Highlights
Changes in ambient concentrations of cloud condensation nuclei (CCN) and ice nuclei (IN) potentially alter cloud properties that may lead to modifications in cloud radiative forcing and/or precipitation
We begin with a comparison between bin and bulk simulations of the potential impact on deep convective cloud development and precipitation as a result of increasing the CCN number concentration
It is important to keep in mind that the purpose of this study is not to predict with great precision the amount of precipitation that may result from the given initial environmental conditions, but instead to numerically determine the extent to which the precipitation patterns and magnitude are altered in response to a modified aerosol loading
Summary
Changes in ambient concentrations of cloud condensation nuclei (CCN) and ice nuclei (IN) potentially alter cloud properties that may lead to modifications in cloud radiative forcing and/or precipitation. Aerosolcloud interactions have been discussed primarily in terms of (IPCC, 2007): (1) The “1st aerosol indirect effect” (Twomey, 1977), in which all else being equal, an increase in the CCN number concentration will result in a higher cloud droplet number concentration and smaller particles. (2) The “2nd aerosol indirect effect” (Albrecht, 1989), in which changes in the CCN number concentration may affect cloud lifetime and precipitation efficiency. An increase in the CCN number concentration will result in smaller cloud droplets, for which the collection kernels and collection efficiencies are substantially smaller in comparison to their larger counterparts, mitigating the collision-coalescence process and suppressing precipitation. The additional CCN particles are hypothesized to increase the longevity of the cloud and reduce the surface heating by shortwave radiation (cooling effect at the surface).
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