Theoretical basis for convective invigoration due to increased aerosol concentration

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 Correspondence to: Z. J. Lebo (zachlebo@caltech.edu) 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.

Observations of aerosol‐induced convective invigoration in the tropical east Atlantic

“…Higher values of COG can result from clouds with greater vertical extent and/or clouds containing more condensate aloft, both of which have been seen in polluted storms (as summarized in Tao et al [2012]). Larger vertical extent would suggest possible convective invigoration, as seen previously [Andreae et al, 2004;Khain et al, 2005;Koren et al, 2005;van den Heever et al, 2006;van den Heever and Cotton, 2007;Lee et al, 2008a;Rosenfeld et al, 2008;Lebo and Seinfeld, 2011;Storer and van den Heever, 2013]. Many studies [e.g., Lynn et al, 2005;Khain et al, 2005;van den Heever et al, 2006;Lee et al, 2008a;Storer et al, 2010;Storer and van den Heever, 2013] have also noted that increased aerosol loading leads to enhanced ice and liquid amounts in deep convective clouds.…”

“…studies [Andreae et al, 2004;Khain et al, 2005;van den Heever et al, 2006;van den Heever and Cotton, 2007;Lee et al, 2008a;Li et al, 2008;Rosenfeld et al, 2008;Lebo and Seinfeld, 2011;Li et al, 2011;Storer and van den Heever, 2013;Fan et al, 2013] showing evidence that increased aerosol concentrations can lead to convective invigoration. These studies are in general agreement that polluted clouds produce less warm rain through the aerosol-induced suppression of the collision and coalescence processes, as the large numbers of small droplets make the warm rain process less efficient.…”

“…Several studies [Andreae et al, 2004;Khain et al, 2005;Koren et al, 2005;van den Heever et al, 2006;van den Heever and Cotton, 2007;Lee et al, 2008a;Li et al, 2008;Rosenfeld et al, 2008;Lebo and Seinfeld, 2011;van den Heever et al, 2011;Storer and van den Heever, 2013;Fan et al, 2013] suggest that increased aerosol concentrations will lead to the invigoration of deep convective storms; however, there is still no clear consensus on this effect, as recently summarized in Tao et al [2012]. It is generally established that in a polluted environment, or one which contains higher number concentrations of aerosols that can act as cloud condensation nuclei (CCN), collision and coalescence in deep convective systems will be less effective due to the increased numbers of smaller cloud droplets, thus leading to a reduction in warm rain production.…”

Uncertainty from the choice of microphysics scheme in convection-permitting models significantly exceeds aerosol effects

“…Similarly, Lebo and Seinfeld (2011) found an opposite response of accumulated surface rain to CCN in idealised supercell simulations using a bulk and bin scheme. Khain and Lynn (2009) found a difference in the response of an idealised supercell to aerosol perturbations when a bin and bulk scheme was used, with the bulk scheme producing stronger updraughts and greater average precipitation than the bin scheme and with the left-moving storm prevailing in the bulk simulation, while the right-moving storm prevailed in the bin simulation.…”

“…Rosenfeld et al, 2008;Stevens and Feingold, 2009) and cloud-resolving (or cloudsystem-resolving) modelling studies (e.g. Fan et al, 2007;Tao et al, 2007;Lebo and Seinfeld, 2011, amongst many others) have suggested that under certain conditions, precipitation suppression in the liquid phase may lead to an invigoration of deep convection and a subsequent enhancement of convective precipitation. The detection of positive correlations between satellite-observed aerosol optical depth (AOD) and precipitation or convective cloud properties (e.g.…”