Aerosol impact on the dynamics and microphysics of deep convective clouds

Abstract

SUMMARYMechanisms through which atmospheric aerosols affect cloud microphysics, dynamics and precipitation are investigated using a spectral microphysics two-dimensional cloud model. A significant effect of aerosols on cloud microphysics and dynamics has been found. Maritime aerosols lead to a rapid formation of raindrops that fall down through cloud updraughts increasing the loading in the lower part of a cloud. This is, supposedly, one of the reasons for comparatively low updraughts in maritime convective clouds. An increase in the concentration of small cloud condensation nuclei (CCN) leads to the formation of a large number of small droplets with a low collision rate, resulting in a time delay of raindrop formation. Such a delay prevents a decrease in the vertical velocity caused by the falling raindrops and thus increases the duration of the diffusion droplet growth stage, increasing latent heat release by condensation. The additional water that rises to the freezing level increases latent heat release by freezing. As a result, clouds developing in continental-type aerosol tend to have larger vertical velocities and to attain higher levels.The results show that a decrease in precipitation efficiency of single cumulus clouds arising in microphysically continental air is attributable to a greater loss of the precipitating mass due to a greater sublimation of ice and evaporation of drops while they are falling from higher levels through a deep layer of dry air outside cloud updraughts. By affecting precipitation, atmospheric aerosols influence the net heating of the atmosphere. Simulations show that aerosols also change the vertical distribution of latent heat release, increasing the level of the heating peak.Clouds arising under continental aerosol conditions produce as a rule stronger downdraughts and stronger convergence in the boundary layer. Being triggered by larger dynamical forcing, secondary clouds arising in microphysically continental air are stronger and can, according to the results of simulations, form a squall line. The squall line formation was simulated both under maritime (GATE-74) and continental (PRE-STORM) thermodynamic conditions. In the maritime aerosol cases, clouds developing under similar thermodynamic conditions do not produce strong downdraughts and do not lead to squall line formation.Thus, the 'aerosol effect' on precipitation can be understood only in combination with the 'dynamical effect' of aerosols. Simulations allow us to suggest that aerosols, which decrease the precipitation efficiency of most single clouds, can contribute to the formation of very intensive convective clouds and thunderstorms (e.g. squall lines, etc.) accompanied by very high precipitation rates. Affecting precipitation, net atmospheric heating and its vertical distribution, as well as cloud depth and cloud coverage, atmospheric aerosols (including anthropogenic ones) influence atmospheric motions and radiation balance at different scales, from convective to, possibly, global ones.

Role of Water Vapor Content in the Effects of Aerosol on the Electrification of Thunderstorms: A Numerical Study

“…In the simulations with a low water vapor content, increasing the aerosol concentration had little effect on the concentration of graupel particles. These results are consistent with previous studies [36][37][38][39][40][41][42] in which increasing the initial CCN concentrations reduced the droplet size, enhanced condensation growth, and reduced the efficiency of the collision and coalescence of droplets. This led to more small droplets being transported above the freezing level and enhanced the growth of ice-phase particles under conditions of ample amounts of water vapor.…”

Inter-Annual Variations of Cloud and Precipitation and their Possible Relationships with Surface Aerosols in Shanghai

“…These results show very weak correlations between surface aerosols (e.g., PM 10 ) and moderate rain. However, higher aerosol loadings could allow more moisture to reach the freezing level (Khain et al, 2005;Lau et al, 2005). Ice nucleation in the mid-troposphere occurs immediately above the freezing level and/or the influence on initial updrafts ruins the low-level warm clouds by aerosol indirect or semidirect effects (Menon et al, 2002;Kaufman et al, 2005;Zhao et al, 2006).…”

Climate diagnostics of three major drought events in the Amazon and illustrations of their seasonal precipitation predictions

“…The possible physical mechanisms leading to the effect of aerosol in cloud formation are summarized as follows. The expressive increase of biomass burning aerosol, which acts as cloud condensation nuclei for cloud droplet formation, can reduce the precipitation formation process efficiency (Rosenfeld, 1999;Ramanathan et al, 2001;Khain et al, 2005;Martins et al, 2009). Even for a moist saturated atmosphere, due to the abundant amount of aerosol, size droplets may not grow enough to precipitate in the form of rain (Twomey, 1974;Silva Dias et al, 2002;Koren et al, 2004).…”