Impacts of Nucleating Aerosol on Florida Storms. Part I: Mesoscale Simulations

Abstract

Toward the end of the Cirrus Regional Study of Tropical Anvils and Cirrus Layer-Florida Area Cirrus Experiment (CRYSTAL-FACE) field campaign held during July 2002, high concentrations of Saharan dust, which can serve as cloud condensation nuclei (CCN), giant CCN (GCCN), and ice-forming nuclei (IFN) were observed over the peninsula of Florida. To investigate the impacts of enhanced aerosol concentrations on the characteristics of convective storms and their subsequent anvil development, sensitivity tests are conducted using the Regional Atmospheric Modeling System (RAMS) model, in which the initialization profiles of CCN, GCCN, and IFN concentrations are varied. These variations are found to have significant effects on the storm dynamics and microphysical processes, as well as on the surface precipitation. Updrafts are consistently stronger as the aerosol concentrations are increased. The anvils cover a smaller area but are better organized and have larger condensate mixing ratio maxima in the cases with greater aerosol concentrations. Cloud water mass tends to increase with increasing aerosol concentrations, with enhanced GCCN concentrations having the most significant influence. Increasing either the GCCN or IFN concentrations produces the most rainfall at the surface whereas enhanced CCN concentrations reduce surface rainfall. Higher IFN concentrations produce ice at warmer temperatures and deeper anvils, but simultaneously increasing the concentrations of CCN and GCCN leads to more supercooled liquid water available for freezing and greater ice mixing ratios. Graupel mixing ratios decrease and hail mixing ratios increase with increasing aerosol concentrations. Higher concentrations of GCCN and IFN result in greater accumulated surface precipitation initially. By the end of the simulation period, however, the accumulated precipitation is the greatest for the case in which the aerosol concentrations are lowest. Such changes in the dynamical and microphysical characteristics of convective storms as a result of the variations in aerosol concentrations have potential climate consequences, both through cloud radiative effects and the hydrological cycle. The impacts of varying CCN, GCCN, and IFN concentrations on the anvils will be discussed more fully in Part II.

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.…”

Dependence of aerosol-precipitation interactions on humidity in a multiple-cloud system

“…The enhancement in evaporation induced by the increase in aerosol concentration develops stronger downdrafts and, when stronger downdrafts descend below the cloud base and collide with environmental flow around the surface, low-level convergence can be intensified. The more intensified domain-averaged low-level convergence generated domain-wide more secondary clouds, inducing the more, stronger domain-averaged updrafts and thus the more domain-averaged condensation, cloud mass and precipitation (Khain et al, 2005;Seifert and Beheng, 2006;van den Heever et al, 2006;van den Heever and Cotton, 2007;Tao et al, 2007;Lee et al, 2008aLee et al, , b, 2009Lee and Feingold, 2010;Storer et al, 2010). Since lowering humidity enables more efficient evaporation, the effect of increases in aerosol concentration on low-level convergence should be stronger at lower humidity.…”

“…Increasing aerosol concentration is known to decrease droplet size and thus increase cloud albedo for a given liquid-water content (the first aerosol indirect effect) (Twomey, 1977). They may also suppress precipitation and, hence, alter cloud-water content and lifetime (the second aerosol indirect effect) (Albrecht, 1989).…”

“…Khain et al (2008) considered cases of a single mixed-phase cloud to examine the relation between humidity and aerosolprecipitation interactions. Evaporation of hydrometeors affects the low-level convergence of a mesoscale cloud ensemble (MCE) where deep convective clouds play an important role in dynamic and energy circulations (Khain et al, 2005;Seifert and Beheng, 2006;van den Heever et al, 2006;van den Heever and Cotton, 2007;Tao et al, 2007;Lee et al, 2008aLee et al, , b, 2009Lee and Feingold, 2010;Storer et al, 2010). Multiple deep convective clouds in the MCE grow above the freezing level to reach the tropopause.…”

Total Lightning Flash Activity Response to Aerosol over China Area

“…Some simulation studies [3][4][5] showed that enhancing the concentration of aerosols resulted in delaying the precipitation start time, stimulating the convection, and forming more big ice particles. Lightning intensity was closely linked with convection intensity.…”