Numerical Simulation Of Aerosol Effects On Cloud And Precipitation Using GRAPES Mesoscale Model

In order to study the effects of aerosols on cloud and precipitation processes usingGRAPES mesoscale model, a two-moment mixed-phase microphysics scheme is employed.A parameterization approach of aerosol activation is firstly introduced into the originalmicrophysics scheme, and relevant microphysical processes are added or modified, makingcloud droplet number concentration a new prognostic variable. Then a summer precipitationprocess and a winter precipitation process are simulated and analyzed.Two experiments with different aerosol concentrations are conducted for eachprecipitation process. The results show that when aerosol concentration increases, the biggestchange of microphysical characteristics is apparent increase of cloud water content. This isbecause of the suppressed conversion from cloud droplets to rain drops when more aerosolsnucleate into more but smaller cloud droplets, which results in more liquid water path andless efficient warm rain processes. On the other hand, with more cloud water left in the air,ice phase particles, especially snow and graupel, can grow through collecting more cloudwater and increased snow can also be collected by graupel. Melted snow and graupeleventually produce more rain. Meanwhile, more latent heat will be released with the increaseof ice particles, which can invigorate cloud updrafts and further promote rain development. Inthe simulation region, aerosol effects on precipitation is spatially non-uniform, resulting fromnon-uniform distribution of warm cloud and cold cloud. In the region where warm clouds arethick, suppressed warm rain processes is the main factor affecting precipitation and surfacerainfall is reduced. While in the region where warm cloud is thin and cold cloud is relativelythicker, although warm rain is reduced, cold rain processes enhancement is dominant andsurface rainfall is increased. Aerosol effects on precipitation is also temporally non-uniformbecause the distribution of warm cloud and cold cloud changes during the development ofcloud and precipitation.Sensitivity experiments across a wide range of aerosol concentrations are conducted forthe winter rain process. The results show that when the aerosol concentration is relatively low,with increasing aerosols cloud water content has a dramatic increase. Accordingly, iceparticles contents increase, leading to the increase of updraft velocity. Melted precipitation from enhanced cold cloud process contributes to increased surface rainfall with increasingaerosols. But when the aerosol concentration is relatively high, most of the cloud propertiesmentioned above become less sensitive to increasing aerosols, mainly resulting from the slowdown of cloud water growth. The extremely suppression of warm rain process leads to thereduced averaged accumulated precipitation in heavily polluted air.