Microphysical, macrophysical, and radiative impacts of aerosol on different cloud systems

Description

The Weather Research and Forecasting (WRF) model was previously adapted to include the aerosol indirect effect via a two-moment bulk microphysics scheme and the aerosol direct effect via a modified Goddard radiation scheme. Three cases from the March 2000 Cloud Intensive Observational Period campaign and three cases from 2009 Routine AAF Clouds with Low Optical Water Depths (CLOWD) Optical Radiative Observations (RACORO) field campaign at the ARM Climate Research Facility’s Southern Great Plains site were examined, including a developing low-pressure system, a cold frontal passage, a series of non-precipitating or weakly precipitating stratus and cumulus, etc. The observed profile of the aerosol concentration from the in situ measurements was used for the control run, and the perturbations are made on the aerosol profile in the sensitivity studies. To investigate the effect of aerosol radiative effect, each profile was run with and once without the modified Goddard scheme for each case. For warm and stratiform precipitation events, generally a reduction of precipitation was found with more aerosols introduced. For mixed-phase and convective precipitation events, generally a nonlinear trend was found where the moderate profiles had the highest rain rates. Including or removing the direct effect proved to have a statistically significant effect of cloud fraction. In all the cases, the liquid water path (LWP) roughly increased with the elevation of aerosol concentrations, corresponding to a decrease in surface shortwave radiation. The outgoing longwave radiation was mainly regulated by the cloud fraction, but had a lower magnitude of change than the shortwave radiation. Therefore, the changes of shortwave radiation induced by aerosols dominated the temperature variation. Results from this study suggest that aerosols play a critical role in macro- and micro-properties of different clouds and the precipitation efficiency. Meanwhile, the direct effect of aerosol has to be taken into account if we aim at an accurate assessment of the aerosol-cloud-radiation interaction in the different cloud systems.