P84 Coupling aerosols to microphysics: sensitivity tests and initial results

Thompson, Greg, and Trude Eidhammer, National Center for Atmospheric Research

A newly coupled aerosol component has been added to the Thompson et al (2008)

microphysics scheme. The scheme was extensively tested with a suite of nine idealized simulations of both warm-rain and ice phase clouds. With these experiments, sensitivity of cloud and precipiation development to assumed aerosol characteristics were evaluated.

Next, the new scheme was tested in a large scale (continental U.S.), high resolution (4 km), and long duration (72 hours) winter storm case. As a test of the model without aerosols, the simulation used a very low constant concentration of cloud droplets (50 per cubic centimeter) followed by a moderately high concentration of 750 per cc. This established approximate bounds for the simulations using explicit aerosols activating as cloud droplets and ice in combination with the new two-moment cloud water scheme that adds prognostic cloud number concentration. As a baseline aerosol simulation, WRF was initialized with current era global, monthly

average aerosol conditions as developed by the GOCART model. Then, a sensitivity experiement with purposefully reduced aerosols, perhaps typical of pre-Industrial era simulated a very "clean" atmosphere and a second sensitivity experiment of greatly increased aerosols simulated a very "polluted" atmosphere. The complete connections between the water and ice species created from aerosols and directly interacting with the RRTMG radiation scheme very clearly illustrates the expected first aerosol indirect effect (Twomey, 197x) as well as the second indirect effect, cloud lifetime (Albrecht, 19xx). These sensitivity experiments and their potential for climate-related impacts will be discussed.