Understanding the mechanisms controlling the vertical aerosol profile

Abstract:

Global climate models tend to overestimate the amount of aerosol aloft, contributing to uncertainty in aerosol radiative forcing. To better understand the reasons for this bias, we 1) compare the vertical profile of aerosol extinction coefficient from several CMIP5 models to that observed from CALIPSO; and 2) calculate linear cross correlations between convective mass flux, precipitation and aerosol vertical dispersivity using MERRA, GPCP and CALIPSO, respectively, to determine the relationships between convective lofting versus wet removal on the vertical aeorosol profile. Relative to CALIPSO, models underestimate aerosol extinction below 500 hPa by 2-11%, but overestimate aerosol extinction by 3-14% above 500 hPa. Results show more convective mass flux is associated with more precipitation over the entire globe. In the tropics, more convective mass flux and precipitation are correlated to more aerosol aloft. On the other hand, more convective mass flux and precipitation anticorrelate with aerosol aloft over the ocean between 30-60N. Aerosol aloft over this region is linked to the convective mass flux over the aerosol emission sources. Our results suggest ambient convective mass flux, leading to both enhanced aerosol lofting and wet removal by convective precipitation, contributing to model bias both below and above 500 hPa. Sensitivity experiment with CAM5 will be performed to try to better understand model deficiencies.