Changes in Ice Water Content Altitude Profiles as a Function of Absorptive Aerosol Optical Depth

Cloud top heights are expected to increase as aerosol optical depths (AODs) increase due to “cloud invigoration” processes (i.e. aerosols change the altitude profile of latent heat release and thereby modify cloud structure). Absorptive aerosol, however, is expected to inhibit convection due to its stabilizing effects on temperature in the first several kilometers of altitude. We use Delanoë and Hogan “DARDAR” Calipso-Cloudsat-Modis ice water content (IWC) data profiles, MODIS AODs, OMI absorptive (AAODs), and MLS CO mixing ratios at 215 hPa, to quantify how cloud structure is modified by absorbing aerosol. CO is a proxy for aerosol, and since CO is produced by combustion, the aerosol associated with CO is sooty and therefore absorptive.

We select 5 IWC thresholds and determine for each tropical region on a profile-by- profile basis the altitudes Ziwc at which the 5 thresholds are exceeded for representative bin ranges of AOD, AAOD, and CO. We calculate changes in the Ziwc altitudes due to changes in MODIS AODs, OMI AAODs, and MLS CO. These changes are then normalized by the AOD, AAOD, and CO increments to calculate derivatives (i.e. in m / 0.10 AOD, m / 0.02 AAOD, and m / 100 ppbv units).

The derivatives associated with the absorptive AODs and MLS CO mixing ratios are frequently negative and smaller in absolute size than those of the positive derivatives associated with the MODIS AODs. This is reasonable, since MODIS AODs include scattering aerosol which promotes invigoration and which is not expected to stabilize temperature profiles as efficiently as the absorbing aerosol.