Positive and Negative CCN Drizzle Relationships in Cumulus and Stratus Clouds

JAMES HUDSON, Stephen Noble, Desert Research Institute

     Abstract Number: 144
     Working Group: Aerosols, Clouds and Climate

Abstract
The second aerosol indirect effect (AIE, cloud lifetime) is due to drizzle suppression by greater cloud condensation nuclei (CCN) concentrations, N_CCN. However, several observations in RICO Caribbean cumuli and POST California stratus revealed opposite positive relationships between N_CCN and drizzle drop concentrations, N_d. The sign and magnitude of these relationships depended on the CCN supersaturation (S), cloud droplet liquid water content threshold that defined cloud parcels, and on drop sizes. In both projects, first AIE (cloud brightness) was distinctly exhibited by strong positive N_CCN-cloud droplet concentration, N_c, relationships. This was true for CCN active at many different S. However, the largest cloud droplets (diameter > ~ 35 µm) showed negative relationships with N_CCN that are consistent with negative N_CCN-N_d.

In RICO strong positive N_CCN-N_d relationships for CCN active at low S (< 0.1%, larger CCN) such as at 0.04%, N_0.04%, seemed to act like giant nuclei, GN, which have been perceived to induce drizzle. Though GN exhibited positive relationship with N_d these relationships were less statistically significant than the positive N_0.04%-N_d relationships. Although N_0.04% does include some GN, most N_0.04% are smaller and in greater concentrations than N_GN. Moreover, unlike N_GN, N_0.04% was not related to low altitude wind speeds that produced the GN. Conventional strong negative N_CCN-N_d were found when exclusively high S N_CCN were considered by subtracting lower S N_CCN such as N_0.80% from N_1.2%. Thus, in RICO second AIE was reduced by positive N_0.04%-N_d relationships even to the extent of some positive N_1.2%-N_d relationships. These results indicate that second AIE is differently affected by N_CCN active at various S.

In the POST stratus clouds these N_CCN-N_d relationships also varied with S. In some cases these relationships were similar to those in RICO, but in other cases N_CCN-N_d relationships were opposite of those of RICO. Opposite N_CCN-N_d relationships were also found between clouds within clean and polluted air masses and between relatively lower and higher altitudes within these stratus clouds.