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Abstract:
The second aerosol indirect effect (AIE, the cloud lifetime effect) is due to drizzle suppression by greater CCN concentrations, NCCN. However, many observations in POST California stratus and RICO Caribbean cumuli revealed opposite positive relationships between NCCN and drizzle drop concentrations, Nd. The sign and magnitude of these relationships depended on the CCN supersaturation, threshold cloud droplet liquid water content that defined cloud parcels, drop sizes and cloud parcel altitude. In POST, first AIE (cloud brightness effect) distinctly exhibited presumed positive NCCN-cloud droplet relationships. In RICO these relationships were often obscured by drizzle. For all 13 POST flights NCCN active at 1.5% supersaturation (N1%S) and active at 0.04% supersaturation (N0.04%S) were negatively correlated with Nd except positively at 800-1400 µm diameter. When only the six clean POST flights are considered most NCCN-Nd relationships are positive, especially at diameters greater than 200 µm. When only the seven POST polluted flights are considered the majority of such relationships are also positive. In RICO strong positive N0.04%S-Nd relationships seemed to reduce the magnitude of the negative N1.5%S-Nd relationships. This was revealed by much stronger negative N1.5-0.80%S-Nd relationships. CCN active at only very high supersaturations, N1.5-0.80%S, are deduced by subtracting N0.80%S from N1.5%S. Thus, in RICO second AIE was reduced by positive N0.04%S-Nd relationships. N0.04%S seemed to function as giant nuclei, GN, but they are smaller and in higher concentrations than GN and they were unrelated to wind speed. These results indicate that second AIE is differently affected by NCCN active at various supersaturations.
