Datensatz

Zusammenfassung

The second aerosol indirect effect (AIE, the cloud lifetime effect) is due to drizzle suppression by greater CCN concentrations, N_CCN. However, many observations in POST California stratus and RICO Caribbean cumuli 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, 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 N_CCN-cloud droplet relationships. In RICO these relationships were often obscured by drizzle. For all 13 POST flights N_CCN active at 1.5% supersaturation (N_1%S) and active at 0.04% supersaturation (N_0.04%S) were negatively correlated with N_d except positively at 800-1400 µm diameter. When only the six clean POST flights are considered most N_CCN-N_d 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 N_0.04%S-N_d relationships seemed to reduce the magnitude of the negative N_1.5%S-N_d relationships. This was revealed by much stronger negative N_1.5-0.80%S-N_d relationships. CCN active at only very high supersaturations, N_1.5-0.80%S, are deduced by subtracting N_0.80%S from N_1.5%S. Thus, in RICO second AIE was reduced by positive N_0.04%S-N_d relationships. N_0.04%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 N_CCN active at various supersaturations.