Low‐altitude summer/winter microphysics, dynamics, and CCN spectra of northeastern Caribbean small cumuli, and comparisons with stratus

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AbstractLow‐altitude cloud microphysics and cloud condensation nuclei (CCN) spectra are presented and analyzed from two cumulus cloud aircraft field campaigns, Ice in Clouds Experiment‐Tropical (ICE‐T) and Rain in Cumulus over the Ocean (RICO), in the same eastern Caribbean location at opposite seasons. These are further compared with two previously analyzed stratus cloud aircraft campaigns off the central California coast, Physics of Stratocumulus Tops (POST) and Marine Stratus/Stratocumulus Experiment (MASE). Multiple regression analysis predictions of cloud droplet concentrations (Nc) based on CCN spectra, NCCN(S) where S is supersaturation, compared much better with measured Nc than single NCCN‐Nc regressions in both cumulus projects. The addition of vertical velocity (W) to the single and multiple regressions showed small improvements. For RICO the multiple regression correlations were also superior to previous adiabatic model predictions of Nc also based on NCCN(S) and mean W. More adiabatic cloud parcels with considerably higher Nc, liquid water contents, and W showed only slightly better correlations than flight‐averaged Nc of all low clouds. Results show the value of more extensive CCN spectra and the relative unimportance of W variations for determining Nc, in these Caribbean cumuli. The fact that flight‐averaged Nc of all low cloud data was almost as well correlated with NCCN(S) as were Nc of more adiabatic cloud parcels indicates that entrainment did not significantly perturb CCN‐Nc relationships. As should be expected higher cloud S were determined for the cumulus clouds than for the stratus clouds. Suppression of cloud S by higher NCCN that had previously been observed in stratus was observed in ICE‐T but not in RICO where the NCCN range may have been too low for cloud S suppression. But ICE‐T and POST even showed this S suppression over the same limited NCCN range as RICO (< 200 cm−3).