Maritime/continental microphysical contrasts in stratus

Abstract

More than 15 h of in situ cloud measurements in Atlantic and Pacific stratus showed cloud droplet concentrations to be correlated with cloud condensation nuclei (CCN) concentrations measured just below cloud. Predictions of droplet concentrations based on complete CCN spectra and measured updraft velocities were better correlated with measured cloud droplet concentrations. This was especially significant for near-adiabatic parcels because these results represent a good level of closure between CCN and cloud microphysics. Over the Atlantic there was a clear dichotomy between maritime and continental cloud microphysics that was related to CCN concentrations: smaller and more numerous droplets in continental clouds with higher CCN concentrations. However, there seemed to be undercounting of continental cloud droplets because many were smaller than the 2 μm diameter threshold of the droplet spectrometer. When averaged over all altitudes, the drizzle (diameter >50 μm) liquid water content was a factor of 2 greater in the maritime clouds. However, as in previously reported small cumuli, drizzle was also a function of cloud depth. Therefore when only thin clouds (depth less than 45 mb) were considered, the maritime clouds had an order of magnitude more drizzle than continental clouds. Drizzle liquid water content was also found to depend on the mean diameter of 2–50 μm diameter cloud droplets. In the thin clouds there was a definite 15 μm mean cloud droplet diameter threshold for the onset of drizzle that was often exceeded in the maritime clouds but almost never exceeded in the continental clouds. There were also higher concentrations of 20–50 μm diameter droplets in the maritime clouds that were commensurate with the maritime/continental drizzle (50–620 μm diameter) contrast. Furthermore, when cloud parcels with equal amounts of cloud droplet liquid water were compared, there was considerably more drizzle in the maritime clouds, especially for larger amounts of cloud water in the thin clouds. All of these maritime/continental contrasts found in thin stratus clouds are very similar to previously reported maritime/continental contrasts in small cumulus clouds. These results support the second indirect aerosol effect (pluvial inhibition) for stratus clouds.