Cloud vertical structure observed from space and ship over the Bay of Bengal...

Zuidema, P., and B. E. Mapes (2008), Cloud vertical structure observed from space and ship over the Bay of Bengal and eastern tropical Pacific, J. Meteor. Soc. Japan, 86A, 205-218.

The cloud vertical structure at two remote tropical oceanic convective locations is characterized using both ship-based (35 GHz) and space-based (94 GHz) cloud radar data. The field experiment data depicted the full diurnal cycle and detail of the vertical structures, while the CloudSat sampling was more extensive, but did not coincide with the field experiments. One location is the monsoonal Bay of Bengal, sampled as part of the Joint Air-Sea Monsoon Interaction Experiment (JASMINE), and the other location is the eastern Pacific intertropical convergence zone, sampled during the Eastern Pacific Investigation of Climate (EPIC) experiment. Both ship- and space-based radar datasets find more high cloudiness (9−14 km altitude with an 11−12 km maximum) over the Bay of Bengal than over the eastern tropical Pacific, postulated to reflect the advection of cirrus by upper-level easterly winds. Low-level cloudiness, with some extending into what may be cumulus congestus, is more characteristic of the eastern tropical Pacific than the Bay of Bengal. CloudSat cloud fractions over the eastern Pacific were only ~ one-third those over the Bay of Bengal, in contrast to comparable cloud fractions within the field experiment datasets. A thin melting-level cloud population was also more apparent in the JASMINE data than in the EPIC field experiment data, but the larger CloudSat dataset showed the opposite. These differences discourage regional generalities based on a few weeks of point sampling. Pre-monsoon-onset conditions included a slight afternoon low cloud amount maximum combined with typically one warm rain event per night. High thin cirrus was common (cloud fraction of ~35% with cloud optical depths of 2 or less and ice water paths typically < 40 g m−2). After monsoon onset, the ship-based cloud radar documented examples of coherent relationships among cloud structure, precipitation, and the larger-scale wind field. CloudSat did not detect much precipitating low cloud during its pre-monsoon time period, but did sample the upper-level clouds at approximately the maximum and minimum of their diurnal range. An intriguing finding is that CloudSat perceived a greater daytime occurrence of the highest clouds with reflectivitives < 10 dBZ at both locations, a finding replicated with a year’s data covering the full, ocean-only, tropical belt. We speculate the daytime high cloud may reflect remains of outflow from previous convection. The comparison also highlights differences in the Rayleigh versus Mie responses of the two radars.