What controls the fractional cloudiness of fair-weather cumuli in a tropical marine environment?

 

Authors

Bruce A. Albrecht — University of Miami
Virendra Prakash Ghate — Argonne National Laboratory
Pavlos Kollias — Stony Brook University

Category

Cloud Properties

Description

The fractional coverage of fair-weather cumuli in marine environments has an important impact on Earth’s surface radiation budget, since these clouds are the most prevalent cloud type over the oceans. Here we make use of ARM observations made from Nauru Island (0.5°S, 167°E) from 1999–2000 during a prolonged period of suppressed conditions at the island. Non-precipitating fair-weather cumuli were the dominant cloud type, and little precipitation was observed. Monthly averages of fractional cloudiness obtained from the ceilometer vary from about 10 to 20% on a seasonal time scale during this period. The fractional cloudiness has a negative correlation (r=-0.84) with the sea surface temperature and the large-scale low-level boundary-layer temperature advection (from NCEP Re-analysis). In this study we examine the processes and parameters responsible for this negative correlation. The cumulus cloud-base mass flux and fractional area coverage of active updrafts are obtained from MMCR Doppler velocity measurements. A mixed-layer model of the subcloud layer based on monthly averaged surface and near-surface temperature and moisture measurements and cloud base height from a ceilometer provides estimates of the convective velocity scale w*. Thermodynamic soundings are used to estimate the static stability in the lower troposphere, the convective inhibition (CIN) at cloud base, and the relative humidity in the cloud layer. These parameters are used to establish the role of various physical processes in regulating the fractional cloudiness of fair-weather cumuli in a marine tropical environment.