Cirrus clouds physical and spatiotemporal features in the Wider Caribbean

B. BARJA, J. C. ANTUÑA

Abstract

Studies on cirrus clouds, their occurrence frequency and the physical characteristics are an actual research topic for the scientific community. Cirrus clouds influence on the radiation budget of the surface-atmosphere system and thus the climate. Moreover, cirrus clouds play a determinant role in the regulation of the water vapor quantity in the upper troposphere and lower stratosphere. The lack of information and studies about cirrus clouds distribution in the Wider Caribbean region is the motivation of the present work. In the present article the atmospheric extinction profiles from the SAGE II sensor onboard the Earth Radiation Budget Satellite (ERBS) have been used to analize the occurrence frequency and the optical and geometrical depth of the optically thin cirrus clouds over the Wider Caribbean region. The extinction ratio between the two wavelengths at 0.525 and 1.02 μm was used to separate the profiles containing only aerosols from thus containing a cloud/aerosols mixture (Kent and McCormick, 1991). This ratio shows a different behavior in the interaction between solar radiation crystal ice and aerosols at both wavelengths. The SAGE II dataset version 6.2 was used, extending 21 years, from October 1984 to August 2005. The results show a seasonal behavior in the cirrus clouds occurrence frequency; with a maximum of 35.5 and 17.2% in the rainy and little rainy periods, respectively. These peaks of cirrus clouds frequencies are located at 14.5 and 12.0 km of altitude. An occurrence of 9% was obtained above the tropopause level. About 75% of the clouds never reached the tropopause height, and 16% are in the tropopause. The isolated cirrus clouds have lesser frequency and optical and geometrical depth than the optically thin clouds attached to a lower opaque clouds. The more frequent geometrical depths of isolated clouds were in the 0.5 and 2.5 km range for the altitudes between 10 to 20 km, with a mean value of 1.96 km, with a mean optical depth of 4.0 x 10-3 at 0.525 μm. The 99% of the clouds were classified as subvisual cirrus, below the threshold value of 3.0 x 10-2. A comparison of the present results with other studies for the whole tropical region was conducted, finding some particular features for the Wider Caribbean region.

Keywords

Optically thin clouds, clouds occurrence frequency, SAGE II.

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