Multispectral analysis of a cloud field observed close to the Argentine coast: a case study

Abstract

A multispectral analysis of the data provided by the Advanced Very-High-Resolution Radiometer (AVHRR) on board the NOAA-14 satellite is used to obtain information about a cloud system located over the Buenos Aires Province (Argentina) and the adjacent Atlantic Ocean, up to about 500 km from the coast. For selected areas of the cloud system, relationships are obtained between satellite-retrieved effective radius ( r e) and cloud top temperature ( T). It is shown that, in the middle-level clouds on the ocean, the cloud top structure is characterized by small drops ( r e≈6–7 μm) at temperatures a few degrees below −5 °C, and by an increase of drop size up to r e≈20–25 μm at temperatures between −15 and −18 °C, showing evidences of mixed-phase growth mechanism and glaciation. However, in the cloud area facing the continent, small particles ( r e<8–10 μm) were found to prevail up to the maximum altitudes, where T≈−20 °C. Comparing Geostationary Operational Environmental Satellite (GOES) images of the same region, taken at time intervals of about half an hour, a possible correlation is suggested between the different top microstructures and cloud ages in the last area. Low-level clouds are observed on the continent, with top temperatures varying between about 12 and −8 °C. In this case, r e slightly increases with altitude up to about the 0 °C level, reaching maximum values close to 10–13 μm; but it decreases above this altitude, to about 5–6 μm in the highest tops. The possibility that the highest tops (where the drop size decreased with altitude) would not belong to an upper independent cloud layer—as in the cases considered by Lensky and Rosenfeld—but to the cloud system below is discussed by comparing their limited optical thickness (RC1≈0.4–0.5) with that of the lower clouds, which, at 0 °C, varies in the more extended interval RC1≈0.4–0.8.