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Size Distribution of Radar Echoes as an Indicator of Growth Mechanisms in Monsoon Clouds around Madras

S. RaghavanCyclone Warning Radar, India Meteorological Department, Port Trust Building. Madras-600 001. India

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T. R. SivarmakrishnanCyclone Warning Radar, India Meteorological Department, Port Trust Building. Madras-600 001. India

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B. RamakrishnanCyclone Warning Radar, India Meteorological Department, Port Trust Building. Madras-600 001. India

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Abstract

A study of the size distribution of radar echoes from precipitating clouds around Madras during the southwest and northeast monsoon seasons shows a preponderance of echo sizes in the D scale (up to 100 km2) with relatively small percentages in the C scale (101 to 1000 km2) and in the B/C scale (>1000 km2). The largest echo size observed was 21 000 km2. If the cumulative percentage frequencies of areas of cells are plotted on logarithmic probability paper, the smaller cells constituting 85–95% of the total population are seen to follow a lognormal distribution. In the larger size ranges, however, systematic deviations on either side of the lognormal graph occur.

The lognormal distribution points to a growth mechanism of convective cells by a process whereby growth at every step is a random proportion of the initial size. The deviations from the lognormal distribution in the land area in the northeast monsoon season indicate limitation of growth after the cells which develop over the sea drift over the land. In the southwest monsoon season and in the sea area during the northeast monsoon, growth is found to occur to very large sizes more often than a lognormal distribution would predict. The deviation from lognormality appears to be due to development of a stratiform mesoscale anvil cloud similar to the model of Leary and Houze in the Global Atmospheric Research Program's (GARP) Atlantic Tropical Experiment (GATE).

Abstract

A study of the size distribution of radar echoes from precipitating clouds around Madras during the southwest and northeast monsoon seasons shows a preponderance of echo sizes in the D scale (up to 100 km2) with relatively small percentages in the C scale (101 to 1000 km2) and in the B/C scale (>1000 km2). The largest echo size observed was 21 000 km2. If the cumulative percentage frequencies of areas of cells are plotted on logarithmic probability paper, the smaller cells constituting 85–95% of the total population are seen to follow a lognormal distribution. In the larger size ranges, however, systematic deviations on either side of the lognormal graph occur.

The lognormal distribution points to a growth mechanism of convective cells by a process whereby growth at every step is a random proportion of the initial size. The deviations from the lognormal distribution in the land area in the northeast monsoon season indicate limitation of growth after the cells which develop over the sea drift over the land. In the southwest monsoon season and in the sea area during the northeast monsoon, growth is found to occur to very large sizes more often than a lognormal distribution would predict. The deviation from lognormality appears to be due to development of a stratiform mesoscale anvil cloud similar to the model of Leary and Houze in the Global Atmospheric Research Program's (GARP) Atlantic Tropical Experiment (GATE).

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