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S. Raghavan
and
S. Rajesh

Contrary to the common perception that tropical cyclones are on the increase, due perhaps to global warming, studies all over the world show that, although there are decadal variations, there is no definite long-term trend in the frequency or intensity of tropical cyclones over the period of about a century for which data are available. There is, nevertheless, a sharp increase in the socio-economic impact of tropical cyclones in the form of increasing property damage. An analysis of cyclones affecting the state of Andhra Pradesh, India, in the last quarter century by normalizing cyclone damage for economic and demographic factors shows that here, as elsewhere, the greater vulnerability is attributable mainly to these factors and not to any increase in frequency or intensity of cyclones. The decrease of alertness in disaster management that often occurs after a few years' lull in occurrence of cyclones, known as the “fading memory syndrome,” also contributes to increases in loss of lives and property damage. This distinction between meteorological and socioeconomic causes for the increased impact is important to avoid a tendency for political and administrative decision makers to blame natural causes. They have to take these realities into account, not just in developing a vigilant disaster management system, but in land-use planning, development of coastal districts, and insurance measures.

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S. Raghavan
,
T. R. Sivarmakrishnan
, and
B. Ramakrishnan

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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D. S. Zrnić
,
R. Raghavan
, and
V. Chandrasekar

Abstract

This paper discusses an application of polarimetric measurements at vertical incidence. In particular, the correlation coefficients between linear copular components are examined, and measurements obtained with the NSSL's and NCAR's polarimetric radars are presented. The data are from two well-defined bright bands. A sharp decrease of the correlation coefficient, confined to a height interval of a few hundred meters, marks the bottom of the bright band.

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