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far away from the coast and not within the radar range, satellite estimated
intensity based on Dvorak's technique (Dvorak, 1984) gets maximum weight.
When the system comes closer to the coast, radar estimated intensity is
considered along with satellite estimated intensity. When the system is very
close to coast or over the land surface, the coastal observations get the highest
preference followed by radar and satellite observations for estimating the
intensity.
2.4
Errors in Estimation of Location and Intensity
during Satellite Era
The average difference in location of a TC based on satellite estimation of
IMD and best track estimates is about 55 km in the deep ocean (Goyal et al.,
2013). The average error in MSW estimation has reduced over the years. During
the pre-satellite era (till 1960), the average error in intensity estimation may be
at least one stage in Beaufort scale (5-15 knots or 3-8 mps upto severe cyclonic
storm stage). There is no classification of intensity between very severe cyclonic
storm and above intensity in Beaufort scale. The error could have reduced
gradually during polar satellite era. It could have been T0.5 (05-20 knots or 3-
10 mps) with the introduction of Dvorak's classification of intensity since 1974.
Based on seven TCs, Mishra and Hem Raj (1975) have shown large difference
between wind speed inferred from synoptic data and those derived from satellite
technique. According to them, the MSW could be under-estimated by 8-17
knots (5-9 mps) in depression/deep depression stage, 26-28 knots (13-14 mps)
in cyclonic storm stage and 37 knots (19 mps) in severe cyclonic storm or
higher stage. Based on recent years data, Goyal et al. (2013) have shown that
there is a difference of T0.5 in the estimation of intensity by satellite method
and best track estimates.
As there is no aircraft reconnaissance in the NIO, Dvorak's technique,
which is based on one minute averaging, has not been verified. Coastal stations,
equipped with cup anemometer and Dynes P.T. anemograph, use three-minute
averaging in Indian region for estimating MSW. However, they have the
limitation to estimate higher intensity. The high wind speed recorders along
the coast of India installed in 2000s are not sufficient to cover the entire coast.
The wind conversion factor for conversion of three-minute wind to one-minute
wind has not been applied to the data over the NIO. Harper et al. (2008)
discussed in details the issues involved with conversion of wind speed based
on different averaging periods.
There are various agencies including IMD, Satellite service division of
NOAA and Joint Typhoon Warning Centre (JTWC) which provide the satellite
estimated location and intensity. However, there is difference in estimation of
location and intensity of CDs by various operational centres due to the
subjectivity involved in the estimation. However, the difference in estimation
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