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2. Remote 
"rvation of 
of India. 
IAPRS & SIS, Vol.34, Part 7, "Resource and Environmental Monitoring", Hyderabad, India, 2002 
REMOTE SENSING OF SEA SURFACE COOLING BY A TROPICAL CYCLONE 
M. V. Rao, B. Jena, I. V. Ramana and M. M. Ali 
Oceanography Division, National Remote Sensing Agency 
Hyderabad-500037, A. P., India 
KEY WORDS: Sea surface cooling, Tropical cyclone, Upwelling, MSMR, and TRMM/TMI. 
ABSTRACT: 
Sea surface temperatures (SSTs) are considerably reduced in response to the passage of a tropical cyclone. Some of the causes for 
this cooling are due to heat release to the atmosphere, the upwelling-taking place due to the cyclonic nature of the winds and due to 
the rainfall. The cooling caused by the passage of hurricane was extensively studied in the Pacific Ocean. The temperature drop in 
the case of hurricane passage has a broad range varying from 1°C to 6°C. The swath of the area affected by the cyclone varies from 
200-400Km. Such a detailed study was not available over the Bay of Bengal where the cyclonic activity is prominent, particularly, 
during the post monsoon periods. In the present study the effect of the October 1999 cyclone in reducing the SSTs is studied. 
Tropical Rainfall Measuring Mission (TRMM) carries a TRMM Microwave Imager (TMI). Besides sea surface wind speeds, 
atmospheric water vapor, cloud liquid water, and precipitation rates, this sensor provides SST with an accuracy of 0.5°K with a 
spatial resolution of 0.25°. Besides; SST is also available from Multi-frequency Scanning Microwave Radiometer (MSMR) onboard 
IRS-P4 with a coarse resolution of 1.5°x1.5°. Bay of Bengal was influenced by two cyclones during October-1999, one during ist 
19" and the other during 25^ to 31*. The first cyclone was classified as the severe cyclonic storm. The second cyclone caused so 
much of damage that a new terminology of ‘Super cyclone’ has emerged. At least 10,000 people died and many more lost their 
homes due to this cyclone. Satellite remote sensing gives an opportunity to study these phenomena in greater detail. 
In this analysis SSTs available from both TMI and MSMR were analyzed from 2"‘October to 25" November 1999. Though the 
microwave sensors have all weather capabilities, SSTs can't be obtained during heavy raining conditions. Hence to over come the 
problem of having no observations during the cyclone day, weekly analysis has been carried out week starting from 3% 9,9 
October. In order to enhance the SST features, weekly SST deviations with respect to the week of 3" to 9" October 1999 were 
analyzed. Similar analysis was carried out during October-November of 1998 and 2000. The analysis of other years was done to 
study the SST deviations during the non-cyclonic periods also. Before the passage of the storm, the SSTs over the cyclonic area are 
0.5° to 1°C more than those in the reference week. After the passage, the SST has reduced to more than 1.5°C. the effect of this 
cooling is spread over 300Km along the coast. Such a reduction is significant over the Indian Ocean and has a dramatic influence on 
the potential fishing zones and air sea interaction. Such an activity is not observed in the other years indicating that the cooling 
observed in 1999 is due to the cyclonic storm. On the other hand, SST anomalies are positive over these regions. The upwelling 
caused by the cyclonic storm has to be balanced by the downwelling at the adjoining place. Since the storm is closer to the coast, the 
downwelling might have probably taken place to the west of the storm, which is evident from the positive SST anomalies towards 
the west of the cyclonic track. The storm also has generated coastally trapped Kelvin waves with cooler waters that propagated 
towards the east coast of India. The extent of these cooled waters near the east coast of India is also prominent in 1999 compared to 
the other years. MSMR observations are not available at two-foot prints closer to the coast. Hence a detailed pattern similar to the 
one observed in TMI is not observed in the SST anomalies. Thus the coastally trapped Kelvin waves could not be reproduced in the 
MSMR-SST anomalies. However, the cooling to the west of the track is clearly seen. 
1. INTRODUCTION constitutes the most hazardous zone of tropical cyclone, as 
winds here are extremely strong, rainfall very heavy, vertical 
Tropical cyclones are large revolving vortices in the velocity very large and the strongest convention arranged in the 
atmosphere extending horizontally from 150 to 1000 km and form of wall cloud in the region. Surround-ing this region, 
vertically from surface to 12-14 km with fierce winds (speeds spiraling winds extends outward to large distances, with 
could reach even 280 km/h) spiraling anti- clockwise around decreasing speeds away from the center. The rate of decrease 
the center. Cyclones generally move 300-500 km per day over of winds may be rapid or gradual. The tropical cyclones are 
the ocean and while doing so pick up energy from the warm divided in to eight classes, namely Low Pressure Area, 
water of the ocean and some of these grow into a devastating Depression, Deep Depression, Cyclonic Storm, Severe 
intensity. There -are four stages in a tropical cyclone Viz; 1. Cyclonic Storm, Very Severe Cyclonic Storm, and Super 
Beginning, 2. Growing, 3. Mature, and 4. Decay. A mature Cyclonic Storm. The analyzed trends and oscillations in 
tropical cyclone consists of a central region of light winds monthly and annual frequencies of tropical cyclones in the 
known as its eye. The eye normally has an average radius of North Indian Ocean from 1877 to 1998 have been presented 
about 20 to 30 km, but it can be 40 to 50 km in large mature (Singh et al, 2000, 2001 and Rao et al 2001). The analysis 
storms. The pressure is the lowest and temperature the highest indicates a slightly decreasing, but non-significant, trend of - 
in this region with either clear to partly cloudy skies. A ring of 0.8/100 years, in tropical cyclone frequency over the North 
very strong winds extending an average upto 30 to 50 km Indian Ocean. 
beyond the centre surrounds the eye. This area is known as 
zone of maximum wind. The zone, though very narrow, 
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