Microwave Imagers

VIS and IR provide proxy variables. Exact positioning of the eye is possible in VIS and IR imagery. Microwave (passive) sensors monitor radiation from below the cirrus shield and provide information on atmospheric WV, cloud liquid water, precipitation, intensity and regions of convective activity. In June of 1987 the first satellite from the Defence Meteorological Satellite Programme (DMSP) carrying a microwave radiometer called Special Sensor Microwave/Image (SSM/I) was launched. It overlapped with the European Research Satellite (ERS-I) launched in July 1991 which carried a scatterometer. The swath width of the ERS-I scatterometer was only 500 km resulting, however, in the less than-complete coverage of the tropical regions each day.

The low horizontal resolution of some current radiometers may limit the usefulness of some parameters but the 15 km resolution of 85 GHz channel of SSM/I provides meso-scale information. This channel provides radar like imagery and is able to discern the circulation centres. Velden et al. (1989) describe the advantage of centre-fixing in TCs using 85 GHz imagery compared to conventional VIS and IR images where centres are covered in cirrus overcast. NASA launched a special satellite that aimed at making new measurements of meteorological quantities in the tropics. The Tropical Rainfall Measuring Mission (TRMM) has completed 3 years of successful data taking in 2000. With the change of its altitude from 350 km to 400 km, it will have enough fuel to provide continuous measurements upto 2005 (Velden and Hawkins, 2002). TRMM Microwave Imager (TMI) provides horizontal resolution of 5-7 km for 85 GHz channel which is 2-3 times better than the SSM/I, and the higher resolution TMI 37 GHz channel; can penetrate deeper into tropical cyclone to reveal additional details.

The Indian Remote Sensing Satellite (IRS-P4) launched in 1999 carried Multichannel Scanning Microwave Radiometer (MSMR). It is providing measurements at 6.6, 10, 18 and 21 GHz frequencies in both H&V polarizations. Attempts have been made to provide estimates of integrated water vapour, liquid water content, precipitation intensity, and SSTs over the global oceans (Gohil et al., 2001).

0 0

Post a comment