Satellite microwave estimation of tropical cyclone rainfall: A case study of Cyclone Joan

The ESMR ( electically scanning microwave radiometer ) sensor of the NIMBUS-6 satellite scanned cyclone Joan at 1530 GMT 5 December 1975. Transformation of the microwave brightness temperature produces a map of the cyclone rainfall intensity at that instant. The path of the cyclone both before and after that date can be deduced from infrared and visible observations made by NOAA-4 and NIMBUS-6. Based on the known cyclone path and the known rainfall at 1530 GMT 5 December 1975, it is possible to predict the expected pluviograph trace at coastal stations in the path of the cyclone. This method is applied to Roebourne and Port Hedland and predicts total cyclonic rainfalls of 56 mm and 98 mm respectively, as opposed to the 42 mm and 176 mm actually recorded. The discrepancy for Port Hedland arises from the 2K observational error in the microwave brightness readings, which makes it almost impossible to accurately determine very high rainfall rates. The 98 mm deduction was based on a peak rainfall rate of 5 mm/h, whereas the pluviograph shows that sustained rainfall rates could exceed 20 mm/h. This implies that peak rainfall determinations based solely on microwave determined rainfall rates will remain unreliable during tropical cyclones until the accuracy of the microwave temperature sensor can be improved.