Calibration of radar rain rate with ground truth data in Sylhet

Abstract

Weather radar collects data to detect reflectivity and reflectivity is usually used 10 estImate rain rate. Conventional weather radar collects data in a format that calculate reflectivity in a continuous pattern of 1 dBZ interval. Bangladesh Meteorological Department (BMD) has 4 radars installed at Dhaka, Rangpur, Cox's Bazar and Khepupara. These radars collect data individually and in composite way. BMD radar data format is different from conventional radar data and not very useful to estimate rain ratc. The data fonual is different in the sense that BMDradar collects data in six different pattems viz; (a) Status 1 = 1 - 4 mm/h, (b) Statusl = 4 - 16IllIllIb,(e) Status 3'" 16 - 32 mm/h, (d) Status 4 = 32 - 64 mmih, (e) Status 5 = 64 - 128mm!h and (f) Status 6 = more than J28 mmlh.ln each Status there is a wide range of rain rate and it is very difficult to extract rain rate for a particular im;\ant from thesc wide range. No method has yet been developed to determine particular rain rate from BMD radar data, which is essential for practical and for different rcscarch purpose. To overcome this problem initiative has been taken to calibratc BMD radar data with I-minute interval raingaugc data collected in a dense raingaugc network in Sylhet. In th,s work it is found that frequency in Status 1 is most dominant for all the stations Amalshid (24.~77"N, 92.476°E), Chhatak (25.041"N, 91.673"E) and Nctro Haor (25.035"N, 92.083°E). Rain rate is calculated ji'om the matching evcnts of radar images and raingauge rainfalL Calibrating radar Status with raingauge rainfall it is found that Status 1 = 0.5 mmJmin to 0.618 mmJmin, Status 2 = 0.5 mmJmin to 0.785 mm/min and Status 3 = 3 mm/min. On avcragc it is found that rain rate is 0.5325 mm/min for Status I, 0.564 mm/min for Status 2 and 3 rom/min for Status 3. Investigating 28 events we found that rain rate lies between 10.47 mmlh to 17.02 mmlh and in average rain rate is 15.84 mmJhin Sylhet.