Simultaneous rainfall analyses using Special Sensor Microwave/Imager (SSM/I) passive microwave measurements and thermal infrared measurements from Meteosat are presented for two storms over northern Italy that caused damaging floods. Upwelling brightness temperatures over precipitating clouds in the mm-cm spectrum are directly associated with precipitation microphysics throughout the cloud column down into the rain layer. In contrast, brightness temperatures in the thermal infrared window, which arise from emission near cloud top, are not directly responsive to precipitation processes. However, because of the diffraction-limited nature of passive microwave detectors, microwave radiometer ground footprints are considerably larger than those characteristic of infrared sensors, even those flown at geosynchronous altitude. Furthermore, passive microwave radiometers are flown on low-earth orbiters, which produce less than ideal sampling rates, whereas optical-infrared radiometers flown on geosynchronous orbiters produce high frequency sampling concomitant with precipitation time scales. In this study, we investigate how these two types of measuring systems could be used in a complementary fashion to improve rainfall estimation. A physically based microwave algorithm is used to estimate rainfall with SSM/I measurements, whereas a two-threshold statistical technique is used for corresponding estimates from Meteosat. Results suggest that an infrared-based analysis of rainfall derived from half-hourly geosynchronous images can be improved with a calibration-transfer approach using a microwave algorithm sensitive to vertical cloud structure.
Investigating a SSM/I microwave algorithm to calibrate Meteosat infrared instantaneous rainrate estimates
PORCU', Federico;PRODI, Franco
1996
Abstract
Simultaneous rainfall analyses using Special Sensor Microwave/Imager (SSM/I) passive microwave measurements and thermal infrared measurements from Meteosat are presented for two storms over northern Italy that caused damaging floods. Upwelling brightness temperatures over precipitating clouds in the mm-cm spectrum are directly associated with precipitation microphysics throughout the cloud column down into the rain layer. In contrast, brightness temperatures in the thermal infrared window, which arise from emission near cloud top, are not directly responsive to precipitation processes. However, because of the diffraction-limited nature of passive microwave detectors, microwave radiometer ground footprints are considerably larger than those characteristic of infrared sensors, even those flown at geosynchronous altitude. Furthermore, passive microwave radiometers are flown on low-earth orbiters, which produce less than ideal sampling rates, whereas optical-infrared radiometers flown on geosynchronous orbiters produce high frequency sampling concomitant with precipitation time scales. In this study, we investigate how these two types of measuring systems could be used in a complementary fashion to improve rainfall estimation. A physically based microwave algorithm is used to estimate rainfall with SSM/I measurements, whereas a two-threshold statistical technique is used for corresponding estimates from Meteosat. Results suggest that an infrared-based analysis of rainfall derived from half-hourly geosynchronous images can be improved with a calibration-transfer approach using a microwave algorithm sensitive to vertical cloud structure.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
