The History of Tropical Cyclone Remote Sensing Via Satellite Microwave Imagers

Satellite-based microwave sensors have been used for 25+ years operationally to assist in global tropical cyclone (TC) monitoring. Microwave frequencies have the potential to mitigate upper-level cloud obscuration that hinders visible and infrared (IR) imagers and provide critical storm rainband and eyewall structure needed to more accurately provide storm location, size, and intensity estimates. Thus, TC warnings around the globe have benefited from a suite of both operational and research and development (R&D) sensors whose data are available in near real-time (1-3 hours).

Operational microwave imagers began with the launch of the Special Sensor Microwave/Imager (SSM/I) onboard the F-8 Defense Meteorological Satellite Program (DMSP) spacecraft in June 1987. The seven channel, four frequency (19.35, 22.235, 37, and 85.5 GHz) imager routinely sampled TCs within its 1400 km swath and became a critical asset for DOD's efforts to map TC characteristics, especially in the western pacific basin where aircraft reconnaissance no longer existed. While F-12's SSM/I failed, the following launches of SSM/Is on F-10, F-11, F-13, F-14, and F-15 (1990-1999) proved invaluable in providing enhanced temporal sampling sorely needed to understand storm structure evolution.

The SSM/I series was followed by the Special Sensor Microwave Imager Sounder (SSMIS) that combined the SSM/I imager channels with the sounder channels from the earlier Special Sensor Microwave Temperature and Moisture (SSM/T1 and T2) sensors. The enlarged 1700 km swath and collocated imager and sounder channels provided enhanced TC monitoring capabilities with the first launch on F-16 in 2003. The conical scan on the SSM/I and SSMIS enables constant resolution across the swath unlike the inherent deficiencies of cross track scanners whose footprint expands away from nadir. Future launches of SSMIS on F-19 and F-20 will continue the TC sampling by these operational mainstays.

The DMSP sensors were greatly augmented with R&D NASA sensors beginning with the launch of the Tropical Rainfall Monitoring Mission (TRMM) and its microwave imager (TMI, 750-800 km swath) and precipitation radar (PR) in 1997. TRMM's non sun synchronous tropical 35 deg inclination provided temporal sampling not feasible with DMSP satellites and increased TC views by 2-4/day. In addition, the lower altitude helped provide 5-6 km 89 GHz spatial resolution, thus 2-3 times better than the 13 km values for the SSM/I and SSMIS sensors. NASA's Aqua spacecraft included the Japanese built Advanced Microwave Scanning Radiometer (AMSR-E), whose 1800 km swath and TMI-like 5-6 km 89 GHz resolution made this a superb TC monitoring sensor. AMSR-E's recent failure now leaves a significant hole in the global TC data sets.

The Navy via the Naval Research Laboratory (NRL) built and launched the Coriolis WindSat polarimetric radiometer in 2003 and this proof of concept sensor has successfully demonstrated the ability to retrieve ocean surface wind vectors via a passive microwave imager. In addition, the large antenna affords excellent 37 GHz imagery that can be used for examining storm's low level circulation center since it responds mainly to cloud liquid water. WindSat data continues nearly a decade after launch and similar sensors might be on follow on DOD spacecraft.

The presentation will summarize the next set of microwave imagers that will shortly become available (Megha Tropiques MADRAS, GCOM-W1 AMSR-2, and NASA's GPM) and highlight how TC sampling will fair with the combination of old sensors falling offline and new ones hopefully providing stable digital data sets.