SSMIS Tropical Cyclone Monitoring Opportunities

The DMSP Special Sensor Microwave Imager Sounder (SSMIS) has untapped potential that could enhance its already significant role in monitoring global cyclone (TC) structures and intensities. As the world's only passive microwave conically scanning imager and sounder, the SSMIS provides all-weather K, Ka, W, and G-band imagery (19, 22, 37, 91, 150, and 183 GHz) with constant resolution across a 1700km swath. This imagery is routinely used by TC warning center analysts and researchers around the world to map rainband organization and inner-core structure. In addition, the SSMIS V-band sounder channels are sensitive to both moisture and temperature at multiple levels throughout the TC environment and several algorithms now produce near real-time estimates of minimum sea level pressure (MLSP) and maximum sustained wind speeds (Vmax). However, the sounder channel applications have been partially limited by the available sounder spatial resolution. A method is proposed here to obtain enhanced sounder resolution via access to “native resolution” data that has not been “averaged” either onboard the spacecraft nor on the ground, thus providing more accurate TC estimates.

The SSMIS imager channels are used to extract surface circulation centers and inner core structure, features unseen with coincident visible/infrared (vis/IR) due to upper-level cloud obscuration ~ 33% of the time. In addition, the imager channels produce multiple environmental data records that benefit the TC community: ocean surface wind speed (used for gale wind radii), rain rate, cloud liquid water and total precipitable water (TPW). TPW maps highlight the TC moisture environment and are especially good at mapping dry air intrusion, whether associated with Saharan Air Layer (SAL) events in the Atlantic of troughs and cold air outbreaks in the Pacific basin.

The SSMIS sounder channels measure brightness temperatures (Tb) that can be converted into atmospheric temperature profiles or used directly to map the extent of the warm core temperature anomaly aloft created by the TC's heat engine. Warm, moist air from the surface is lifted in the eyewall to the storm top and then flows out anticylonically, forming a warm core that is highly correlated with the storm intensity. The CIMSS sounder algorithm uses the Tb values in channels 3, 4, and 5 to estimate MSLP and Vmax using a method similar to their earlier efforts applied to the cross-track scanning Advanced Microwave Sounding Unit (AMSU) flown on NOAA low earth orbiting (LEO) operational satellites.

Microwave sounders vary in their instantaneous field of view (IFOV) or spatial resolution due to a combination of scan geometry, sensor altitude, frequency, and antenna size. Very early efforts using the Microwave Sounder Unit (MSU) were severely limited by its coarse resolution. AMSU (48x48km) enabled TC intensity algorithms to reach an accuracy viable for operational utility by the Joint Typhoon Warning Center (JTWC, Hawaii), the National Hurricane Center (NHC, Miami), and the Central Pacific Hurricane Center (CPHC, Hawaii). The SSMIS (37x37km) has improved resolution and its conical scanning keeps the IFOV constant across the entire 1700 km swath rather than dramatically degraded resolution near the swath edges with cross-track scanning, thus mitigating one of the inherent issues with AMSU TC applications. However, the SSMIS sensor data is averaged onboard the spacecraft and also within the ground processing system to create the 37x37km footprint. The “native resolution” sounder data is 17.6x27.3km, a significant advantage over the currently available data when one considers ~ 63% of TC eyewalls have diameters < 40 km. A highlight of this paper is a V-band antenna brightness temperature image of Cyclone Anja in 2009 taken at native resolution during an SSMIS payload orbital calibration. This image is the highest resolution V-band image ever produced from space of an active TC and clearly illustrates the advantages of higher resolution. This paper also describes the impact of higher resolution on TCI estimation and an approach to make it available operationally, such that enhanced TC intensity estimates can be provided to both the operational and research community.