Passive Microwave Remote Sensing of Tropical Cyclones

By measuring the microwave emission (and reflection) of the Earth's surface and atmosphere over a range of frequencies, conically-scanning space-borne passive microwave radiometers offer two classes of products that are indispensable to the hurricane forecasting and analysis communities: raw brightness temperatures and geophysical retrievals. High resolution brightness temperature measurements at 37 and 89 GHz have traditionally been used to determine storm eye locations and infer storm structures and intensities, while brightness temperature inversion algorithms calculate useful quantities such as sea-surface temperature, water vapor, surface wind speed, cloud water, and precipitation. New capabilities have been recently developed and deployed on conically-scanning satellite radiometers, namely polarimetry and sounding, that will improve the ability to determine tropical cyclone intensities and structures.

For the past three decades, the primary technology for sensing ocean surface vector winds has been the scatterometer. These reliable instruments are able to measure near-surface winds in a wide range of atmospheric conditions and have aided hurricane forecasters in determining eye locations and track forecasts. Scatterometers generally measure only a single quantity--wind vectors--and the sensitivities of these measurements decrease at high winds. Also, the large amounts of precipitation present in tropical cyclones, usually correlated with the highest of winds, interfere with surface measurements. Polarimetric radiometers, such as WindSat, introduce the ability to passively measure both surface wind speed and wind direction. While less sensitive than scatterometers to wind direction at low wind speeds, polarimetric radiometers are well suited for measuring high wind conditions due to the increased sensitivities to both magnitude and direction at high wind speeds. Additionally, the multi-frequency design allows for the coincident retrieval of other environmental parameters, such as precipitation. As with scatterometers, rain contamination results in erroneous surface measurements; however, the ability to perform coincident precipitation retrievals and to measure surface and atmospheric conditions over a range of frequencies offers the potential to mitigate unwanted environmental contamination.

Cross-track scanning microwave sounders like AMSU-A and MHS provide high quality temperature and humidity profile measurements, respectively, that are a primary data source for weather prediction models. Additionally, cross-track sounding measurements can be used to estimate tropical cyclone intensity. One drawback to the cross-track scanning geometry, however, is the variation of the measurement geometry over a scan--path length, incidence angle, resolution, and polarization are all functions of scan position. The Defense Meteorological Satellite Program (DMSP) recently upgraded its conically-scanning microwave imagers to include sounding channels by deploying the Special Sensor Microwave Imager/Sounder (SSMI/S). The conical configuration results in a consistent geometry across the entire scan, with a slight improvement in resolution over cross-track sounder nadir resolutions.

This paper will encompass the capabilities of conically-scanning passive microwave radiometers that are of interest to hurricane forecasters and analysts. The benefits of coincident multi-frequency measurements will be discussed, including the interdependence of the measurable environmental parameters and the effect of this interdependence on retrievals. Additionally, improvements in passive sensing of surface vector winds in hurricane conditions will be presented.