Measuring the structure of hurricanes with a microwave sounder

Analysis of data from hurricane aircraft experiments has shown that there is a very high correlation between microwave sounder observations and radar observations of convective systems. The correlation is so strong that it is possible to develop a model function that can be used to transform the radiometric brightness temperatures into vertical profiles of equivalent radar reflectivity. This new method makes it possible to use current and future satellite sounders to generate proxy precipitation and cloud radar data. This in turn will enable a wide range of analyses of convective systems and processes using algorithms and methodologies developed for the radar systems.

We show results from the joint NASA-NOAA Tropical Cloud Systems and Processes (TCSP) field campaign in 2005. In that experiment NASA deployed a suite of remote sensing instruments on the high-altitude ER-2 aircraft that included a precipitation radar system and a microwave sounder. The sounder, the High Altitude MMIC Sounding Radiometer (HAMSR), was developed at the Jet Propulsion Laboratory under the NASA Instrument Incubator Program with new technology and is functionally similar to the Advanced Microwave Sounding Unit (AMSU) that is now operating on several NOAA weather satellites. Comparisons of HAMSR observations with those from the ER-2 Doppler Radar (EDOP) shows that height resolved EDOP-equivalent radar reflectivity can be derived from the HAMSR brightness temperatures with good accuracy, although at lower vertical resolution than is possible with the radar. In addition, since HAMSR is a cross-track scanning sensor (like AMSU), reflectivity can be estimated for the entire 3D volume of the atmosphere observed by HAMSR – unlike EDOP, which is a nadir-only sensor. With this new method HAMSR can be used to map out the convective structure of tropical convection from the surface to 15 km with a vertical resolution of roughly 1-2 km.

Efforts to extend this method to the AMSU satellite instruments are under way, and preliminary results are very promising. This will open up a new avenue for hurricane analysis. In particular, it will be possible to get a picture of the internal structure of hurricanes as the sensors pass overhead. Even though the spatial resolution of the satellite sensors is relatively poor, 15-50 km, the additional information – which is otherwise only available from a very few radar systems – is expected to be of high value for nowcasting and forecasting as well as retrospective analysis. This approach will be of particular interest when applied to a geostationary microwave sounder, with its ability to monitor hurricanes continuously throughout their life cycles. Such a system is now under development in response to the National Academy of Sciences' recommendation, in its recent “decadal survey” of earth satellite missions, to develop the Precipitation and All-weather Temperature and Humidity (PATH) mission. Key technology required for such a sensor has been developed at the Jet Propulsion Laboratory, and the “Geostationary Synthetic Thinned Aperture Radiometer” (GeoSTAR) is ready for implementation. A possible joint NASA-NOAA mission, perhaps flying GeoSTAR as a demonstration payload on one of the new GOES-R/S/T satellites, is being explored.