Measuring the structure of hurricanes and severe storms with a microwave sounder

Analysis of data from hurricane aircraft experiments has revealed a very strong correlation between microwave sounder observations and radar observations of convective systems - so strong that it has been possible to develop a retrieval algorithm that can be used to transform the radiometric brightness temperatures into vertical profiles of equivalent radar reflectivity. This new method, which is also applicable to satellite observations, makes it possible to use a number of current and future satellite sounders to generate proxy radar data, which enables a wide range of analyses of convective systems and processes using additional algorithms and methodologies developed for the radar systems. Efforts are now under way to make this new tool available for both weather forecasting and research. This development is based on analysis of observations 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 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 reflectivity can be derived from the HAMSR brightness temperatures with good accuracy. Preliminary results from efforts to determine the averaging kernels indicate high vertical resolution – significantly higher than for conventional soundings. In addition, HAMSR is a cross-track scanning sensor (like AMSU), and reflectivity can therefore be estimated for the entire 3D volume of the atmosphere observed by HAMSR. 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 analysis and prediction of hurricane and other convective systems. In particular, it will be possible to get a picture of the internal structure of these storms as the satellite sensors pass overhead. Even though the spatial resolution of the satellite microwave sounders is relatively poor, 15-50 km, the additional information – which is otherwise only available from a very few satellite 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 storms 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, described in a separate paper) will soon be 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.

Copyright 2010 California Institute of Technology. Government sponsorship acknowledged.