Developing a GeoSTAR science mission

Abstract

The geostationary synthetic thinned aperture radiometer (GeoSTAR) is a new instrument design that has been under development at the Jet Propulsion Laboratory in the form of a proof-of-concept prototype. It is intended to fill a serious gap in our Earth remote sensing capabilities - namely the lack of a microwave atmospheric sounder in geostationary orbit. Such sensors have long been part of low-earth-orbiting (LEO) operational weather satellites and research satellites and have had a major impact ranging from numerical weather prediction to climate research. A similar capability in GEO is highly desired because of the advantageous observing point GEO offers, with continuous views of the entire visible Earth disc - crucial for the observation of hurricanes and other rapidly evolving atmospheric phenomena. GEO also enables full resolution of the diurnal cycle, which is particularly important in the study of atmospheric processes and climate variability where clouds and convection play a role, since those phenomena are known to have strong diurnal variability and are difficult to sample properly with sun synchronous LEO satellites. The GeoSTAR prototype produced the first interferometric radiometric images obtained at sounding frequencies in early 2005, and subsequent tests have demonstrated that the system exhibits excellent stability, accuracy and sensitivity and performs even better than predicted. This can be characterized as a breakthrough development. The technology required to implement GeoSTAR is at a level of maturity that a space mission can be contemplated. Such a mission is recommended by the U.S. National Research Council in its recent Decadal Survey of Earth missions and is being considered by both NASA and NOAA for the coming decade. Recent studies indicate that it is indeed feasible to implement a GeoSTAR mission in the 2014-16 time frame. We discuss possible mission scenarios as well as the science benefits that would ensue. The benefits are particularly significant in the area of tropical cyclones and severe storms, where there currently is a dearth of observations. With a geostationary microwave sounder it is possible to obtain the 3-dimensional distribution of temperature, water vapor and liquid water continuously and regardless of cloud cover, and atmospheric stability indices such as lifted index (LI) and convective available potential energy (CAPE) can be derived nearly everywhere. That will make it possible, for example, to detect severe-storm precursor conditions even if the area is under cloud cover. Recent progress in radiative transfer models now also makes it possible to obtain those parameters in the presence of moderate precipitation, and rain rates and snow rates can be derived as well. Aircraft based field campaign observations have also shown that a microwave sounder can be used to derive measures of convective intensity and precipitation in deep-convective systems from scattering due to ice particles formed by such systems. This can be used to estimate the intensity of tropical cyclones and can be used to detect sudden intensification and weakening in near-real time.