Considerations for thermal satellite infrared sounder applications

Satellite passive infrared/microwave (IR/MW) sounders have become an indispensable tool for weather forecasting, both in terms of numerical model assimilation and in providing real-time soundings for operational forecasters and nowcasting applications. While IR/MW sounders were originally designed with the weather forecasting application in mind, thermal IR Earth emission spectra contain a plenitude of additional information about atmospheric state parameters. This chapter overviews the development, validation, and application of satellite sounder retrieval systems for atmospheric monitoring and weather forecasting. The success of IR/MW sounder-retrieved products in operational applications depends strongly on knowledge of their accuracy and precision, which are determined using statistical intercomparison methods and accurate in situ field measurements, especially radiosondes, but also ozonesondes and other instrumentation discussed in earlier chapters of this book. In both assimilation and retrieval systems, these field data are needed for the forward problem (i.e., forward radiance calculations), the inverse problem (i.e., the geophysical retrievals, which require training and/or tuning data), and last, but not least, the validation problem (i.e., the error characterization of the end-to-end sounding system). Knowledge of the three-dimensional nature and complexity of atmospheric processes across space and time advances only through exploiting many available instruments and observing systems.