Deriving convective structure from microwave sounder observations

Microwave sounders have been used for many years to measure the thermodynamic state of the atmosphere, i.e. the three-dimensional distribution of temperature, water vapor and liquid water, both from space, from aircraft and from the ground. Currently, the most advanced satellite based microwave sounding system is the Advanced Microwave Sounding Unit (AMSU-A and-B) operating on a number of weather satellites, and the current state of the art in " retrieval " systems is represented by the Microwave Integrated Retrieval System (MIRS) [Boukabara, 2007]. Data from these systems are routinely used in numerical weather prediction to great effect. Microwave sounders have been particularly effective because the observations are only minimally affected by clouds. In contrast, infrared sounders, which are otherwise both more accurate and have higher resolution are only able to penetrate very thin clouds. Atmospheric sounding [Janssen, 1993] is based on the fact that the thermal radiation received by a radiometer originates at wavelength-dependent depths in the atmosphere. This is caused by a non-uniform absorption spectrum, particularly by molecular absorption lines. At wavelengths near the peak of such a line, absorption may be so strong that most of the underlying atmosphere is opaque, and only the top of the atmosphere is " seen ". Conversely, at wavelengths far away from the lines, often called a " window " region, the atmosphere may be nearly transparent, and the surface or the bottom of the atmosphere is seen instead. Through spectral sampling, i.e. by measuring narrow spectral bands, or " channels " , it is then possible to probe into different depths of the atmosphere. A " weighting function " describes which portion of a channel's signal strength originates from different depths. For a radiometer looking down into the atmosphere from a high-flying aircraft or a satellite, a typical weighting function reaches a peak at a certain depth, which is characteristic for that channel. A set of channels is selected so that the respective weighting functions are evenly distributed through the atmosphere. Fig. 1 shows the MW absorption spectrum in the sounding region (left) and the nominal AMSU-A temperature weighting functions in the 50-GHz band (right). Liquid droplets and ice particles make most clouds completely opaque in the infrared band, but in the microwave region they are partially transparent. The microwave spectral absorption features of liquid water therefore make it possible to determine the vertical distribution of the liquid water in clouds …