On the use of spectral polarimetry to observe ice cloud microphysics with radar

Abstract

The formation of rainfall is a complex process. Warm rain is formed under conditions where the temperature is above the freezing point of water. Raindrops then result from collision and coalescence. In cold rain also ice particles and super-cooled water droplets at temperatures below the freezing point come into play. The microphysical transition of ice particles into raindrops is understood in common terms, but details of this and the impact on rainfall rates are still largely unknown. Aircraft measurements of ice particles are difficult and mostly lead to snapshot information, whereas continuous observations of the entire transition process are needed. This is where remote sensing enters the scene. Weather radars are very useful tools to study the entire rain cell and advanced systems, combining Doppler and polarization capabilities, can deliver a wealth of information. The interpretation of radar observations of ice precipitation is, however, quite complex. A typical ice precipitation event can consist of a mixture of different particle types, such as pristine ice particles, aggregates, or graupel, and therefore one has to take into account the differences in scattering of the radar waves due to this variety. This has led to a number of reported relationships between the radar reflectivity and ice water content; see for example Sekhon and Srivastava (1970), Smith (1984), Matrosov (1992). There is, however, not a unique one. Radar techniques to discriminate ice crystal types have been developed. These techniques are based on a conceptual idea of stratiform rain as a layered structure, where each layer is homogeneous in terms of type of hydrometeor. In other words: mixtures of ice crystal types do not occur at a given latitude. Matrosov (1998) has investigated the use of dual-wavelength radar for estimation of snow parameters. They have shown that using measurements taken at two wavelengths, where at least one of them is located in a non-Rayleigh region, one can estimate parameters of a particle size distribution. In these studies, it was assumed that there is only one type of particles present in the observation volume. In a similar vain, Matrosov et al. (1996) have shown that by using dual-polarization radar measurements taken at several elevation angles, it is possible to discriminate various types of ice particles, such as planar crystals, columnar crystals and aggregates in a homogeneous cloud. The use of VHF profiler measurements for the retrieval of the size distribution of ice particles above the freezing level in the stratiform region of a tropical squall line was demonstrated by Rajopadhyaya et al. (1994). This technique is based on a velocity-diameter relationship (e.g., Langleben 1954 or Locatelli and Hobbs 1974) and vertically pointing Doppler observations. These methods deliver information about the ice crystals averaged over the radar volume in a cloud layer at a given altitude. However, since the radar volume is very large and given the fact that it is not uncommon that different types of ice crystals occur within the cloud layer, the radar signal may be due to different categories of ice crystals in the resolution volume. To alleviate this problem, the spectral dual-polarization method has been developed (Moisseev et al. 2004). It was shown that a combination of Doppler measurements and dual-polarization observations can be used to distinguish between different types of ice hydrometeors within a radar volume. In this chapter, we will expand this concept, using observations of the spectral differential reflectivity.