Some notes on scattering of radiowaves by clouds

In two recent articles Erkelens et al. (1999a and 1999b) showed that coherent particle scatter (scatter of radiowaves by spatial variations in particle mass density) can explain the dual wavelength radar measurements made by Knight and Miller (1998) and dual wavelength measurements of a smoke plume made by Roger and Brown (1997). The 1999articles also showed quantitatively that coherent particle scatter can be stronger in clouds than incoherent particle scatter. When the spatial variations in humidity and in liquid water are of equal size, coherent particle scatter would be insignificant compared to coherent air scatter – also called clear air or Bragg scatter – (Gossard and Strauch, 1983). The Erkelens et al. papers gave some qualitative reasons why the spatial variations in liquid water can be expected to be larger than those in humidity. The arguments above make a strong case for the significance of coherent particle scatter for radar measurements of clouds. That is why this article will take the first steps in exploring the importance of this scattering mechanism for radar cloud studies. As quantitative data is sparse, these notes must be labelled as tentative, but this is not continuously stressed throughout the article for readability. In the first part (sections 2, 3 and 4) of this article the physical basis of coherent particle scatter will be explained and developed further. In the second part it will be indicated for which radar wavelengths and for which cloud types and atmospheric conditions one can theoretically expect coherent particle scatter to be significant (section 6). Furthermore, it will be shown quantitatively that it is indeed reasonable to assume that coherent particle scatter will dominate coherent air scatter in cumulus clouds. To make this discussion somewhat less academic, this discussion will refer to some cloud measurements in section 5. In the last section some recommendations for further research steps are presented.