Coherent Particle Scatter in Clouds: Reflection Calculations Based on In-situ Measurements

Well-known radar scattering mechanisms are Rayleigh scattering (incoherent particle scattering), and clear-air scattering (coherent air scattering). This article discusses the importance in clouds of these two mechanisms compared to a third scattering mechanism: coherent particle scattering, which is caused by spatial variations in the liquid water content of the clouds. We will argue that for radars with a wavelength larger than a centimetre, coherent particle scatter can dominate Rayleigh scattering from individual droplets, both for cumulus and stratiform clouds. Coherent air scatter in clouds is probably less important than previously thought. Incoherent particle scatter is due to particles that are randomly distributed within the radar volume. It is most important in atmospheric radars with a small wavelength. Coherent air scatter is caused by variations in the refractive index of atmospheric gases. It is normally dominated by spatial variations in humidity (Gossard and Strauch, 1980) and occurs mainly in radar measurements using a long wavelength (cmwaves or longer). Erkelens et al. (2000) recently showed that coherent particle scatter, which is caused by variations in the liquid water content (LWC) of clouds, may be important in clouds. Like coherent air scatter this mechanism is mainly important in atmospheric radars with a long wavelength. Erkelens et al. showed that coherent particle scatter can explain the results of dual-wavelength radar measurements of cumulus clouds performed by Knight and Miller (1998) and those of a smoke plume done by Rogers and Brown (1997). In this article we explore the importance of this scattering mechanism for other radar systems and cloud types. Coherent particle scatter is compared in strength to the other two scattering mechanisms. As quantitative data is sparse, the discussion must be labelled as tentative, but for the sake of readability this is not stressed continuously.