The influence of deep convective motions on the variability of Z-R relations

Abstract Effects of deep convection on the precipitation rate R leading to variations in radar meteorological Z – R relations are studied. The basic contributions to this subject come from vertical and horizontal air motions as well as decreasing air density with height. Their influence on Z – R relations is investigated both with an analytical approach from cloud microphysics distinguishing between two characteristic spectral forms, and a mesoscale bulk model case study of a single cumulonimbus cloud. The precipitation rate is strongly affected by deep convective motions leading to increased mean value and standard deviation of the prefactor a in Z – R relations Z = aR b . To a lesser extent, density stratification tends to diminish the prefactor. The exponent b , which can, without deep convection, vary from b =1 to b =7/4 depending on characteristic spectral form, remains unaffected by any of the dynamical effects studied here. Values of b can only be altered by such changes of the particle spectra, which affect the distribution of terminal velocity with hydrometeor size: in practice, this implies phase changes or variations in composition of the mixed-type hydrometeor ensemble. In spite of the variations in Z – R relations found in the present study, when performing an average over the whole cloud and precipitation volume , standard Z – R relations proposed for stagnant air still hold in a statistical sense. Furthermore, the effects of vertical air density gradients can be compensated, which should also help to improve quantitative rainfall estimates at large ranges from the radar site.