Relationships between layer-mean radar reflectivity and columnar effective radius of warm cloud: Numerical study using a cloud microphysical bin model

Abstract

The relationship between the layer-mean radar reflectivity, Ze, and the columnar effective particle radius, Re, in evolving shallow warm clouds was investigated by numerical experiments using a hybrid cloud microphysical model and a forward simulator of satellite measurements. Changes in the cloud/rain droplet size distributions were traced in a kinematically driven warm cloud for various values of number concentration of cloud condensation nuclei (CCN) and maximum updraft velocity. In contrast to previous interpretations of the observed data, we found four paths for the relationships between Ze and Re during the lifetime of a warm cloud. In the first path, both Ze and Re increase with an approximate sixth-power dependency, indicating a stage of condensational growth of droplets without raindrops in the cloud. In the second path, only Ze increases rapidly, while Re remains almost constant (Re second ), indicating a stage in which few raindrops emerge in the cloud before appreciable precipitation occurs at the surface. This second path was newly identified in this study. In the third path, Re increases rapidly while Ze does not change greatly, indicating a stage of coalescence of droplets. Precipitation reaches the surface at the end of the third path. In the fourth path, both Ze and Re decrease, indicating a greater contribution of raindrop evaporation and weakening or termination of precipitation. The maximum values of Ze and Re and the constant value of Re second for the second stage depend on the CCN number concentration and the updraft velocity.

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“…Grabowski (2016) recently proposed a "piggybacking" approach that separates dynamical and microphysical impacts on deep convection. Kuba et al (2014Kuba et al ( , 2015) used a two-dimensional kinematic driver model (Shipway and Hill 2012) to study shallow cloud properties simulated by a spectral bin cloud microphysics scheme and examined the differences between NDW6 and the bin scheme; the bin scheme examined here is a cloud microphysical model with a two-moment bin method based on Kuba and Fujiyoshi (2006) and modified by Kuba and Murakami (2010), and will be referred to as the KF-bin scheme. To investigate the relationships between cloud microphysical processes and satellite observational data, Suzuki and Stephens (2008) examined the relationships between mean radar reflectivity, Z e , and mean particle effective radius, R e , values for shallow clouds retrieved from satellite observations and used this information to identify warm cloud microphysical processes.…”

“…Suzuki et al (2010b) further analyzed the contoured frequency by optical depth (CFODD) for vertical profiles of radar reflectivity as a function of cloud optical depth from cloud top to investigate cloud microphysical processes based on a methodology adapted from Nakajima et al (2010). Kuba et al (2014) reproduced a diagram proposed by Suzuki and Stephens (2008) using a kinematic driver model (Shipway and Hill 2012) with the KF-bin scheme and a forward simulator of satellite measurements. Changes in the size distribution of cloud droplets and raindrops were traced in the two-dimensional kinematic driver model (Shipway and Hill 2012) for a warm cloud.…”

“…Changes in the size distribution of cloud droplets and raindrops were traced in the two-dimensional kinematic driver model (Shipway and Hill 2012) for a warm cloud. Kuba et al (2014) calculated radar reflectivity and optical thickness using the numerical data output from this model as the input for the radar simulator developed by Masunaga and Kummerow (2005) in the Joint Simulator for Satellite Sensors (Hashino et al 2013).…”

“…In the Z e -R e diagram reproduced by Kuba et al (2014), the relationships between Z e and R e follow four paths during the lifetime of warm clouds. In the first path, both Z e and R e increase with an approximate sixth-power dependency, indicating a stage of condensational growth of droplets without raindrops in the clouds.…”

“…Kuba et al 2015followed Kuba et al (2014) and investigated the relationships between horizontally averaged radar reflectivity, Z m , and cloud optical depth from cloud top, τ d , to examine cloud microphysical processes inferred from the CFODD by Suzuki et al (2010b) and Sato et al (2012). Kuba et al (2015) used the same kinematic model with the KF-bin scheme and the satellite simulator to depict vertical profiles of radar reflectivity of warm clouds in the same way as Kuba et al (2014). Figs.…”