On the Parameterization of Evaporation of Raindrops as Simulated by a One-Dimensional Rainshaft Model

Abstract

The process of evaporation of raindrops below cloud base is investigated by numerical simulations using a one-dimensional rainshaft model with bin microphysics. The simulations reveal a high variability of the shape of the raindrop size distributions, which has important implications for the efficiency of evaporation below cloud base.A new parameterization of the shape of the raindrop size distribution as a function of the mean volume diameter is suggested and applied in a two-moment microphysical scheme. In addition, the effect of evaporation on the number concentration of raindrops is parameterized. A comparison of results of the revised two-moment scheme and the bin microphysics rainshaft model shows that the two-moment scheme is able to reproduce the results of the reference model in a wide parameter range.

The Parallelized Large-Eddy Simulation Model (PALM) version 4.0 for atmospheric and oceanic flows: model formulation, recent developments, and future perspectives

“…Since q and θ l are conserved quantities for wet adiabatic processes, condensation/evaporation is not considered for these variables. Liquid-phase microphysics are parametrized following the two-moment scheme of Seifert andBeheng (2001, 2006), which is based on the separation of the droplet spectrum into droplets with radii < 40 µm (cloud droplets) and droplets with radii ≥ 40 µm (rain droplets). The model predicts the first two moments of these partial droplet spectra, namely cloud and rain droplet number concentration (N c and N r , respectively) as well as cloud-and rainwater specific humidity (q c and q r , respectively).…”

“…The moments' corresponding microphysical tendencies are derived by assuming the partial droplet spectra to follow a gamma distribution that can be described by the predicted quantities and empirical relationships for the distribution's slope and shape parameters. For a detailed derivation of these terms, see Seifert andBeheng (2001, 2006).…”

“…The evaporation of rain drops in subsaturated air (relative water supersaturation S < 0) is parametrized following Seifert (2008):…”

Diagnosis of systematic differences between multiple parametrizations of warm rain microphysics using a kinematic framework

“…In an effort to overcome some of these issues and aid development and testing of microphysics schemes some researchers have taken a 'reductionist' approach and made use of kinematic frameworks (e.g. Clark, 1974;Petch et al, 1997;Morrison and Grabowski, 2007;Seifert, 2008;Seifert and Stevens, 2010). In a kinematic framework the flow is prescribed, allowing advective transport and particle sedimentation while avoiding the complexity caused by feedbacks between dynamics and microphysics.…”

Improving Collisional Growth in Lagrangian Cloud Models: Development and Verification of a New Splitting Algorithm

“…where M k is the k-th moment of the mass density distribution (see Seifert, 2008). The calculation of these moments in the LCM framework will be described in section 4.1.1.…”