Turbulent effects on the microphysics and initiation of warm rain in deep convective clouds: 2‐D simulations by a spectral mixed‐phase microphysics cloud model

Abstract

[1] The spectral bin microphysics Hebrew University cloud model with the spatial resolution of 50 Â 50 m is used to simulate the evolution of isolated deep mixed phase convective clouds under different meteorological conditions and at different aerosol concentrations. The model takes into account the effects of turbulence on droplet collision rate. Turbulent collision kernels are calculated at each time step and at each grid point. The turbulence-induced collision rate enhancement is determined by means of lookup tables calculated in the recent studies for different values of turbulent dissipation rate and the Taylor microscale Reynolds numbers. Deep convective clouds observed during the Large-Scale Biosphere-Atmosphere Experiment in Amazonia-Smoke, Aerosols, Clouds, Rainfall and Climate campaign in the Amazon region are simulated at different aerosol concentrations. Turbulence intensity in the simulated clouds is spatially inhomogeneous and reaches its maximum at the tops of multiple bubbles forming the clouds. It is shown that polluted clouds are more turbulent than those developing in the clean atmosphere. An agreement of the calculated droplet size distributions with those measured in situ is demonstrated. It is shown that turbulence accelerates formation of raindrops, especially in polluted clouds. At the same time, turbulence-induced collision enhancement lessens the amount of ice and leads to a decrease in the net accumulated rain from mixed-phase clouds. To a certain degree the effects of turbulence on precipitation counteract the aerosol effects. Since no turbulence effects on collisions of drops larger than 22 mm in radius as well as on drop ice and ice collisions are considered in this study, and taking into account that a 2-D model geometry is used, the results of the study should be considered as preliminary. Additional numerical and theoretical investigations are required to quantify the turbulent effects.Citation: Benmoshe, N., M. Pinsky, A. Pokrovsky, and A. Khain (2012), Turbulent effects on the microphysics and initiation of warm rain in deep convective clouds: 2-D simulations by a spectral mixed-phase microphysics cloud model,

Formation of highly porous aerosol particles by atmospheric freeze-drying in ice clouds

“…This potential longevity may affect the aerosols' radiative effect and its prolonged involvement in ice nucleation, cloud activation, and precipitation. The particle density decrease may also change how an aerosol affects turbulence in a cloud, further influencing cloud development and precipitation (52). Moreover, the particles' larger and corrugated surface area may increase gas adsorption and rates of light-induced heterogeneous chemical processes (53).…”

Simultaneous and synergistic profiling of cloud and drizzle properties using ground-based observations

“…This choice of threshold is motivated by the recognition that a characteristic or critical droplet radius exists, above which intense droplet coalescence triggers rapid drizzle formation. This radius is found to be 12-14 µm as shown by satellite and ground-based observations (Suzuki et al, 2010;Rosenfeld, 2000;Rosenfeld and Gutman, 1994), aircraft measurements (Gerber, 1996;Boers et al, 1998;Freud and Rosenfeld, 2012), and numerical simulations (Magaritz et al, 2009;Pinsky and Khain, 2002;Benmoshe et al, 2012). This retrieval technique allows us to retrieve not only drizzle microphysical properties below the cloud base but also within the cloud at the same time.…”

The effects of turbulence on the microphysics of mixed‐phase deep convective clouds investigated with a 2‐D cloud model with spectral bin microphysics

“…[], the values of ε in deep cumulus clouds may range from several hundred cm 2 s − 3 to ~ 2 ⋅ 10 3 cm 2 s − 3 . Simulations of mixed‐phase deep convective clouds performed by Benmoshe et al [] using the Hebrew University cloud model (HUCM) supported the validity of these estimations. It was shown that the intensity of turbulence is distributed over the cloud in a highly inhomogeneous manner, indicating the presence of so‐called large‐scale turbulence intermittency.…”

“…Under such intense turbulence, collision kernels can exceed gravitational kernels by a factor as high as 5–15 [ Pinsky et al , ]. Benmoshe et al [] investigated the role of turbulence in formation of first raindrops in deep convective clouds under different aerosol loadings. The values of ε and Re λ were calculated at each model grid point using the equation for turbulence kinetic energy, as well as some assumptions concerning the maximum scales of turbulent vortices following from the theory of cloud top entrainment instability proposed by Grabowski and Clark [, ].…”