Description of the Cloud Model

A 2-D mixed phase Hebrew University cloud model (HUCM) with spectral bin microphysics (Khain et al., 2004, 2005, 2008) has been used to investigate whether an increase in aerosol concentration in the zone of maritime tropical convection can change the cloud microphysical structure of individual clouds to make it suitable for lightning formation. The HUCM model microphysics is based on solving a kinetic equations system for size distribution functions for water drops, ice crystals (plate-, columnar- and branch types), aggregates, graupel and hail/frozen drops, as well as atmospheric APs. Each size distribution is described using 43 doubling mass bins, allowing simulation of graupel and hail with sizes up to 4 cm in diameter. The model is specially designed to take into account the effects of AP on the cloud microphysics, dynamics, and precipitation. The initial (at t = 0) CCN size distribution is calculated using the empirical dependence

using the procedure described by Khain et al. (2000). In (1) N is the concentration of activated AP (nucleated droplets) at supersaturation S1 (in %) is with respect to water, N0 and k are the measured constants. At t > 0 the prognostic equation for the size distribution of non-activated AP is solved. Using the value of S1 calculated at each time step, the critical AP radius is calculated according to the Kohler theory. The APs with radii exceeding the critical value are activated and new droplets are nucleated. The corresponding bins of the CCN size distributions become empty.

Primary nucleation of each type of ice crystals is performed within its own temperature range following Takahashi et al. (1991). The dependence of the ice nuclei concentration on supersaturation with respect to ice is described using an empirical expression suggested by Meyers et al. (1992) and applied using a semi-lagrangian approach (Khain et al. 2000) allowing the utilization of the diagnostic relationship in the time dependent framework. The diffusional growth/evaporation of droplets and the deposition/sublimation of ice particles are calculated using analytical solutions for supersaturation with respect to water and ice. An efficient and accurate method of solving the stochastic kinetic equation for collisions (Bott, 1998) was extended to a system of stochastic kinetic equations calculating waterice and ice-ice collisions. The model uses height dependent drop-drop and drop-graupel collision kernels following Khain et al. (2001) and Pinsky et al. (2001). Iceice collection rates are assumed to be temperature dependent (Pruppacher and Klett, 1997). An increase in the water-water and water-ice collision kernels caused by the turbulent/inertia mechanism was taken into account according to Pinsky and Khain (1998) and Pinsky et al. (2007). Advection of scalar values is performed using the positively defined conservative scheme proposed by Bott (1989). The computational domain is 178 km x 16 km with the resolution of250 m and 125 m in the horizontal and vertical directions, respectively.

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