Effect of ice on mixed-phase cloud dynamics

Description

High sensitivity of mixed-phase cloud properties to ice number concentration, Ni, found in previous modeling studies is investigated here using simulations of a stratiform Arctic cloud observed on 26 April 2008 during flight 31 of the Indirect and Semi-Direct Aerosol Campaign (ISDAC). A 3D large-eddy simulation model with a spectral bin microphysics treatment is shown to lose its ability to maintain the liquid phase in the mixed layer when Ni increases from the observed value of 0.5 1/L to 2 1/L. In order to better understand this highly non-linear model response to changes in Ni, the simulations are compared to a model run with liquid phase only, which produces a cloud with the largest and growing liquid water path (LWP) because the depth of a mixed layer is increasing and more moisture is kept in that layer due to lack of precipitation. The simulations with smaller ice concentration stabilize the LWP near the initial value, while in the higher Ni simulation, the cloud starts losing liquid water almost immediately and the LWP is reduced by half in less than two hours. The changes in liquid water are accompanied by corresponding reduction in the radiative cooling of the layer and a slowdown in the vertical mixing, confirming the important role of interactions among microphysics, radiation, and dynamics in this type of clouds. It is shown that at early stages, changes in liquid and ice water as well as in radiative cooling/heating rates are proportional to the Ni change, while changes in the vertical buoyancy flux are highly nonlinear, and, at some levels, differ even in sign between lower and higher Ni simulations. At higher Ni, the degree of reduction in positive buoyancy flux within and below the liquid cloud layer is large enough to slow down the circulation, making vertical motions too weak to sustain continuous formation of liquid water in the considered case.