PNNL

Abstract

Previous modeling studies have shown a high sensitivity of simulated properties of mixed-phase clouds to ice number concentration, Ni, with many models losing their ability to maintain the liquid phase as Ni increases. Although models differ widely at what Ni the mixed-phase cloud becomes unstable, the transition from a mixed-phase to an ice only cloud in many cases occurs over a narrow range of ice concentration. To gain better understanding of this non-linear model behavior, in this study, we analyze simulations of a mixed-phase stratiform Artic cloud observed on 26 April 2008 during recent Indirect and Semi-Direct Aerosol Campaign (ISDAC). The BASE simulation, in which Ni is constrained to match the measured value, produces a long-lived cloud in a quasi steady state similar to that observed. The simulation without the ice (NO_ICE) produces a comparable but slightly thicker cloud because more moisture is kept in the mixed layer due to lack of precipitation. When Ni is quadrupled relative to BASE (HI_ICE), the cloud starts loosing liquid water almost immediately and the liquid water path 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 slow down in the vertical mixing, confirming the important role of interactions among microphysics, radiation and dynamics in this type of clouds. Deviations of BASE and HI_ICE from NO_ICE are used to explore the linearity of the model response to variation in Ni. 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 qualitatively different in HI_ICE compared to BASE. Thus, while the positive feedback between the liquid water path and radiative cooling of the cloud layer is essential for glaciation of the cloud at higher Ni, the non-linear (with respect to Ni) reduction in positive buoyancy flux within and below the liquid cloud layer emerges as the process that influences the initial response of the mixed layer dynamics to the appearance of ice and subsequently determines the sustainability of liquid water in the cloud in this case.