Possible sufficiency of conventional ice nucleation mechanisms in a case study of arctic stratus: April 8th during ISDAC

Mixed-phase stratus clouds are common during winter and transition seasons in the Arctic and through various feedback mechanisms are expected to exert a strong influence on Arctic climate. Despite their important role, their representation in cloud models remains problematic. In particular, models of all types experience difficulties reproducing observed ice crystal number concentrations and liquid-ice phase partitioning in these clouds. . The supply rate of heterogeneous ice nuclei (IN) to the boundary layer from air aloft is typically far too slow -- relative to the sedimentation sink at the surface -- to explain observed number concentrations of ice crystals (only counting sizes that are expected to minimize shattering artifacts), as has been found by a number of studies. This longstanding discrepancy has driven consideration of alternative ice nucleation mechanisms or IN sources that may elude detection by the standard portable IN measurement method (the continuous flow diffusion chamber). Recent advances in the design of probe tips for instruments measuring ice-crystal size distributions greatly reduce shattering artifacts and provide greater confidence in measured ice concentrations. These improved tips were deployed during the Indirect and Semi-Direct Aerosol Campaign (ISDAC) in April, 2008. A well-studied case study from another field project (M-PACE) has been found by a number of modeling groups to require alternative mechanisms or undetected IN sources in order to explain the observed number concentrations of ice crystals. Representation of the ice in that case was hampered by the presence of a variety of ice crystal habits with different mass growth rates and fall-speed relationships or complicated shapes for which the fall-speeds are not well constrained. The April 8 case study from ISDAC presents a contrasting case. The in-situ observations indicate predominance of well-characterized habits -- pristine (and aggregates) of dendritic ice crystals with no riming -- which have slow fall-speeds and thus provide a slow removal rate of ice concentration by number, which favors the possibility that IN entrained into the boundary layer from aloft -- at concentrations far greater than found in the M-PACE case -- can maintain sufficient ice concentrations. Also, the full suite of instrumentation on the Convair-580 was operational for this case, and the aircraft overflew the Barrow ground site, allowing intercomparison of measurements and greater constraints on model simulations. We use large-eddy simulations with size-resolved microphysics to determine whether conventional ice nucleation mechanisms using the observed IN concentration can account for the observed cloud properties within the uncertainties in IN concentrations, ice nucleation mechanisms, ice crystal habits, and large-scale forcing.