On the Microphysical Representation of Observed Arctic Mixed-Phase Clouds

Articulating Arctic cloud microphysics is important towards modeling future radiative feedbacks upon Arctic climate. Updated analyses of in situ microphysical properties of three Arctic cloud systems sampled by aircraft in July 1998 during SHEBA/FIRE.ACE are examined in detail and compared with surface-based millimeter Doppler radar. The clouds were at °N over a melting ice surface, in distinctly different yet typical synoptic conditions. The cases comprise a mid-level all-ice cloud on 8 July, a deep, weakly forced, layered, mixed-phase stratus cloud system with pockets of drizzle, large dendrites, rimed ice and aggregates on 18 July, and a deep, mixed-phase cloud system with embedded convection on 28 July followed by an all-water boundary-layer cloud on 29 July. The new observations include measured ice water contents exceeding 2 g m-3 on 18 and 28 July, 3-cm snowflakes and 5-mm graupel particles on 28 July, unexpected in clouds close to the North Pole. The cloud cases help provide insight into the cloud changes that accompany recent changes in Arctic climate. The observed snow particle size distributions are then analyzed with the intent of developing exponential fits suitable for use in in one-moment microphysical schemes. Exponential fits for the deep mixed-phase clouds are generally in agreement with previous observations of mid-latitude frontal systems, but differ for shallow mixed-phase clouds. This suggests that cloud microphysical one-moment schemes applied to the Arctic climate need to find ways to take the observed variability into account.