11-12 March 2013 NSA Cloud Phase Partitioning Modeling Case Study

Description

The 11-12 March 2013 case study from the North Slope of Alaska site is a current focus of the ASR Cloud Phase Partitioning research group due to the occurrence of a mixed-phase Arctic stratocumulus that rapidly transitions to a cloud system with very little precipitation. In this study, a process model is used to investigate feedbacks that determine cloud phase partitioning in Arctic mixed-phase stratocumulus. Limited area model simulations with the WRF model forced by ECMWF reanalysis indicate that the observed rapid transition in precipitation is primarily due to changes in synoptic conditions where three different scenarios are identified that determine the cloud phase at the NSA site. The first is large-scale convergence over open water with fractional sea-ice that causes strong snow-bands to form that are advected to Barrow from the west. The second scenario is the formation of cloud streets that form as a ridge structure causes cold, dry air to be advected over the open water that then is advected to Barrow from the southwest. The third scenario is the advection of cold, dry air below ~0.4km to Barrow from the south that causes the near-surface layer to be stably stratified and reduces the formation of cloud ice while the cloud liquid is still maintained by cold-top cooling. The three scenarios result in different liquid and ice water path relationships. Interestingly, the model shows a more rapid decrease in liquid water path at the NSA site compared to the observations, indicating a potential bias in the model representation of Arctic mixed-phase stratocumulus with low cloud ice production. Sensitivity to microphysical parameterizations of cloud ice characteristics will be presented and discussed.