NUMERICAL-SIMULATION OF CLOUD PLUMES EMANATING FROM ARCTIC LEADS

A two-dimensional, steady state boundary layer model is used to invest igate the formation of cloud plumes over open Arctic leads. Satellite observations from the period of an intense storm in the Beaufort Sea d uring April 1992 are used to document lead plume formation [Fett et al ., 1997]. These observations show a marked variability of open leads w ith and without cloud plumes dependent on synoptic weather conditions. Model simulations are conducted (using soundings before and after the storm's passage) which show extensive plumes forming in the postfront al environment but not in the prefrontal atmosphere. To better isolate important factors contributing to plume development and plume dynamic s, a large set of idealized model experiments is performed. This matri x of model simulations permits exploration of the parameter space of p lume formation. The model is found capable of forecasting the onset of steam fog over a lead in a manner which agrees with empirical relatio nships. Such steam fog, however, dissipates quickly upon advecting ove r the surrounding ice and does not therefore appear generally responsi ble for extensive plumes observed to emanate from leads. Instead, exte nded cloud plumes form at the top of the convective thermal internal b oundary layer (TIBL) that develops over a lead. These elevated plumes are quite sensitive to the relative humidity of the environmental air. Unlike steam fog, the elevated plumes also are found to be sensitive to the lapse rate of the environmental air. Changes in wind speed have little impact on the integrated heat and moisture amounts transferred to the atmosphere from a lead (provided advection balances flux diver gence) but do impact the cloud plumes by altering shear-driven entrain ment. For wide leads, indications are that lead refreezing must be in an advanced stage before having a major impact on the plume formation process. Additionally, we investigate the conditions under which a clo ud plume that is detached from its parent lead may form downwind of a lead. Often, the plumes are predominantly water clouds near the lead, with the ice crystal fraction increasing as the plume cools downstream . A simple formula for estimating plume penetration depth is found to agree reasonably well with the model simulations for 1 km wide leads b ut deviates considerably for wider leads.