Horizontal Variations in the Net Heat Flux of a Springtime Freezing Lead

The first month of the FIRE.ACE overflights of SHEBA, which took place in April and May, corresponded with a period of increased lead activity. Leads varying in width from15-500 m were observed during a 2 week period beginning 27 Apr 98. Rapid ice production was observed to occur within the leads at air temperatures between -25 and -15 C with frazil ice production being evident for extended periods of time in the widest leads. In one case a 400 m wide lead, with the orientation/wind direction resulting in minimum fetch, was completely covered with new ice in less than 6 hours. While open water was exposed plumes of steam were observed above and downwind of this lead reaching heights of 5-10 m immediately downwind.

This study documents the evolution of the surface energy budget and characteristics of several leads observed during SHEBA. In addition, the atmospheric footprint of a 400 m wide lead is described using observations from several in situ observing platforms located downwind. Aircraft measurements of skin temperature, albedo and ice characteristics obtained with the video are used to characterize the horizontal variability across the lead. The components of the surface energy budget are used to force an ice growth model. Observations of ice growth and horizontal variability in ice thickness and temperature are used to evaluate the parameterized processes in the model. The model is used to determine the area-average heat and moisture fluxes into the atmosphere which is correlated with the observed atmospheric footprint.