ICE PACK AND LEAD SURFACE-ENERGY BUDGETS DURING LEADEX-1992

During a 1-month deployment for the Arctic Leads Experiment (LEADEX) in March and April 1992 on the Arctic ice cap roughly 200 km north of Prudhoe Bay, Alaska, surface-based mean meteorological and flux instruments plus a variety of remote sensors were operated at the main base camp. Identical systems were also deployed by helicopter on the upwind and downwind edges of several Arctic leads, two of which we describe in this paper. At the base camp the diurnal amplitude for sensible heat flux was +/-10 W m(-2) about a mean of -3 W m(-2), net radiation was +/-30 W m(-2) about a mean of -15 W m(-2), and net surface energy flux was +/-20 W m(-2) about a mean of -12 W m(-2). The mean latent heat flux was +1 W m(-2) with a diurnal variation of about +/-1.5 W m(-2). Mean values for the momentum and sensible heat transfer coefficients were C-D = (1.20 +/- 0.20) x 10(-3) and C-H= (0.75 +/- 0.25) x 10(-3) at a 10-m reference height with only modest diurnal variations. Two lead deployments were examined. Lead 3 was approximately 1 km across. Only limited meteorological data were obtained for about 6 hours at the end of April 7 and beginning of April 8 when the lead was covered with about 10 cm of ice. Downwind of the lead, the sensible heat flux increased to about 170 W m(-2) and the stress doubled, suggesting an ice-covered lead 10-m drag coefficient of 2.2 x 10(-3). More than 36 hours of data were obtained upwind and downwind of lead 4, which varied in width from 80 to 120 m. Doppler minisodars upwind and downwind of the lead indicated a doubling in the depth (5 to 10 m) of the shear-driven turbulent surface layer downwind of the lead and an intensification of intermittent wave interactions exceeding 60 m (sodar range maximum). Three prominent waves with strong downward motion were observed in this period, apparently causing increases in the downwind stress magnitude. Various sources of data were used to compute estimates over a 36-hour period of the net surface heat flux Q(g) over the lead, the adjacent pack ice, and any open water that might have occurred in the lead. The results indicate that once significant ice forms, the sun is increasingly more effective in reducing the surface freezing rate and in shutting off convective mixing in the ocean under the lead. Over the period of observations the average net surface heat flux was -75 W m(-2) over the pack ice, -130 W m(-2) over the lead, and -250 W m(-2) over the open water.