HR: 09:30h
AN: C31B-07    [Abstracts]
TI: Effects of Clouds and Sea Ice on the Solar Energy Budget of the Southern Ocean
AU: Fitzpatrick, M F
EM: fitz@atmos.washington.edu
AF: University of Washington, Department of Earth and Space Sciences, Box 351310, Seattle, WA 98195 United States
AU: * Warren, S G
EM: sgw@atmos.washington.edu
AF: University of Washington, Department of Earth and Space Sciences, Box 351310, Seattle, WA 98195 United States
AB: Downward solar irradiance at the sea surface, measured on 18 voyages of an icebreaker in the Southern Ocean, is used to infer transmittance of solar radiation by clouds. Together with surface albedo estimated from coincident sea-ice reports, downward and net shortwave cloud radiative forcings are computed. The net forcing at a solar zenith angle of 60 degrees is typically -250 watts per square meter over open ocean, but only half this value over sea ice because of the higher surface albedo and less-frequent occurrence of clouds. Frequency distributions of effective optical depth are fitted by decaying exponentials, giving a characteristic optical depth of 15 at 47 S, increasing to 24 in the region of maximum cloud cover at 58 S, and decreasing to 11 at 67 S near the coast of Antarctica. This effective optical depth is a convenient intermediate quantity that can be used to compute what the transmittance of this same cloud field would be under different conditions of solar illumination and surface albedo. The distributions of optical depth are used, together with distributions of surface albedo from Brandt et al. (2005), to estimate the geographical and seasonal variations of shortwave irradiance and cloud radiative forcing at the surface, both for the present climate and for altered surface and cloud conditions. Poleward of 67 S in spring, ice causes a greater reduction of solar energy input to the surface than does cloud. However, in summer the clouds are more important than ice at all latitudes in the Southern Ocean. In the present climate the clouds are optically thicker over open water than over sea ice, suggesting a possible negative feedback if the sea-ice area shrinks with climatic warming. The changes to the solar energy budget caused by removing sea ice are therefore computed both with and without cloud-thickness changes. Compared to the present climate in spring, removing sea ice results in an increase in irradiance reaching the ocean surface, regardless of the type of cloud remaining. However, in summer the removal of ice results in higher irradiance at the surface only if clouds remain unchanged. If clouds become as thick as those presently over the ocean at 55-60 S, irradiance reaching the ground in summer decreases poleward of 65 S. References: Brandt, R.E., S.G. Warren, A.P. Worby, and T.C. Grenfell, 2005: Surface albedo of the Antarctic sea-ice zone. J. Climate, 18, 3606-3622. Fitzpatrick, M.F., R.E. Brandt, and S.G. Warren, 2004: Transmission of solar radiation by clouds over snow and ice surfaces: A parameterization in terms of optical depth, solar zenith angle, and surface albedo. J. Climate, 17, 266-275. Fitzpatrick, M.F., and S.G. Warren, 2005: Transmission of solar radiation by clouds over snow and ice surfaces, Part 2: Cloud optical depth and shortwave radiative forcing from pyranometer measurements in the Southern Ocean. J. Climate, in press. Fitzpatrick, M.F., and S.G. Warren, 2005: The relative importance of clouds and sea ice for the solar energy budget of the Southern Ocean. Submitted to J. Climate.
DE: 0321 Cloud/radiation interaction
DE: 0750 Sea ice (4540)
DE: 1621 Cryospheric change (0776)
DE: 3310 Clouds and cloud feedbacks
DE: 9310 Antarctica (4207)
SC: Cryosphere [C]
MN: Fall Meeting 2005