MEASUREMENT OF SURFACE RADIATION FLUXES AND CLOUD OPTICAL-PROPERTIES DURING THE 1994 ARCTIC-OCEAN SECTION

During a voyage to the north pole from Alaska by the icebreakers USCGC Polar Sea and Canadian CGC Louis S. St.-Laurent (the 1994 Arctic Ocea n Section, July 24 to September 3) an atmospheric radiation and remote sensing experiment measured downwelling shortwave and longwave radiat ion reaching the sea ice surface. The experiment included a Fourier tr ansform infrared (FTIR) spectroradiometer which measured zenith radian ce at 1 cm(-1) resolution in the middle infrared wavelength range 5-20 mu m, an Eppley pyranometer measuring most of the downwelling shortwa ve flux (0.28-2.80 mu m), an Eppley pyranometer measuring the downwell ing near-infrared flux (0.78-2.80 mu m), and an Eppley pyrgeometer mea suring the downwelling longwave flux. In conjunction with a discrete-o rdinates radiative transfer model, the FTIR emission spectra are used to estimate 8-12 mu m cloud emissivity and effective radius of the clo ud droplet size distribution. The broadband shortwave flux measurement s are used to estimate shortwave cloud scattering optical depth. Most of the FTIR emission spectra recorded under overcast skies are consist ent with cloud effective radius in the range 10-12 mu m, but 27% of th e spectra are more consistent with the range 4-6 mu m, suggesting an o ccasional continental aerosol influence to Arctic cloud microphysics. The average daily shortwave cloud-scattering optical depth ranged from 2 to 46, which is similar to a range inferred from radiometer data re corded at Barrow, Alaska, during the same season. The downwelling shor twave flux measurements and estimates of net surface flux are generall y consistent with a four-decade Russian climatology but also suggest t hat the frequency of cloud cover sampled during the 1994 Arctic Ocean Section was somewhat larger than the climatological average. These rad iation measurement data from the 1994 Arctic Ocean Section should be u seful for examining the treatment of atmospheric radiation and surface energy input in Arctic climate model simulations.