Cloud radiative forcing at the top of the atmosphere during FIRE ACE derived from AVHRR data

Cloud radiative forcing at the top of the atmosphere is derived from narrow band visible and infrared radiances from NOAA-12 and NOAA-14 advanced very high resolution radiometer (AVHRR) data taken over the Arctic Ocean during the First ISCCP Regional Experiment Arctic Cloud Experiment (FIRE ACE) duri ng spring and summer 1998. Shortwave and longwave fluxes at the top of the atmosphere (TOA) were computed using narrowband-to-broadband conversion for mulae based on coincident Earth Radiation Budget Experiment (ERBE) broadban d and AVHRR narrowband radiances. The NOAA-12/NOAA-14 broadband data were v alidated using model calculations and coincident broadband flux radiometer data from the Surface Heat Budget of the Arctic Ocean experiment and from a ircraft data. The AVHRR TOA albedos agreed with the surface- and aircraft-b ased albedos to within one standard deviation of +/-0.029 on an instantaneo us basis. Mean differences ranged from -0.012 to 0.023 depending on the rad iometer and platform. AVHRR-derived longwave fluxes differed from the model calculations using aircraft- and surface-based fluxes by -0.2 to -0.3 W in -, on average, when the atmospheric profiles were adjusted to force agreeme nt between the observed and the calculated downwelling fluxes. The standard deviations of the differences were less than 2%. Mean total TOA albedo for the domain between 72 degreesN and 80 degreesN and between 150 degreesW an d 180 degreesW changed from 0.695 in May to 0.510 during July, while the lo ngwave flux increased from 217 to 228 W m(-2). Net radiation increased from -89 to -2 W m-2 for the same period. Net cloud forcing varied from -15 W m (-2) in May to -31 W m(-2) during July, while longwave cloud forcing was ne arly constant at similar to8 W m(-2). Shortwave cloud forcing dominated the cloud effect, ranging from -22 W m(-2) during May to -40 W m(-2) in July. The mean albedos and fluxes are consistent with previous measurements from the ERBE, except during May when the albedo and longwave flux were greater than the maximum ERBE values. The cloud-forcing results, while similar to s ome earlier estimates, are the most accurate values hitherto obtained for r egions in the Arctic. When no significant melting was present, the clear-sk y longwave flux showed a diurnal variation similar to that over land under clear skies. These data should be valuable for understanding, the Arctic en ergy budget and for constraining models of atmosphere and ocean processes i n the Arctic.