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Cloud Radiative Forcing From AVHRR Data Over the Arctic During FIRE ACE |
Analytical Services & Materials
NASA-Langley Research Center
Analytical Services & Materials
Analytical Services & Materials
Clouds have similar visible brightnesses and infrared temperatures as the underlying snow making it difficult to detect them in satellite imagery. This has made observational cloud forcing estimates in the arctic difficult. Advanced Very High Resolution Radiometer (AVHRR) multispectral data allows us to identify clouds by using the temperature difference between the 3.7um and 10.8um channels, which is higher for clouds than clear sky. Mutlichannel scene identification thresholds were derived on an image by image basis. NOAA-12 and NOAA-14 AVHRR narrowband visible (0.65um) and infrared (10.8um) radiances are used to derive shortwave (0.2-5.0um) and longwave (5-50um) broadband fluxes over the arctic during the First ISCCP Regional Experiment (FIRE) Arctic Cloud Experiment (ACE). Emperical formulae were based on coincident Earth Radiation Budget Experiment (ERBE) broadband fluxes and AVHRR narrowband fluxes from 1986 over the same arctic region.
Satellite cloud amounts are validated against SHEBA ship observations, SHEBA cloud radar, and Barrow surface observations. Aircraft and surface-based radiometer measurements are used to verify the AVHRR based broadband satellite fluxes. The data sampling of the AVHRR HRPT images taken during FIRE-ACE provides a good diurnal estimate of cloud radiative forcing, except for early evening. Cloud forcing analysis will be presented from March through October 1998 over the arctic ocean to include both the SHEBA and Barrow, AK sites. Cloud forcing estimates computed for the SHEBA domain encompass the area of 72°N to 80°N and 180°W to 150°W. The Barrow domain will be of similar size. Results for the SHEBA domain for May-July 1998 show shortwave cloud forcing to be 3 to 4 times the longwave forcing, with typical net forcing in the -16 to -21 W/m2 range. The clear-sky longwave flux before the onset of melting shows a diurnal cycle similar to that over land and is also manifested in tower flux instruments.
