Cloud coverage and height during FIRE ACE derived from AVHRR data

Cloud cover and height are derived from NOAA-12 and NOAA-14 advanced very h igh resolution radiometer (AVHRR) data taken over the Arctic Ocean for an 8 degrees latitude by 30 degrees longitude domain centered on the Surface He at Budget of the Arctic Ocean (SHEBA) ship Des Groseilliers. Multispectral thresholds were determined subjectively and applied to each image, providin g excellent temporal coverage during the May-July 1998 First ISCCP Regional Experiment Arctic Clouds Experiment (FIRE ACE). Mean cloud amounts were ne ar 70% for the entire period but varied regionally from 55 to 85%. On the b asis of a limited climatology of ship observations, these values appear to be typical for this part of the Arctic, suggesting that most of FIRE ACE wa s conducted in representative cloud conditions. A diurnal cycle of mean clo ud amount was found for the domain during June and July having a range of 1 0% with a middle-to-late morning maximum. The AVHRR-derived cloud amounts a re in good agreement with visual and radar measurements taken from the Des Groseilliers, except for a few subvisual and low cloud cases. Average AVHRR -derived cloudiness differ from the mean values obtained at the surface by -1 to +3%; this represents a significant improvement over previous satellit e retrievals. The satellite-derived cloud heights are very accurate for mos t of the low cloud cases. Higher cloud altitudes are less certain because c loud optical depths were not available to adjust the temperature observed f or the optically thin high clouds, and the radiating temperature of many of the high clouds is representative of some altitude deep in the cloud rathe r than the highest altitude level of condensate. The development of a more accurate automated algorithm for detecting polar clouds at AVHRR wavelength s will require inclusion of variable thresholds to account for the angular dependence of the surface reflectance as well as the seasonally changing al bedos of the ice pack. The use of a 1.6-mum channel on the AVHRR, or other complement of instruments, will greatly enhance the capabilities for detect ing clouds over poles during summer.