HIGH-RESOLUTION SATELLITE ANALYSIS AND MODEL EVALUATION OF CLOUDS ANDRADIATION OVER THE MACKENZIE BASIN USING AVHRR DATA

Both the issues of high-resolution satellite analysis and model evalua tion for a region centered on the Arctic Circle (60 degrees-75 degrees N) are addressed. Model cloud fraction, cloud height; and outgoing ra diation are compared to corresponding satellite observations using a m odel-to-satellite approach (calculated radiances from model state). Th e dataset consists of forecasts run at 15-km resolution up to;30 h and nearly coincident Advanced Very High Resolution Radiometer (AVHRR) im agery during the Beaufort and Arctic Storm Experiment over the Mackenz ie Basin for a monthly period in the fall of 1994. A cloud detection a lgorithm is designed for day and night application using the 11-mu cha nnel of AVHRR along with available information on atmospheric and grou nd temperatures. The satellite and model estimates of cloud fraction a re also compared to observations at 20 ground stations. A significant result of the validation is that the model has a higher frequency of l ow cloud tops and a lower frequency of midlevel cloud tops than the ob servations. On a monthly basis, the model 11-mu outgoing brightness te mperature (TB) is consequently higher than observed by about 4.4 K at all forecast times, which corresponds to a deficit of 760 m in mean cl oud-top height and about 10 W m(-2) in outgoing Aux at the top of the atmosphere. Possible errors in the parameterization of ice or water cl oud emissivity are evaluated but ruled out as the dominant cause for t he warm TB bias in the model. Rather, the problem is attributed to low clouds being trapped in the boundary layer, whereas high clouds appea r to be reasonably well modeled. The role of thin ice clouds is furthe r evaluated by comparing distributions of observed and modeled Il-p mi nus 12-mu TB differences, DIF45 (channel 4 minus channel 5). The relat ionship between the true height of the clouds and the effective height observed by satellite is modeled from forecast outputs as a function of DIF45. The quality of daily estimates is evaluated from time series at various locations. The time series shows that there was a marked d rop in DIF45 during the month, which is attributed to a decrease in th e occurrence of cirrus clouds. Finally. the diurnal cycle of TB and cl oud fraction is found to be relatively large with average monthly 0600 -1800 UTC TB differences of both signs of the order of 4-8 K in broad sectors and cloud fraction differences of 10%-30%. Where low clouds pr evail, the cloud fraction tends to decrease at night and TB increases. Overall, model-observation differences are dominated by differences i n the vertical distribution of clouds. A reduction of this effect impl ies a modification of the ''preferred'' model climatology in terms of its vertical distribution of humidity and cloud water.