High-resolution daytime cloud observations for northwestern Mexico from GOES-7 satellite observations

The first stage in a program of research to develop a regional model capabl e of describing the hydrology of semiarid areas of northwest Mexico and sou thwest United States, using remotely sensed data, is described in this pape r. Finescale information on cloud cover is required to provide the radiatio n forcing for making simple, near-real-time estimates of daytime evaporatio n in hydrologic models, and frequent satellite observations have the potent ial to document cloud variability at high spatial and temporal resolutions. In this study, the operational framework for obtaining information on clou d cover was developed and applied, using hourly sampled, 1-km resolution GO ES-7 data as received in real time in Obregon, Mexico. These satellite data were collected and analyzed from 1 July 1993 to 31 July 1994 for an approx imately 10(6) km(2) rectangular area in northwest Mexico. An efficient meth od was devised to provide clear-sky radiance images for the study area, at 4 km x 4 km resolution, and updated at monthly intervals, by applying thres holds indexed to the locally appropriate clear-sky radiance, thereby allowi ng for spatial and temporal changes in surface conditions. Manual image ins pection and comparison with ground-based measurements of cloud cover and su rface solar radiation provided reassurance that the high-resolution cloud-s creening algorithm gave satisfactory results. This algorithm was applied to investigate the effects of temporal sampling frequency on estimates of daytime-average cloud cover and to document aspec ts of the cloud characteristics for the study area. The high-resolution alg orithm proved to be efficient and reliable and bodes well for its future us e in providing high-resolution estimates of surface solar radiation for use in a hydrologic model. Monthly clear-sky composite images were consistentl y generated, showing little evidence of contamination by persistent clouds, and tracked the seasonal evolution in surface radiance. Comparison with gr ound-based measurements gave confidence in the credibility of the satellite estimates and revealed weaknesses in the Campbell-Stokes solarimeter. The seasonal evolution of spatial patterns of cloud and its diurnal cycle were investigated. The average cloudiness for the study area is 0.25, with a sub stantial annual variation from 0.19 in April to 0.40 in December. Persisten t cloudy conditions throughout the year were detected over the Pacific Ocea n west of Baja California. The derived high-resolution cloud estimates, whe n compared with similar estimates from the International Satellite Cloud Cl imatology Project (ISCCP D1), were about half those obtained with the low-r esolution data, indicating that, in this complex study area where land and water boundaries are in close proximity, low-resolution satellite observati ons of clouds may not be able to depict the true cloud cover.