Features of Clouds over the Tropical Pacific during Northern Hemispheric Winter Derived from Split Window Measurements

Abstract

Cloud covers for six cloud types have been studied over the tropical Pacific Ocean from split window data of the advanced very high resolution radiometer (AVHRR) on board NOAA-9 during the period January 11 to February 9, 1987. Six cloud types, including cirrus clouds and low-level cumulus/stratocumulus clouds which were hard to detect without visible data, are classified by the radiative properties of the 10μm window region. Comparisons between cloud types classified by the split window method and visible data show consistency in optical thickness. Cumulus-type clouds classified by the method indicate higher reflectivity than cirrus-type clouds.The thirty-day mean cloud cover defined in 2.5° latitude/longitude areas over the tropical Pacific for total cloudiness, total optically thin clouds (cirrus-type clouds), . optically thick high-level clouds (cumulonimbus-type cloud) and optically thick low-level clouds (cumulus/stratocumulus-type clouds) have coverage of 54%, 31%, 6% and 9%, respectively. It is found that cirrus-type clouds are the major cloud over the tropics. Another feature is that cumulonimbus-type clouds occupy only about 46% within the so-called high-level cloud (colder than -20°C) defined by single infrared data.Cloud features over the western Pacific and the eastern Pacific divided along 150°W, show significant differences. The total cloudiness over the western Pacific ocean area was 61%, and 41% over the eastern Pacific. Cumulonimbus-type cloud and cirrus-type cloud appear over the western Pacific twice as often as over the eastern Pacific. In contrast, cloud cover of cumulus/stratocumulus-type clouds over the eastern Pacific is twice as much as over the western Pacific.The spatial distribution of high cloud amount area of cumulonimbus-type cloud over the western Pacific shows overall correspondence to a warmer sea surface temperature (SST)(>29°C) area, while the distribution of high cloud amount area of cumulus/stratocumulus-type clouds is confined to over the cooler SST (26°C) ocean area from the scatter diagram of SST derived from the split window data and cloud cover of cumulonimbus-type cloud, and also from the scatter diagram of SST and mean brightness temperature of cumulonimbus-type cloud.