Seasonal Variation of Liquid and Ice Water Path in Nonprecipitating Clouds over Oceans

Abstract

Seasonal variations of liquid and ice water paths (LWP and IWP) in nonprecipitating clouds over oceans are estimated for 4 months by combining the International Satellite Cloud Climatology Project (ISCCP) and Special Sensor Microwave/Imager (SSM/I) data. The ISCCP data are used to separate clear/cloudy skies and warm/cold clouds and to determine cloud optical thickness, cloud-top temperature, and sea surface temperature. SSM/I data are used to separate precipitating and nonprecipitating clouds and to determine LWP. About 93% of all clouds are nonprecipitating clouds, and about half of nonprecipitating clouds are warm (cloud-top temperature > 0°C). The average LWP for warm nonprecipitating clouds is about 6 mg cm⁻². The values of total water path obtained from the ISCCP values of optical thickness for cold nonprecipitating clouds are larger than the LWP values from SSM/I, which the authors explain in terms of IWP. The average IWP for cold nonprecipitating clouds is about 7 mg cm⁻², with LWP being about 5 Mg cm⁻². Tropical and cold hemisphere clouds have higher IWP values (around 10 mg cm⁻²) than those in warm hemispheres; where LWP values for warm nonprecipitating clouds vary little with latitude or season. Ice fractions, IWP/(LWP + IWP), in cold nonprecipitating clouds increase systematically with decreasing cloud-top temperatures, reaching 50% at about −15°C but ranging from about −5° to −10°C in the northern midlatitudes in autumn and the Tropics year-round to about −25°C in the southern midlatitudes in summer. The ratio of IWP to LWP in cold nonprecipitating clouds reaches almost 3 in the northern midlatitudes in autumn and falls as low as 0.6 in the southern midlatitudes in spring-summer. Combining warm and cold nonprecipitating clouds gives a global ratio of IWP to LWP that is about 0.7 over oceans.