Observations of the Global Characteristics and Regional Radiative Effects of Marine Cloud Liquid Water

Abstract

The large-scale spatial distribution and temporal variability of cloud liquid water path (LWP) over the world's oceans and the relationship of cloud LWP to temperature and the radiation budget are investigated using recent satellite measurements from the Special Sensor Microwave/Imager(SSM/1),the Earth Radiation Budget Experiment (ERBE), and the International Satellite Cloud Climatology Project (ISCCP). Observations of cloud liquid water on a 2.5° × 2.5° grid are used over a 53-month period beginning July 1987 and ending in December 1991. The highest values of cloud liquid water (greater than 0.13 kg m−2) occur largely along principal routes of northern midlatitude storm and in area dominated by tropical convection. The zonally averaged structure is distinctly trimodal, where maxima appear in the midlatitudes and near the equator. The avenge marine cloud LWP over the globe is estimated to he about 0.113 kg m−2. Its highest seasonal variability is typically between 15% and 25% of the annual mean but in certain locations can exceed 30%. Comparisons of cloud LWP to temperature for low clouds during JJA and DJF of 1990 show significant positive correlations at colder temperatures and negative correlations at warmer temperatures. The correlations also exhibit strong seasonal and regional variation. Coincident and collocated observations of cloud LWP from the SSM/I and albedo measurements from the Earth Radiation Budget Satellite (ERBS) and the NOAA-10 satellite are compared for low clouds in the North Pacific and North Atlantic. The observed albedo-LWP relationships correspond reasonably well with theory, where the average cloud effective radius (re) is 11.1 μm and the standard deviation is 5.2 μm. The large variability in the inferred values of re suggests that other factors may be important in the albedo-LWP relationships. In terms of the effect of the LWP on the net cloud forcing, the authors find that a 0.05 kg m−2 increase in LWP (for LWP >0.2 kg m−2) results in a −25 W m−2 change in the net cloud forcing at a solar zenith angle of 75°.