Interactions between North Atlantic clouds and the large-scale environment

The European Centre for Medium Range Weather Forecasting initialized analyses and the US Air Force Three-Dimensional Nephanalysis are employed to construct a joint time series of gridpoint values of cloudiness and large-scale meteorological fields, including heat and moisture budgets, for January 1979. Interpretation of cloud in the context of the large-scale flow is given for the monthly average situation, disturbed and undisturbed conditions, and a longitudinal cross section through a baroclinic wave. The relationship of cloud fraction to the large-scale humidity field is examined, along with several diagnostic parameterizations of cloud fraction currently employed in general circulation models. The grid-scale threshold relative humidity below which cloud, on average, does not occur was determined to show a strong decrease with height. It was shown that appropriate "tuning' of a diagnostic relative humidity-based parameterization can result in accurate parameterized mean monthly total cloud amount for the region, and layer cloud fractions to within 5% of observed layer cloud fractions. -from Authors