Interactions between North Atlantic Clouds and the Large-Scale Environment

Abstract

This paper addresses the problem of understanding and predicting the presence of clouds and their effects on the atmosphere in the midlatitudes of the North Atlantic Ocean. The European Centre for Medium Range Weather Forecasting initialized analyses and the U.S. 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. In general, middle clouds are formed primarily due to the benefit from large-scale three-dimensional moisture convergence; and low cloud formation depends on surface moisture flux and the static stability. Upper-level moisture was deemed to be sufficiently unreliable so that no inferences regarding high clouds could be made. Comparison of the relative humidity field with cloud cover in a cross section of a baroclinic wave shows that peak cloud fractions are displaced approximately 3.5° latitude to the east of the peak relative humidities. From concurrent examination of the residual heat and moisture sources, it is suggested that clouds do not respond instantaneously to the large-scale relative humidity field, but take a period of time on the order of hours to adjust in terms of evaporation and condensation. 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. However, this type of cloud fraction parameterization appears to be unable to diagnose layer cloud fraction on the smaller time and space scales that are undoubtedly required for obtaining the correct local cloud radiative and hydrological feedback with the dynamics.