Air-Sea Fluxes and Numerical Weather Prediction

Glenn White

Global Modeling Branch
Environmental Prediction Center
National Centers for Environmental Prediction
National Weather Service/NOAA/Dept. Of Commerce
Camp Springs, MD

This study assesses the ability of NCEP s global forecast system to produce realistic estimates of transfers of water, energy and momentum across the air-sea boundary in its analysis cycle, short and medium range forecasts and longer-range forecasts. Air-sea fluxes can serve as a good diagnostic of the global model s parameterisations of the boundary layer, radiation and cloudiness and are essential for coupling atmosphere and ocean models, a key element of dynamical seasonal forecasting.

The study reviews recent activity in air-sea fluxes and summarizes an inter-comparison of air-sea fluxes from global reanalyses and independent estimates. Present knowledge of air-sea fluxes is far from complete; recent independent estimates of the surface energy balance do not produce a reasonable surface energy balance over the ocean. The magnitude of surface evaporation is a major uncertainty in our knowledge of the ocean s surface energy balance. No one reanalysis is superior in all aspects; surface stress and sensible and latent heat fluxes from reanalyses may be as realistic as other independent global estimates. Independent estimates of precipitation and surface radiation, however, appear superior to the reanalyses estimates. Cloudiness appears to be one of the major problems in improving air-sea fluxes in numerical weather prediction.

Since the NCEP/NCAR reanalysis, surface short wave fluxes, equatorial surface stress and the precipitation pattern have improved in the NCEP global forecast system; however, sensible heat flux appears to be too low and the NCEP system still has problems with low-level oceanic stratus clouds.