Satellite Estimates of Ocean–Air Heat Fluxes During Cold Air Outbreaks

Abstract

Nomograms of mean column heating, due to surface sensible and latent heat fluxes, have been developed from Stage and Businger's (1981a,b) boundary-layer model for cold air outbreaks over warm water. Mean sensible heating of the cloud-free region is related to the cloud-free path (CFP, the distance from the shore to the first cloud formation) and the difference between land-air and sea-surface temperatures ?1 and ?0, respectively. Mean latent heating is related to the CFP and the difference between land-air and sea-surface specific humidities q1 and q0, respectively. Results are also applicable to any path within the cloud-free region. Corresponding heat fluxes may be obtained by multiplying the mean heating by the mean wind speed in the boundary layer. The sensible heating, estimated by the present method, is found to be in good agreement with that computed from the bulk transfer formula. The sensitivity of the solutions to the variations in the initial coastal soundings and large-scale subsidence is also investigated. The results are not sensitive to divergence, but are affected by the initial lapse rate of potential temperature; the greater the stability, the smaller the heating, other factors being equal. Unless one knows the lapse rate at the shore, this requires another independent measurement. For this purpose, we propose to use the downwind slope of the square of the boundary layer height, the mean value of which is also directly proportional to the mean sensible heating. The height of the boundary layer should be measurable by future spaceborne lidar systems. The general behavior of the mean sensible heating, the potential temperature, and the height of the boundary layer as a function of downwind distance within the cloud-free region, and their relations to several important parameters are studied analytically in the Appendix. By-products include the finding that the sensible (latent) heat flux is virtually linear with the contrast in land-air and sea-surface temperatures (specific humidities), thus providing a new kind of flux parameterization in lieu of the classical bulk transfer formulas. The applicability of the results to lake-effect snowstorms is also noted. Finally, the method can be used in reverse to check the validity of boundary-layer models.