Sea-surface roughness lengths in the midlatitude coastal zone

Abstract

This study summarizes midlatitude coastal-zone observations of the roughness lengths for describing the turbulent air–sea exchange of momentum, heat and moisture. The roughness lengths for momentum and sensible heat appear to be smaller than those given by widely used open-ocean models (TOGA COARE algorithm) in conditions of weak to moderate winds. The reduction in roughness, compared to the open-ocean model, is largest for moisture transport. The less efficient transfer of moisture compared to heat for these datasets is apparently related to the dynamic nature of temperature and the fact that temperature fluctuations dominate buoyancy generation of turbulence, which was not observed in the western tropical Pacific warm pool data used to develop the original COARE algorithm. Unlike the usual formula, these data suggest that the scalar roughness length ratio (zoh divided by zoq) is larger than unity for weak to moderate winds and less than or near unity for stronger winds. Reduced latent heat flux is supported by five different datasets from the coastal zone, including measurements from aircraft, a tower and a ship. Latent heat fluxes were found to be 25 to 50% smaller than predicted by the COARE algorithm, depending on the dataset. The generality of these results, in particular their application to the open ocean or to situations where wave state effects are important, is not known. Copyright © 2010 Royal Meteorological Society