Abstract

The atmospheric surface layer is the lowest few tens of meters of the atmosphere. The vertical fluxes of horizontal momentum, of heat, and of humidity are the basic statistical quantities describing turbulence in the surface layer. These three fluxes are sufficient information to determine the stability of the horizontally homogeneous surface layer (i.e., determine the Richardson number or equivalently the Monin-Obukov length), with the humidity flux being of secondary importance compared with the heat flux. These fluxes then determine the height profiles of many other surface-layer characteristics, such as the gradients of mean temperature and humidity, temperature variance and structure parameter (or those of humidity), vertical velocity variance and velocity structure parameter, and energy dissipation rate. Basically, all surface layer turbulence statistics that obey Monin-Obukov similarity are predicted from these three fluxes, with the humidity flux usually being unimportant for those quantities not involving averages of humidity fluctuations. An example of a quantity that does not obey Monin-Obukov similarity is the variance of horizontal wind speed fluctuations in the unstable surface layer; this quantity is affected by the large eddies of the mixed layer above the surface layer. By way of terminology: unstable conditions occur when part of the turbulence energy is generated by upward convection of heat; neutral conditions occur when the turbulence is generated by wind shear near the ground with convection providing no energy; finally, stable conditions occur when part of the turbulence energy is damped by stable stratification, but turbulence persists because of wind shear.

© 1983 Optical Society of America