Divergent wind analyses in the oceanic boundary layer

A method of analyzing the divergent part of the marine boundary layer flow is presented in the context of two-dimensional statistical interpolation. Instead of assuming geostrophic balance of observed minus background differences, a linearized Ekman balance is assumed. This results in correlation functions that are rotated with respect to the geostrophic correlation functions by a turning angle that is precisely the difference between the actual and geostrophic wind at mean sea level. The turning angle is also related to the degree to which divergent flow due to friction is analysed; the larger the turning angle, the larger the extent of the analysis of divergent flow. Case studies comparing geostrophic and ageostrophic analyses show that changes to the pressure field occur when the turning angle is large and wind observations are sufficiently dense. Ageostrophic analyses are shown to contain divergent analysis increments and consequently, realistic wind analysis increments result.