Anti-Clockwise Rotation of the Wind Hodograph. Part I: Theoretical Study

Abstract

In a first theoretical study, the reasons for anti-clockwise rotation (clockwise rotation in the Southern Hemisphere) of the wind hodograph in the boundary layer are investigated. As observations of wind hodographs show two different kinds of anti-clockwise rotation (ACR), one frequently observed, which is highly irregular with partly ACR and the other with clear ACR, two methods have been applied. At first a two-dimensional nonlinear model which includes sea-land breezes as well as a mountain reveal partly ACR at the lee of the mountain. By analyzing the different terms in the equation for the wind vector rotation, it is shown that the pressure-gradient term is usually the leading one, and the advection term is very small. But the latter becomes important in cases of ACR. Second, a linear model is solved analytically showing that inclusion of a rotating thermal force in the ACR sense generates clear ACR. It is shown that a critical value for the phase shift between the thermal forces in the horizontal directions exists. At that value, transition to ACR occurs. The critical curves are drawn as function of latitude, friction and relative amplitudes of the thermal forces. Common features of hodographs obtained by observations such as eccentricity, shape, tilt and sense of rotation might be explained in view of the theory.