Anti-Clockwise Rotation of the Wind Hodograph and Its Relation to Thermal Forcing

Abstract

In a first theoretical study, the reasons for anticlockwise rotation (clockwise rotation in the southern hemisphere) of the wind hodograph in the boundary layer are investigated. As observations of wind hodographs show two kinds of anti-clockwise rotation (ACR, in brevity) one frequently observed, which is highly irregular with partly ACR and the other with clear ACR, two methods have been applied. At first two-dimensional nonlinear model (described by Alpert et al., 1982) including sea-land breeze as well as a mountain reveal at the lee of the mountain partly ACR. By analyzing the different terms in the equation for the wind vector rotation (see Neumann, 1977) it is shown that usually the pressure-gradient term is the leading one and the advection term is very small. But the latter becomes important in case of ACR. Secondly, a linear model is solved analytically showing that inclusion of a rotating heating forcing in the ACR sense generates clear ACR.

This talk was based upon a cooperative study with Dr. M. Kusuda which was recently published in JAS (Vol. 40), p. 487–499.