Anticlockwise Rotation, Eccentricity and Tilt Angle of the Wind Hodograph. Part II: An Observational Study

Abstract

Following the theory of Kusuda and Alpert (Part I), expressions for the eccentricity and tilt angle of the surface-wind hodograph at different latitudes are derived as functions of the amplitude and phase shift of the horizontal thermal force and also of some reasonable frictional parameter. Further, analyses of 47 hodographs in Washington, Oregon and California give the observed tilt angles, eccentricities and senses of rotation. The tilt angle is nearly linearly dependent on latitude. Eccentricities are in general high (0.9?1) and higher friction leads usually to higher eccentricity. However, eccentricity may sometimes be very small giving a circular hodograph) even with high friction and also very high with low friction; the latter case is primarily due to a phase shift between the horizontal thermal forces. It is shown that although in general the frequency of ACR (anticlockwise rotation) hodographs decrease northward in the Northern Hemisphere, lower friction may lead to a minimum in ACR frequency at some latitude and this (minimum) latitude moves northward as friction increases. ACR hodographs tend to have slightly higher eccentricities relative to those with clockwise rotation. The limitations of the Rayleigh friction parameterization are discussed and the horizontal momentum equations are solved with the more accurate K-theory friction parameterization. It is shown that near-surface ACR hodographs turn to become clockwise with increasing altitude.