An Easy Method for Estimation of Q-Vectors from Weather Maps

Abstract

It is shown how to estimate the directions and relative magnitudes of Q-vectors from a map of isobars and isotherms. The divergence of this vector field represents the forcing function in the quasi-geostrophic omega-equation. The direction of the Q-vector at a point is determined by the rate of change of the geostrophic wind vector taken along the isotherms, with the colder air to the left in the Northern Hemisphere. Its direction is 90° to the right of this vector change of wind. The strength of the Q-vector is proportional to the magnitude of the rate of vector wind change, and to the magnitude of the temperature gradient. Application to an actual situation is shown and compared with the traditional inferences from advections of temperature and vorticity. General agreement is found. Patterns of Q-vectors and associated vertical motion are sketched for idealized patterns of surface lows and highs and for upper-level troughs and ridges. Examples of confluent frontogenesis are shown, for a lower-tropospheric col and for a upper-level jet entrance. Patterns of Q-vectors and vertical circulations are noted for frontogenetical and frontolytical situations.