Richardson number limited shear zones in the free atmosphere

Two case studies are presented of the detailed structure of frontal zones with strong vertical shear. Temperature structure has been obtained from sequential radiosonde ascents. Wind structure, including the fields of horizontal divergence and deformation, has been obtained on scales of 1 to 10 km by means of Doppler radar techniques. The results are consistent with the frontal zones being at least partly controlled by shearing instability, in so far as there was a balance between a wind field tending to diminish the Richardson number (Ri) and shearing instability tending to impose a lower limit to Ri. One of the main factors tending to diminish Ri was stretching deformation in a direction normal to the local shear vector. A typical rate of decrease of Ri which this would have produced in the absence of shearing instability was 10−4 s−1. The Richardson number itself within the frontal zones was mainly between 0.2 and 0.4 when averaged vertically over 200 m and horizontally over circles of diameter 10 km; however, Ri diminished to about half of these values locally on horizontal scales of about 1 km. As a result shearing instability is thought to have led to turbulent breakdown in only isolated patches rather than in a sustained fashion over the entire depth of the frontal zones.