Diagnosis and prediction of middle level cloud development associated with cold fronts over Southern Australia

Broad scale vertical velocities have been used to predict the development, over southern Australia, of clouds associated with cold fronts. The vertical velocities were determined by solving explicitly the quasi-geostrophic omega equation expressed in terms of Q-vectors. The frontal cloud band was located within the area where up-motion was diagnosed, with the deepest cloud in the vicinity of the strongest upward velocities, and the rear of the band coinciding closely with the change from up- to down-motion. On those occasions when a major cloud mass evolved the region of up-motion extended to low latitudes as a clearly defined band. This band was indicative of a well developed warm conveyor belt associated with the frontal system. Areas of diagnosed down-motion were free of deep, organised cloud masses. Stronger upward motion did not produce an immediate increase in the cloudiness; typically, there was a delay of 12 to 24 hr before the corresponding cloud development became apparent.