Quasi-geostrophy, Moist Convection and Tropical Transition

Usually, when one computes the ascent attributable to quasi-geostrophic (QG) motions, the result is an unimpressive “few cm/s” or so.  Vertical motion associated with heavy precipitation, on the other hand, is usually several m/s or more.   At first glance, it would seem that QG offers little quantitative guidance about precipitation and diabatic effects.  On the other hand, QG ascent often tells us where it will rain, if not how much.  Considerable work in synoptic-scale meteorology has been devoted to understanding how slow, quasi-geostrophic lifting is produced.  But, clearly, this motion is not solely responsible for sensible weather.  What useful information is present in the QG dynamics when it comes to moist convection?

One example where both QG vertical motions and deep moist convection are present is tropical transition (TT), which is the formation of a tropical cyclone from a baroclinic disturbance.  In its weakest form, TT is preceded not by a baroclinic cyclone, but rather can be initiated by slow, quasi-geostrophic ascent.  This talk briefly reviews work on TT in which Lance Bosart has been heavily involved.  We also review some recent theoretical work on the relationship between adiabatic QG motions and the total diabatic response.  This recent work offers some insight as to how impulses of QG ascent lead to a large diabatic response and how scale selection operates, favoring baroclinic disturbances roughly near the scales associated with TT events.  This work also points to the importance of lower-tropospheric or middle-tropospheric QG perturbations, thus making somewhat tenuous the linkage between TT and transient features on the dynamic tropopause.