The role of vorticity random stretching in an idealized tropical depression genesis problem

Abstract:

The formation of a tropical depression from a mesoscale convective system (MCS) is studied with a one-and-a-half layer shallow water model. Random local mass sink is preferentially seeded to a circular region to mimic convection in the MCS. The vorticity patches produced by convective stretching are self-rearranged into a large vortex with quasi-monotonic and axisymmetric vorticity structure, which can be indirectly predicted by the vorticity probability distribution function (PDF). The PDF is found to be mainly driven by the repetitive convective stretching on the existing vorticity patch. This process can be approximately represented by a weighted Poisson process which depicts the “diffusion” of parcels on a set of quantum vorticity levels. Among the updraft parameters, the layer thickness loss in a single updraft which is interpreted as convective intermittency is a much more dominant factor than updraft size and duration time for vorticity PDF. More intermittent convection means an individual convective event induces more stretching. It leads to a wider PDF with a “high vorticity tail”, and therefore a more compact vortex with higher maximum wind. Higher intermittency also means convection is less frequent, so there is lower predictability of the PDF and the tropical depression intensity.