Analysis of tropical cyclogenesis within the framework of quasi-steady flow boundaries: select PREDICT cases

Tropical cyclogenesis is characterized by the aggregation and local amplification of moist entropy and vorticity. A protected flow region is needed for this aggregation to take place because the tropical atmosphere is, in general, hostile to cyclogenesis. The recirculation area associated with the nonlinear critical layer of tropical waves provides such a favorable environment.

Outside of convective regions, the flow in the tropics is, to a good approximation, layer-wise horizontal. In the frame of reference moving with a precursor wave, the recirculating flow can be approximated as quasi-steady. Within this framework of quasi-steady and layer-wise horizontal motion, we calculate the flow's manifolds, i.e. distinguished streamlines that divide the flow into distinct regions. Snapshots of the horizontal flow as represented by global numerical prediction models are employed. The manifolds often delineate well the air masses with elevated moist-entropy of the precursor disturbance from the low moist-entropy environment. In such cases, the manifolds provide guidance on how the air mass distribution evolves during the next 1-2 days. The examination of the flow's manifolds superposed on the distribution of moist entropy provides a simple, yet useful framework to analyze tropical cyclogenesis on the meso-scale.

Two specific examples of this methodology are presented from the PRE-Depression Investigation of Cloud systems in the Tropics (PREDICT) 2010 field experiment. First, during the genesis of Karl, a low-level circulation center develops in a region of high surface fluxes displaced by a few hundred kilometers from the circulation center at 700 hPa. The low-level center is located underneath the advection of low moist-entropy at mid-levels and genesis does not occur before the respective circulations align vertically. Secondly, the remnants of ex-Gaston are characterized by a well-protected low-level circulation containing air with steadily increasing values of moist-entropy. At mid-levels, however, high moist-entropy air is continuously exported into the environment hindering the re-development of the system.