Aircraft Observations of the Formation of an Aligned Circulation in Weak Tropical Cyclones

Characterizing the inner core structure of weak tropical cyclones (weak TCs; defined here as ranging in intensity from pre-tropical depressions to tropical storms), at the onset of genesis or intensification (termed “development” here) is an important factor in identifying whether development is likely to occur. The formation of a deep cyclonic circulation that reaches the surface is a key step in this development process. Such a circulation can form either locally, in the case of genesis, or through the alignment of a low- and mid-level circulation, in the case of intensification of a tropical depression or storm. Understanding how this aligned circulation forms is challenging, however, since weak TCs have a weak rotational constraint, are dominated by divergent flow (largely driven by latent heating in precipitation), and are more vulnerable to hostile environments (e.g., dry air, vertical shear) than stronger TCs.

This work will present aircraft observations of the structural characteristics of weak TCs that align and compare them with those that do not. Emphasis will be placed on the structures of the cyclonic circulations and the resultant precipitation structure, divergence, and mass flux profiles. Notable differences in these structures will point toward physical hypotheses that have been advanced to explain the alignment of these systems.