The Thermodynamic Environment of a Mid-Level Vortex and Tropical Cyclogenesis

Evidence is accumulating that small deviations in both the temperature and humidity profiles from standard tropical conditions can greatly affect deep convection. Rainfall rate increases with increasing moisture (perhaps not too surprising). However slightly cooler temperatures at low levels and slightly warmer temperatures in the upper troposphere both increase the precipitation rate and make the convective mass flux profile more bottom-heavy. This result is demonstrated in both observations from the TCS08 and PREDICT field programs on tropical cyclogenesis and in numerical modeling of convection. (See the abstract submitted by Gjorgjievska et al.)

Many observations and numerical calculations show that tropical cyclone spinup at low levels is preceded by the formation of a strong, mid-level vortex. However, isolating the mechanism by which this vortex assists low-level spinup has been difficult. A balanced virtual temperature anomaly accompanies a mid-level vortex; it consists of air cooler than ambient below the vortex and warmer above. This anomaly structure is precisely what is needed to produce the bottom-heavy convective mass flux profiles associated with low-level spinup.

We therefore hypothesize that the mid-level vortex often observed in spinup cases makes tropical cyclogenesis possible by providing a thermodynamic environment favorable to the bottom-heavy convective mass flux profiles that produce vorticity convergence at low levels.

In this talk we summarize the above ideas and present nonlinear balance potential vorticity inversion calculations which show that the temperature dipole observed in developing cyclones is indeed a balanced response to a mid-level vortex. This eliminates the possibility that the observed temperature dipole is solely a result of the convection itself.