The creation of an equivalent potential temperature reservoir in the eye of a deepening tropical cyclone

In mature tropical cyclones (TCs) it is common to observe the highest equivalent potential temperature (theta-e) in the lower portion of the eye. The high wind annulus found under and just radially outward of the eyewall contains the largest fluxes from the sea, and is believed to be the primary source for this reservoir of warm theta-e. Height-radial distance cross-sections of theta-e for mature, axisymmetric TCs reveal a steady increase from the outer edge of the high wind annulus to the quiescent center (Hawkins and Imbembo 1976, Jorgensen 1984, Schneider and Barnes 2005).

In tropical storms there is evidence that the warmest theta-e is also found in the nascent circulation center. However, this warm theta-e appears first in the light wind region of the tropical storm center and does not appear to be collocated with the developing ring of strong surface winds (Molinari et al. 2004, Heymsfield et al. 2006). There is no annulus that contains a steady increase of theta-e that adjoins the convective cells of the partial eyewall. How does this reservoir of warm theta-e form in the early stages of a TC and what might be its' role in the intensification of the tropical storm is the focus of this presentation.

To explore the creation of the warm theta-e dome found in the center during the early stages of TC development we shall use the Global Positioning System (GPS) sondes deployed in Humberto (2001) over the course of three days. There were over 200 GPS sondes jettisoned from the NOAA WP-3Ds and the NASA DC-8 and ER-2. The NOAA aircraft also deployed airborne expendable bathythermographs, collected in-situ measurements, and sampled the reflectivity structure with the lower fuselage and tail radars.

Horizontal maps created from the GPS sondes reveal that Humberto is highly asymmetric. The eyewall, based on the reflectivity observations during the second day, is croissant-shaped and occupies less than half of the ring of high winds. The inflow to the nascent eyewall is found in a single broad channel that originates in the SE sector of the storm and wraps cyclonically into the eyewall, entering from the N. In this inflow channel are convective cells that inhibit the increase of theta-e. There is no annulus adjoining the eyewall where theta-e undergoes a noticeable increase. The western flank of the inflow channel does not feed the convective cells of the eyewall. Instead this part of the inflow wraps into the circulation center where it is trapped by the strong inversion found in the nascent eye. This air circles the center, and acquires energy from the sea eventually producing a reservoir of theta-e that is 5-7 K higher than the air feeding the eyewall and is about 750 m thick. The air must circle the eyewall for about 6-8 h, making 1-2 circuits of the circulation center to achieve what we call eye excess energy (Barnes and Fuentes 2010).

Many TCs are highly asymmetric in their initial stages of development. The details that we see with the Humberto dataset allow us to speculate on aspects of genesis and the role of the reservoir of warm theta-e in the intensification of a TC. While the warm theta-e is created by the sea-air fluxes it is the strong inversion and the inertial stability of the low-level flow that prevents this air from being ingested into the eyewall during this early stage of TC development. The results will be placed in the context of other tropical storm analyses, most notably Danny (1997, Molinari et al. 2004) and Chantal (2001, Heymsfield et al. 2006).