The evolution of thermodynamic structures in the inner core of Humberto (2001)

During 3 consecutive days in September 2001, over 200 Global Positioning System dropwindsondes (GPS sondes) were deployed from NOAA and NASA aircraft as tropical storm Humberto intensified to a category 2 hurricane and then weakened to a category 1. The 2 NOAA WP-3Ds deployed GPS sondes from altitudes of 1.5 km and 4 km and were concentrated within 200 km of the circulation center. The NASA DC-8 and ER-2 sampled both the inner core and the environment, launching GPS sondes from above 11.5 km and 16 km, respectively. This combined effort has made Humberto the most densely sampled hurricane with GPS sondes to date.

We intend to compare the structures seen in Humberto, as it evolves over the three days of sampling, with the structures that should be present if the WISHE theory is responsible for intensification. According to the WISHE theory (Emanuel 1986, 1991), the maximum temperature perturbation should be found in the upper atmosphere. Past observations of mature tropical cyclones (TCs, Hawkins and Rubsam 1964) confirm that maximum temperature perturbation exists in the upper troposphere.

We have developed multiple horizontal fields based on the GPS sonde data using cubic splines, objective analysis techniques and subjective analysis and have combined these fields with the lower fuselage radar data from the NOAA WP-3Ds. Developed from the horizontal fields, vertical cross sections of various thermodynamic variables are constructed for each of the three days of sampling. Radial-height cross sections of the temperature perturbation and theta_e are compared to the structures expected from WISHE theory. Inflow path-height sections of theta_e reveal where the boundary layer is accumulating energy. Convective available potential energy (CAPE) and convective inhibition (CIN) are also calculated.

On the 22nd, when Humberto is a tropical storm, a maximum temperature perturbation of 6 K is found between 1 and 3 km, under a region of stratiform precipitation. The upper troposphere shows minimal warming of about 1 K. Equivalent potential temperature in the nascent eyewall is found to be sourced from the boundary layer under the low level warm core. On the 23rd, as Humberto strengthens to category 2, the maximum warming is located between 3 and 5 km and has a magnitude of about 10 K. This is in contradistinction to what would be expected if the intensification of the storm was viewed from the perspective of WISHE theory. The largest temperature perturbations in the core are found adjacent to the northern eyewall convection. To the south, where eyewall convection is absent, CIN values are high, forming a cap. Streamlines reveal that the air gains energy in this region before it is lifted into the convection. Values of CAPE also increase in this area. As Humberto weakens and becomes a category 1 storm on the 24th, the maximum warming is again found in the lower troposphere.

The temperature structures found as Humberto evolves over the three days show that other processes besides WISHE contribute to intensification. Although the upper atmosphere does show moderate warming when Humberto is above hurricane intensity, the maximum temperature perturbations are found in the lower troposphere. Factors that may be responsible for the location of the temperature perturbation in the lower and mid troposphere include wind shear that may be eroding the upper level warm core and the arc-shaped arrangement of the convective elements. The Humberto dataset offers a detailed view of a highly asymmetric TC that differs from the axisymmetric view often used by numeric models.