The evolution of Humberto in a sheared environment

The causes of intensity change of tropical cyclones (TCs) have remained a major challenge for the meteorological community (Marks et al. 1998, AMS statement on Hurricane Research and Forecasting 2006, National Science Board Report 2006, Holland and Lukas 2006). A contributing factor to our lack of understanding about intensity variations is the inadequate sampling of a TC throughout its lifecycle. In 2001 NOAA and NASA marshaled their resources to collect a more complete dataset that can be utilized to address the evolutionary aspects of a TC. The unprecedented sampling of TC Humberto during the Convection and Moisture Experiment (CAMEX-4) provides an opportunity to examine the details of changes in intensity as tropical storm Humberto intensified to a category 2 hurricane and then weakened to a category 1. Rarely do we get a view of storm evolution over 3 successive days with multiple aircraft, over 200 Global Positioning System dropwindsondes (GPS sondes) deployed, and airborne expendable bathythermographs (AXBT's).

The focus of this talk will be on how the large-scale environmental changes are affecting the meso-scale structure and intensity of the TC. Environmental shear, SSTs, and 500 hPa relative humidity varied substantially over the 3 days of sampling. Reflectivity fields from the NOAA WP-3Ds reveal that an eyewall existed only on the down shear left side of the circulation center. Strong radial inflows were associated with the down shear sector of the storm with moderate outflow on the up shear side. Tangential winds also had strong asymmetries perpendicular to the inflow-outflow couplet. The dense GPS sonde distribution achieved during the Humberto study will allow us to present vertical cross-sections of the aforementioned variables that will clarify the relationship between the convection, the kinematic variables, and the large scale environment.

Maps of CAPE, CIN, and reflectivity show the connection between convective structures and changes in thermodynamic conditions. Plan views and vertical cross-sections of equivalent potential temperature display how the internal thermodynamic structures of Humberto have responded to the environmental changes that took place. All of the above mentioned variables will be shown in the context of how the warm core evolves. Vertical cross-sections of the warm core show that the warm core had the highest temperature perturbation in the lower troposphere, contrary to what WISHE theory would predict. Hydrostatic pressure perturbations reveal that the majority of the pressure drop was occurring due to warming in the lower troposphere, a possible reason for why Humberto intensified in an increasingly hostile environment of higher shear, lowering SSTs, and dry air intrusion.

The temperature perturbation from the idealized, axisymmetric, WISHE model will be compared with the actual temperature perturbation cross-sections from Humberto. The differences in the temperature perturbation fields are likely due to the high wind shear. Radial inflow, CAPE, CIN, and the distribution of equivalent potential temperature in the planetary boundary layer along with vertical cross-sections of equivalent potential temperature in the context of environmental conditions allow for speculation about the structure of the warm core and why it differs from what is expected from WISHE.