An Investigation of the Eyewall Boundary Layer and Stability Indices in a Sheared Tropical Cyclone

In 2001 the National Oceanic and Atmospheric Administration (NOAA) and the National Aeronautical and Space Administration (NASA) marshaled their resources to collect a more complete dataset that may 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 the opportunity to investigate the asymmetric structure of a TC in a higher sheared environment. A comparison of the structures in Humberto with past observations of more intense low latitude storms, developing in a more homogeneous environment, reveal important structural differences between these two types of systems. Past observations and modeling studies exhibit evidence that in moderate to high shear, eyewall convection has a quasi-stationary wavenumber-1 asymmetry in mesoscale vertical velocity and reflectivity. A mesoscale ascent maximum is located downshear with a mesoscale precipitation maximum located left of the downshear vector. While these studies have focused on reflectivity and vertical velocity in the eyewall, Humberto offers an opportunity to investigate the boundary layer (BL) and stability in and around the eyewall. As the vertical shear of the horizontal wind (VWS) increases and Humberto becomes a TC, the kinematic fields demonstrate persistent relationships with the VWS vector. The highest tangential winds are located to the left of the VWS vector. The radial winds have inflow-outflow couplets aligned with the VWS vector with the strongest convergence located in the downshear quadrant of the storm. Convergence covers more area around the eyewall as the storm intensifies and has a persistent relationship with the VWS and motion vector. Intriguingly, on the second consecutive day of sampling convergence is initiated in the upshear quadrant of the storm. This is contrary to the idea that upward motions in the eyewall are caused by the differential advection of vorticity with height. Lower equivalent potential temperature, lower CAPE, and high CIN are located in the upshear quadrant and effectively suppress convection in the eyewall on the upshear side of the storm. The opposite is found on the down shear side of the eyewall. These observations support past research showing a mesoscale ascent maximum located downshear in the eyewall with a minimum in the upshear quadrants. The warm core also exhibits high asymmetries in the upper and lower troposphere. In the upper troposphere the higher temperature gradients are associated with the upshear quadrant and vice versa. In the lower troposphere the gradients are reversed.