Kinematic and microphysical aspects of mesovortices in Hurricane Ike (2008)

Modeling studies published over the last decade (e.g., Schubert et al 1999, Kossin et al 2002, Rozoff et al 2006, and Hendricks et al 2009) have shown that vorticity mixing in the tropical cyclone inner core can lead to the formation of mesoscale vortices and can have an impact on storm intensity. However, few direct observations of mesovortices in the inner core region have been made. This study presents a look at features observed during the landfall of Hurricane Ike (2008) from the University of Alabama in Huntsville's Mobile Alabama X-band (MAX) dual polarization radar. Ike made landfall in the US around 07 UTC on 13 September 2008 near Galveston Island, Texas. The MAX was deployed near the Anahuac Airport in Anahuac, TX, about 53 km from the Houston/Galveston National Weather Service NEXRAD 88D radar, and was located within the eyewall of Ike for about four hours. During this time, many small vortices were observed near the MAX. The closest vortices came within 10-15 km of the MAX. These features varied in size, with diameters ranging about 5-10 km.

The proximity of the MAX to the KHGX radar allows for a dual-Doppler analysis. Calculated fields of horizontal winds, vertical velocity, vorticity, and divergence present the kinematic attributes of the vortices. Vorticity values for the features are on the order of 5 x 10^-3 s^-1, and the derived horizontal wind fields show directional variations associated with the embedded mesovortices. Findings from this analysis are compared to results from simulation studies published over the last decade to reach an understanding of the mesovortices in the context of the larger circulation. Using the dual-polarimetric data obtained with the MAX, characteristics of hydrometeors in the inner core region can be analyzed. A look at the distribution and variation in observed differential reflectivity and correlation coefficient is used to assess particle types and shapes. Specific differential phase is also considered very useful in this study because it is much less sensitive to errors, for example problems that may arise from beam blockage. Analysis of the dual polarization parameters presents the microphysical features associated with the mesovortices.

As a next step for this work, preliminary thermodynamic retrievals will also be discussed. With methods similar to those detailed by Gal-Chen (1978), Roux (1985), and Lee et al (2000) the pressure perturbation field can be retrieved by manipulating the momentum and thermodynamic equations. The retrieved perturbations will be used to compute the pressure field, and the angular momentum will be retrieved using the gradient wind equation. Additionally, the temperature field will be computed using retrieved temperature perturbations, along the lines of the method presented in Viltard and Roux (1998). The derived pressure and temperature fields will be compared to surface data to begin addressing the evolution of Ike's thermodynamic structure during landfall.