Investigating the Kinematic and Thermodynamic Environment of a Landfalling Stratiform Rainband using Radar and Ground-Based Remote Sensing

The diagnosis of the three-dimensional wind field through the use of single and dual-Doppler analysis is a powerful tool in determining the kinematic properties of a storm system. Countless studies have been conducted (Zipser, 1969; Gamache and Houze, 1982; Johnson and Kriete, 1982; Keenan and Rutledge, 1993) using these tools to analyze the kinematic environments of mesoscale convective systems (MCS). Other efforts have also employed these methods to provide insight into the internal structure of hurricanes (Jorgensen et al., 1982; Marks, 1985; Barnes et al., 1983), specifically, their stratiform regions. It has become evident that the convective centers present in the eyewall of a hurricane act much like that of the leading edge of a squall line or MCS in terms of the production of precipitation within the system. Like an MCS, the convection within the eye acts to propel huge amounts of ice upward. Unlike in an MCS, the upper-level synoptic environment of a hurricane acts to transport these particles outward radially where they fall through the expansive mesoscale updraft areas and begin to grow through vapor deposition. As the ice hydrometeors descend below the melting level, cooling by melting and evaporation produces the observed mesoscale downdraft, which is the main focus of this study

Hurricane Ivan (2004) is used as the case for this study. The analysis domain spans portions of extreme northwestern Florida, much of southern Alabama within 100 km of the coastline and extreme southeast Mississippi. Ivan made landfall southeast of Mobile, Alabama at approximately 0650 UTC 16 September 2004 with maximum sustained winds of about 50 m s-1. Ivan experienced a weakening trend that ensued just six hours prior to landfall. The minimum central pressure rose from 931 hPa at 0000 UTC to 946 hPa just seven hours later at the time of landfall. The cause of this weakening trend was assumed to be very dry, low θe mid-level air to the west and northwest of the system. An alternate hypothesis that we pursue in this study is that the production of cool, low θe air within the mesoscale downdraft region of landfalling stratiform rainbands can contribute to the system's weakening given that the rainband makes landfall well in advance of the center of circulation. The sublimation and/or evaporation of precipitation into low θe air just below cloud base allows air to become negatively buoyant. Additionally, melting of ice as it falls through the cloud allows for a cooling of ~1-10°C h-1 in the melting layer. It has been well documented that mesoscale downdrafts are common within MCS's, and also in the rainbands of tropical cyclones, but the full effect of the downdrafts (over land) on storm intensity has yet to be fully investigated. The goal of this research is to quantify the strength and aerial extent of the mesoscale downdrafts and associated cooling.

The stratiform rainband of interest made landfall at approximately 1800 UTC 15 September 2004. Profiles of vertical motion and divergence within the landfalling stratiform rainband are presented, along with results derived from dual-Doppler analyses, surface observations and a WRF model simulation of the storm. Vertical profiles are created using the VAD/EVAD technique (Browning and Wexler, 1968; Matejka and Srivastava, 1991) as applied to the Mobile, Alabama WSR-88D radar and Doppler on Wheels (DOW) radar data. Ground based instrumentation, such as the UAH-Mobile Integrated Profiling System (UAH-MIPS) will provide surface observations for comparison and validation purposes as well as vertical profiles of numerous meteorological variables. Models results will provide validation for information gained from the EVAD/dual-Doppler analysis. Results obtained from dual-Doppler analyses with data from the Mobile and DOW radars will serve to support and verify results obtained from the multiple EVAD analyses while also aiding in the quantification of mesoscale downdrafts in regions inaccessible though the EVAD technique.