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Brian D. Hirth, John L. Schroeder, Christopher C. Weiss, Douglas A. Smith, and Michael I. Biggerstaff

Abstract

The structure of the coastal internal boundary layer (IBL) during a landfalling hurricane has important ramifications on operational forecasting, structural design, and poststorm damage assessment. Despite these important issues, the mean IBL structure at the coastline during landfall is poorly understood. Knowledge of the vertical kinematic structure within tropical cyclones over water has improved greatly through aircraft reconnaissance missions and the advent of GPS dropsondes and stepped frequency microwave radiometers. Unfortunately, reconnaissance and research aircraft are limited to overwater missions, resulting in a poor understanding of vertical kinematic structure near the coastal interface, where changes in IBL structure are expected due to changes in surface roughness. Composite single- and dual-Doppler radar observations collected by the Shared Mobile Atmospheric Research and Teaching Radars during the landfall of Hurricane Frances (2004) are presented. Data analyses from the Cape Canaveral, Florida, region reveal a pronounced IBL throughout the data collection period. As a result, significant variability in the analyzed wind speed and direction are found across and near the coastal interface. IBL height is found to be suppressed when compared to an accepted empirical growth model, while multiple abrupt roughness transitions associated with the Cape Canaveral region contribute to a complex mean IBL structure.

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Jennifer L. Palucki, Michael I. Biggerstaff, Donald R. MacGorman, and Terry Schuur

Abstract

Two small multicellular convective areas within a larger mesoscale convective system that occurred on 20 June 2004 were examined to assess vertical motion, radar reflectivity, and dual-polarimetric signatures between flash and non-flash-producing convection. Both of the convective areas had similar life cycles and general structures. Yet, one case produced two flashes, one of which may have been a cloud-to-ground flash, while the other convective area produced no flashes. The non-lightning-producing case had a higher peak reflectivity up to 6 km. Hence, if a reflectivity-based threshold were used as a precursor to lightning, it would have yielded misleading results. The peak upward motion in the mixed-phase region for both cases was 8 m s−1 or less. However, the lightning-producing storm contained a wider region where the updraft exceeded 5 m s−1. Consistent with the broader updraft region, the lightning-producing case exhibited a distinct graupel signature over a broader region than the non-lightning-producing convection. Slight differences in vertical velocity affected the quantity of graupel present in the mixed-phase region, thereby providing the subtle differences in polarimetric signatures that were associated with lightning activity. If the results here are generally applicable, then graupel volume may be a better precursor to a lightning flash than radar reflectivity. With the dual-polarimetric upgrade to the national observing radar network, it should be possible to better distinguish between lightning- and non-lightning-producing areas in weak convective systems that pose a potential safety hazard to the public.

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