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  • Author or Editor: Michael I. Biggerstaff x
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Michael I. Biggerstaff and Robert A. Houze Jr.

Abstract

A high-resolution composite analysis covering the entire breadth of the northern portion of a mature leading-line, trailing stratiform squall-line system reveals that mean subsidence observed in the transition zone consisted of two different types of average downdraft: one at upper levels that was mechanically forced and one at lower levels that was microphysically forced. Both the upper-level and lower-level mean downdrafts in the transition zone appeared to be the average effect of convective-scale vertical drafts associated with convective structures that moved relative to the front edge of the convective line. The structure of individual upper-level convective-scale downdrafts suggested that they may have been partially composed of gravity waves excited by the interaction of the penetrative convective updrafts of the mature and dissipating convective cells with the stable ambient flow. The lower-level mean downdraft extended from midlevels to near the surface but was maximum near the melting level and was associated with air of low equivalent potential temperature. It was likely microphysically driven by cooling associated with melting and evaporation.

The upper-level and lower-level subsidence in the transition zone had little effect on the radar reflectivity minimum observed at middle to low levels in the transition zone. The primary microphysical process affecting the development of the reflectivity minimum appears to have been the inability of small ice crystals to form, grow, or persist at midlevels in the transition zone. Consequently, less aggregation could occur in the transition zone just above the melting level than in the secondary band at the same altitude.

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A. Addison Alford, Jun A. Zhang, Michael I. Biggerstaff, Peter Dodge, Frank D. Marks, and David J. Bodine

Abstract

The hurricane boundary layer (HBL) has been observed in great detail through aircraft investigations of tropical cyclones over the open ocean, but the coastal transition of the HBL has been less frequently observed. During the landfall of Hurricane Irene (2011), research and operational aircraft over water sampled the open-ocean HBL simultaneously with ground-based research and operational Doppler radars onshore. The location of the radars afforded 13 h of dual-Doppler analysis over the coastal region. Thus, the HBL from the coastal waterways, through the coastal transition, and onshore was observed in great detail for the first time. Three regimes of HBL structure were found. The outer bands were characterized by temporal perturbations of the HBL structure with attendant low-level wind maxima in the vicinity of rainbands. The inner core, in contrast, did not produce such perturbations, but did see a reduction of the height of the maximum wind and a more jet-like HBL wind profile. In the eyewall, a tangential wind maximum was observed within the HBL over water as in past studies and above the HBL onshore. However, the transition of the tangential wind maximum through the coastal transition showed that the maximum continued to reside in the HBL through 5 km inland, which has not been observed previously. It is shown that the adjustment of the HBL to the coastal surface roughness discontinuity does not immediately mix out the residual high-momentum jet aloft. Thus, communities closest to the coast are likely to experience the strongest winds onshore prior to the complete adjustment of the HBL.

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