Boundary Layer Structure and Dynamics in Outer Hurricane Rainbands. Part I: Mesoscale Rainfall and Kinematic Structure

Abstract

Results of hurricane boundary layer experiments conducted in outer rainbands of Hurricanes Josephine (1984) and Earl (1986) are presented. Comparisons of precipitation and kinematic structures in these storms and in Hurricane Floyd (1981) indicate that principal rainbands have common characteristic mesoscale and convective-scale features in the boundary layer. The two-dimensional mesoscale structure suggests that these rainbands are made up of a linear aggregate of cellular reflectivity elements (on the inner, upshear side of the band) and stratiform rain (on the outer downshear side). The bands are oriented perpendicular to the shear above the boundary layer and cells move downband at about 85% of the density-weighted mean wind speed of the 0.2?6 km layer. The boundary-layer wind field is strongly influenced by the rainband with alongband and crossband wind maxima located on the outer side of the band axis, and minima 4?8 km to the inner side. Maximum crossband convergence and cyclonic shear vorticity are also found to the inner side of the rainband axis. Updrafts and downdrafts are preferentially located on the inner side of the band axis, with some downdrafts spreading out at the surface. The band-relative positions of the updraft and perturbation pressure minimum suggest that the minimum may be produced by interaction of the wind shear and the updraft. Outer hurricane rainbands show many similarities to tropical squall lines, major differences are associated with propagation and the structure of the leading and trailing edges.