Boundary layer structure and dynamics in outer hurricane rainbands

Results of hurricane boundary layer experiments conducted in outer rainbands of Hurricanes Josephine (1984) and Earl (1986) are presented. Comparisons of precipitation, kinematic, and thermodynamic 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 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 band is oriented perpendicular to the shear above the boundary layer and cells move downband at about 80% of the maximum wind. Alongband and crossband wind maxima, and maximum equivalent potential temperatures are located on the outer side of the band axis, with minima 4-8 km to the inner side. Updrafts and downdrafts are preferentially located on the inner side of the band axis, along with maximum crossband convergence, cyclonic shear vorticity, and minimum equivalent potential temperatures.; Downdraft transport of cool and dry air from middle levels on the inner side of the rainband was responsible for modifying mixed layer structure adjacent to the band on alongband scales of 100 km. An undisturbed mixed layer of 500 m was present on the outer side of the band while a variety of structures were observed on the inner side indicative of both disturbed and recovering mixed layers. Application of a mixed layer model to low level flow trajectories from the outer rainband to the eyewall indicates that under some conditions, the mixed layer may not recover sufficiently and low surface equivalent potential temperature air may reach the eyewall. These conditions are associated with suppressed flow in a region of positive divergence with moderate rainfall from a middle level anvil cloud. Incomplete recovery was most evident when a recovering mixed layer exhibited a negative jump in water vapor mixing ratio. Differential evaporation cooling over the transition layer drives entrainment of dry air from above which overcomes any evaporation moistening, resulting in a drier mixed layer (with lower surface equivalent potential temperature). Depending on the humidity profile and spatial scale of the initial disturbed mixed layer, the model results suggest that incomplete recovery may be responsible for transitional changes in hurricane intensity.