Marine Boundary-layer Structure and Circulation in the Region of Offshore Redevelopment of a Cyclone during GALE

Abstract

Marine boundary-layer structure and circulation is documented for the 24 February 1986 case of offshore redevelopment of a cyclone during the Genesis of Atlantic Lows Experiment (GALE) Intensive Observing Period (IOP) 9. Mesoscale and satellite information emphasize that the onshore cyclone is not well organized as it moves offshore to the cold shelf waters with redevelopment occurring later over the Gulf Stream region. Within hours of redevelopment, low-level aircraft data were obtained in the region. Vertical aircraft profiles down by the National Center for Atmospheric Research (NCAR) King Air in the vicinity of redevelopment over the Gulf Stream, as well as the midshelf front region and cold shelf waters, reveal two distinct boundary layers. Over the Gulf Stream region approximately 50 km south-southwest of the redeveloping cyclone, the near-neutral marine boundary layer (−h/L = 6.6) capped by layered stratocumulus is characterized by a low cloud base (360 m), relatively thick stratocumulus cloud layer (800–1200 m) and strong subcloud-layer winds (8–9 m s-1). Associated with the developing cyclone near the Gulf Stream is shallow cyclonic flow with convergence and subsequent acceleration of the wind near the western edge. Closer to the coast over the cold shelf waters and the midshelf front region, the relatively cloud-free boundary layer (h/L = 44.4) is characterized by a slightly shallower, new-neutral boundary layer (h = 700 to 755 m) with very light and variable winds. Boundary layer flow is strongly divergent west of the midshelf front. Them two regions are approximately 150–200 km west of the Gulf Stream region or redevelopment. Flux profiles agree with results from other marine boundary layers under similar cloud and stability conditions and emphasize the warming and moistening of the subcloud layer from new the western edge of the Gulf Stream eastward. Temperature and moisture turbulence structure appear less well organized. The mean momentum budget emphasizes the strong baroclinicity in the MABL and the importance of horizontal advection near the western edge of the Gulf Stream. Comparison turbulent kinetic energy (TKE) budgets over the Gulf Stream and over the midshelf front show shear production and dissipation to dominate over the Gulf Stream with strong winds. Turbulent transport over the Gulf Stream is a significant term due primarily to the flux of horizontal velocity variance, which is approximately 5 times that of the flux of vertical velocity variance. Over the midshelf front, all normalized terms in the TKE budget are less active in producing, dissipating and transferring TKE for a given heat flux as compared to the Gulf Stream region, where the effects of the developing cyclone are evident.