Mesoscale Coastal Processes during GALE IOP 2

Abstract

During Intensive Observation Period 2 of the Genesis of Atlantic Lows Experiment, a number of mesoscale phenomena were observed with special and conventional observing systems over the land and coastal waters. This study involved analysis of these data for the period 24–26 January 1986 in order to define the structure and dynamics of three features: the coastal front; a shallow cyclone that propagated along the coastal front, modifying it as it moved northward; and a low-level jet that formed in the strong coastal pressure-gradient field. The coastal front formed in an existing pressure trough over the Gulf Stream as a result of both ageostrophic deformation and differential diabatic heating. There existed considerable variability in the frontal strength and position on both the mesoalpha and mesobeta scales. The level of strongest frontogenesis was near the surface, with frontolysis calculated above 950 mb. The marine atmospheric boundary layer (MABL) over the Gulf Stream was conducive to cyclone formation. Latent and sensible heat fluxes of up to 800 W m−2 and 400 W m−2 respectively, were calculated early in the study period, and a deep, moist conditionally unstable boundary layer was present. Calculation of the vorticity tendency associated with the sensible heating yielded a narrow band of positive values to the east of the coastline. As a weak midtropospheric wave reached this favorable region to the cut of Florida, a shallow cyclone formed along the coastal front. As the cyclone tracked northeastward along the front, geostrophic deformation ahead of it strengthened the front while strong cold-air advection to its rear displaced the coastal front to the east, leaving behind a dry, stable MABL with low-level, cold-air advection and weak descent. As the cyclone moved northward along the front, conditionally unstable, moist, low-level air ahead was forced by the southeasterly flow to rise along the coastal front and its extension over the cold air near the coastline, causing enhanced precipitation. A low-level northeasterly jet was also observed over the Carolinas, and formed as a result of the strong low- level pressure gradient created by the proximity of the cold continental air over land and the warm air of the Gulf Stream MABL near the coast. This jet, with a maximum near 960 mb, showed a diurnal variation of up to 20 m s−1 which likely resulted from day/night variations in mixing at jet level, an inertial oscillation with the frictional decoupling of the low-level flow at sunset, and isallobaric accelerations.