The effects of the sea surface temperature (s.s.t.) front at the edge of the Gulf Stream on the marine atmospheric boundary layer (MABL) are investigated using a numerical model to study the modification effects of an oceanic front on the MABL structure. The situation simulated is flow from over cold shelf water to over the warm water of the Gulf Stream. The initial temperature and humidity profiles of the air are specified to be near neutral over the cold water and are therefore typical of undisturbed conditions. The differential in s.s.t. across the oceanic front creates a horizontal variation in the surface perturbation pressure and the stability. The surface perturbation pressure and turbulent fluxes modulate the flow and produce horizontal variations in horizontal wind components with associated vertical motions. A thermally direct cell is produced as a result of the s.s.t. difference across the front. The isotherms slope upward towards the warm water. Entrainment of inversion layer air and upward vertical motion over the warm water cause the MABL to be deeper there. A layer of cloud forms over warm water and is associated with mixed layer deepening rather than lowering of the condensation level. Turbulent fluxes in the MABL show considerable spatial variation. Surface stress is much larger over the front and over the warm water than over the cold water. This is mostly caused by wind speed changes associated with the front. Changes in the drag coefficient due to changes in surface roughness and stability are much less important.Mean budgets for temperature and total water indicate that there is a balance between horizontal advection and turbulent flux divergence. The U momentum budget shows that once the geostrophic balance terms are subtracted, the balance is mainly between the pressure gradient force associated with the induced temperature field and turbulent friction, with horizontal advection and the Coriolis force acting on the geostrophic departure playing minor roles. The V momentum budget shows a balance between horizontal advection, Coriolis force and friction.Although there are few data for comparison, the results are in qualitative agreement with observations in the area. This study shows that the s.s.t. front at the Gulf Stream edge produces marked local changes in the nearby atmospheric surface layer. 29

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A complete yearly record (1988) of surface measurements is used to examine the atmospheric diurnal secondary circulations over the entire area of Hong Kong in conjunction with spatial and temporal variations of surface temperature, wind speed and rainfall. Evidence of atmospheric diurnal secondary circulations is found at 10 sites. The occurrence of a summer morning rainfall maximum over the coast results from the interaction ofthe large-scale summer monsoon and local mesoscale secondary circulations. The afternoon onshore secondary circulation accelerates the advection of warm, humid unstable air and, coupled with the upward orographic lifting, produces enhanced rainfall along windward mountain ridges.Dynamical and scaling considerations suggest that the blocking effect is negligible and the primary forcing mechanism is land-sea temperature difference, but terrain effects are also important. Although the secondary circulation system's strength and timing vary the circulation behaves like a classic sea-land breeze circulation, complicated by superimposed mountain-valley breezes.

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