Dynamic significance of Hudson valley channeling

Observations from the Hudson Valley Ambient Meteorology Study (HVAMS) indicate that channeling in the Hudson valley (New York, USA) boundary layer extends roughly to the height of surrounding terrain. Channeling continues in a very shallow nocturnal SBL, in which a shallow jet-like sheet of air moves along the valley axis. We report on a series of mesoscale model simulations (MESO-NH model) that aim to clarify the dynamic meaning of the channeling observed in HVAMS. During the period of interest (Oct 6-11, 2003), the geostrophic wind shifted from westerly to easterly, and the boundary layer flow shifted from southerly to northerly. The model was run with two nested domains, with 2 km x 2 km x 3 m (200 x 270 x 85 grid points) resolution in the smaller domain (Fig. 1). Special attention is made to evolution of the nocturnal cold pool and its modulation of the nocturnal along-valley jet. We present detailed comparisons of modeled and observed profiles of boundary layer structure at one-minute intervals at more than twenty surface sites and compare simulated and observed aircraft ‘flights'. The simulations are interpreted to identify the dominant forcing terms in the momentum budget and scalar continuity equation. We follow the simulation through several consecutive days to describe the accumulated effects of diurnal cycle of convective boundary layer (CBL) development and decay in the presence of the slower rotation of the geostrophic wind during the period. We interpret the subsequent shift of surface winds from southerly channeling to northerly channeling in the terms of the prognostic equation for boundary layer wind direction.