Response of the Hudson Valley boundary layer to diurnal forcing during a case of rotating geostrophic wind

Previous studies have demonstrated the connection wind in a valley is related to the geostrophic flow aloft. Often these studies consider nearly steady environmental geostrophic wind forcing. As the background geostrophic wind rotates, there a critical angles at which the along-valley wind component shifts discontinuously. We focus on a case study of observations obtained during the Hudson Valley Ambient Meteorology Study (HVAMS). Our aim is better understanding of the dynamic meaning of channeling in valleys. The Hudson Valley axis is oriented approximately N-S and is flat in its southern reaches; there are not up- or down-valley circulations, motions down the large-scale pressure gradient component occur primarily along the valley axis. During the period of interest (Oct 7-12, 2003), the ambient wind at 1500 m altitude shifted from westerly to easterly, with the passage of a weak front. Effects provoked by the rotating geostrophic wind are superimposed on the diurnal cycle of convective boundary layer (CBL) development and decay. Research questions addressed are: • Are there observable differences in the surface pressure field between cases for which the geostrophic wind is aligned to the valley (‘forced channeling') and those for which it is normal to the valley axis (‘pressure-driven channeling')? • What is the relative importance of momentum mixing during the collapse of the CBL and the rapidly adjusting surface mesoscale pressure gradient perturbation in determining wind speed and direction in the residual and stable layers? Addressing these issues leads us to consider how baroclinic the boundary layer was during the period. We use the HVAMS observation network to address these issues. Wind profilers operated at two ends of a 100 km stretch of the valley. Special launches complemented the 0 and 12Z operational radiosonde releases at Albany. 18 surface weather stations (9 of them with surface flux capability, ISSF, NCAR) provide an estimate of the surface thermodynamic, wind and pressure fields. Results from Eta model simulations made during this period are presented for comparison. Turbulent momentum fluxes observed during the early morning and CBL decay period with the Wyoming King Air research plane complete the data used.