Diurnal Dynamics of Hudson Valley Boundary Layer Winds: a Second Look

Combining long-term and case study observations we examine mechanisms that determine the Hudson Valley boundary layer wind vector over the typical fair-weather diurnal cycle. First, we present a seasonal climatology of stable boundary layer channeling as evidenced by high resolution 1200 UT (0700 LT) operational soundings at Albany NY from 2011-present. In fair weather, topographic influences demand that winds follow the Valley axis, the direction indicated by the along-valley surface pressure gradient available from existing National Weather Service Automated Surface Observation System stations at Albany and Poughkeepsie (115 km to the south).

Second, we re-examine data from the Hudson Valley Ambient Meteorology Study (HVAMS) during September and October 2003. The HVAMS surface network included nine integrated surface flux system stations from the National Science Foundation's Lower Atmosphere Observing Facilities and five conventional meteorological stations encompassing the Hudson Valley for 100 km from Albany southward. Additional Albany radiosondes along with data from radar wind profilers at Schenectady and Kingston airports provided additional wind information through the depth of the planetary boundary layer. At dawn and dusk, an the University of Wyoming's King Air made detailed soundings of the stable boundary layer by making ‘close approaches' at small airports.

After very briefly recounting our efforts of a decade ago, analysis proceeds by estimating the momentum balance in the afternoon convective boundary layer (CBL); we use the suite of HVAMS observations to infer vetrical momentum exchanges in the decaying CBL seen during the early-evening transition. After this transition, the wind at ≈150 m (the top of the eventual stable boundary layer SBL) often presents the direction earlier seen above zi ≈ 1200m. We re-examine the idea that decaying turbulence in the CBL, even while weaking rapidly, may still be sufficient to mix momentum and other scalar quantities down to the top of the ensuing SBL, the hodograph indicating the remnant CBL winds spiraling toward geostrophic balance. During this period, and after the establishment of the shallow (100-200 m) SBL, we estimate momentum balances using a simple bulk model that includes a tilted ‘mixed' layer to counter the Coriolis acceleration. This model treats the effects of the ‘cold pool' in the valley and its modulation of the nocturnal along-valley ‘jet'.