Mechanisms and Climatology of an Observed Complex Convective Boundary Layer in the Hudson Valley of New York

Daytime convective boundary layer (CBL) structure in valleys exhibits distinctive horizontal and vertical variability that greatly influences vertical mixing and horizontal transport of air masses both above and below the CBL. In the Hudson Valley of New York, cross-valley horizontal advection and along-valley channeling leads to a baroclinic convective boundary layer structure that features directional shear and multiple mixed layers. Our previous work identified three principal mechanisms operating in tandem responsible for the complex CBL structure as observed during the Hudson Valley Ambient Meteorology Study (HVAMS): 1) channeled flow within the valley; 2) advection of warmer and drier air from higher elevation land adjacent to the valley; and 3) fog formation or pooling of cooler air on the valley floor during the overnight hours. Although high-resolution HVAMS observations showed multiple mixed layers occurred on almost half the fair weather (CBL) days during the intensive field campaign (lasting 6 weeks), no climatological studies had been performed regarding the frequency and seasonality of this structure. Until recently, standard National Weather Service (NWS) soundings lacked sufficient vertical resolution to depict this feature convincingly. Here, we present an overview of the mechanisms responsible for this complex CBL structure, illustrated using high-resolution Global Positioning System soundings now available from the NWS Albany NY Weather Forecast Office, and present a multi-year climatology of its frequency and seasonality.