Convective boundary layer structure during the Hudson Valley Ambient Meteorology Study

The structure and dynamics of the convective boundary layer (CBL) are well understood, but the presence of topography alters certain features. Here, we present case studies and a composite of unusual convective boundary layer structures observed during the intensive field campaign (IFC) of the Hudson Valley Ambient Meteorology Study (“HVAMS”). The HVAMS IFC deployment occurred during the fall of 2003, and featured a dense network of high quality high temporal resolution surface stations and remote sensing platforms, including periodic and continuous measurements of vertical profiles of winds, temperature, and humidity.

In the IFC study area (between Kingston and Albany), valley walls range from 200 – 800 m. Despite vigorous convective mixing, significant directional wind shear is frequently noted above 0.5zi, and appreciable vertical scalar gradients are observed. In the extreme, cross-valley horizontal advection and along-valley channeling leads to an afternoon double inversion within the CBL. These differential advection effects have not previously been extensively documented nor are they accounted for in mesoscale forecasting or air quality models. The persistence of the multiple BL structure depends on the intensity of surface buoyancy flux, which tends to destroy the midlevel inversion, and horizontal thermal advection that tends to enhance it. The upper layer loses turbulent kinetic energy as it advects across the valley, similar to what happens to the CBL during the evening transition.

To identify conditions favorable for development and maintenance of the double inversion, observations taken from a variety of remote sensing and observational platforms were made during the HVAMS IFC. These include CBL structure flights on the instrumented University of Wyoming King Air aircraft, vertical profiles of temperature, humidity, and winds from the Tethered Atmospheric Observing System (TAOS), Albany National Weather Service rawinsonde data, a NOAA Environmental Technology Lab wind profiler, a sodar, and the Mobile Integrated Profiling System (MIPS) from the University of Alabama at Huntsville.