Mechanisms responsible for complex structure in a convective boundary layer

In the Hudson Valley of New York, cross-valley horizontal advection and along-valley channeling leads to a complex structure in the convective boundary layer (CBL; see attached figure). This variability greatly influences vertical mixing and horizontal transport of air masses above and below the CBL. These differential advection effects have not been extensively documented nor are they accounted for in mesoscale forecasting or air quality models. Previous studies have found anecdotal evidence of multiple mixed layers but no explanation of the mechanism(s) behind the origin and maintenance of such a structure has been offered. For this presentation, multiple cases of double mixed layers observed during the Hudson Valley Ambient Meteorology Study (HVAMS) are documented. Through high resolution time series and heat, moisture, and trace gas budget analysis, mechanisms responsible for the complex CBL structure are proffered.

As part of HVAMS , an intensive field campaign (IFC) was conducted during the fall of 2003. The IFC featured the deployment of 9 Integrated Surface Flux Facility (ISFF) stations and the Tethered Atmospheric Observation System (TAOS) from NCAR; the Mobile Integrated Sounding Unit (MIPS) from the University of Alabama at Huntsville; the University of Wyoming King Air instrumented aircraft; NOAA’s ETL wind profiler at Schenectady Airport; a sodar on the river at Schodack Island State Park; and additional rawinsonde launches at the NWS WFO Albany. Stations not part of the IFC deployment but nevertheless used as part of long-term data analysis for the HVAMS project include NWS Automated Surface Observing System (ASOS) and Cooperative Observer (COOP) stations.

One goal of HVAMS was to capture air mass modification sequences, where local exchangeprocesses come to dominate CBL concentration tendencies (heat and moisture) after the first day following a frontal passage (Freedman and Fitzjarrald 2001). During HVAMS several such sequences did occur, and the day after a frontal passage is when the complex CBL structure becomes evident. Three principal mechanisms operating separately or in tandem lead to the devel

opment of the multiple mixed layer structure: 1) the presence of early morning fog which reduces the total available buoyant energy for boundary layer growth; 2) advection of warmer air from the Catskill Plateau over the Valley and 3) channeling of winds within the Valley that serves to maintain low-level ambient conditions (temperature and humidity).

The role of fog. Fog dissipation diverts energy that would normally initially be used to dissipate the early morning surface inversion and drive mixed layer growth. Analysis of surface flux and insolation data at the ISFF sites, surface visibility observations from the Albany (ALB) and Poughkeepsie (POU) Automated Surface Observation System (ASOS) stations, and visual observations from the King-Air indicate that fog, when present, persisted until about 0900 LT. During the IFC, maximum buoyancy fluxes reached about 150 W m^-2, inversion depths were about 200 m, and Δθv averaged about 10 K. Calculations using the integral method (Garratt 1992) indicate that it should take approximately 3.7 hours for the surface inversion to dissipate. Sunrise during the IFC varied from 0650 LT to 0720LT. On days without fog the surface inversion dissipated by mid-morning (approximately 1030 LT), in agreement with integral method estimates. Soundings on days fog was present indicate that the surface inversion did not fully erode until noon or shortly thereafter. Thus, with morning fog, only about 2 - 4 hours of positive buoyancy flux is available on the Valley floor to drive CBL growth before shadows and low sun angle result in convective conditions decaying around 1600 LT. This in itself, however, is not the only factor contributing to the complex boundary layer structures observed.

Advection of air from the Catskill Plateau. Just to the west of the Hudson Valley lies the Catskill Plateau, elevated terrain which rises abruptly to over 1000 m near the IFC study area (seeattached figure). The Plateau extends irregularly westward about 200 km, with most of the terrainaveraging about 600 m in elevation.

Following a frontal passage, the atmosphere encompassing both the Plateau and the Hudson Valley is rather homogeneous and well-mixed, so there is little variation in temperature or scalarconcentrations. Subsequently, local processes (surface heating) and circulations (valley channeling) begin to dominate, and by day two the air over the Catskill Plateau is several degreeswarmer than corresponding heights over the Valley. With the prevailing synoptic flow,this elevated mixed layer advects eastward over the Valley, strengthening the remnant subsidence inversion and producing a “double” mixed layer that persists for the remainder of the sequence. Time series of cross-valley flights over the King Air show decreasing turbulence from west-to-east, suggesting that the elevated mixed layer remains decoupled from the valley surface. Valley processes discussed above (i.e. fog/radiational cooling and channeling) serve to maintain two (ormore, in some cases) distinct daytime mixed layers until the next frontal system moves through.

Figure. 1: Perspective plot of the Hudson Valley (looking north) and schematic of mechanisms responsible for doublemixed layer.