Post-frontal air mass modification

During the spring-summer growing season, a procession of frontal systems moves across the northeastern U.S. ushering in drier and usually cooler air masses. Forests locally modify these air masses by changing the local and regional energy partition through evapotranspiration, reducing the vapor pressure deficit (VPD) and surface temperature and facilitating the formation of boundary layer cumulus clouds. The relatively frequent passage of fronts has a cumulative impact on forest-atmosphere exchange, in that the replacement of overlying air masses minimizes the potential for environmental stresses such as high surface temperatures and excessive VPDs.

Previous studies of forest-atmoshere exchange typically examine a suite of unrelated days without considering the impact of diurnal sequences and air mass modification. Thus, composites of "golden" days, chosen to accommodate remote sensing and data processing needs, are biased towards "best" weather days, which usually occur just after a frontal passage when a fresh air mass moves into a region. In this study, the cumulative (multi-day) effect of local and regional air mass modification, including the feedbacks associated with the presence of boundary layer cumulus clouds (BLcu), is analyzed. A multi-day model sensitivity analysis is presented to examine the relationships among the surface, mixed layer, and the cloud layer.

A composite (5-day length) of sequences featuring the daily appearance of BLcu indicates a diminished role for entrainment and other external forcings due to the daily occurrence of a rapid growth phase in ML diurnal evolution subsequent to day 1. From the sequence minimum (day 2) in temperature and moisture, surface flux convergence accounts for about 50% of the overall net moistening and heating of the mixed layer. BLcu, by modulating the exchange of heat, and moisture between the surface and the ML, and mass, heat, and moisture between the ML and the free atmosphere above, act to maintain a nearly constant afternoon relative humidity during the latter part of the sequence.This explains the climatological tendency during mid-summer for the surface Bowen ratio to approach some multiple of the equilibrium Bowen ratio (e.g., 1/eRH)

Model sensitivity tests show that changes in subsidence and stability above the mixed layer affect ML processes most on day 1; during subsequent days, the rapid growth phase dominates the ML growth equation, and reduces the impact of these external terms, confirming the observations referred to above. Finally, increasing the regional surface Bowen ratio (Breg)to 3.5 reduces BLcu fraction to < 20% and produces little net moistening of the ML; whereas reducing Breg by 30% increases sequence BLcu coverage by 30 - 80%.

Northeastern forests are acclimated to frequent passages of cold or dry fronts during the growing season. This provides a regular series of cooler and drier air masses to the region; local and regional air mass modification then allows for the formation of BLcu. Any change in local land use patterns (or changes in the air mass source regions), that produce a decrease in the frequency of BLcu (which tend to mitigate extreme conditions and enhance net carbon uptake), may adversely impact the health of the forest ecosystem, and permanently alter the regional climate.