During the spring and summer months, there is a regular procession of frontal passages across the mid- and high-latitudes of North America. During a typical frontal sequence, a warm, moist air mass is replaced by a drier, and usually cooler high pressure system. The relatively frequent passage of fronts has a cumulative impact on the forest-atmosphere system, in that the regular arrival of fresh air masses minimizes the potential for prolonged periods of enhanced vapor pressure deficit or reduced evapotranspiration. Furthermore, forests may locally modify these air masses to enhance favorable meteorological conditions by injecting moisture into the boundary layer, thereby facilitating the formation of boundary layer clouds. Several studies suggest that boundary layer clouds, by modifying the light environment and tempering the buildup of heat (for example, the vapor pressure deficit), may provide an optimum condition for carbon uptake.
Boundary layer cumulus usually form one or two days after frontal passage. For the next few days, the mixed layer functions as a heat and moisture reservoir. The additional moistening usually results in the lowering of the lifting condensation level, and an increase in the probability of boundary layer cumulus formation. Under certain conditions, however, clouds are virtually absent during the entire post-frontal sequence. Factors that may inhibit boundary layer cloud formation include depletion of soil moisture, an increase in canopy resistance, inversion strength, the thermodynamic profile, subsidence, and entrainment. The absence of clouds may represent a positive feedback, further reducing the probability of boundary layer cloud formation.
Previous studies involving forest-atmosphere exchange have focused on seasonal time scales, particular days, or a single diurnal event in examining the heat, moisture and carbon budgets for the boundary layer-forest system. Using data from both the BOREAS project and the Harvard Forest Long Term Ecological Research site in central Massachusetts, synoptic analyses and model interpolated temperature and moisture fields, we identify frontal cases for each site and
surrounding region and analyze the boundary layer heat and moisture budgets to determine the relative contributions from local, advective, and entrainment processes over the course of several days. We also investigate trends in carbon uptake during frontal events. A temporal composite is then presented from at least 10 frontal cases. Finally, a boundary layer model coupled with an analytical scheme depicting cumulus onset and cloud fraction is used to test the forest-atmosphere-boundary layer cloud relationship by examining system sensitivity to canopy resistance, vapor pressure deficit,
entrainment, and subsidence, and to explore feedback relationships among surface and inversion fluxes and surface radiation variables