| A study of vegetation-atmosphere interactions and boundary layer cumulus clouds (“BLcu”) in terms of seasonal trends (i.e., heat and moisture tendencies) and short-term events (specifically the modification of in situ air masses) is presented. In the northeastern U.S., in response to increasing insolation and sensible heat flux, both the mixed layer height (zi) and lifting condensation level (LCL) peak (∼1300 and 1700 m) just before the start of the growing season. With the commencement of transpiration, the Bowen ratio (β) falls abruptly (from greater than 3 to less than 1) as additional moisture is transpired into the boundary layer, and zi and the LCL decrease. By late spring, boundary layer cumulus cloud frequency increases sharply, as the mixed layer approaches a new equilibrium.; At Harvard Forest during 1995, afternoon net carbon uptake (Fco2 ) was 52% greater on days with boundary layer cumulus clouds than clear days. For 1996–1998, afternoon Fco2 was also enhanced, especially during dry periods. The same enhancement, albeit reduced, was observed at a northern jack pine site during the BOREAS project, despite very different phenological, hydrological, and climatological regimes.; Sixteen frontal sequences affecting the northeastern U.S. were analyzed in terms of local and regional contributions to the temperature and moisture tendency equations. A composite 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.; Model sensitivity tests show that changes in subsidence and γ &thetas;ν affect ML processes most on day 1; dining 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 Bowen ratio (βreg) to 3.5 reduces BLcu fraction to <20% and produces little net moistening of the ML; whereas reducing βreg by 30% increases sequence BLcu coverage by 30–80%.; Changes in land-use resulting in a loss of forest cover may diminish BLcu frequency due to a reduction in evapotranspiration and consequent higher LCLs. This positive feedback, whereby reduced BLcu frequency leads to higher surface temperatures (and LCLs), may also significantly reduce net C uptake, which has important implications for existing and potential climate change scenarios. |
