Postfrontal Airmass Modification

Abstract

The northeastern United States is subject to relatively frequent passages of frontal systems during the growing season. After a frontal passage, the newly arrived air mass is gradually modified by the underlying, mostly vegetated landscape. For the 1995–98 growing seasons, 25 frontal sequences with at least 4 days between frontal passages were identified; 16 had sufficient data continuity for rigorous analysis. A composite of sequences featuring the daily appearance of boundary layer cumulus clouds (BLcu) indicates a diminished role for entrainment and other external forcings because of the daily occurrence of a rapid growth phase in the mixed-layer (ML) diurnal evolution subsequent to day 1. Between frontal passages, net heat and moisture flux convergence in the ML is near zero, but during the warming and moistening phase, the surface flux terms, through a net radiation–BLcu feedback, are the principal controls on the tendencies of the ML temperature θ and specific humidity q. The combination of the θ and q tendencies leads to a nearly constant lifting condensation level, relative humidity, and BLcu cloud fraction during the latter part of the sequences. The presence of BLcu enhances water use efficiency and net afternoon carbon uptake throughout the sequence, with day 4 featuring optimal conditions. A multiday box model was used to perform sensitivity studies on subsidence, the lapse rate γθυ above the ML, cloud mass flux, and the regional surface Bowen ratio βreg. The effects of subsidence and γθυ on ML processes are most conspicuous on day 1; during subsequent days, the rapid growth phase dominates the ML growth equation and reduces the impact of these external terms. Increasing βreg to 3.5 reduces BLcu fraction to less than 20% and produces little net moistening of the ML, whereas reducing βreg by 30% increases sequence BLcu coverage by 30%–80%. In sum, the presence of a net radiation–BLcu feedback allows for the establishment of an equilibrium in the ML heat and moisture tendencies and ensures the appearance of BLcu on each day of the sequence, thus sustaining favorable conditions for forest–atmosphere exchange (i.e., carbon uptake).