The role of entrainment in surface-atmosphere interactions over the boreal forest

We present a description of the evolution of the convective boundary layer (CBL) over the boreal forests of Saskatchewan and Manitoba, as observed by the national center for atmospheric research (NCAR) Electra research aircraft during the 1994 boreal ecosystem-atmosphere study (BOREAS). All observations were made between 1530 and 2230 UT (0930-1630 local solar time, LST). We show that the CBL flux divergence often led to drying of the CBL over the course of the day, with the greatest drying (approaching 0.5 gkg"-"1hr"-"1) observed in the morning, 1000-1200 LST, and decreasing over time to nearly no drying (0 to 0.1 gkg"-"1hr"-"1) by midafternoon (1500-1600 LST). The maximum warming (0.45 Khr"-"1) also occurred in the morning and decreased slightly to about 0.4 Khr"-"1 by midafternoon. The CBL vapor pressure deficit (VPD) increases over the course of the day. A significant portion of this increase can be explained by the vertical flux divergence, though horizontal advection also appears to be important. We suggest a linkage between boundary layer growth, the vertical flux divergences, and boundary layer cloud formation, with cloud activity peaking at midday in response to rapid CBL growth, then decreasing somewhat later in the day in response to CBL warming and decreased growth. We also see evidence of feedback between increasing VPD and stomatal control. (orig.)Available from TIB Hannover: RR 6341(85) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman