ROLE OF ENTRAINMENT IN SURFACE-ATMOSPHERE INTERACTIONS OVER THE BOREAL FOREST

We present a description of the evolution of the convective boundary l ayer (CBL) over the boreal forests of Saskatchewan and Manitoba, as ob served by the National Center for Atmospheric Research (NCAR) Electra research aircraft during the 1994 Boreal Ecosystem-Atmosphere Study (B OREAS). All observations were made between 1530 and 2230 UT (0930-1630 local solar time (LST)). We show that the CBL flux divergence often l ed to drying of the CBL over the course of the day, with the greatest drying (approaching 0.5 g kg(-1) h(-1)) observed in the morning, 1000- 1200 LST, and decreasing over time to nearly no drying (0-0.1 g kg(-1) h(-1)) by midafternoon (1500-1600 LST). The maximum warming (0.45 K h (-1)) also occurred in the morning and decreased slightly to about 0.4 K h(-1) by midafternoon. The CBL vapor pressure deficit (VPD) increas ed over the course of the day. A significant portion of this increase can be explained by the vertical flux divergence, though horizontal ad vection also appears to be important. We suggest a linkage among bound ary layer growth, the vertical flux divergences, and boundary layer cl oud formation, with cloud activity peaking at midday in response to ra pid 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. We use eddy-covariance fl ux measurements from the Electra to compute the virtual temperature en trainment ratio A(r). The computed mean value of 0.08 +/- 0.12 is some what lower than the commonly assumed value of 0.2, as well as with oth er estimates from BOREAS. This value is very sensitive to the determin ation of CBL depth. We find that A(r) increases with an increasing jum p in mean wind across the CBL top. The entrainment flux of water vapor is found to be most dependent on time of day (negative correlation). The ratio of entrainment to surface flux of water vapor is 1.57 +/- 0. 25. Airborne lidar observations of the CBL top reveal a CBL top ''thic kness'' that is smaller than would be expected from simple theory but consistent with past lidar observations. The normalized thickness is f ound to have a very consistent value, (h) over bar/h(0) - 1 = 0.116 +/ - 0.008, where 12 cases were examined. A new method of computing the v ariability of the CBL top is illustrated, and we show that this varian ce in the CBL depth also scales with the depth but that the value of t his normalized variance differs substantially from the ''thickness'' d efined in past literature.