CONVECTIVE BOUNDARY-LAYER BUDGETS OF MOISTURE AND SENSIBLE HEAT OVER AN UNSTRESSED PRAIRIE

Convective boundary layer budgets of sensible heat and moisture were evaluated for 2 days over the unstressed vegetation of the tallgrass Konza National Prairie during IFC 1 of the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) 87. On June 1, 1987, winds were strong, while on the other day, June 4, 1987, winds were light. Data for the budgets came from morning and afternoon missions of the NCAR King Air aircraft. In addition to the budget evaluation the study had these objectives: to estimate the area-averaged surface fluxes and compare them to independent, ground-based measurements, to estimate the near surface evaporative fraction, and to compare various evaluations of the ratio of surface to inversion fluxes (the entrainment parameter). The budget analyses suggest that horizontal and vertical advection were significant terms in the budget and cannot be ignored. Given the uncertainties in both aircraft and ground-based instruments and methods, the surface sensible heat flux comparisons were reasonable as was the comparison for evaporative fraction. However, comparison of latent heat fluxes was unsatisfactory. In contrast to other FIFE studies, consistent underestimation of surface fluxes by the aircraft was not found. The direct evaluation of the entrainment parameter k, was problematic and did not compare well with other studies; however. using other methods of estimation, k was greater than 0.2. Bowen ratio estimates from level aircraft runs were low compared to sounding estimates. Error analyses and the budget evaluations emphasized the fact that the FIFE area was too small for accurate estimates of gradients and near-inversion fluxes. Since larger sites may make adequate deployment of surface property measurements prohibitive, a suggestion for improvement in experimental design of aircraft missions is presented. Both days and a third (June 6, 1987) studied previously, showed morning and afternoon episodes of drying throughout the mixed layer and surface layer. A mixed layer growth model is presented which predicts that the time rate of change of potential temperature in the superadiabatic layer is about 40% of the surface temperature rate. Variations in surface evaporation rate and leaf vapor pressure deficit appeared to be strongly associated with solar angle with no modulation by drying episodes, suggesting that atmospheric forcing of plant moisture cycling was minimal during this period.