Convective boundary layer budgets of moisture and sensible heat over an unstressed prairie

Abstract

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 nstruments 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.

Dynamics of Sheared Convective Boundary Layer Entrainment. Part I: Methodological Background and Large-Eddy Simulations

“…Others have focused on the CBL turbulence structure in CBLs with wind shear at the surface (LeMone 1973;Pennel and LeMone 1974). Some of the more recent studies have specifically estimated entrainment flux ratios under conditions of broadly varying surface heating and shear (Grossman 1992;Betts et al 1992;Betts and Ball 1994;Betts and Barr 1996;Barr and Strong 1996;Davis et al 1997;Flamant et al 1997;Angevine 1999;Margulis and Entekhabi 2004), but the methods of analysis varied, and the scatter in their estimates of entrainment flux ratio as a function of entrainment zone shear indicates that other factors, not specifically addressed in those analyses, also influence the entrainment flux ratio. Some of the more recent studies have specifically estimated entrainment flux ratios under conditions of broadly varying surface heating and shear (Grossman 1992;Betts et al 1992;Betts and Ball 1994;Betts and Barr 1996;Barr and Strong 1996;Davis et al 1997;Flamant et al 1997;Angevine 1999;Margulis and Entekhabi 2004), but the methods of analysis varied, and the scatter in their estimates of entrainment flux ratio as a function of entrainment zone shear indicates that other factors, not specifically addressed in those analyses, also influence the entrainment flux ratio.…”

Mean and Flux Horizontal Variability of Virtual Potential Temperature, Moisture, and Carbon Dioxide: Aircraft Observations and LES Study

“…The LES results reveal that the moisture fluxes decrease over time, whereas the buoyancy and CO 2 fluxes are closely clustered around a single curve. Grossman (1992), Davis et al (1997), andLeMone et al (2002) also found that the moisture exchange decreases during the daytime. This can be explained by the fact that in the morning, the ABL specific humidity decreases due to entrainment of dry air, which has a larger effect than the release of moisture at the surface.…”

“…Estimates of ␤ based on aircraft measurements in similar conditions are similar to our LES value, but typically larger than the aircraft value. Observed estimates show large variations, ranging from 0.08 Ϯ 0.12 in Davis et al (1997) to Ϫ0.03-0.8 in Grossman (1992) and 0.6 in Vilà-Guerau de Arellano et al (2004). While in our study, we focus on one particular day, Davis et al (1997) calculated an average ␤ value from 19 days that consists of morning and afternoon flights during the Boreal Ecosystem-Atmosphere Study (BOREAS-94) campaign.…”

“…Notice that the (left) buoyancy and (right) carbon dioxide flux profiles are clustered around a single curve due to the scaling presentation. Grossman (1992) studied 2 days of the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE-87), and by using the linear extrapolation method to the top of the ABL, they found Ϫ0.03 for the day with light winds (Ͻ3 m s Ϫ1 ) and 0.19 for the windy day (8-10 m s Ϫ1 ) at around 1100 CDT. Furthermore, Vilà-Guerau de Arellano et al (2004) found 0.6 from their analysis of 1 day at 1300 UTC during the European Union's Regional Assessment and Monitoring of the Carbon Balance within Europe (RECAB-02) project, in which the aircraft tracks were flown repeatedly at three levels in the ABL.…”

Entrainment results including advection and case studies from the Flatland boundary layer experiments

“…These studies all used series of radiosonde flights to determine the budget quantities. In addition, Betts et al [1992], Grossman [ 1992], and Davis et al [1997] used aircraft measurements for budget studies. Theoretical studies were made by Stull [ 1976] and Tennekes [1973] among others.…”