ESTIMATION OF SURFACE HEAT AND MOISTURE FLUXES OVER A PRAIRIE GRASSLAND .1. INSITU ENERGY BUDGET MEASUREMENTS INCORPORATING A COOLED MIRROR DEW-POINT HYGROMETER

This paper focuses on in situ measurements obtained during the First ISLSCP Field Experiment (FIFE) needed to support the development and validation of a biosphere model. Seasonal time series of surface flux measurements obtained from two surface radiation and energy budget stations (SREBS) used to support the FIFE surface flux measurement subprogram are analyzed. Data collection and processing procedures are presented along with the measurement analysis for the entire 1987 experimental period. The two Florida State University (FSU) stations, which were part of a 22-station surface flux network, used a cooled mirror dew point hygrometer to measure surface layer moisture gradients. Evapotranspiration was determined using the Bowen ratio method. A case study of FIFE Golden Day 2 illustrates the capabilities of a SREBS system. Sensible and latent heat fluxes are compared among themselves and against the 22-site domain means. The FSU site intercomparison demonstrates clear evidence of site-to-site differences, but a lengthy time series is required to interpret them correctly. Consistent relationships are observed between the SREBS fluxes and the all-site means on the FIFE Golden Days. The 1987 surface flux data show variations on annual, intraseasonal, synoptic, and diurnal time scales. The annual time scale, arising from a continually changing solar declination, is evident in the available heating although cloudiness variability serves to disguise the signal until late summer. The intraseasonal time scale is observed to be a response to the large scale rainfall pattern of 1987 (alternating wet and dry conditions during the summer months resulted in daytime daily mean Bowen ratios ranging from 0.1 to 0.7, with highest values occurring during plant senescence in October). Fluxes are also modulated on a synoptic time scale (5-7 days) resulting mostly from cloudiness perturbations which have a direct impact on surface available heating. The diurnal time scale, which results from the continually changing solar zenith angle, is modulated by a variety of soil moisture and plant physiology factors but is most sensitive to the intermittent behavior of cloudiness. These characteristics of the surface fluxes emphasize the strong feedback cloud and precipitation have on virtually all aspects of the atmospheric and surface energy budgets.