Ricardo K. Sakai

Importance of Low-Frequency Contributions to Eddy Fluxes Observed over Rough Surfaces

Abstract Eddy covariance flux observations at a deciduous temperate forest site (83 days) and at a boreal forest site (21 days) are analyzed for midday periods (1100–1400 LT). Approximate stationarity of the time series is demonstrated, and the ensemble-averaged roughness sublayer cospectra are presented. Spectral and cospectral forms in the roughness sublayer are more peaked than those found in an inertial sublayer. They exhibit similar forms dependent on (z − d)/(h − d), where d is the displacement height and h is the canopy height. The inertial-layer spectral forms are recovered when observations are made where this scaled height is approximately 4. For a sample summer at the midlatitude deciduous forest, large eddies with periods from 4 to 30 min contribute about 17% to surface eddy fluxes of heat, water vapor, and carbon dioxide (CO2). Much larger contributions can occur in light-wind conditions. This effect, likely caused by the passage of convective boundary layer eddies, is not observed when using...

Seasonal Variation in Radiative and Turbulent Exchange at a Deciduous Forest in Central Massachusetts

Abstract Temperate deciduous forests exhibit dramatic seasonal changes in surface exchange properties following on the seasonal changes in leaf area index. Nearly continuous measurements of turbulent and radiative fluxes above and below the canopy of a red oak forest in central Massachusetts have been ongoing since the summer of 1991. Several seasonal trends are obvious. Global solar albedo and photosynthetically active radiation (PAR) albedo both are good indicators of the spring leaf emergence and autumnal defoliation of the canopy. The solar albedo decreases throughout the summer, a change attributed to decreasing near-infrared reflectance since the PAR reflectance remains the same. Biweekly satellite composite images in visible and near-infrared wavelengths confirm these trends. The thermal emissions from the canopy relative to the net radiation follow a separate trend with a maximum in the midsummer and minima in spring and fall. The thermal response number computed from the change in radiation tempe...

Boundary Layer Clouds and Vegetation–Atmosphere Feedbacks

Abstract An analysis of boundary layer cumulus clouds and their impact on land surface–atmosphere exchange is presented. Seasonal trends indicate that in response to increasing insolation and sensible heat flux, both the mixed-layer height (zi) and the lifting condensation level (LCL) peak (∼1250 and 1700 m) just before the growing season commences. With the commencement of transpiration, the Bowen ratio falls abruptly in response to the infusion of additional moisture into the boundary layer, and zi and LCL decrease. By late spring, boundary layer cumulus cloud frequency increases sharply, as the mixed layer approaches a new equilibrium, with zi and LCL remaining relatively constant (∼1100 and 1500 m) through the summer. Boundary layer cloud time fraction peaks during the growing season, reaching values greater than 40% over most of the eastern United States by June. At an Automated Surface Observing System (ASOS) station in central Massachusetts, a growing season peak is apparent during 1995–98 but reve...

Detecting leaf area and surface resistance during transition seasons

Abstract In this study we seek empirical relationships among canopy resistance to water vapor transport, the time-varying leaf area index (LAI), in situ radiative flux observations, and a satellite-based estimate of leaf state (NDVI, the normalized difference vegetation index) from a leafless deciduous forest to a covered canopy and vice versa. These relationships can be used in numerical models such as verification in global climate models. They also can be useful tools for developing remote sensing techniques. LAI was found through analysis of frequent video images of canopy evolution in spring and autumn during 1992 and 1993 at a deciduous forest in central Massachusetts. We examined the impact of leaf presence on water vapor transport during spring and autumn using an LAI time series during leaf emergence and leaf fall for the four study seasons. The canopy resistance to water vapor transport ( r c ) decreased abruptly at leaf emergence in each year but then also continued to decrease slowly during the remaining growing season, owing to slowly increasing LAI. One remarkable result is that a single linear relationship between r c . and LAI during leaf emergence can be used to estimate the minimum seasonal r c associated with the maximum foliage cover. Canopy resistance and PAR-albedo (albedo from photosynthetically active radiation (PAR) instruments) began to increase about 1 month before leaf fall with the diminishment of CO 2 gradient above the canopy as well, at which time evaporation began to be controlled as if the canopy were leafless. We present empirical linear regressions relating NDVI, r c , and PAR-albedo. The NDVI linear regressions with surface measurements indicate that tower-based measurements can represent at least a satellite pixel region. These results reinforce the notion that relationships among these parameters are scale independent from tower-based measurements spatial scale to a satellite pixel resolution (1.1 km X 1.1 km area)., at least.

Growing season water balance at a boreal jack pine forest

Measurements of energy and CO2 fluxes were made over the growing seasons of 1994 and 1996 in a northern jack pine forest as part of the Boreal Ecosystem-Atmosphere Study (BOREAS). Simultaneous measurements of soil water potential and content, rainfall, leaf wetness, and air specific humidity by our group and others allowed us to construct a complete water balance at this site for the 1994 season. The longer-term (spanning weeks) rate of loss of soil moisture in the upper 0.25 m of soil matched the total evapotranspiration, measured by eddy covariance. Evapotranspiration (measured as QE, latent heat flux) was just 24% of the net radiation, a result that has been found in many boreal forest types, regardless of the canopy coverage. Low canopy conductances (typically 2–4 mm s−1) probably represent an adaptation to the extremely low soil moistures and poor nutrient status of the site. Net radiation was the best single-variable predictor of evapotranspiration, having a correlation coefficient of 0.8 with QE for the 1994 season. Afternoons with sustained (>4 continuous hours) cloudless conditions resulted in water stress detectable as reduced QE relative to what would be predicted from the net radiation alone. The open canopy at our site promoted the role of the lichen-covered surface in the overall water vapor exchange; subcanopy QE was 26% of the total.