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Ricardo K. Sakai, David R. Fitzjarrald, and Kathleen E. Moore

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 (zd)/(hd), 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 many currently popular averaging procedures. Several running-mean periods have been used to assess the effect of the mean removal procedure on flux estimates. Given the assumption that large eddies would have been sampled at the towers had an ensemble measurement been possible, a correction is proposed based primarily on the mean wind speed to adjust fluxes obtained using short averaging intervals. This correction is successful in achieving observational energy-balance closure at two dissimilar forested sites. Cospectral similarity is found for all scalars studied. Daytime fluxes of CO2, for example, can be underestimated at standard flux towers by 10%–40%, depending on wind speed.

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David R. Fitzjarrald, Otávio C. Acevedo, and Kathleen E. Moore

Abstract

At the time of leaf emergence in deciduous forests, markedly enhanced evapotranspiration leads to a rapid drop in the Bowen ratio. A small fraction of this surface flux alteration converges into the boundary layer, and this can be detected in the mean temperature and humidity daily increments at the surface. A simple technique is presented for identifying this response in surface climate data and extracting time series for the date of spring onset and for the “spring intensity,” a measure of surface energy budget partition change in spring. A tendency Bowen ratio B′ is found from changes in the daily increment of temperature and humidity in multidecadal averages. The spring date determined using this criterion for stations along the U.S. east coast corresponds to the date at which the normalized difference vegetation index (NDVI) reaches 80% of its seasonal maximum. Northward movement of the vernal front is similar to that obtained using Hopkins’ empirical rule; it is linearly related to leaf emergence and flowering dates from the North American lilac phenology network. Spring intensity increases northward; the states from Virginia north exhibit distinctly higher values. There has been a trend in the most recent decades toward earlier spring dates, except for regions in Virginia and North Carolina. The same analyses performed using the small subset of stations with longer-term records indicate that a trend toward an earlier spring date is confined to recent decades. An inverse relationship between the spring date and spring average temperature was found for the Midwest but is inadequate for the northeast. Spring intensity has generally increased in northeastern North America throughout the twentieth century. However, large oscillations with an approximate 20-yr period distinguish the northeastern United States from the Midwest, indicating that the intensity of spring is not a simple function of spring temperature or of forest cover fraction.

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Jeffrey M. Freedman, David R. Fitzjarrald, Kathleen E. Moore, and Ricardo K. Sakai

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 (z i) 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 z i and LCL decrease. By late spring, boundary layer cumulus cloud frequency increases sharply, as the mixed layer approaches a new equilibrium, with z i 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 reveals large variations in monthly frequency due to periods of drought or excessive wetness. Light–cloud cover regression relationships developed from ASOS ceilometer reports for Orange, Massachusetts, and Harvard Forest insolation data show a good linear fit (r 2 = 0.83) for overall cloud cover versus insolation, and a reasonable quadratic fit (r 2 = 0.48) for cloud cover versus the standard deviation of insolation, which is an indicator of sky type. Diffuse fraction (the ratio of diffuse to global insolation) shows a very good correlation (r 2 = 0.79) with cloud cover. The sky type–insolation relationships are then used to analyze the impact that boundary layer clouds have on the forest ecosystem, specifically net carbon uptake (FCO2), evapotranspiration (ET), and water use efficiency (WUE). During 1995, afternoon FCO2 was 52% greater on days with boundary layer cumulus clouds than on clear days, although ET was the same, indicating greater light use efficiency and WUE on partly cloudy days. For 1996–98, afternoon FCO2 was also enhanced, especially during dry periods. Further analysis indicates that the vapor pressure deficit (VPD) was significantly greater (>8 hPa) during 1995 and parts of 1996–98 on clear days as compared with partly cloudy days. A long-term drought combined with abnormally warm weather likely contributed to the high VPDs, reduced FCO2, ET, and the dearth of clouds observed during 1995. In general, the presence of boundary layer cumulus clouds enhances net carbon uptake, as compared with clear days.

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Kathleen E. Moore, David R. Fitzjarrald, Ricardo K. Sakai, Michael L. Goulden, J. William Munger, and Steven C. Wofsy

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 temperature relative to the net radiation is directly related to the Bowen ratio or energy partition. The subcanopy space follows a different pattern dictated by the presence of the canopy; there the midday sensible heat flux is a maximum in spring and fall when the canopy is leafless, while subcanopy CO2 flux is maximum in midsummer. Subcanopy evapotranspiration did not have a distinct seaasonal peak in spring, summer, or fall. The temperature dependence of the respiration rate estimated from the eddy correlation subcanopy CO2 flux is comparable to that found using nocturnal flux measurements.

The surface energy balance follows a seasonal pattern in which the ratio of turbulent sensible heat flux to the net radiation (QH/Q*) is a maximum in the spring and fall (0.5–0.6), while the latent heat flux (QE) peaks in midsummer (QH/Q* = 0.5). This pattern gives rise to a parabolic growing season shape to the Bowen ratio with a minimum in early August. Growing season changes in the canopy resistance (Rc), related to the trends in the Bowen ratio, are more likely to be predicted using the thermal channels of remote sensing instruments than the shorter-wavelength bands.

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