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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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Israel Lopez-Coto, Micheal Hicks, Anna Karion, Ricardo K. Sakai, Belay Demoz, Kuldeep Prasad, and James Whetstone

Abstract

Accurate simulation of planetary boundary layer height (PBLH) is key to greenhouse gas emission estimation, air quality prediction, and weather forecasting. This paper describes an extensive performance assessment of several Weather Research and Forecasting (WRF) Model configurations in which novel observations from ceilometers, surface stations, and a flux tower were used to study their ability to reproduce the PBLH and the impact that the urban heat island (UHI) has on the modeled PBLHs in the greater Washington, D.C., area. In addition, CO2 measurements at two urban towers were compared with tracer transport simulations. The ensemble of models used four PBL parameterizations, two sources of initial and boundary conditions, and one configuration including the building energy parameterization urban canopy model. Results have shown low biases over the whole domain and period for wind speed, wind direction, and temperature, with no drastic differences between meteorological drivers. We find that PBLH errors are mostly positively correlated with sensible heat flux errors and that modeled positive UHI intensities are associated with deeper modeled PBLs over the urban areas. In addition, we find that modeled PBLHs are typically biased low during nighttime for most of the configurations with the exception of those using the MYNN parameterization, and these biases directly translate to tracer biases. Overall, the configurations using the MYNN scheme performed the best, reproducing the PBLH and CO2 molar fractions reasonably well during all hours and thus opening the door to future nighttime inverse modeling.

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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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