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Song-Lak Kang and Kenneth J. Davis
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Song-Lak Kang and Kenneth J. Davis

Abstract

Large-eddy simulation (LES) is used to examine the impact of heterogeneity in the surface energy balance on the mesoscale and microscale structure of the convective atmospheric boundary layer (ABL). A long (16 or 32 km) and narrow (5 km) domain of the convective ABL is forced with an imposed surface heat flux consisting of a constant background flux of 0.20 K m s−1 (250 W m−2) added to a sinusoidal perturbation of 16 or 32 km and whose amplitude varies from 0.02 to 0.20 K m s−1 (25–250 W m−2). The output is analyzed using a spatial filter, spectral analyses, and a wave-cutoff filter to show how the mesoscale and microscale components of the ABL respond to surface heterogeneity.

The ABL response is divided by amplitude of heterogeneity into oscillatory and nonoscillatory mesoscale flows, with amplitudes of 0.08 K m s−1 (100 W m−2) and greater being oscillatory. Although mean ABL structure is disturbed relative to the homogeneous case for all heterogeneous cases, the microscale structure of the ABL in the quasi-steady flows retains characteristics of mixed-layer similarity. The vertical sensible heat flux is dominated in all cases by the microscale flux, with an interscale term becoming significant for high-amplitude cases and the mesoscale flux remaining small in all cases. Prior observations of ABLs over heterogeneous surfaces are consistent with the lower-amplitude cases. These results contradict past studies that suggest that heterogeneous surfaces lead to large mesoscale fluxes. The interscale flux and oscillatory microscale structures raise questions about the ability of mesoscale models to properly simulate the ABL in high-amplitude heterogeneity.

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Yuning Shi, Kenneth J. Davis, Fuqing Zhang, and Christopher J. Duffy

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Land surface models (LSMs) and hydrologic models are parameterized models. The number of involved parameters is often large. Sensitivity analysis (SA) is a key step to understand the complex relationships between parameters and between state variables and parameters. SA is also critical to understand system dynamics and to examine the parameter identifiability. In this paper, parameter SA for a fully coupled, physically based, distributed land surface hydrologic model, namely, the Flux–Penn State Integrated Hydrologic Model (Flux–PIHM), is performed. Multiparameter and single-parameter tests are performed to examine the three dimensions of identifiability: distinguishability, observability, and simplicity. Results show that Flux–PIHM model predictions of discharge, water table depth, soil moisture, land surface temperature, and surface heat fluxes are very sensitive to the selection of parameter values. Parameter uncertainties produce large uncertainties in hydrologic and land surface variable predictions. The van Genuchten parameters α and β and the Zilitinkevich parameter C zil are the most identifiable among the 20 tested parameters. Results indicate that the land surface and the subsurface are closely coupled. Hydrologic parameters have significant influence on land surface simulations. At the same time, land surface parameters have considerable impacts on hydrologic simulations; the evapotranspiration prediction prior to a strong precipitation event is critical for initializing accurate prediction of discharge peaks. Results also show that parameter identifiability depends on seasons and canopy wetness. Parameter identifiability at high and low flow conditions can be extremely different. Complex system dynamics have been revealed during the SA.

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Yuning Shi, Kenneth J. Davis, Christopher J. Duffy, and Xuan Yu

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A fully coupled land surface hydrologic model, Flux-PIHM, is developed by incorporating a land surface scheme into the Penn State Integrated Hydrologic Model (PIHM). The land surface scheme is adapted from the Noah land surface model. Because PIHM is capable of simulating lateral water flow and deep groundwater at spatial resolutions sufficient to resolve upland stream networks, Flux-PIHM is able to represent heterogeneities due to topography and soils at high resolution, including spatial structure in the link between groundwater and the surface energy balance (SEB). Flux-PIHM has been implemented at the Shale Hills watershed (0.08 km2) in central Pennsylvania. Multistate observations of discharge, water table depth, soil moisture, soil temperature, and sensible and latent heat fluxes in June and July 2009 are used to manually calibrate Flux-PIHM at hourly temporal resolution. Model predictions from 1 March to 1 December 2009 are evaluated. Both hydrologic predictions and SEB predictions show good agreement with observations. Comparisons of model predictions between Flux-PIHM and the original PIHM show that the inclusion of the complex SEB simulation only brings slight improvement in hourly model discharge predictions. Flux-PIHM adds the ability of simulating SEB to PIHM and does improve the prediction of hourly evapotranspiration, the prediction of total runoff (discharge), and the predictions of some peak discharge events, especially after extended dry periods. Model results reveal that annual average sensible and latent heat fluxes are strongly correlated with water table depth, and the correlation is especially strong for the model grids near the stream.

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Wayne M. Angevine, Peter S. Bakwin, and Kenneth J. Davis

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A 915-MHz boundary layer wind profiler with radio acoustic sounding system (RASS) was sited 8 km from a very tall (450 m) television transmitting tower in north-central Wisconsin during the spring, summer, and autumn of 1995. The profiler measured wind means and variances, and the RASS attachment measured virtual temperature. These quantities are compared to measurements from cup and sonic anemometers and a thermometer/hygrometer at 396 m above ground level on the tower. The precision of hour-averaged profiler winds is better than 1 m s−1, and the precision of the RASS virtual temperature is better than 0.9 K. Corrections to the virtual temperature measured by the RASS are discussed, and a new virtual temperature retrieval method is proposed. Vertical velocity variance correlation is similar to a previous study, and the fact that bias is small indicates that the calculation method used is reliable.

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Song-Lak Kang, Kenneth J. Davis, and Margaret LeMone

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This study analyzes data collected by aircraft and surface flux sites over a 60-km north–south-oriented aircraft track for five fair-weather days during the International H2O Project (IHOP_2002) to investigate the atmospheric boundary layer (ABL) structures over a heterogeneous land surface under different background weather conditions. The surface skin temperature distribution over the aircraft track in this case is mostly explained by the soil thermal properties and soil moisture, and corresponds to the observed ABL depths except one day having a weak surface temperature gradient and a weak capping inversion. For the other four days, the blending height of the surface heterogeneity likely exceeds the ABL depth and thus the ABL establishes equilibrium with local surface conditions.

Among the four days, two days having relatively small Obukhov lengths are evaluated to show the background weather conditions under which small-scale surface heterogeneity can influence the entire ABL. In fact, on one of these two days, relatively small-scale features of the surface temperature distribution can be seen in the ABL depth distribution. On the two small Obukhov length days multiresolution spectra and joint probability distributions, which are applied to the data collected from repeated low-level aircraft passes, both imply the existence of surface-heterogeneity-generated mesoscale circulations on scales of 10 km or more. Also on these two small Obukhov length days, the vertical profiles of dimensionless variances of velocity, temperature, and moisture show large deviations from the similarity curves, which also imply the existence of mesoscale circulations.

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Youngjean Choi, Song-Lak Kang, Jinkyu Hong, Sue Grimmond, and Kenneth J. Davis
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Bradford W. Berger, Kenneth J. Davis, Chuixiang Yi, Peter S. Bakwin, and Cong Long Zhao

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Methodology for determining fluxes of CO2 and H2O vapor with the eddy-covariance method using data from instruments on a 447-m tower in the forest of northern Wisconsin is addressed. The primary goal of this study is the validation of the methods used to determine the net ecosystem exchange of CO2. Two-day least squares fits coupled with 30-day running averages limit calibration error of infrared gas analyzers for CO2 and H2O signals to ≈2%–3%. Sonic anemometers are aligned with local streamlines by fitting a sine function to tilt and wind direction averages, and fitting a third-order polynomial to the residual. Lag times are determined by selecting the peak in lagged covariance with an error of ≈1.5%–2% for CO2 and ≈1% for H2O vapor. Theory and a spectral fit method allow determination of the underestimation in CO2 flux (<5% daytime, <12% nighttime) and H2O vapor flux (<21%), which is due to spectral degradation induced by long air-sampling tubes. Scale analysis finds 0.5-h flux averaging periods are sufficient to measure all flux scales at 30-m height, but 1 h is necessary at higher levels, and random errors in the flux measurements due to limited sampling of atmospheric turbulence are fairly large (≈15%–20% for CO2 and ≈20%–40% for H2O vapor at lower levels for a 1-h period).

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Weiguo Wang, Kenneth J. Davis, Daniel M. Ricciuto, and Martha P. Butler

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An explicit footprint model for flux measurements of passive scalars in the lower part of the convective boundary layer (CBL) is introduced. A simple footprint model is derived analytically in an idealized CBL. The simple model can simulate the overall characteristics of the flux footprint. Then a method is proposed to adjust the analytical solutions to those from a Lagrangian stochastic model that considers more realistic atmospheric conditions in the vertical direction. The adjusted footprint model is a function of Monin–Obukhov length (L), roughness length, receptor height, and CBL depth (h). Comparison between the results from the adjusted footprint model and stochastic model suggests that the adjusted footprint model can well simulate the streamwise extent of the footprint within the dimensionless upwind distance X < 1, which accounts for a majority of the footprint. The model applies to stabilities of –L/h between 0.01 and 0.1 and roughness lengths between 10−5 and 2 × 10−3 h in the lower part of the mixed layer (from 0.1h to 0.6h).

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Andreas Giez, Gerhard Ehret, Ronald L. Schwiesow, Kenneth J. Davis, and Donald H. Lenschow

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For the first time, two lidar systems were used to measure the vertical water vapor flux in a convective boundary layer by means of eddy correlation. This was achieved by combining a water vapor differential absorption lidar and a heterodyne wind lidar in a ground-based experiment.

The results prove that the combined lidar system can determine vertical flux profiles with a height resolution of approximately 100 m. Vertical averaging over a greater height interval reduces the error sufficiently that the changes in flux occurring throughout the day as a result of solar heating can be resolved. Horizontal and, for the first time, vertical integral scales were calculated from the lidar signals. The error analysis based on these results indicates that instrumental white noise and sampling error are the main sources of the statistical error in the flux measurement. Since the lidars measure simultaneously at many levels throughout the boundary layer, these errors can be reduced by vertical averaging to less than 50% for a 40-min time series, depending on how much vertical resolution is required in the flux profile.

The combined lidar system was used to measure the height-resolved water vapor flux associated with boundary layer circulations induced by active fair-weather cumulus clouds. A cloud-modulated flux of up to 300 W m−2 was observed in the upper third of the boundary layer. The measurement also showed the breakdown of that flux during the transition from active to passive cumulus clouds.

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