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W. Gao and M. L. Wesely

Abstract

Turbulent fluxes of chemically reactive trace gases in the neutral atmospheric boundary layer (ABL) were simulated with a one-dimensional, coupled diffusion-chemistry model. The effects of rapid chemical reactions were included with a suite of second-order turbulence equations in which additional chemical terms were used to describe contributions to flux by rapid chemical production and loss. A total of 69 chemical reactions were incorporated to describe basic atmospheric photochemistry coupled with chemistry for isoprene and its oxidation products. Daytime flux Profiles of O3, NO, N02, OH, isoprene, and other depositing gases were simulated with assumed rates of NO emission from soil, isoprene emission rates appropriate for a deciduous forest, and initial concentrations of various chemical species typical of a remote area. Results show that chemical reactions can influence vertical fluxes by producing sources or sinks in the atmosphere and by changing mean concentrations. Magnitudes of NO and NO2 fluxes decrease with height at a much greater rate than predicted by a nonreactive model. The NO emitted from soil can quickly be converted to N02, and the upward NO flux can decrease by as much as 80% at a height of 100 m. The magnitude of NO2 flux decreases sharply with height because of the NO-to-NO2 conversion, but NO2 deposition near the surface tends to be enhanced by an increase in NO2 concentration near the surface NO emission source. The Profile of 03 flux simulated with forced entrainment at the top of the ABL closely matches the profile derived from a field experiment, and the flux throughout the ABL increases slightly because mean O3 concentrations are increased by chemical production associated with isoprene emissions. Simulated profiles of isoprene flux closely agree with results of a nonreactive model and appear to be controlled primarily by surface emission and vertical turbulent mixing. Chemical reactions appear to have a substantial effect on vertical concentration gradients, diffusivities, and deposition velocities for NO2, NO3, and N2O5. The reactions have a negligible effect on the deposition velocities for O3, HCHO, CH3OOH, HNO2, H2O2, and HNO3.

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R. L. Coulter and M. L. Wesely

Abstract

Measurements of acoustic backscatter in the lower planetary boundary layer and optical line-of-sight scintillation in the surface layer are each used to compute sensible heat fluxes in the unstable surface layer. Comparisons with simultaneous low-level point measurements by eddy correlation show good agreement, indicating that remote-sensing methods can be successful over less homogeneous terrain where conventional surface layer measurement techniques are less accurate. Corrections to take into account the effects of humidity fluctuations are found necessary in order to achieve accuracies within 10%. Free convection is assumed to permit interpretation of the sodar data, while either forced or free convection is assumed for the scintillation data. A systematic overestimate of heat fluxes is found from sodar measurements made during the morning, when the height of the convectively mixed layer is increasing rapidly.

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I. Y. Lee and M. L. Wesely

Abstract

Simulations have been carried out with a numerical model describing air chemistry, aerosol microphysics, and turbulent mixing, in order to study the behavior of fine sulfate particles in the atmospheric surface layer over wet surfaces. Achievement of local equilibrium of sulfuric acid vapor between the gas phase and particles is rapid and can overpower turbulent mixing in controlling local particle size distributions. Numerical results clearly indicate that in regions of relative humidifies lower than about 80% the large submicron particles increase in number at the expense of concentrations of smaller particles. Simulations that incorporate turbulent mixing and surface dry deposition above wet surfaces show rapid change of vertical flux with height and tend to product downward fluxes for particles larger than 0.1 μm in radius and upward fluxes for particles smaller than about 0.05 μm in radius. This tendency has been seen in fluxes measured by eddy correlation at heights of several meters above surfaces of varying wetness. However, the model has not reproduced certain other observations, such as strong upward fluxes of the larger particles above the sea and persistence of upward fluxes of the smaller particles above coniferous forests for several hours after surface wetness disappears. In addition, the numerical indications that fluxes of the smaller particles tend to change direction within a meter of a wet surface have yet to be supported by field experiments.

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G. W. Thurtell, C. B. Tanner, and M. L. Wesely

Abstract

A rugged and stable pressure-sphere anemometer system is described which Provides an accurate measurement of wind velocity and direction within a meter of the ground. The horizontal wind velocity, (u 2 + v 2)½, agreed very closely with cup anemometer measurements, indicating good accuracy in the measurement of the dominant term u. Eddy correlation measurements of shear stress with the pressure sphere agreed very well with Davis shear-stress meter measurements and satisfactory agreement was found with data obtained from wind velocity profiles and from wind measurements using a drag coefficient. Ratios of σw/u * during neutral periods were found to be in excellent agreement with values derived by Panofsky and Lettau, providing further indication of the accuracy obtainable with the pressure-sphere system.

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M. L. Wesely, G. W. Thurtell, and C. B. Tanner

Abstract

A three-dimensional pressure-sphere anemometer and fast thermometer system (P.S.A.T.) was used to measure vertical heat flux density in the atmospheric surface layer at 1–4 m above alta fescue and snap beans. Good agreement with independent measurements was obtained, which shows the the P.S.A.T. is sufficiently small and has adequate high-frequency response and accuracy for eddy correlation measurements within 1 m of the surface. Also obtained with the P.S.A.T. were (uT)/(wT), r u,T, r w,T and σT/T*, and their dependence upon stability. When the atmosphere was thermally stable, slow wave motions frequently increased σT even though turbulent mixing was lacking.

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W. Gao, R. L. Coulter, B. M. Lesht, J. Qiu, and M. L. Wesely

Abstract

The authors compared methods for estimating surface fluxes under clear-sky conditions over a large heterogeneous area from a limited number of ground measurements and from satellite observations using data obtained from the southern Great Plains Cloud and Radiation Testbed (CART) site, an area of approximately 350 km × 400 km located in Kansas and Oklahoma. In situ measurements from 10 energy balance Bowen ratio (EBBR) stations showed large spatial variations in latent and sensible heat fluxes across the site because of differences in vegetation and soil conditions. This variation was reproduced by a model for parameterization of subgrid- scale (PASS) surface fluxes that was developed previously and extended in the present study to include a distribution of soil moisture inferred from combined visible and thermal infrared remote sensing data. In the framework of the PASS model, the satellite-derived normalized difference vegetation index and surface temperature were used to derive essential surface parameters including surface albedo, surface conductance, soil heat flux ratio, surface roughness length, and soil moisture, which were then used to calculate a surface energy budget at satellite-pixel scales with pixel-specific surface meteorological conditions appropriately distributed from their mean values using a distribution algorithm. Although the derived soil moisture may be influenced by various uncertainty factors involved in the satellite data and the model, spatial variations of soil moisture derived from the multichannel data from the Advanced Very High Resolution Radiometers on the NOAA-14 satellite appeared to have some correlation (the correlation coefficient is as large as 0.6) with the amount of accumulated previous rainfall measured at the 58 Oklahoma Mesonet stations located within the CART area. Surface net radiation, soil heat flux, and latent and sensible heat fluxes calculated at a spatial resolution of 1 km (the size of a satellite pixel) were evaluated directly by comparing with flux measurements from the EBBR stations and indirectly by comparing air temperature and humidity inferred from calculated sensible and latent heat fluxes with corresponding values measured at 1.5 m above the 58 meteorological stations. In calculating regional fluxes, biases caused by the sampling uncertainty associated with point measurements may be corrected by application of the satellite data.

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J. Song, M. L. Wesely, M. A. LeMone, and R. L. Grossman

Abstract

The second part of the parameterization of subgrid-scale surface fluxes model (PASS2) has been developed to estimate long-term evapotranspiration rates over extended areas at a high spatial resolution by using satellite remote sensing data and limited, but continuous, surface meteorological measurements. Other required inputs include data on initial root-zone available moisture (RAM) content computed by PASS1 for each pixel at the time of clear-sky satellite overpasses, normalized difference vegetation index (NDVI) from the overpasses, and databases on available water capacity and land-use classes. Site-specific PASS2 parameterizations evaluate surface albedo, roughness length, and ground heat flux for each pixel, and special functions distribute areally representative observations of wind speed, temperature, and water vapor pressure to individual pixels. The surface temperature for each pixel and each time increment is computed with an approximation involving the surface energy budget, and the evapotranspiration rates are computed via a bulk aerodynamic formulation. Results from PASS2 were compared with observations made during the 1997 Cooperative Atmosphere–Surface Exchange Study field campaign in Kansas. The modeled diurnal variation of RAM content, latent heat flux, and daily evapotranspiration rate were realistic in comparison to measurements at eight surface sites. With the limited resolution of the NDVI data, however, model results deviated from the observations at locations where the measurement sites were in fields with surface vegetative conditions notably different than surrounding fields. Comparisons with aircraft-based flux measurements suggested that the evapotranspiration rates over distances of tens of kilometers were modeled without significant bias.

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J. Song, M. L. Wesely, R. L. Coulter, and E. A. Brandes

Abstract

A model framework for parameterized subgrid-scale surface fluxes (PASS) has been modified and applied as PASS1 to use satellite data, models, and limited surface observations to infer root-zone available moisture (RAM) content with high spatial resolution over large terrestrial areas. Data collected during the 1997 Cooperative Atmosphere–Surface Exchange Study field campaign at the Atmospheric Boundary Layer Experiments site in the Walnut River watershed in Kansas were used to evaluate applications of the PASS1 approach to infer soil moisture content at times of satellite overpasses during cloudless conditions. Data from Advanced Very High Resolution Radiometers on the NOAA-14 satellite were collected and then adjusted for atmospheric effects by using LOWTRAN7 and local atmospheric profile data from radiosondes. The input variables for PASS1 consisted of normalized difference vegetation index and surface radiant temperature, together with representative observations of downwelling solar irradiance, air temperature, relative humidity, and wind speed. Surface parameters, including roughness length, albedo, surface conductance for water vapor, and the ratio of soil heat flux to net radiation, were estimated with parameterizations suitable for the area using satellite data and land-use information;pixel-specific near-surface meteorological conditions such as air temperature, vapor pressure, and wind speed were adjusted according to local surface forcing; and RAM content was estimated using surface energy balance and aerodynamic methods. Comparisons with radar cumulative precipitation observations and in situ soil moisture estimates indicated that the spatial and temporal variations of RAM at the times of satellite overpasses were simulated reasonably well by PASS1.

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J. C. Doran, M. L. Wesely, R. T. McMillen, and W. D. Neff

Abstract

Measurements of heat and momentum fluxes along the valley floor of Brush Creek in Colorado are described. The measurements were taken in the fall of 1984 as part of the Department of Energy's Atmospheric Studies in Complex Terrain field program. The sensible heat flux to ground decreased from approximately 40–60 W m−2 prior to midnight to about 10–25 W m−2 in the morning hours. Surface friction velocities u * ranged from approximately 20–15 cm s−1 during the corresponding time periods. Considerable site-to-site variability in flux values was found, and disturbances of the upwind flow appear to be a significant contributing cause.

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J. Song, M. L. Wesely, D. J. Holdridge, D. R. Cook, and J. Klazura

Abstract

Estimates of the hydrological budget in the Walnut River Watershed (WRW; ∼5000 km2) of southern Kansas were made with a parameterized subgrid-scale surface (PASS) model for the period 1996–2002. With its subgrid-scale distribution scheme, the PASS model couples surface meteorological observations with satellite remote sensing data to update root-zone available moisture and to simulate surface evapotranspiration rates at high resolution over extended areas. The PASS model is observationally driven, making use of extensive parameterizations of surface properties and processes. Heterogeneities in surface conditions are spatially resolved to an extent determined primarily by the satellite data pixel size. The purpose of modeling the spatial and interannual variability of water budget components at the regional scale is to evaluate the PASS model's ability to bridge a large grid cell of a climate model with its subgrid-scale variation. Modeled results indicate that annual total evapotranspiration at the WRW is about 66%–88% of annual precipitation—reasonable values for southeastern Kansas—and that it varies spatially and temporally. Seasonal distribution of precipitation plays an important role in evapotranspiration estimates. Comparison of modeled runoff with stream gauge measurements demonstrated close agreement and verified the accuracy of modeled evapotranspiration at the regional scale. In situ measurements of energy fluxes compare favorably with the modeled values for corresponding grid cells, and measured surface soil moisture corresponds with modeled root-zone available moisture in terms of temporal variability despite very heterogeneous surface conditions. With its ability to couple remote sensing data with surface meteorology data and its computational efficiency, PASS is easily used for modeling surface hydrological components over an extended region and in real time. Thus, it can fill a gap in evaluations of climate model output using limited field observations.

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