John P. Schieldge

THE EJECTION-SWEEP CHARACTER OF SCALAR FLUXES IN THE UNSTABLE SURFACE LAYER

In the atmospheric surface layer, it is widely accepted that ejection andsweep eddy motions, typically associated with coherent structures, areresponsible for much of the land-surface evaporation, sensible heat, andmomentum fluxes. The present study analyzes the ejection-sweep propertiesusing velocity and scalar fluctuation measurements over tall natural grassand bare soil surfaces. It is shown that momentum ejections and sweeps occurat equal frequencies (D eject ≈ D sweep ≈ 0.29) irrespective of surfaceroughness length or atmospheric stability conditions. Also, their magnitudesare comparable to values reported from open channel velocity measurements (Dsweep ≈ 0.33; D eject ≈: 0.30). The scalar D eject is constant andsimilar in magnitude to the momentum D eject( ≈ 0.29) over both surfacesand for a wide range of atmospheric stability conditions, in contrast to thescalar D sweep. The scalar sweep frequency is shown to depend on the scalarskewness for the dynamic convective and free convective sublayers, but isidentical to D eject for the dynamic sublayer. The threshold scalar skewnessat which the D sweep dependence occurs is 0.25, in agreement with theaccepted temperature skewness value at near-neutral conditions. In contrastto a previous surface-layer experiment, this investigation demonstrates thatthe third-order cumulant expansion method (CEM) reproduces the measuredrelative flux contribution of ejections and sweeps (ΔS0) for momentumand scalars at both sites. Furthermore, a linkage between ΔS0 and thescalar variance budget is derived via the third-order CEM in analogy tomomentum. It is shown that ΔS0 can be related to the flux divergenceterm and that such a relationship can be estimated from surface-layersimilarity theory, and the three sublayer model of Kader and Yaglom andproposed similarity functions.

Skin temperature perturbations induced by surface layer turbulence above a grass surface

High-frequency (5 Hz) atmospheric surface layer (ASL) turbulent velocity (u′) and infrared skin temperature perturbations (T′s) were measured above a grass-covered forest clearing and analyzed for cloud free conditions. These measurements were used to investigate mechanisms responsible for the production of large short-lived T′s perturbations caused by rapid excursions in u′. To quantify the effects of u′ on rapid surface cooling, wavelet spectra of u′ and T′s and cospectra of u′T′s were computed. The u′ wavelet power spectra were then analyzed using Townsends [1961, 1976] hypothesis. Townsends hypothesis states that ASL eddy motion can be decomposed into an active component, which is a function of the ground shear stress (u*) and height (z) above the zero plane displacement, and an inactive component, which is produced in the atmospheric boundary layer (ABL) outer region. A −1 power law in the u′ power spectrum was used as a signature for inactive eddy motion. Therefore the −1 power law was used to identify wavenumber ranges (about 1.5 decades) associated with inactive eddy motion. The measured T′s wavelet spectra and u′T′s cospectra identified with this wavenumber range demonstrate that much of the T′s energy and 〈u′T′s〉 are due to inactive eddy motion, where the angle brackets indicate time averaging. Hence, in contrast to the laboratory experiments of Owen and Thomson [1963], it is argued that skin temperature perturbations at the canopy-atmosphere interface of a grass-covered surface (small thermal inertia) are strongly dependent on the inactive eddy motion produced in the outer layer of the ABL.

Estimation of Momentum and Heat Fluxes Using Dissipation and Flux-Variance Methods in the Unstable Surface Layer

Dissipation and flux-variance methods, derived from the turbulent kinetic energy and temperature variance budget equations in conjunction with Monin-Obukov similarity theory, were used to estimate surface fluxes of momentum and sensible heat. To examine the performance of these two methods, direct eddy correlation measurements were carried out above a nonuniform grass-covered forest clearing in Durham, North Carolina. The dissipation method sensible heat flux predictions were in good agreement with eddy correlation measurements. Also, the flux-variance method reproduced the measured sensible heat flux well following an adjustment to the similarity constant. However, the momentum flux (or friction velocity) estimated by the dissipation and flux-variance methods were both inferior to those for sensible heat flux. The data from this experiment indicated that the above two methods are sensitive to the dimensionless wind shear (ϕm) and temperature standard deviation (ϕθ) functions. On the basis of dimensional analysis and the temperature variance budget equation a new dissipation approach for estimating sensible heat flux was derived. The similarity constant for this new approach was shown to be around 1.6 for uniform surfaces and from the data of this experiment.

Estimation of area-average sensible heat flux using a large-aperture scintillometer during the Semi-Arid Land-Surface-Atmosphere (SALSA) Experiment

The use of a large-aperture scintillometer to estimate sensible heat flux has been successfully tested by several investigators. Most of these investigations, however, have been confined to homogeneous or to sparse with single vegetation-type surfaces. The use of the scintillometer over surfaces made up of contrasting vegetation types is problematic because it requires estimates of effective roughness length and effective displacement height in order to derive area-average sensible heat from measurements of the refractive index. In this study an approach based on a combination of scintillometer measurements and an aggregation scheme has been used to derive area-average sensible heat flux over a transect spanning two adjacent and contrasting vegetation patches: grass and mesquite. The performance of this approach has been assessed using data collected during the 1997 Semi-Arid Land-Surface-Atmosphere field campaign. The results show that the combined approach performed remarkably well, and the correlation coefficient between measured and simulated area-average sensible heat flux was ∼0.95. This is of interest because this approach offers a reliable means for validating remotely sensed estimates of surface fluxes at comparable spatial scales.

Geologic mapping using thermal images

Abstract In the past, remote sensing from aircraft and satellite for geologic mapping concentrated on the visible and reflective infrared parts of the spectrum, because of the availability of Landsat and aircraft multispectral scanners operating in this spectral range. With the launch of the Heat Capacity Mapping Mission (HCMM) satellite, regional thermal image data also became available. We have examined the HCMM data for geologic information over two desert areas in southern California, the Trona area and the Pisgah area. Three techniques were used for displaying and combining thermal data and visible and near infrared, including color additive composites, principal components, and calculation of thermal inertia images. Use of the color additive composite image was simplest and allowed for simultaneous display of both thermal and reflectance properties. Thermal data were found to provide additional geologic information, unavailable from Landsat data or from aircraft visible and near-infrared data alone. The addition of these data relating to thermal properties allowed separation of rock types with differing thermal properties but with similar reflectance characteristics.

A numerical simulation of soil temperature and moisture variations for a bare field

We simulated the diurnal variations of soil temperature and moisture content for a bare agricultural field in the San Joaquin Valley in California. The simulation pertained to the first 72 hours of drying, from saturation, of a sandy, clay loam soil. The results were compared with measurements of soil temperature and moisture content made at the field. Calculated and measured values of soil temperature trends agreed in general, but model results of moisture trends did not replicate observed diurnal effects evident at depths 4 centimeters or more below the surface.

Sensitivity of thermal inertia calculations to variations in environmental factors

Abstract The sensitivity of thermal inertia (TI) calculations to errors in the measurement or parameterization of a number of environmental factors is considered here. The factors include effects of radiative transfer in the atmosphere, surface albedo and emissivity, variations in surface turbulent heat flux density, cloud cover, vegetative cover, and topography. The error analysis is based upon data from the Heat Capacity Mapping Mission (HCMM) satellite for July 1978 at three separate test sites in the deserts of the western United States. Results show that typical errors in atmospheric radiative transfer, cloud cover, and vegetative cover can individually cause root-mean-square (RMS) errors of about 10% (with atmospheric effects sometimes as large as 30–40%) in HCMM-derived thermal inertia images of 20,000-200,000 pixels.

Use Of Thermal-Inertia Properties For Material Identification

A knowledge of the thermal inertia of the Earths surface can be used in geologic mapping as a complement to surface reflectance data as provided by Landsat. Thermal inertia, a body property, cannot be determined directly but can be inferred from radiation temperature measurements made at various times in the diurnal heating cycle, combined with a model of the surface heating processes. We have developed such a model and applied it along with temperature measurements made in the field and from satellite to determine thermal properties of surface materials. An example from a test site in western Nevada is used to illustrate the utility of this technique.