William E. Eichinger

Seasonal estimates of riparian evapotranspiration using remote and in situ measurements

In many semi-arid basins during extended periods when surface snowmelt or storm runoff is absent, groundwater constitutes the primary water source for human habitation, agriculture and riparian ecosystems. Utilizing regional groundwater models in the management of these water resources requires accurate estimates of basin boundary conditions. A critical groundwater boundary condition that is closely coupled to atmospheric processes and is typically known with little certainty is seasonal riparian evapotranspiration (ET). This quantity can often be a significant factor in the basin water balance in semi-arid regions yet is very difficult to estimate over a large area. Better understanding and quantification of seasonal, large-area riparian ET is a primary objective of the Semi-Arid Land-Surface-Atmosphere (SALSA) Program. To address this objective, a series of interdisciplinary experimental campaigns were conducted in 1997 in the San Pedro Basin in southeastern Arizona. The riparian system in this basin is primarily made up of three vegetation communities: mesquite (Prosopis velutina), sacaton grasses (Sporobolus wrightii), and a cottonwood (Populus fremontii)/willow (Salix goodingii) forest gallery. Micrometeorological measurement techniques were used to estimate ET from the mesquite and grasses. These techniques could not be utilized to estimate fluxes from the cottonwood/willow (C/W) forest gallery due to the height (20‐30 m) and non-uniform linear nature of the forest gallery. Short-term (2‐4 days) sap flux measurements were made to estimate canopy transpiration over several periods of the riparian growing season. Simultaneous remote sensing measurements were used to spatially extrapolate tree and stand measurements. Scaled C/W stand level sap flux estimates were utilized to calibrate a Penman‐Monteith model to enable temporal extrapolation between synoptic measurement periods. With this model and set of measurements, seasonal riparian vegetation water use estimates for the riparian corridor were obtained. To validate these models, a 90-day pre-monsoon water balance over a 10 km section of the river was carried out. All components of the water balance, including riparian ET, were

A Priori Field Study of the Subgrid-Scale Heat Fluxes and Dissipation in the Atmospheric Surface Layer

Field measurements are carried out to study statistical properties of the subgrid-scale (SGS) heat fluxes and SGS dissipation of temperature variance in the atmospheric surface layer, and to evaluate the ability of several SGS models to reproduce these properties. The models considered are the traditional eddy-diffusion model, the nonlinear (gradient) model, and a mixed model that is a linear combination of the other two. High-resolution wind velocity and temperature fields are obtained from arrays of 3D sonic anemometers placed in the surface layer. The basic setup consists of two horizontal parallel arrays (seven sensors in the lower array and five sensors in the upper array) at different heights (2.4 and 2.9 m, respectively). Data from this setup are used to compute the SGS heat flux and dissipation of temperature variance by means of 2D filtering in horizontal planes, invoking Taylor’s hypothesis. Model coefficients are measured from the data by requiring the real and modeled timeaveraged dissipation rates to match. Various other experimental setups that differ mainly in the separation between the sensors are utilized to show that filter size has a considerable effect on the various model coefficients near the ground. For the basic setup, conditional averaging is used to study the relation between large-scale coherent structures (sweeps and ejections) and the SGS quantities. It is found that under unstable conditions, negative SGS dissipation, indicative of backscatter of temperature variance from the subgrid scales to the resolved field, is most important during the onset of ejections transporting relatively warm air upward. Large positive SGS dissipation of temperature variance is associated with the end of ejections (and/or the onset of sweeps) characterized by strong drops in temperature and vertical velocity under unstable conditions. These results are also supported by conditionally sampled 2D (streamwise and vertical) velocity and temperature distributions, obtained using an additional setup consisting of the 12 anemometers placed in a vertical array. The nonlinear and mixed model reproduce the observations better than the eddy-diffusion model.

Regional scale evaporation and the atmospheric boundary layer

Note: 33(1): 99-124 Reference EFLUM-ARTICLE-1995-006doi:10.1029/94RG03112 Record created on 2005-09-08, modified on 2017-02-23

Tower and Aircraft Eddy Covariance Measurements of Water Vapor, Energy, and Carbon Dioxide Fluxes during SMACEX

Abstract A network of eddy covariance (EC) and micrometeorological flux (METFLUX) stations over corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] canopies was established as part of the Soil Moisture–Atmosphere Coupling Experiment (SMACEX) in central Iowa during the summer of 2002 to measure fluxes of heat, water vapor, and carbon dioxide (CO2) during the growing season. Additionally, EC measurements of water vapor and CO2 fluxes from an aircraft platform complemented the tower-based measurements. Sensible heat, water vapor, and CO2 fluxes showed the greatest spatial and temporal variability during the early crop growth stage. Differences in all of the energy balance components were detectable between corn and soybean as well as within similar crops throughout the study period. Tower network–averaged fluxes of sensible heat, water vapor, and CO2 were observed to be in good agreement with area-averaged aircraft flux measurements.

Preface paper to the Semi-Arid Land-Surface-Atmosphere (SALSA) Program special issue.

The Semi-Arid Land-Surface-Atmosphere Program (SALSA) is a multi-agency, multi-national research effort that seeks to evaluate the consequences of natural and human-induced environmental change in semi-arid regions. The ultimate goal of SALSA is to advance scientific understanding of the semi-arid portion of the hydrosphere-biosphere interface in order to provide reliable information for environmental decision making. SALSA approaches this goal through a program of long-term, integrated observations, process research, modeling, assessment, and information management that is sustained by cooperation among scientists and information users. In this preface to the SALSA special issue, general program background information and the critical nature of semi-arid regions is presented. A brief description of the Upper San Pedro River Basin, the initial location for focused SALSA research follows. Several overarching research objectives under which much of the interdisciplinary research contained in the special issue was undertaken are discussed. Principal methods, primary research sites and data collection used by numerous investigators during 1997-1999 are then presented. Scientists from about 20 US, five European (four French and one Dutch), and three Mexican agencies and institutions have collaborated closely to make the research leading to this special issue a reality. The SALSA Program has served as a model of interagency cooperation by breaking new ground in the approach to large scale interdisciplinary science with relatively limited resources.

An Experimental Study of Small-Scale Variability of Radar Reflectivity Using Disdrometer Observations

Abstract Analysis of data collected by four disdrometers deployed in a 1-km2 area is presented with the intent of quantifying the spatial variability of radar reflectivity at small spatial scales. Spatial variability of radar reflectivity within the radar beam is a key source of error in radar-rainfall estimation because of the assumption that drops are uniformly distributed within the radar-sensing volume. Common experience tells one that, in fact, drops are not uniformly distributed, and, although some work has been done to examine the small-scale spatial variability of rain rates, little experimental work has been done to explore the variability of radar reflectivity. The four disdrometers used for this study include a two-dimensional video disdrometer, an X-band radar-based disdrometer, an impact-type disdrometer, and an optical spectropluviometer. Although instrumental differences were expected, the magnitude of these differences clouds the natural variability of interest. An algorithm is applied to ...

Analysis of Ducted Motions in the Stable Nocturnal Boundary Layer during CASES-99

Abstract Data obtained with multiple instruments at the main site of the 1999 Cooperative Atmosphere–Surface Exchange Study (CASES-99) are employed to examine the character and variability of wave motions occurring in the stable nocturnal boundary layer during the night of 14 October 1999. The predominant motions are surprisingly similar in character throughout the night, exhibiting largely westward propagation, horizontal wavelengths of ∼1 to 10 km, phase speeds slightly greater than the mean wind in the direction of propagation, and highly coherent vertical motions with no apparent phase progression with altitude. Additionally, vertical and horizontal velocities are in approximate quadrature and the largest amplitudes occur at elevated altitudes of maximum stratification. These motions are interpreted as ducted gravity waves that propagate along maxima of stratification and mean wind and that are evanescent above, and occasionally below, the altitudes at which they are ducted. Modal structures for ducte...

Structure of the atmosphere in an urban planetary boundary layer from lidar and radiosonde observations

The planetary boundary layer (PBL) over Mexico City was probed with a scanning backscatter lidar to characterize and evaluate the multidimensional structure of the atmosphere. Comparisons were made between radiosonde and lidar-derived PBL heights which showed the two techniques to be in close agreement. The spatial properties of the free atmosphere-PBL interface were found to be approximately the same size as the entrainment zone thickness. Below the interface the lidar observed spatially resolved structures, such as thermal plumes, convective eddies, low-level jets, and entrainment into the PBL. These structures were spatially correlated with the local diabatic condition and wind stress. One highly unstable atmosphere contains a lidar-visualized convective structure rising to a height of 0.45 the inversion base, which was predicted from earlier turbulence models. Other features, such as low-level jets, were found to be associated with neutral atmospheres in the mixing layer. The analysis indicates that the transport of pollutants is not a continuous and gradient-driven process, but low frequency and spatially discontinuous. The high spatial and temporal resolution afforded by the scanning lidar depicts surface-atmosphere interactions which are neither spatially homogeneous nor horizontally uniform.

Subgrid-Scale Dissipation in the Atmospheric Surface Layer: Effects of Stability and Filter Dimension

Field measurements are undertaken with the specific purpose of addressing open issues in subgrid-scale (SGS) modeling of turbulence for large eddy simulation. Wind velocity and temperature signals are obtained using a horizontal linear array of six three-dimensional sonic anemometers placed at a height of 2.15 m in the surface layer over a grass field. From these data, the SGS heat flux and a two-dimensional surrogate of the SGS dissipation of temperature variance (x) are computed by means of two-dimensional horizontal filtering and by invoking Taylor’s hypothesis. Conditional averaging is used to isolate the effects of large-scale structures (sweeps and ejections) of the flow on the SGS dissipation under different stability conditions. During flow events associated with strong increments of vertical velocity (possibly associated with the onset of ejection events), negative values of x, indicative of transfer of temperature variance from the small scales to the resolved field (backscatter), have an important relative contribution regardless of atmospheric stability. Strong drops in the vertical velocity (possibly associated with the onset of sweeps) are accompanied by large positive values of the SGS dissipation. The two-dimensional SGS dissipation is compared with a one-dimensional surrogate based on a single sensor used in earlier work. The one- and two-dimensional results show qualitatively the same trends. Quantitative differences underscore the advantages of a two-dimensional approach based on the sensor array used in this work.

The average dissipation rate of turbulent kinetic energy in the neutral and unstable atmospheric surface layer

The mean rate of dissipation of turbulent kinetic energy is related to the surface fluxes of momentum and heat through the turbulent kinetic energy budget equation. This relationship may be used to estimate surface fluxes from measurements of the dissipation rates. The success of recent applications of the approach has been limited by uncertainties surrounding the functional relationship between the dimensionless dissipation rates and the atmospheric stability parameter. A pair of field experiments was designed and carried out in the atmospheric surface layer to identify this functional relationship over a broad range of neutral and convective flows, covering greater than 3 orders of magnitude in the stability parameter. Mean dissipation rates were computed using Fourier power spectra, second-order structure functions, and third-order structure functions. Arguments are presented for the superiority of the third-order approach. A three-sublayer conceptual model is invoked to guide the dimensional analysis, and the resulting dissipation rates are shown to scale uniquely in the three sublayers. Near the wall, in the dynamic sublayer, dissipation is significantly less than production, as energy is transported up to the more convective regions, where an equality between dissipation and production is achieved.