Russell J. Qualls

Effect of Vegetation Density on the Parameterization of Scalar Roughness to Estimate Spatially Distributed Sensible Heat Fluxes

The First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) was initiated in part to explore methods to improve our ability to estimate spatially distributed heat fluxes over hilly prairie. In this research, spatially distributed, remotely sensed surface temperatures and surface-based measurements of leaf area index, wind speed, and air temperature were used in a Monin-Obukhov type similarity formulation to calculate sensible heat fluxes (Hc). Use of Monin-Obukhov type similarity required that a parameterization for scalar roughness of sensible heat be developed. Several methods were examined including a linearized version, with respect to leaf area index, of an earlier theoretical expression of scalar roughness for grass-like vegetation. Comparison between measured and parameterized scalar roughness values gave a correlation of r=0.828. This parameterization was used with data measured on July 11, 1987, under conditions of strong solar heating, high winds, and fairly uniform soil moisture, to calculate Hc values at an array of 10 surface flux stations. A spatial comparison between Hc and measured sensible heat fluxes yielded good agreement with a correlation coefficient of 0.878 and a root-mean-square error of 31.1 Wm−2.

Influence of components of the advection-aridity approach on evapotranspiration estimation

Abstract Several methods have been proposed to estimate areal evapotranspiration from common meteorological data. The advection-aridity approach is one such method which requires no site-calibrated parameters, and uses measurements at a single elevation which are commonly available from a meteorological station. The method is based on a complementary relationship between actual and potential evapotranspiration which postulates that a decrease in actual evapotranspiration will result in a complementary or symmetrical increase in potential evapotranspiration for a given energy input. In this paper, the advection-aridity equations were rewritten to isolate terms which account for available energy, advection or drying power of the air, and sensible heat flux. The advection-aridity method was tested on a daily basis with data for 43 days collected from a flat, semi-arid grassland over a 4 month period. The influence of each of the terms was examined to determine its relative influence in generating the complementary or symmetrical structure between actual and potential evapotranspiration. Advection and sensible heat flux were found to be very significant, whereas available energy was not. A comparison between reference values of evapotranspiration E en obtained from the energy budget with measurements of net radiation, ground heat flux, and sensible heat fluxes obtained by the eddy correlation method, and evapotranspiration estimates from the advection-aridity method, E aa , produced a small root mean square error of 13.1 W m −2 . However, significant bias was present in that E aa overestimated E en for large values of E en , but underestimated E en for small values. Measurement error and external energy sources not accounted for in the complementary relationship or the advection-aridity approach are discussed as potential causes of this bias.

A Cooperative Atmosphere–Surface Exchange Study (CASES) Dataset for Analyzing and Parameterizing the Effects of Land Surface Heterogeneity on Area-Averaged Surface Heat Fluxes

Abstract A multiscale dataset that includes atmospheric, surface, and subsurface observations obtained from an observation network covering a region that has a scale order comparable to mesoscale and general circulation models is described and analyzed. The dataset is half-hourly time series of forcing and flux response data developed from the one-month Cooperative Atmosphere–Surface Exchange Study (CASES-97) experiment, located in the Walnut Watershed near Wichita, Kansas. The horizontal complexity of this dataset was analyzed by looking at the sensible and latent heat flux response of station data from the three main land surface types of winter wheat, grass/pastureland, and bare soil/sparse vegetation. The variability in the heat flux response at and among the different sites points to the need for a spatially distributed, time-varying atmospheric-forcing dataset for use in land surface modeling experiments. Such a dataset at horizontal spacings of 1, 5, and 10 km was developed from the station data an...

Directional radiometric temperature profiles within a grass canopy

Abstract The scalar roughness for sensible heat flux, z 0h , which appears in Monin and Obukhov similarity theory has been shown to exhibit substantial variability. Brutsaert and Sugita (cf. W. Brutsaert and M. Sugita, J. Atmos. Sci. 53 (1996) 209–16) derived an expression for z 0h to account for variability in z 0h as a result anisothermal vegetation skin temperature, which assumes an exponential profile of canopy skin temperature. We examined this assumption through directional radiometric measurements of temperature profiles in a relatively uniform grass canopy. The temperature gradient steepened monotonely throughout the morning and was modeled successfully with an exponential profile. The root-mean-square error (RMSE) between measured and modeled profiles was small and ranged between 0.038°C and 0.530°C. One of the most significant findings of this research is that the decay coefficient, b , of the exponential profile model was approximately constant with time for this canopy. This may provide a useful simplification in applying Brutsaert and Sugitas (loc. cit.) parameterization for z 0h .

A calibration‐free formulation of the complementary relationship of evaporation for continental‐scale hydrology

An important scaling consideration is introduced into the formulation of the complementary relationship (CR) of land surface evapotranspiration (ET) by specifying the maximum possible evaporation rate (Epmax) of a small water body (or wet patch) as a result of adiabatic drying from the prevailing near-neutral atmospheric conditions. In dimensionless form the CR therefore becomes yB = f( Epmax−EpEpmax−EwxB) = f(X) = 2X2 − X3, where yB = ET/Ep, xB = Ew/Ep. Ew is the wet-environment evaporation rate as given by the Priestley-Taylor equation, Ep is the evaporation rate of the same small wet surface for which Epmax is specified and estimated by the Penman equation. With the help of North American Regional Reanalysis data, the CR this way yields better continental-scale performance than earlier, calibrated versions of it and is on par with current land surface model results, the latter requiring vegetation, soil information and soil moisture bookkeeping. Validation has been performed by Parameter-Elevation Regressions on Independent Slopes Model precipitation and United States Geological Survey runoff data. A novel approach is also introduced to calculate the value of the Priestley-Taylor parameter to be used with continental-scale data, making the new formulation of the CR completely calibration free.

Rescaling the complementary relationship for land surface evaporation

Recent research into the complementary relationship (CR) between actual and apparent potential evaporation has resulted in numerous alternative forms for the CR. Inspired by Brutsaert [2015], who derived a general CR in the form y=function(x), where x is the ratio of potential evaporation to apparent potential evaporation and y is the ratio of actual to apparent potential evaporation, an equation is proposed to calculate the value of x at which y goes to zero, denoted xmin. The value of xmin varies even at an individual observation site, but can be calculated using only the data required for the Penman (1948) equation as expressed here, so no calibration of xmin is required. It is shown that the scatter in x-y plots using experimental data is reduced when x is replaced by X=(x-xmin)/(1-xmin). This rescaling results in data falling along the line y=X, which is proposed as a new version of the CR. While a reinterpretation of the fundamental boundary conditions proposed by Brutsaert [2015] is required, the physical constraints behind them are still met. An alternative formulation relating y to X is also discussed. This article is protected by copyright. All rights reserved.

Evaluation of Spatially Distributed Ground-Based and Remotely Sensed Data to Estimate Spatially Distributed Sensible Heat Fluxes

The First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) was initiated partly to improve our ability to model spatial distributions of surface-atmospheric fluxes over hilly prairie. Monin-Obukhov similarity was used to calculate sensible heat fluxes (Hc) at an array of ten FIFE flux measurement sites for comparison with measured sensible heat fluxes (Hm). Data were collected within the dynamic sublayer on a clear morning when there was strong solar heating of the surface, winds in excess of 5 m s−1, and uniformly wet soil conditions. The sensitivity of correlations between Hc and Hm to spatial variability of air (Ta) and aircraft-based (thermal infrared multispectral scanner (TIMS)) remotely sensed surface (Tr) temperatures, wind speed )u), and an atmospheric stability parameterization (ψh) was examined. Hc was found to depend on the spatial variability of Tr and u but not on Ta and ψh. Furthermore, approximately half the discrepancy between Hc and Hm may be attributed to uncertainty in Hm.

Modeling of long‐wave and net radiation energy distribution within a homogeneous plant canopy via multiple scattering processes

Received 13 September 2005; revised 20 April 2006; accepted 28 April 2006; published 30 August 2006. [1] This paper presents a newly developed multiple-layer, long-wave radiation scattering model for use in homogeneous vegetation canopies. The model is able to simulate the radiation distribution within and the outgoing radiation above the canopy. This model differs from the shortwave model developed earlier by the authors owing to the complexity introduced by the fact that leaves within and soil below the canopy emit long-wave radiation in accordance with their surface temperature. Combined, the short- and long-wave models are able to simulate net radiation distribution above and within the canopy sublayers and at the soil surface. The model represents multiple scatterings of radiation reflected, transmitted, and emitted from leaf surfaces and penetrating through gaps as infinite series equations, which are reduced analytically to simple forms. In stand-alone mode this model has the limitation that it requires canopy temperature profile data as input in order to simulate outgoing long-wave radiation and net radiation. However, once we or other users couple this model to a turbulent transfer model, canopy temperature profiles will be produced as model output, making this a useful tool for remote sensing data assimilation. The model was tested against measurements collected in a wheat field in 2002. Satisfactory agreement was obtained between the modeled outgoing long-wave radiation above a wheat canopy and observed long-wave radiation measured with an Eppley precision infrared radiometer (PIR), both for daily total values and diurnal variation of 20-min averages. The root-mean-square error (RMSE) of daily total values of outgoing long-wave radiation, with respect to measurements, was only 1.1% of the mean of the measurements. Comparison of the modeled net radiation with two independent measurements produced RMSE values equal to 3.7% of the mean measured daily total net radiation.

Modeling of the short wave radiation distribution in an agroforestry system

In an agroforestry system, short wave radiation distribution under the canopy of the trees is important to the activity of the annual crops growing beneath. A three-dimensional model is proposed to describe the short wave radiation distribution under the tree canopy in an agroforestry system. In the model, the agroforestry system is assumed to be planted in regular arrays and have spherical crowns. The short wave radiation that reaches a prescribed point on the ground is computed as the integration of radiation from all the differential directions of the whole hemisphere. The canopy depth in each differential direction is computed, and the extinction coefficient of the tree canopy is estimated by computer analysis of photograph images of the tree crowns taken by a conventional camera. A major advantage of this method is that the laborious leaf area index (LAI) and leaf inclination measurement can be eliminated. The model is able to predict both the short wave radiation distributions below the canopy of an agroforestry system at a prescribed time and the diurnal variation of the total hemispherical short wave radiation at a prescribed point below the canopy. Comparison of the modeled results with the measured values showed that the proposed model describes the daily patterns of the short wave radiation under the tree canopy quite well for both discontinuous canopy and overlapping canopy and for different shading conditions. The difference between the modeled daily total values of the short wave radiation under the canopy and the measured daily total values is usually less than 5% of the global radiation. Results from sensitivity analyses of the model to crown radius and canopy gap fraction are reported.

Use of land surface temperature to estimate surface energy fluxes: Contributions of Wilfried Brutsaert and collaborators

Land surface temperature (LST) plays a key role in governing the land surface energy budget, and measurements or estimates of LST are an integral part of many land surface models and methods to estimate land surface sensible heat (H) and latent heat fluxes. In particular, the LST “anchors” the potential temperature profile in Monin-Obukhov similarity theory, from which H can be derived. Brutsaert has made important contributions to our understanding the nature of surface temperature measurements as well as the practical but theoretically sound use of LST in this framework. His work has coincided with the wide-spread availability of remotely sensed LST measurements. Use of remotely sensed LST estimates inevitably involves complicating factors, such as: varying spatial and temporal scales in measurements, theory, and models; spatial variability of LST and H; the relationship between measurements of LST and the temperature “felt” by the atmosphere; and the need to correct satellite-based radiometric LST measurements for the radiative effects of the atmosphere. This paper reviews the progress made in research in these areas by tracing and commenting on Brutsaerts contributions.