H21H-0826
Preliminary Evaluation of Sensible Heat Flux Measurements From a Large Aperture Scintillometer Using Lysimetric Data
The path integrating capabilities of scintillometers over several kilometers make it a potential tool that can bridge the gap between primary point based observations (lysimeters, Bowen ratio, or eddy covariance) and the demand for large-scale spatially averaged surface fluxes. Further, the spatial scale of sensible heat flux data collected from a scintillometer is comparable to the spatial resolution of satellite images. Therefore, scintillometer data may be useful for validating evapotranspiration maps based on satellite data. Numerous studies have evaluated the measurement accuracy of scintillometers using eddy covariance systems; however, the latter has energy balance closure problems up to 30%. The main objective of this study is to test the Large Aperture Scintillometer (LAS) using lysimetric data. The LAS monitors the sensible heat flux (H) and water vapor flux (LE) is calculated as a residual of the surface energy balance equation by monitoring net radiation (Rn) and soil heat flux (G) (LE=Rn+G-H). A Large Aperture Scintillometer (LAS) was deployed across two lysimeter fields planted with grain sorghum under dryland management conditions. The orientation of LAS was selected to have the path of the LAS perpendicular to the predominant wind direction and to avoid direct sun light on the lenses. The refractive index of air was monitored during the 2007 cropping season at 15-min. intervals, synchronized with weather station and lysimeter measurements. In addition, a net radiometer and three soil heat flux plates were installed near both the receiver and transmitter of the scintillometer as well as on two large monolithic lysimeters. Predicted water vapor fluxes from the scintillometer-net radiometer-heat flux plate setup were compared with lysimeter data. Preliminary results (three months of data) indicate that the LAS is a promising tool for deriving water vapor fluxes. However, further evaluation is needed under a variety of crop/weather conditions to fully assess its capability to accurately estimate spatially distributed water vapor fluxes.
H21H-0827 INVITED
Scintillometry for Evaporation Measurements - Between Pragmatism and Perfectionism
During the last twenty years, scintillometers have been discovered as powerful tools for the measurement of turbulent heat exchanges in meteorology and related sciences, such as hydrology and agriculture. A rapidly growing number of scintillometer installations access energy balances and determine evaporation over large spatial scales. Scientists target at refining satellite remote sensing schemes and optimizing irrigation to save valuable water resources. Short-path and long-path scintillometers are available with their distinct features, capabilities and applications. The ability of scintillometers to measure path averaged turbulence at high temporal resolution resulted in data sets delivering striking evidence of the particular advantages of the scintillation method. The theory, however, is quite complex and there are still areas of uncertainty. More experimental and theoretical efforts are required or being done to improve our knowledge about turbulence spectra near the dissipation range, about optical scattering in strong turbulence, about similarity relations and about the effect of inhomogeneities and entrainment on scintillation measurements. Instrumental design is being improved with respect to the accuracy of the physical parameters involved, the implementation of new theoretical findings and methods, as well as the optimization of user interfaces to facilitate set-up and operation. Scintillometry fascinates and has gained wide acceptance. Today, even with a long list of questions still remaining open, an overwhelming amount of impressive measurement results satisfies every pragmatic and challenges every perfectionist.
H21H-0828 INVITED
Validating regional-scale surface energy balance models
One of the major challenges in developing reliable regional surface flux models is the relative paucity of scale- appropriate validation data. Direct comparisons between coarse-resolution model flux estimates and flux tower data can often be dominated by sub-pixel heterogeneity effects, making it difficult to assess the intrinsic model accuracy. Several possible approaches to validating regional energy balance models are presented, including using high-resolution remote sensing data to disaggregate to the observation scale, or alternatively using dense tower networks or aircraft-based flux sensors to improve the spatial coverage of the observations. Such techniques, however, typically have limited temporal coverage, often providing only snapshots of surface conditions. Scintillometry provides a unique means for acquiring time-continuous flux measurements at the scale of the regional model pixel, facilitating rigorous validation over heterogeneous landscapes. Paired short- and microwave scintillometers allow simultaneous measurement of area-averaged sensible and latent heating, and will be invaluable in working towards robust continental and ultimately global models of the surface energy balance.
H21H-0829
Estimation of Evapotranspiration Using Optical Scintillometry
The estimation of spatial and temporal distributions of evapotranspiration is a long standing challenge for hydrologic science. In this study we present our experiences with the new technology of scintillometry for estimation of evapotranspiration over transects with lengths between 200 and 5,000 m. Large Aperture Scintillometers operating at optical wavelengths are employed to measure the sensible heat flux over dry desert grasslands in the Middle Rio Grande Valley and over moist grasslands in the Valles Caldera. Net radiation and soil heat fluxes are measured with a 4 component net radiation meter and soil heat flux plates, respectively, and evaluated with remote sensing algorithms. The evapotranspiration is estimated as the difference between net radiation minus the sum of sensible heat flux and soil heat flux. The estimated daily and instantaneous evapotranspiration rates are compared with eddy covariance measurements. We will evaluate the potential of scintilllometers for reliable and robust evaluation of evapotranspiration rates for hydrologists.
H21H-0830 INVITED
Area-Averaged Fluxes from Field to Kilometer Scale with Optical and Microwave Scintillometers
Scintillometry has proven to be a suitable method to obtain surface fluxes over heterogeneous areas over spatial scales of up to 10 km. We will present two of many field-studies conducted by the Meteorology and Air Quality Group of Wageningen University to illustrate this point. Different scintillometer types have been tested. Optical scintillometers yield the structure parameter of temperature, CT2, for long-path Large Aperture Scintillometers (LAS) and both CT2 and the dissipation rate of turbulent kinetic energy, e, for short-path laser scintillometers. CT2 and e are related to the surface fluxes of heat, H, and momentum, t, by virtue of Monin-Obukhov similarity theory. For the LAS - that provides CT2 only - t is obtained from additional wind speed measurements and an estimate of the roughness length. An optical scintillometer in combination with a millimeter-wave scintillometer (MWS) yields both CT2 and Cq2, the structure parameter of humidity, from which the sensible and the latent heat flux can be determined. The following two scintillometer field experiments will be discussed: EVAGRIPS, Lindenberg, Germany 2003. This study deals with a LAS and a MWS (94 GHz) installed over path length of 5 km at 45 m height over a heterogeneous flat agricultural terrain consisting of a mix of lakes, forest and agriculture fields over undulating terrain. The concept of an effective scintillometer height will be introduced, which needs to be applied when the scintillometer height is not constant over the path. RAPID, Idaho, USA, 1999. This study deals with the estimation of evapotranspiration using a LAS and laser scintillometer installed at field scale (~500m) over irrigated alfalfa in an area affected by advection of warm and dry desert air. In these conditions the sensible heat becomes negative and the water vapor deficit is increased, both enhancing evapotranspiration. References: De Bruin, H.A.R.: 2002, ‘Introduction, renaissance of scintillometry', Boundary-Layer Meteorol. 105, 1-4. De Bruin, H.A.R., Hartogensis, O.K., Allen, R.G., and Kramer, J.W.J.L., 2004: ‘Note on the Regional Advection Perturbations in an Irrigated Desert (RAPID) Experiment', Theor. Appl. Climatol. 80, 143-152. Hartogensis, O.K., De Bruin, H.A.R., Van De Wiel, B.J.H.: 2002, ‘Displaced-Beam Small Aperture Scintillometer Test. Part II: Cases-99 Stable Boundary-Layer Experiment', Boundary-Layer Meteorol. 105, 149-176. Meijninger, W.M.L.; Beyrich, F.; Lüdi, A.; Kohsiek, W.; De Bruin, H.A.R.: 2005, ‘Scintillometer-Based Turbulent Fluxes of Sensible and Latent Heat Over a Heterogeneous Land Surface : a Contribution to Litfass-2003 Boundary-Layer Meteorol. 121, 89-110.
H21H-0831
Variation in Scintillometer and Eddy Tower Footprints and Implications for the Validation of Satellite Derived Energy Fluxes in Heterogeneous Terrain
The large aperture scintillometer (LAS) has emerged as one of the best tools for quantifying areal averaged fluxes over heterogeneous land surfaces. This is particularly useful as a validation of surface energy fluxes derived from satellite sources. We examine how changes in surface source area contributing to the scintillometer and eddy covariance measurements relates to satellite derived estimates of sensible and latent heat flux. Field data was collected on the Konza Prairie in Northeastern Kansas, includes data from two eddy covariance towers: one located on an upland, relatively flat homogeneous area, and the second located in a lowland area with generally higher biomass and moisture conditions. The large aperture scintillometer spans both the upland and lowland areas and operates with a path length of approximately 1 km specifically to compare to MODIS derived estimates of surface fluxes. Data from the MODIS sensor was used on a daily basis to compute fluxes using the 'triangle method' which combines the remotely sensed data with a soil-vegetation-atmosphere-transfer scheme and a fully developed atmospheric boundary layer model. As wind direction varies, the relative contribution of upland and lowland sources contributing to the LAS measurements varies while the MODIS pixel contribution remains relatively constant. Via examining relative contributions of upland and lowland areas to the total LAS measured fluxes we are able to evaluate the relationship between the LAS observations and the remotely sensed estimates of the surface energy balance. General implications for validation of remotely sensed data in heterogeneous terrain will also be discussed.
H21H-0832
Mobile scintillometry to study heat advection over heterogeneous surfaces
Large Aperture Scintillometer (LAS) receivers measure the structure parameter of the refractive index from intensity fluctuations of the transmitter beam. Due to the spatial averaging over 1-4 km employed by this emerging technique the constraints for long temporal averaging (15-30 min) and associated uncertainties that have to be met by other flux measurement techniques do not apply for LASs. In this paper the constraints for temporal averaging of LASs will be examined as a function of environmental conditions and transect geometry. Moreover, analysis of data from a mobile LAS measurement across a surface gradient from rough and dry to smoother and wet will be presented. In this experiment the LAS was mounted on a pickup truck, allowing for quick redeployment of the transect after meaurement. The potential for the use of LAS to study local advection of heat in riparian or irrigated areas in the semi-arid southwest will be evaluated.
H21H-0833
Estimating Sensible Heat in the Great Basin, NV using a Large Aperture Scintillometer
In arid regions such as the southwestern US, where net evaporation exceeds net precipitation, communities are faced with the challenge or providing sufficient water resources to meet the increasing demand. Recent population growth in major cities such as Las Vegas, Nevada, coupled with a continued long-term drought, has led water managers to seek additional water from greater distances. In 2004 a study was initiated to investigate two valleys in the Great Basin, Nevada that could potentially provide additional water to Las Vegas. The goal of the study was to close a water balance on each basin scaling from leaf level measurements to basin wide estimates. The evapotranspiration (ETa) component of the water budget was estimated using an energy balance approach. A large aperture scintillometer (Scintec BLS 5000) was used to measure sensible heat flux with a path length of 1500 m and within the footprint of an eddy flux tower. Additionally, micro-meteorological stations were used to estimate net radiation (Rn) and soil heat flux (G) at two mid point locations along the scintillometer path length. An adjusted R2 of 0.93 between scintillometer and eddy flux sensible heat estimates was found during a diurnal measurement in early spring 2004. Additional runs also accounted for significant amounts of the variation between these two techniques, but at lower R2 values. These results suggest that a scintillometer can be an effective tool for estimating sensible heat over long path lengths of heterogeneous vegetation.
H21H-0834
VALIDATION OF SEBAL HEAT FLUXES WITH SCINTILLOMETRY
The Surface Energy Balance Algorithm for Land (SEBAL) is being used worldwide for the estimation of latent heat fluxes from MODIS and Landsat images. Our study objective is to investigate how large aperture scintillometers can be best employed for the validation of SEBAL heat fluxes. In this study, Landsat and MODIS images of the Valles Caldera National Preserve (New Mexico, USA) and North-Eastern Ghana were selected to estimate sensible and latent heat fluxes using SEBAL. These fluxes will be compared against direct ground measurements of the heat fluxes using scintillometers for sensible heat flux and eddy covariance systems for sensible and latent heat fluxes.