Henk Bruin

Estimation of sensible heat flux using the Surface Energy Balance System (SEBS) and ATSR measurements

This paper describes a modified version of the Surface Energy Balance System (SEBS) as regards the use of radiometric data from space and presents the results of a large area validation study on estimated sensible heat flux, extended over several months. The improvements were made possible by the characteristics of the Along Track Scanning Radiometer (ATSR-2) on board the European Remote Sensing satellite (ERS-2) and relate to: (a) the use of bi-angular radiometric data in two thermal infrared channels to estimate column atmospheric water vapor: (b) the use of bi-angular radiometric data in four spectral channels in the 550-1600 nm spectral regions to estimate aerosols optical depth: (c) determination of bottom of atmosphere (BOA) spectral reflectance using column water vapor, aerosols optical depth and a two-stream radiative transfer scheme to relate BOA spectral reflectance to top of atmosphere spectral radiance (d) direct and inverse modeling of radiative transfer in a vegetation canopy to relate BOA spectral reflectance to canopy properties, such as spectrally integrated hemispherical reflectance (albedo). A parameterization of the aerodynamic resistance for heat transfer (in term of kB(-1)) was applied for the first time at large spatial scales. For such large area analyses SEBS requires wind speed, potential temperature and humidity of air at an appropriate reference height. The latter was taken as being the height of the planetary boundary layer (PBL) and the data used were fields generated by an advanced numerical weather prediction model, i.e. regional atmospheric climate model (RACMO), integrated over the PBL. Validation of estimated sensible heat flux H obtained with the ATSR radiometric data was done using long-range, line-averaged measurements of H done with large aperture scintillometers (LAS) located at three sites in Spain and operated continuously between April and September 1999. The root mean square deviation of SEBS H estimates from LAS H measurements was 25.5 W m(-2)

Fluxes of carbon dioxide and water vapour from a Sahelian savanna

Abstract Simultaneous measurements of atmospheric CO 2 flux, F c , and latent heat flux, E , from a shrub savanna in Niger, West Africa, were made by eddy correlation. The vegetation at the study site consisted of scattered shrubs with an understorey of grasses and herbs. The measurements made available some of the first data on CO 2 and H 2 O exchange for an semi-natural, mixed plant community, growing in the semi-arid tropics. Such data are necessary for the development of improved soil-vegetation-atmosphere models, able to describe the complex interplay between atmospheric CO 2 , vegetation conductance and the surface energy balance of the Sahel in global climate models. In this framework, the effect of the extreme and highly variable environment, represented by the saturation deficit, D , and integrated water content of the upper soil layers, Φ, was discussed. F c and E were measured throughout the transition from the wet to the dry season (September–October) during the HAPEX-Sahel Intensive Observation period, in 1992. At the same time, leaf stomatal conductances, g 1 , of the shrubs and two understorey herb species was measured. Daily totals of F c decreased by 50% (peak values declined from −10 to −5 μmol m −2 s −1 ) over 3 weeks following the last rainfall of the wet season. During the same period, g 1 decreased roughly four-fold for all sampled species. D appeared to be the main controlling parameter in the exchange of CO 2 and H 2 O. g 1 and ‘water use efficiency’, | F c E |, were well correlated with D , which also considerably influenced the response of F c to photosynthetically active radiation, Q p . Simultaneously, a decreasing Φ caused lower values of g 1 and F c . However, soil moisture had little effect on the empirical relationships found between atmospheric variables ( D or Q p ) and g 1 or F c .

Oxygen sensitivity of krypton and Lyman-a hygrometers

Abstract The oxygen sensitivity of krypton and Lyman-α hygrometers is studied. Using a dewpoint generator and a controlled nitrogen/oxygen flow the extinction coefficients of five hygrometers associated with the third-order Taylor expansion of the Lambert–Beer law around reference conditions for oxygen and for water vapor were measured. Latent heat flux corrections for cross-talk of the sensible heat flux are given as a function of the Bowen ratio. The spread observed in oxygen sensitivities calls for individual oxygen calibrations for each apparatus. It is found that the separation between the tubes is the crucial parameter with respect to the oxygen sensitivity. The oxygen-related nonlinearities in the Lambert–Beer law are stronger than those associated with water vapor. This leads to a strong reduction of the oxygen sensitivity at a more or less constant water vapor sensitivity when the separation between the tubes is doubled from 1.3 to 2.6 cm. This characteristic is supported by a sensitivity analysi...

Scintillometer-Based Estimates of Sensible Heat Flux Using Lidar-Derived Surface Roughness

AbstractThe estimation of sensible heat flux, H, using large aperture scintillometer (LAS) under varying surface heterogeneity conditions was investigated. Surface roughness features characterized by variable topography and vegetation height were represented using data derived from the highly accurate light detection and range (lidar) techniques as well as from traditional vegetation survey and topographic map methods. The study was conducted at the Cibola National Wildlife Refuge, Southern California, over a riparian zone covered with natural vegetation dominated by tamarisk trees interspersed with bare soil in a region characterized by arid to semiarid climatic conditions. Estimates of H were obtained using different representations of surface roughness features derived from both traditional and lidar methods to estimate LAS beam height [z(u)] at each increment u along its path, vegetation height (hc), displacement height (d), and roughness length (z0) combined with the LAS weighing function, W(u), alon...

ESTIMATING SURFACE FLUXES IN IRRIGATED AREAS WITH SCINTILLOMETERS

Scintillometry has proven to be a good alternative method to obtain surface fluxes over heterogeneous areas over spatial scales of up to 10 km and in non-stationary conditions in the stable surface layer (see e.g. [1]). This study concerns agro- hydrological scintillometer applications of estimating evaporation over homogeneous irrigated areas on a scale of 50 to 500 m. Two types of scintillometers will be considered, notably the displaced beam small aperture scintillometer (DBSAS) and the large aperture scintillometer (LAS) deployed in two field campaigns in Idaho, USA in 1999 [2] and in the Yaqui Valley, Sonora, Mexico in 2000 [3]. The DBSAS and the LAS are optical instruments that consist of a transmitter and receiver. The receiver records intensity fluctuations of the light beam emitted by the transmitter, which are caused by refraction of the beam upon its passage through the turbulent surface layer. These intensity fluctuations are a measure of the structure parameter of temperature, CT2. The DBSAS obtains also the dissipation rate of turbulent kinetic energy, e, from the correlation between the two displaced beams. 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. Evapotranspiration can then be estimated from net radiation and the soil heat flux measurements. In both field campaigns the irrigated agricultural area was surrounded by a desert. In these conditions dry, warm desert air can be advected over the cool evaporating surface by which sensible heat becomes negative and the water vapor deficit is increased, both enhancing evapotranspiration. As a result the surface layer is stably stratified and wind shear is the only turbulence generating mechanism. The DBSAS directly gives information on this process, the LAS does not. We will outline the potential of scintillometers of obtaining principle turbulence parameters (CT2 and e) and fluxes of latent and sensible heat, and compare these with eddy covariance method based estimates for the two experiments. We will present evidence that scintillometers have advantages over the eddy covariance (EC) method in the often non-stationary stable surface layer, since they can be used for short flux-averaging periods as they average turbulence not only in time but also in space. Furthermore, scintillometers require less complex data processing and quality control procedures. Last, the transmitter and receiver of the instrument can be installed at the borders of the field by which the instrument does not interfere with the farmer’s activities in the field. Finally, the medium aperture scintillometer (MAS) will be discussed, which has an aperture size that includes features of both the DBSAS and the LAS. Result of MAS measurements over grass at Cabauw, Netherlands will be shown. References [1] De Bruin, H.A.R.: 2002, ‘Introduction, renaissance of scintillometry’, Boundary- Layer Meteorol. 105, 1-4. and the papers of this special issue on scintillometry. [2] 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. [3] Hoedjes, J.C.B., Zuurbier, R.M., and Watts, C.J.: 2002, ‘Large aperture scintillometer used over a homogeneous irrigated are partly affected by regional advection’, Boundary-Layer Meteorol. 105, 99-117. [4] 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.