O.K. Hartogensis

Determination of Area-Averaged Sensible Heat Fluxes with a Large Aperture Scintillometer over a Heterogeneous Surface – Flevoland Field Experiment

To test the applicability of the scintillation method over a heterogeneous area an experiment was carried out in the summer of 1998 in Flevoland (The Netherlands). In the patchy area only four crops were grown namely sugar beet, potatoes, wheat and onions. From eddy covariance measurements it was found that the heterogeneity was mainly caused by differences in thermal properties. No variations in the aerodynamics roughness length were observed. Two large aperture scintillometers were installed at a height of 11.6 and 20.4 m. A good resemblance was found between the sensible heat fluxes derived from both LAS instruments and the area-averaged fluxes obtained from the in-situ eddy covariance measurements. The slight underestimation of the lower LAS could be assessed using a blending height model and an analytical footprint model. The results also indicated that when scintillometer measurements are made below the blending height the violation to Monin–Obukhov Similarity Theory is small and that reasonable fluxes can be obtained from path-averaged structure parameters.

Intermittent Turbulence in the Stable Boundary Layer over Land. Part III: A Classification for Observations during CASES-99

Abstract In this paper a classification of stable boundary layer regimes is presented based on observations of near-surface turbulence during the Cooperative Atmosphere–Surface Exchange Study-1999 (CASES-99). It is found that the different nights can be divided into three subclasses: a turbulent regime, an intermittent regime, and a radiative regime, which confirms the findings of two companion papers that use a simplified theoretical model (it is noted that its simpliflied structure limits the model generality to near-surface flows). The papers predict the occurrence of stable boundary layer regimes in terms of external forcing parameters such as the (effective) pressure gradient and radiative forcing. The classification in the present work supports these predictions and shows that the predictions are robust in a qualitative sense. As such, it is, for example, shown that intermittent turbulence is most likely to occur in clear-sky conditions with a moderately weak effective pressure gradient. The quantit...

Determination of area averaged water vapour fluxes with large aperture and radio wave scintillometers over a heterogeneous surface - Flevoland field experiment

A large aperture scintillometer (LAS) andradio wave scintillometer (RWS)were installed over a heterogeneous areato test the applicability of the scintillation method.The heterogeneity in the area, whichconsisted of many plots, was mainly caused bydifferences in thermal properties ofthe crops; the variations in theaerodynamic roughness lengthwere small. The water vapour fluxesderived from the combined LAS-RWSsystem, also known as the two-wavelengthmethod, agreed fairly well with the aggregatedwater vapour fluxes derived from in-situeddy covariance measurements. The water vapourfluxes derived from a stand-alone LASare also presented. It was found that a single LASand an estimate of the area averagedavailable energy (using a simple parameterisationscheme) can provide also reasonablearea-averaged water vapour fluxes.

Derivation of an Effective Height for Scintillometers: La Poza Experiment in Northwest Mexico

The large-aperture scintillometer (LAS) is by now a generally accepted device for routinely obtaining the area-averaged sensible heat flux, H, on a scale of up to 10 km. It is an optical instrument that consists of a transmitter and receiver. In practice, the LAS beam height often varies along the path due to a variety of reasons. This study will explain what effective height to use in such situations, when analyzing scintillometer data to derive H. Several aspects are covered: a slanted path over flat terrain, structured terrain, and varying path height due to the curvature of the earth’s surface. To test the derived effective height formulation the authors present LAS data taken in September and October 1996 at a rangeland site in Sonora, Mexico. In experiment 1, the LAS was set up over a slant path, ranging roughly between 10 and 45 m above the surface over a 3200-m path. In experiment 2, a horizontal LAS path was used at approximately 30 m over a pathlength of 1100 m. The resulting sensible heat fluxes were compared with eddy-covariance data and show satisfactory results for both the full and one of the approximate formulations of the effective height.

Displaced-beam small aperture scintillometer test. Part II: CASES-99 stable boundary layer experiment

The performance of the Scintec displaced-beam small aperture scintillometer (DBSAS) in the stable boundary layer (SBL) is investigated using data gathered during the CASES-99 experiment in Kansas, U.S.A. The DBSAS is superior to the eddy-covariance method in determining vertical fluxes of sensible heat and momentumclose to the ground and/or over short (< 1 min) averaging intervals. Both aspects are of importance in the shallow and non-stationary SBL.The friction velocity, u*, the temperature scale, θ*, and from these the sensible heat flux, H, were calculated from the indirectly determined dissipation rate, ∈, and the structure parameter of temperature, CT2, by the DBSAS, which was operated over a path length of 112 m. All these variables are compared with eddy-covariance data for 10-minute time averages. Previously reported systematic errors in the DBSAS, overestimation of u* for low u* values and underestimation of u* for high u* values, have in part been dealt with by adjusting the beam displacement distance from 2.7 mm to 2.6 mm in the calculations. The latter adjustment is presented as a working hypothesis, not a general solution.

An extra large aperture scintillometer for long range applications

An incoherent scintillometer with 0.31-m aperture was testedalong a 9.8-km path over grassland. Scintillometer derived heat fluxes were comparedwith in situ eddy covariance measurements. Albeit with considerable scatter, the fluxescompared well during daytime. During nighttime credible fluxes are also obtained. The scintillometerfunctioned satisfactory for 96% of the 7-week period.

Comparison of sensible heat flux estimates using AVHRR with scintillometer measurements over semi-arid grassland in northwest Mexico

The problems associated with the validation of satellite-derived estimates of the surface fluxes are discussed and the possibility of using the large aperture scintillometer is investigated. Simple models are described to derive surface temperature and sensible heat flux from the advanced very high resolution radiometer (AVHRR). Data were collected over an extensive site of semi-arid grassland in northwest Mexico during the summer of 1997 as part of the semi-arid land-surface-atmosphere (SALSA) program. Comparison of surface temperature derived from AVHRR with that derived from a ground-based infrared thermometer showed an RMSE of around 2°C, while estimates of sensible heat flux derived from AVHRR compared well with measurements using either eddy correlation or a large aperture scintillometer.

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.

Confronting the WRF and RAMS mesoscale models with innovative observations in the Netherlands: Evaluating the boundary layer heat budget

The Weather Research and Forecasting Model (WRF) and the The Weather Research and Forecasting Model (WRF) and the Regional Atmospheric Mesoscale Model System (RAMS) are frequently used for (regional) weather, climate and air quality studies. This paper covers an evaluation of these models for a windy and calm episode against Cabauw tower observations (The Netherlands), with a special focus on the representation of the physical processes in the atmospheric boundary layer (ABL). In ad-dition, area averaged sensible heat flux observations by scintillometry are utilized which enables evaluation of grid scale model fluxes and flux observations at the same horizontal scale. Also, novel ABL height observations by ceilometry and of the near surface longwave radiation divergence are utilized. It appears that WRF in its basic set-up shows satisfactory model results for nearly all atmospheric near surface variables compared to field observations, while RAMS needed refining of its ABL scheme. An important inconsistency was found regarding the ABL daytime heat budget: Both model versions are only able to correctly forecast the ABL thermodynamic structure when the modeled surface sensible heat flux is much larger than both the eddy-covariance and scintillometer observations indicate. In order to clarify this discrepancy, model results for each term of the heat budget equation is evaluated against field observations. Sensitivity studies and evaluation of radiative tendencies and entrainment reveal that possible errors in these variables cannot explain the overestimation of the sensible heat flux within the current model infrastructure.

Developments in Scintillometry

Thirty scientists from five nations discussed developments in the growing field of scintillometry—the study of wave propagation in the atmospheric surface layer that can be used to measure and decipher low-level turbulence.