The applicability of the scintillation method over heterogeneous areas

Surface fluxes at a scale of several kilometers are required in many meteorological studies. The scintillation technique is one of the few methods that can provide fluxes at these scales (1 - 10 km). Since usually the earth's surface is heterogeneous at these scales the question arises whether the scintillation method, which is based on the Monin-Obukhov Similarity Theory (MOST), can be applied. To test the applicability of the method over a heterogeneous area an experiment was carried out in Flevoland (The Netherlands). The patchy area consisted of many rectangular plots (500 x 250 m) where four crops were grown namely, sugar beet, potatoes, wheat and onions. Each crop covered 25 % of the area independent of the wind direction (i.e. isotropic conditions). Eddy covariance instruments were placed at four plots, each covered by a different crop, to provide independant surface flux measurements. Based on the eddy covariance measurements it was found that the heterogeneity in the area was caused by variations in thermal properties (i.e. H and LvE). No variation in the surface roughness for momentum was observed. Two Large Aperture Scintillometers (LAS) were placed on two windmills at a height of 11.6 and 20.4 m. The path length of both scintillometers was 2.2 km. A good resemblance was found between the sensible heat fluxes derived from the upper LAS and the area averaged sensible heat fluxes derived from the aggregated in-situ eddy covariance measurements. The slightly lower fluxes from the LAS at 11.6 m could be assessed by using a blending height and a footprint model. After accounting for the spatial distribution of the surface fluxes of the crops in the source area of the LAS the results agreed fairly well. The results have demonstrated that the scintillation method is applicable over heterogenous areas. Also when the scintillometer is measuring below the blending height the violation of the MOST relationship between path averaged structure parameters and fluxes is small and reasonable fluxes can be obtaine

Monin-Obukhov Similarity Functions of the Structure Parameter of Temperature and Turbulent Kinetic Energy Dissipation Rate in the Stable Boundary Layer

The Monin-Obukhov similarity theory (MOST) functions fepsi; and fT, of the dissipation rate of turbulent kinetic energy (TKE), ¿, and the structure parameter of temperature, CT2, were determined for the stable atmospheric surface layer using data gathered in the context of CASES-99. These data cover a relatively wide stability range, i.e. ¿ = z/L of up to 10, where z is the height and L the Obukhov length. The best fits were given by f¿ = 0.8 + 2.5¿ and fT= 4.7[1 + 1.6(¿)2/3], which differ somewhat from previously published functions. ¿ was obtained from spectra of the longitudinal wind velocity using a time series model (ARMA) method instead of the traditional Fourier transform. The neutral limit f¿ = 0.8 implies that there is an imbalance between TKE production and dissipation in the simplified TKE budget equation. Similarly, we found a production-dissipation imbalance for the temperature fluctuation budget equation. Correcting for the production-dissipation imbalance, the 'standard' MOST functions for dimensionless wind speed and temperature gradients (Øm and Øh) were determined from f¿ and compared with the Øm and Øh formulations of Businger and others. We found good agreement with the Beljaars and Holtslag [J. Appl. Meteorol. 30, 327-341 (1991)] relations. Lastly, the flux and gradient Richardson numbers are discussed also in terms of f¿ and fT

Variance Method to Determine Turbulent Fluxes of Momentum And Sensible Heat in The Stable Atmospheric Surface Layer

Evidence is presented that in the stable atmospheric surface layer turbulent fluxes of heat and momentum can be determined from the standard deviations of longitudinal wind velocity and temperature, ¿u and ¿T respectively, measured at a single level. An attractive aspect of this method is that it yields fluxes from measurements that can be obtained with two-dimensional sonic anemometers. These instruments are increasingly being used at official weather stations, where they replace the standard cup anemometer-wind vane system. With methods such as the one described in this note, a widespread, good quality, flux network can be established, which would greatly benefit the modelling community. It is shown that a 'variance' dimensionless height (¿¿) defined from ¿uand ¿Tis highly related to the 'conventional' dimensionless stability parameter ¿ = z/L, where is height and L is the Obukhov length. Empirical functions for ¿¿ are proposed that allow direct calculation of heat and momentum fluxes from ¿u and ¿F. The method performs fairly well also during a night of intermittent turbulence

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

In this study we investigated the performance of a displaced-beam small aperture scintillometer (DBSAS) - operated over a path length of 112 m - under stable conditions using data gathered during the CASES-99 experiment in Kansas, USA. The DBSAS has the advantage over the eddy covariance method that it can determine fluxes of sensible heat and momentum close to the surface and/or over short (< 1 minute) averaging intervals. Both aspects are of importance in the often shallow and non-stationary stable boundary layer (SBL). From raw DBSAS measurements the dissipation rate of turbulent kinetic energy and the structure parameter of temperature can be deduced, which follow Monin-Obukhov similarity theory (MOST) to give the friction velocity, u* the temperature scale, T* and subsequently the sensible heat flux, H. All these variables were compared with eddy covariance data for 10-minute time averages. Systematic errors were found for the DBSAS u*; i.e. overestimation for low u* values and underestimation for high u* values. It seems these errors cannot be attributed to the use of MOST, since they are already present in the dissipation rate data. In determining the dissipation rate, a form of the temperature spectrum in the dissipation range has to be assumed. The way the DBSAS weights this spectrum will be discussed. Furthermore, the sensitivity of the DBSAS method to small off-sets in instrumental parameters will be discussed. It was found that an adjustment of the beam displacement distance, d that is within the accuracy at which d is determined, removes some of the systematic errors. This adjustment is presented as a working hypothesis, not a general solutio

Fluxes and Gradients in the Convective Surface Layer and the Possible Role of Boundary-Layer Depth and Entrainment Flux

We study the relation between fluxes and gradients in the very unstable surface layer by comparing recent proposals in the literature with the well-known Businger¿Dyer functions. The recent proposals include results from large-eddy simulation (LES), which account for entrainment effects and effects of the boundary-layer depth. A comparison of the relationships is made with experimental data. The LES-based gradient functions show the impact of entrainment in the surface layer, but the scatter in the field data is too large to confirm this. Therefore this result is preliminary and future tests against new observations are recommended. It appears that the Businger¿Dyer relationship behaves differently to the alternatives, and that it deviates from observations for large stability

Long range scintillometry

In the past years there has been a renewed interest in the use of scintillometers for the measurement of sensible and latent heat flux. This interest is partly invoked by the need to infer the energy fluxes over areas that match in size with satellite pixels or the grid of a numerical model. In this respect it is of importance to exploit the path limits of a scintillometer. One of the problems encountered in extending the path length is saturation of scintillation. To avoid saturation one may use large apertures, or choose a longer wavelength such as microwave or radiowave, or install the scintillometer at a higher level above the ground. Here our experiences with an extra large aperture scintillometer (XLAS) operating over a path of 9.8 km are reported. The instrument has transmitter and receiver apertures of 31 cm and uses a 0.94 m, 100 mW light emitting diode as light source. The height of the beam over the surface is 40 m. The transmitter was installed in the TV tower near the town of IJsselstein and the receiver in the KNMI meteorological tower near Cabauw, both at 40 m height. The path crosses mainly pasture land and some low built-up area, mainly situated near the transmitter. The scintillometer was first installed in August 2000, dismantled in October of that year, and re-installed in October 2001 with the aim to get a full year of data. From publications in the literature it can be concluded that in our set-up saturation gets significant at a sensible heat flux of 100 Wm-2 or more. Corrections may be made for higher heat fluxes. We compare the scintillometer fluxes with eddy correlation fluxes obtained near the receiver end of the optical path. In doing so, one faces the issue of comparing a local measurement with an area-averaged one. To gain insight in the degree of homogeneity of the area, we will analyse thermal pictures of the surface beneath the scintillometer path. In this way we hope to get better to grips with the performance of the scintillometer as a device for measuring area-averaged heat fluxe

Performance of HIRLAM in a semiarid heterogeneous region: Evaluation of the land surface and boundary layer description using EFEDA observations

Observations from the European Field Experiment in a Desertification-threatened Area (EFEDA) are used to evaluate the performance of the radiation, land surface, and boundary layer description of the numerical weather prediction (NWP) system High-Resolution Limited Area Model (HIRLAM) in semiarid conditions. Model analysis and 6-h forecast data of the fully coupled three-dimensional model are compared with the comprehensive dataset of a case study representing a sample of 22 days of anticyclonic conditions. Distributed micrometeorological surface stations, radiosondes, flux aircraft, and airborne lidar provide a unique validation dataset of the diurnal cycle of surface and boundary layer processes. The model surface, soil, and boundary layer are found to be too moist and slightly too cold during most of the diurnal cycle. The model radiation and surface energy budgets are biased toward more humid conditions. Model shortcomings are identified essentially in four areas. These are the moisture data assimilation, the land-use and soil classification with its associated physiographic database, the aerosol parameterization in the radiation code, and the boundary layer vertical resolution and entrainment description. Practical steps for immediate improvement of the model performance are proposed. They focus on the use of a land-use and soil classification and physiographic database adapted to Mediterranean landscapes, in combination with the inclusion of aerosol parameters in the radiation scheme, that account for the typically higher aerosol load of arid and semiarid environments

Energy balance closure for the LITFASS-2003 experiment

In the first part, this paper synthesises the main results from a series of previous studies on the closure of the local energy balance at low-vegetation sites during the LITFASS-2003 experiment. A residual of up to 25% of the available energy has been found which cannot be fully explained either by the measurement uncertainty of the single components of the surface energy balance or by the length of the flux-averaging period. In the second part, secondary circulations due to heterogeneities in the surface characteristics (roughness, thermal and moisture properties) are discussed as a possible cause for the observed energy balance non-closure. This hypothesis seems to be supported from the fluxes derived from area-averaging measurement techniques (scintillometers, aircraft)