H. A. R. De Bruin

A system to measure surface fluxes of momentum, sensible heat, water vapour and carbon dioxide

An eddy covariance system is described which has been developed jointly at a number of European laboratories and which was used widely in HAPEX-Sahel. The system uses commercially available instrumentation: a three-axis sonic anemometer and an IR gas analyser which is used in a closed-path mode, i.e. air is brought to the optical bench by being ducted down a sampling tube from a point near the sonic anemometer. The system is controlled by specially written software which calculates the surface fluxes of momentum, sensible and latent heat and carbon dioxide, and displays them in real time. The raw turbulent records can be stored for post-processing. Up to five additional analogue instruments can be sampled at up to 10 Hz and digitised by the sonic anemometer. The instruments are described and details of their operation and connection are presented. The system has relatively low power consumption and can operate from appropriate solar cells or rechargeable batteries. Calibration of the gas analyser needs to be performed typically every 2 or 3 days, and, given that the system requires minimal maintenance and is weather insensitive, it can be operated for the routine collection of surface flux data for extended periods. There are a number of corrections which have to be applied in any eddy covariance system and we describe the system of transfer functions which define our system. Some representative results showing the potential of the system are presented.

A verification of some methods to determine the fluxes of momentum, sensible heat, and water vapour using standard deviation and structure parameter of scalar meteorological quantities

A set of micro-meteorological data collected over a horizontal, uniform terrain (the plain of La Crau, France) in June 1987 is analysed. Conditions were predominantly sunny and arid, while due to the “Mistral” the wind speed could exceed 10 m/s. Verification of several methods to evaluate surface fluxes of heat, momentum and water vapour from the standard deviation of temperature, wind and specific humidity is presented. Also, a similar approach using the structure parameter of temperature is considered. These methods are all based on Monin-Obukhov (M-O) similarity theory. It is found that the standard deviation of temperature, vertical and horizontal wind speed as well as the structure parameter for temperature behave according to M-O similarity. It is shown that the sensible heat flux and friction velocity can be determined from a fast response thermometer and a cup anemometer. Also, it appears that the analytic solution of the set of governing equations as derived by the first author yields good results. M-O theory does not appear to work for the standard deviation of specific humidity. This may be due to the relative importance of large eddies.

Some Practical Notes on the Parameter kB−1 for Sparse Vegetation

This paper deals with the parameter kB21, the logarithm of the ratio between momentum and heat roughness length, of sparsely vegetated surfaces and bare soil. The bare soil surface is included as a reference, since it is fairly homogenous and smooth, having no distinguishable roughness elements. The mean value of kB21 is about 8 for the vineyard and 12 for the savannah. These values are significantly greater than kB 21 5 2, which is usually assumed to hold for vegetation. The mean value of kB21 for bare soil is small and negative, which agrees with the literature. A large variation of kB 21 during the day is measured for all three surfaces. This behavior has been observed for sparse vegetation in previous studies. Some authors explained the phenomenon with a vertical movement of the source of heat through the day as solar angle varies, or with the use of an inappropriate value of effective surface temperature to calculate kB21. For the first time, this diurnal variation is measured for a smooth surface, the bare soil, for which neither explanation is valid. A sensitivity study reveals that the calculated kB21 is very sensitive to measuring errors in the micrometeorological variables and errors in the roughness length for momentum. This explains the large range in observed kB21 values for one particular surface type. In addition, several semiempirical expressions for kB21 from the literature are tested. Two well-established formulas, both based on a simple combination of Reynolds and Prandtl numbers, appear to produce the best estimates of daily averaged kB21 values. None of the formulas are able to describe the diurnal variation. The authors conclude that the concept of kB21 is questionable as it is based upon extrapolating a theoretical profile through a region where this profile does not hold, toward a ‘‘surface temperature’’ that is difficult to define and to measure. It should therefore be avoided in meteorological models, for example, by applying canopy boundary layer resistances. Unfortunately, in remote sensing, the bulk transfer equations are up to now the only option, which therefore requires the use of kB21.

The scintillation method tested over a dry vineyard area

Measurements of a scintillometer device mounted at 4 m above a dry vineyard area in La Mancha, Spain, are used to obtain an average sensible heat flux densityH. Averaging is over a rectangular area determined by the distance between the scintillometer light source and receptor (875 m) and some upwind distance governed by the horizontal wind speed perpendicular to that line. Using similarity relations obtained from La Crau, a good correspondence betweenH measured with the scintillometer and an eddy-correlation device in the centre of a vineyard is obtained. The friction velocityu* was either measured directly using a sonic anemometer or obtained indirectly from two wind speeds and known values of the roughness length zo and displacementd. The free convection formulation underestimates the sensible heat flux by about 30%. This is due to a significant contribution of mechanically generated turbulence to the total turbulent transport, which was caused by relatively strong winds and rough terrain.

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.

Intermittent Turbulence and Oscillations in the Stable Boundary Layer over Land. Part I: A Bulk Model

In the stable boundary layer (SBL) it is observed often that turbulence is not continuous in space and time. This discontinuous, intermittent turbulence causes alternations from the mean evolution of the stratified atmospheric boundary layer, which may result in an oscillatory type of behavior of the near-surface wind speed and temperature. It is well known that intermittent turbulence in the SBL can be generated by various mechanisms. This paper focuses on an intermittency generating mechanism that arises from a direct interaction of the lower atmosphere (first tens of meters) with the vegetation surface, without interaction with the air aloft. It is shown that the essence of this mechanism can be captured by a 1D bulk model of three coupled nonlinear differential equations. In the present paper, numerical runs with the model show that intermittent turbulence is most likely to occur over land surfaces with low vegetation during clear-sky conditions in the presence of a moderate to low synoptical pressure gradient. The existence of a vegetation layer has a strong influence on intermittency dynamics. Due to its small heat capacity, the vegetation temperature is able to quickly respond to rapidly changing conditions. This in turn affects the stability of the lower atmosphere, causing an important feedback mechanism. In addition, it was found that intermittent behavior of SBL models occurs for various first-order closure schemes with different stability functions. However, stability functions that allow turbulent transport beyond the critical Richardson number effectively suppress intermittent‐oscillatory behavior. Currently, the latter type of formulations is often used in numerical weather prediction to prevent excessive SBL cooling in very stable conditions. The advantage of using a simplified SBL model, as proposed in the present paper, is that it allows an analytical study of the system, which, in turn, allows theoretical predictions about the occurrence of intermittent SBL behavior (see the companion paper).

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.

Results from One-Year Continuous Operation of a Large Aperture Scintillometer over a Heterogeneous Land Surface

A large-aperture scintillometer (LAS) was operated continuouslyduring a period of more than one year over a heterogeneous land surface in Central Europeat the transition between marine and continental climates. The LAS measurements of the refractiveindex structure parameter, CN2, were used to estimate the sensible heat flux. Thiswas possible for about 60to 80% of the time under daytime conditions during thesummer, with lower values obtained for the cold season (October to March). Using datafrom a three-week long field experiment, the LAS-based heat flux was compared with a weighedaverage of local heat flux measurements over the main land use classes (forest, agriculture,water) in the area, resulting in reasonable agreement. LAS-based heat fluxes were then used forcomparison with the heat flux values of a numerical weather prediction model. An over-predictionof the model heat flux was found in summer but the modelled values were lower than the LASderived data during the cold season.