- L. Chandrapala (1)
- J.M. Cohard (1)
- H.A.R. Debruin (2)
- J.G. Evans (1)
- T. Foken (1)
- M. Gockede (1)
- M. Gosset (1)
- C.S.B. Grimmond (1)
- O.K. Hartogensis (3)
- H.M. Hemakumara (1)
- A.A.M. Holtslag (1)
- W. Kohsiek (1)
- R. Kouznetsov (1)
- J.P. Leps (1)
- H. Lohse (1)
- A. Lüdi (1)
- W.M.L. Meijninger (1)
- A.F. Moene (4)
- S.H. Richter (1)
- J.C. Rodriguez (1)
- R. Uijlenhoet (1)
- R. Vogt (1)
- H.C. Ward (1)
- C.J. Watts (1)
- U. Weisensee (2)
Similarity Relations for C_T^2 in the Unstable Atmospheric Surface Layer: Dependence on Regression Approach, Observation Height and Stability Range
Braam, M. ; Moene, A.F. ; Beyrich, F. ; Holtslag, A.A.M. - \ 2014
Boundary-Layer Meteorology 153 (2014)1. - ISSN 0006-8314 - p. 63 - 87.
nocturnal boundary-layer - aperture scintillometer - structure parameters - sonic anemometer - sensible heat - fluxes - temperature - scintillation - cases-99 - momentum
A great variety of similarity functions for the structure parameter of temperature ( C 2 T ) have been proposed in the literature. They differ in the way they were derived from the data and in the characteristics of the dataset used for their derivation (surface type, observation level, stability range). In this study, we use one single dataset (CASES-99 experiment) and investigate the impact on the similarity functions of applying various regression approaches, and measuring at different heights and within different stability ranges. We limit ourselves to similarity functions under unstable conditions, and evaluate only the most common shape that describes the relation with two coefficients ( f(z/L)=c 1 (1-c 2 z/L) -2/3 , where z is the height, and L is the Obukhov length and a measure of the stability, and c 1 and c 2 are the regression coefficients). The results show that applying various regression approaches has an impact on the regression coefficients c 1 and c 2 . Thus studies should always specify the regression approach when presenting similarity relations. We suggest use of an orthogonal distance regression method such that uncertainties in -z/L are also taken into account, to apply this to the logarithmic transformation of both dimensionless groups, and to use a weighted dataset such that unreliable data points have a smaller influence on the fit. Dividing the dataset into eight height ( z ) and eight stability ( -1/L classes) classes, we show that the observation height and the stability range has an impact on the coefficients too. This implies that variations in c 1 and c 2 found in the literature may result from variations in the height and stability ranges among the datasets. Furthermore, application of the coefficients on a dataset obtained at a different height or within a different stability range has to be made with care. Finally, the variation in the coefficients between the classes indicates that the Monin–Obukhov similarity function for C 2 T is not sufficiently described by the two-coefficient function used here.
A critical revision of the estimation of the latent heat flux from two-wavelength scintillometry
Ward, H.C. ; Evans, J.G. ; Hartogensis, O.K. ; Moene, A.F. ; Debruin, H.A.R. ; Grimmond, C.S.B. - \ 2013
Quarterly Journal of the Royal Meteorological Society 139 (2013)676. - ISSN 0035-9009 - p. 1912 - 1922.
large-aperture scintillometer - heterogeneous land-surface - temperature-humidity correlation - refractive-index - water-vapor - structure parameters - regional advection - eddy-covariance - scintillation - fluctuations
Simultaneous scintillometer measurements at multiple wavelengths (pairing visible or infrared with millimetre or radio waves) have the potential to provide estimates of path-averaged surface fluxes of sensible and latent heat. Traditionally, the equations to deduce fluxes from measurements of the refractive index structure parameter at the two wavelengths have been formulated in terms of absolute humidity. Here, it is shown that formulation in terms of specific humidity has several advantages. Specific humidity satisfies the requirement for a conserved variable in similarity theory and inherently accounts for density effects misapportioned through the use of absolute humidity. The validity and interpretation of both formulations are assessed and the analogy with open-path infrared gas analyser density corrections is discussed. Original derivations using absolute humidity to represent the influence of water vapour are shown to misrepresent the latent heat flux. The errors in the flux, which depend on the Bowen ratio (larger for drier conditions), may be of the order of 10%. The sensible heat flux is shown to remain unchanged. It is also verified that use of a single scintillometer at optical wavelengths is essentially unaffected by these new formulations. Where it may not be possible to reprocess two-wavelength results, a density correction to the latent heat flux is proposed for scintillometry, which can be applied retrospectively to reduce the error.
Path-average rainfall estimation from optical extinction measurements using a large-aperture scintillometer
Uijlenhoet, R. ; Cohard, J.M. ; Gosset, M. - \ 2011
Journal of Hydrometeorology 12 (2011)5. - ISSN 1525-755X - p. 955 - 972.
multiple-scattering corrections - raindrop size distribution - sensible heat fluxes - heterogeneous land-surface - flevoland field experiment - beer-lambert law - area - attenuation - precipitation - scintillation
The potential of a near-infrared large-aperture boundary layer scintillometer as path-average rain gauge is investigated. The instrument was installed over a 2.4-km path in Benin as part of the African Monsoon Multidisciplinary Analysis (AMMA) Enhanced Observation Period during 2006 and 2007. Measurements of the one-minute-average received signal intensity were collected for 6 rainfall events during the dry season and 16 events during the rainy season. Using estimates of the signal base level just before the onset of the rainfall events, the optical extinction coefficient is estimated from the path-integrated attenuation for each minute. The corresponding path-average rain rates are computed using a power-law relation between the optical extinction coefficient and rain rate obtained from measurements of raindrop size distributions with an optical spectropluviometer and a scaling-law formalism for describing raindrop size distribution variations. Comparisons of five-minute rainfall estimates with measurements from two nearby rain gauges show that the temporal dynamics are generally captured well by the scintillometer. However, the instrument has a tendency to underestimate rain rates and event total rain amounts with respect to the gauges. It is shown that this underestimation can be explained partly by systematic differences between the actual and the employed mean power-law relation between rain rate and specific attenuation, partly by unresolved spatial and temporal rainfall variations along the scintillometer path. Occasionally, the signal may even be lost completely. It is demonstrated that if these effects are properly accounted for by employing appropriate relations between rain rate and specific attenuation and by adapting the pathlength to the local rainfall climatology, scintillometer-based rainfall estimates can be within 20% of those estimated using rain gauges. These results demonstrate the potential of large-aperture scintillometers to estimate path-average rain rates at hydrologically relevant scales.
Structure parameters for temperature and humidity from simultaneous eddy-covariance and scintillometer measurements
Beyrich, F. ; Kouznetsov, R. ; Leps, J.P. ; Lüdi, A. ; Meijninger, W.M.L. ; Weisensee, U. - \ 2005
Meteorologische Zeitschrift 14 (2005)5. - ISSN 0941-2948 - p. 641 - 649.
large-aperture scintillometer - flevoland field experiment - refractive-index - surface-layer - heterogeneous surface - land-surface - fluxes - scintillation - heat - turbulence
Line-averaged values of the structure parameters of temperature and humidity, CT2 and Cq2, were estimated from simultaneous measurements with an optical and a microwave scintillometer over a path of 4.7 km length at the Meteorological Observatory Lindenberg during the LITFASS-2003 experiment. By cross-correlating the detected signals of the two scintillometers, the temperature-humidity structure parameter, CTq, and the temperature-humidity correlation, rTq, were also derived directly from the measurements. Comparison with corresponding values obtained from local measurements with an eddy-covariance system on a meteorological tower show a consistent behaviour in time (with some exceptions especially for Cq2 and rTq during nighttime). The deviations are of a magnitude between 20 % and 35 % of the typical daytime values of the structure parameters.
Monin-Obukhov Similarity Functions of the Structure Parameter of Temperature and Turbulent Kinetic Energy Dissipation Rate in the Stable Boundary Layer
Hartogensis, O.K. ; Debruin, H.A.R. - \ 2005
Boundary-Layer Meteorology 116 (2005)2. - ISSN 0006-8314 - p. 253 - 276.
atmospheric surface-layer - aperture scintillometer test - convective conditions - sonic anemometer - fluxes - scintillation - algorithms - cases-99 - validity - models
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.
Derivation of an effective height for scintillometers: La Poza Experiment in Northwest Mexico
Hartogensis, O.K. ; Watts, C.J. ; Rodriguez, J.C. ; Bruin, H.A.R. de - \ 2003
Journal of Hydrometeorology 4 (2003)5. - ISSN 1525-755X - p. 915 - 928.
large-aperture scintillometer - flevoland field experiment - sensible heat fluxes - refractive-index - heterogeneous surface - area - scintillation - fluctuations
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.
Effects of water vapour on the structure parameter of the refractive index for near-infrared radiation
Moene, A.F. - \ 2003
Boundary-Layer Meteorology 107 (2003). - ISSN 0006-8314 - p. 635 - 653.
surface-layer turbulence - humidity fluctuations - boundary-layer - temperature - scintillation - fluxes - similarity - quantities - spectrum - moisture
The refractive index of air (n) mainly depends on temperature and water vapour content. For near-infrared radiation, temperature is the main determining factor. To determine the structure parameter of temperature (C-T(2)) from the structure parameter of the refractive index (C-n(2)), the influence of water vapour content on n needs to be taken into account as a correction. Three levels of approximation are presented. The first involves the standard deviations of T and q (sigma(T) and sigma(q)) as well as the correlation coefficient between T and q (R-Tq). The second approximation involves R-Tq and the Bowen ratio (beta), and the last uses only the Bowen ratio. The latter is the classical Bowen ratio correction. Evaluation of the validity of the assumptions used in the derivation reveals that a large error may be introduced (for situations with R-Tq1, the correction is small, and all three approximations give errors of less than 1% in C-T(2). When C-T(2) is used to compute the sensible heat flux, the influence of the quality of the correction for water vapour fluctuations on the measured energy balance is small: for small \\beta\\, the correction is large, but the absolute value of the heat flux is small, whereas for large \\beta\\, the correction is insignificant.
Evapotranspiration fluxes over mixed vegetation areas measured from large aperture scintillometer
Hemakumara, H.M. ; Chandrapala, L. ; Moene, A.F. - \ 2003
Agricultural Water Management 58 (2003). - ISSN 0378-3774 - p. 109 - 122.
surface-layer - sensible heat - scintillation - index
Routine measurement of spatially averaged surface fluxes of sensible heat (H) in river basins is now feasible. These fluxes, when combined with net radiation estimates, can be used to derive areally averaged actual evapotranspiration (ET). The scintillation method is shown to be promising method for estimating areally averaged sensible heat fluxes. The large aperture scintillometer (LAS) is an optical device used to monitor fluctuations in refractive index of the turbulent atmosphere over a relatively large area. The study reported here has estimated ET fluxes for an area of mixed vegetation at Horana, a field site about 40 km southeast of Colombo, Sri Lanka. ET estimates derived from the scintillometer and net radiometer were compared with estimates obtained from a remote sensing based surface energy balance algorithm for land (SEBAL). The SEBAL estimating of ET were derived using NOAA satellite images without any a priori calibration. The average deviation of ET estimates between SEBAL and LAS for 10-day periods was 17%. However, this deviation fell to 1% when monthly estimates were considered. This suggests regional ET values derived from remote sensing are reasonable estimates, however, the LAS was used in only one agro-ecosystem to validate the SEBAL model. The LAS is a low cost alternative to other methods of estimating heat fluxes for use in basin scale water use studies. (C) 2002 Elsevier Science B.V. All rights reserved.
Experimental determination of turbulent fluxes over the heterogeneous LITFASS area: Selected results from the LITFASS-98 experiment
Beyrich, F. ; Richter, S.H. ; Weisensee, U. ; Kohsiek, W. ; Lohse, H. ; Bruin, H.A.R. de; Foken, T. ; Gockede, M. ; Berger, F. ; Vogt, R. ; Batchvarova, E. - \ 2002
Theoretical and Applied Climatology 73 (2002)39479. - ISSN 0177-798X - p. 19 - 34.
sensible heat - land-surface - structure parameter - natural conditions - momentum - scintillation - inhomogeneity - aircraft - exchange - forest
During the LITFASS-98 experiment, local flux measurements were performed over five different types of underlying surface (grass, barley, triticale, pine forest, water) in a heterogeneous landscape using eddy covariance and profile techniques over a three week time period in June, 1998. Estimates of the area-integrated sensible heat flux during daytime were obtained from continuous measurements with a large aperture scintillometer (LAS) along a 4.7¿km path. The calculation of a mean diurnal cycle of the fluxes during the experiment revealed significant differences between the main land use classes. A land-use weighted average of the sensible heat flux was found to be in good agreement with the LAS based estimate, which in turn was supported by other regionally integrated flux estimates from budget considerations and aircraft measurements for a few case studies. The profiles of turbulent quantities measured along a 99¿m-tower significantly deviate from ¿idealised¿ profiles measured over homogeneous terrain. Peculiarities in the profile structure could be attributed to the heterogeneity of the terrain, namely to the differences in the surface characteristics of the footprint areas for the different tower levels