Analysis of the Systematic Errors Found in the Kipp & Zonen Large-Aperture Scintillometer

Studies have shown a systematic error in the Kipp & Zonen large-aperture scintillometer (K&ZLAS) measurements of the sensible heat flux, H. We improved on these studies and compared four K&ZLASs with a Wageningen large-aperture scintillometer at the Chilbolton Observatory. The scintillometers were installed such that their footprints were the same and independent flux measurements were made along the measurement path. This allowed us to compare H and the direct scintillometer output, the refractive index structure parameter, C2n. Furthermore, spectral analysis was performed on the raw scintillometer signal to investigate the characteristics of the error. Firstly, correlation coefficients =0.99 confirm the robustness of the scintillometer method, and secondly we discovered two systematic errors: the low-C2n error and the high-C2n error. The low-C2n error is a non-linear error that is caused by high-frequency noise, and we suspect the error to be caused by the calibration circuit in the receiver. It varies between each K&ZLAS, is significant for H = 50 W m-2, and we propose a solution to remove this error using the demodulated signal. The high-C2n error identified by us is the systematic error found in previous studies.We suspect this error to be caused by poor focal alignment of the receiver detector and the transmitter light-emitting diode that causes ineffective use of the Fresnel lens in the current Kipp & Zonen design. It varies between each K&ZLAS (35% up to 240%) and can only be removed by comparing with a reference scintillometer in the field

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

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

WindVisions: first phase final report

It is the objective of this project to develop a Wind and Visibility Monitoring System (WindVisions) at Mainport Schiphol. WindVisions will consist of a crosswind scintillometer, which is a horizontal long range wind and visibility sensor, and a SODAR (Sound Detecting And Ranging), a vertical scanning wind sensor. The area of interest to monitor is the landing and take-off course of airplanes ranging from the surface to about 300m height along a runway

Estimating ET using scintillometers and satellites in an irrigated vineyard in the Costa De Hermosillo, Sonora, Mexico

Observation techniques for surface energy balancecomponents on kilometer scale. Several methods have been proposed to estimate ET overlarge areas which combine Earth Observation Satellite datawith standard climate data. Here we use the Makkinkapproach where incoming solar radiation is obtained bycalculating exoatmospheric incoming solar radiation, usingvisible data from GOES West to estimate cloudiness and16-day composite MODIS Enhanced Vegetation Index data toestimate fractional vegetation cover. This methodology can beused operationally at a spatial resolution of 1 km2 butvalidation data are required at a similar spatial scale. LargeAperture Scintillometers transmit and receive near infraredradiation over distances of several kilometers and providemeasurements of the structure parameter for the refractiveindex of air which is related by Monin-Obukhov SimilarityTheory to the surface sensible heat flux (H), which requiresmeasurements of air temperature, pressure and wind speed.ET can then be obtained indirectly as the residual of theenergy balance, ET = Rn – G – H so that we need estimates ofNet Radiation (Rn) and Soil Heat Flux (G) as well. Ascintillometer (Scintec BLS-450) was installed over anirrigated vineyard with area of 72 hectares (1200m x 600m) inJune 6th 2009. Net Radiation was measured in the field andsoil heat flux was estimated using G = A * Rn, where theparameter A was obtained from 8-day composite MODIS LandSurface Temperature data. Comparison of ET derived fromsatellite and scintillometer for June 2009 showed considerablescatter with r2 = 0.81 and ETSAT = 1.12 * ETBLS. Thisapparent overestimation from the satellite-based ET is similarto that found in previous work. However, in these initialcalculations it was assumed that unstable conditions wouldprevail during the daytime but it appears that stableconditions often occur in the late afternoon. Secondly, theestimation of G needs to be carefully revised since this canhave a large effect on ET. These factors are being included inthe analysis of data over an entire growing season to assessthe seasonal behavior of the model

The BLLAST field experiment: Boundary-Layer Late Afternoon and Sunset Turbulence

Due to the major role of the sun in heating the earth's surface, the atmospheric planetary boundary layer over land is inherently marked by a diurnal cycle. The afternoon transition, the period of the day that connects the daytime dry convective boundary layer to the night-time stable boundary layer, still has a number of unanswered scientific questions. This phase of the diurnal cycle is challenging from both modelling and observational perspectives: it is transitory, most of the forcings are small or null and the turbulence regime changes from fully convective, close to homogeneous and isotropic, toward a more heterogeneous and intermittent state. These issues motivated the BLLAST (Boundary-Layer Late Afternoon and Sunset Turbulence) field campaign that was conducted from 14 June to 8 July 2011 in southern France, in an area of complex and heterogeneous terrain. A wide range of instrumented platforms including full-size aircraft, remotely piloted aircraft systems, remote-sensing instruments, radiosoundings, tethered balloons, surface flux stations and various meteorological towers were deployed over different surface types. The boundary layer, from the earth's surface to the free troposphere, was probed during the entire day, with a focus and intense observation periods that were conducted from midday until sunset. The BLLAST field campaign also provided an opportunity to test innovative measurement systems, such as new miniaturized sensors, and a new technique for frequent radiosoundings of the low troposphere. Twelve fair weather days displaying various meteorological conditions were extensively documented during the field experiment. The boundary-layer growth varied from one day to another depending on many contributions including stability, advection, subsidence, the state of the previous day's residual layer, as well as local, meso- or synoptic scale conditions. Ground-based measurements combined with tethered-balloon and airborne observations captured the turbulence decay from the surface throughout the whole boundary layer and documented the evolution of the turbulence characteristic length scales during the transition period. Closely integrated with the field experiment, numerical studies are now underway with a complete hierarchy of models to support the data interpretation and improve the model representations

Role of the residual layer and large-scale subsidence on the development and evolution of the convective boundary layer

Observations, mixed-layer theory and the Dutch Large-Eddy Simulation model (DALES) are used to analyze the dynamics of the boundary layer during an intensive operational period (1 July 2011) of the Boundary Layer Late Afternoon and Sunset Turbulence campaign. Continuous measurements made by remote sensing and in situ instruments in combination with radio soundings, and measurements done by remotely piloted aircraft systems and two manned aircrafts probed the vertical structure and the temporal evolution of the boundary layer during the campaign. The initial vertical profiles of potential temperature, specific humidity and wind, and the temporal evolution of the surface heat and moisture fluxes prescribed in the models runs are inspired by some of these observations. The research focuses on the role played by the residual layer during the morning transition and by the large-scale subsidence on the evolution of the boundary layer. By using DALES, we show the importance of the dynamics of the boundary layer during the previous night in the development of the boundary layer at the morning. DALES numerical experiments including the residual layer are capable of modeling the observed sudden increase of the boundary-layer depth during the morning transition and the subsequent evolution of the boundary layer. These simulations show a large increase of the entrainment buoyancy flux when the residual layer is incorporated into the mixed layer. We also examine how the inclusion of the residual layer above a shallow convective boundary layer modifies the turbulent kinetic energy budget. Large-scale subsidence mainly acts when the boundary layer is fully developed, and, for the studied day, it is necessary to be considered to reproduce the afternoon observations. Finally, we also investigate how carbon dioxide (CO2) mixing ratio stored the previous night in the residual layer plays a fundamental role in the evolution of the CO2 mixing ratio during the following day