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

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

Spatial and temporal patterns of surface-atmosphere energy exchange in a dense urban environment using scintillometry

Spatially-integrated measurements of the surface energy balance (SEB) are needed in urban areas to evaluate urban climate models and satellite observations. Scintillometers allow observations of sensible heat flux (QH) over much larger areas than techniques such as eddy covariance (EC), however methods are needed to partition between remaining unmeasured SEB terms. This is the first study to use observed spatial and temporal patterns of QH from a scintillometer network to constrain estimates of remaining SEB terms in a dense, heterogeneous urban environment. Results show that QH dominates the surface energy balance in central London throughout the year, with expected diurnal courses and seasonal trends in QH magnitude related to solar radiation input. Measurements also reveal a clear anthropogenic component of QH with winter (summer) weekday QH values 11.7% (5.1%) higher than weekends. Spatially, QH magnitude is correlated with vegetation and building land cover fraction in the measurement source areas. Spatial analysis provides additional evidence of anthropogenic influence with highest weekday/weekend ratios (1.55) from the City of London. Spatial differences are used to estimate horizontal advection and a novel method to estimate monthly latent heat flux is developed based on observed land cover and wet-dry surface variations in normalized QH. Annual anthropogenic heat emissions are estimated to be 46.3 W m−2 using an energy balance residual approach. The methods presented here have potential to significantly enhance understanding of urban areas, particularly in areas with tall buildings where there is little observational data

The energy balance experiment EBEX-2000. Part III: Behaviour and quality of the radiation measurements

An important part of the Energy Balance Experiment (EBEX-2000) was the measurement of the net radiation and its components. Since the terrain, an irrigated cotton field, could not be considered homogeneous, radiation measurements were made at nine sites using a variety of radiation instruments, including pyranometers, pyrgeometers and net radiometers. At several of these sites multiple instruments were employed, which enabled us to compare instruments and assess accuracies. At all sites the outgoing longwave and shortwave radiation and the net radiation were measured, while the incoming radiation was supposed to be uniformly distributed over the field and was therefore measured at three sites only. Net radiation was calculated for all sites from the sum of its four components, and compared with the direct measurement of net radiometers. The main conclusions were: (a) the outgoing shortwave radiation showed differences of up to 30 W m-2 over the field; the differences were not clearly related to the irrigation events; (b) the outgoing longwave radiation showed differences of up to 50 W m-2; the differences increased during the periods of irrigation; (c) the net radiation showed differences of several tens of W m-2 across the field, rising to 50 W m-2 or more during the periods of irrigation; (d) the net radiation is preferably to be inferred from its four components, rather than measured directly, and (e) attention should be paid to the characteristics of pyranometers that measure the outgoing radiation, and thus are mounted upside down, while they are commonly calibrated in the upward position. The error in the net radiation at EBEX-2000 is estimated at max (25 W m-2, 5%) per site during the day and 10 W m-2 at nigh

The energy balance experiment EBEX-2000. Part II: Intercomparison of eddy-covariance sensors and post-field data processing methods

The eddy-covariance method is the primary way of measuring turbulent fluxes directly. Many investigators have found that these flux measurements often do not satisfy a fundamental criterion¿closure of the surface energy balance. This study investigates to what extent the eddy-covariance measurement technology can be made responsible for this deficiency, in particular the effects of instrumentation or of the post-field data processing. Therefore, current eddy-covariance sensors and several post-field data processing methods were compared. The differences in methodology resulted in deviations of 10% for the sensible heat flux and of 15% for the latent heat flux for an averaging time of 30 min. These disparities were mostly due to different sensor separation corrections and a linear detrending of the data. The impact of different instrumentation on the resulting heat flux estimates was significantly higher. Large deviations from the reference system of up to 50% were found for some sensor combinations. However, very good measurement quality was found for a CSAT3 sonic together with a KH20 krypton hygrometer and also for a UW sonic together with a KH20. If these systems are well calibrated and maintained, an accuracy of better than 5% can be achieved for 30-min values of sensible and latent heat flux measurements. The results from the sonic anemometers Gill Solent-HS, ATI-K, Metek USA-1, and R.M. Young 81000 showed more or less larger deviations from the reference system. The LI-COR LI-7500 open-path H2O/CO2 gas analyser in the test was one of the first serial numbers of this sensor type and had technical problems regarding direct solar radiation sensitivity and signal delay. These problems are known by the manufacturer and improvements of the sensor have since been made

Multi-scale sensible heat fluxes in the urban environment from large aperture scintillometry and eddy covariance

Sensible heat fluxes (QH) are determined using scintillometry and eddy covariance over a suburban area. Two large aperture scintillometers provide spatially integrated fluxes across path lengths of 2.8 km and 5.5 km over Swindon, UK. The shorter scintillometer path spans newly built residential areas and has an approximate source area of 2-4 km2, whilst the long path extends from the rural outskirts to the town centre and has a source area of around 5-10 km2. These large-scale heat fluxes are compared with local-scale eddy covariance measurements. Clear seasonal trends are revealed by the long duration of this dataset and variability in monthly QH is related to the meteorological conditions. At shorter time scales the response of QH to solar radiation often gives rise to close agreement between the measurements, but during times of rapidly changing cloud cover spatial differences in the net radiation (Q*) coincide with greater differences between heat fluxes. For clear days QH lags Q*, thus the ratio of QH to Q* increases throughout the day. In summer the observed energy partitioning is related to the vegetation fraction through use of a footprint model. The results demonstrate the value of scintillometry for integrating surface heterogeneity and offer improved understanding of the influence of anthropogenic materials on surface-atmosphere interactions