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

Open data and smallholder food and nutritional security

This report was commissioned CTA as a member of the GODAN initiative. It aims to provide a better understanding of the actual impact of the open data movement on the food and nutrition security of smallholders and highlight the areas of potential unfilled opportunity

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

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

Scintillometry in urban and complex environments: a review

Knowledge of turbulent exchange in complex environments is relevant to a wide range of hydro-meteorological applications. Observations are required to improve understanding and inform model parameterisations but the very nature of complex environments presents challenges for measurements. Scintillometry offers several advantages as a technique for providing spatially-integrated turbulence data (structure parameters and fluxes), particularly in areas that would be impracticable to monitor using eddy covariance, such as across a valley, above a city or over heterogeneous landscapes. Despite much of scintillometry theory assuming flat, homogeneous surfaces and ideal conditions, over the last 20 years scintillometers have been deployed in increasingly complex locations, including urban and mountainous areas. This review draws together fundamental and applied research in complex environments, to assess what has been learnt, summarise the state-of-the-art and identify key areas for future research. Particular attention is given to evidence, or relative lack thereof, of the impact of complex environments on scintillometer data. Practical and theoretical considerations to account for the effects of complexity are discussed, with the aim of developing measurement capability towards more reliable and accurate observations in future. The usefulness of structure parameter measurements (in addition to fluxes, which must be derived using similarity theory) should not be overlooked, particularly when comparing or combining scintillometry with other measurement techniques and model simulations