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

High density NOAA time series of ET in the Gediz Basin, Turkey

An evapotranspiration method comparison was carried out by the International Water Management Institute (IWMI, Sri Lanka), at two locations in the Gediz Basin, Turkey, in the period from May to September 1998. In the IWMI study a number of ground-based techniques were compared with results obtained by remote sensing methods. Recently, a search of the satellite active archive yielded over 70 high quality level 1b images from NOAA/AVHRR over the same time period. The processing of these images with the SEBAL algorithm enabled us to build up a detailed time series of sensible and latent heat fluxes for a period of 120 days. In this paper a comparison is made between the sensible and latent heat fluxes determined from the present series of NOAA-14/AVHRR images and the results obtained earlier from various other prediction methods applied during the 1998 IWMI project. Specifically, the NOAA/SEBAL results are assessed against the scintillometer and temperature fluctuation methods. The results show that the NOAA derived evapotranspiration values follow the seasonal irrigation cycle quite well and correspond closely to the Landsat derived values, although they are lower than the results obtained with the traditional crop factor and Penman¿Monteith methods

Methodology to define and map the sub-region entities urban, peri-urban and rural areas

KB36-0063-02

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

Energy balance closure for the LITFASS-2003 experiment

In the first part, this paper synthesises the main results from a series of previous studies on the closure of the local energy balance at low-vegetation sites during the LITFASS-2003 experiment. A residual of up to 25% of the available energy has been found which cannot be fully explained either by the measurement uncertainty of the single components of the surface energy balance or by the length of the flux-averaging period. In the second part, secondary circulations due to heterogeneities in the surface characteristics (roughness, thermal and moisture properties) are discussed as a possible cause for the observed energy balance non-closure. This hypothesis seems to be supported from the fluxes derived from area-averaging measurement techniques (scintillometers, aircraft)