First Results of an Optical and Millimeter Wave Scintillometer system at the Chilbolton test range

Scintillometry has become a generally accepted technique to obtain turbulent fluxes at the 0.1 – 10 km scale. Optical large aperture scintillometers (LAS) that yield the sensible heat flux (H) have been tested under a wide range of circumstances (De Bruin, 2002) and are commercially available. With a millimeter wave scintillometer (MWS) in combination with an optical scintillometer the evapotranspiration (L_vE), can be determined. The aim is to develop such a combined Optical Millimeter Wave Scintillometer (OMS) system for operational use.

In this study we will present the scientific challenges we face in developing the OMS and present the first results of an experiment we conducted at the Chilbolton test-range of the Rutherford Appleton Laboratory (RAL) in the UK in the summer of 2007. At this experiment we deployed a prototype MWS built by RAL in combination with two types of LAS's.

For an operational OMS several scientific issues have to be solved, e.g. scintillations caused by water vapor absorption, direct observation of the correlation between atmospheric temperature and humidity fluctuations, saturation of the LAS signal for long paths and the sensitivity of the OMS to mast vibrations.

The 500m long Chilbolton test range acts as an outdoor laboratory with highly controlled conditions, perfect for testing scintillometer prototypes: the instrument cabins are placed at exactly the same height (4m) and the benches are supported by large concrete pillars, thereby ruling out sensor vibration.

We deployed a mirror based LAS built by Wageningen University and a Fresnel lens based LAS, which is sold by Kipp and Zonen, Delft, the Netherlands. The MWS prototype built by RAL in collaboration with the Centre for Ecology and Hydrology, Wallingford, U.K., operates at 94 GHz and has the special feature that it uses a GPS receiver as a reference to lock the transmitter and receiver frequencies. This makes the RAL94 a low power MWS, as no temperature controlled crystal is needed for frequency synthesis. All raw signals of the scintillometers were measured and stored at 500Hz, providing maximum freedom in the data processing, e.g. allowing spectral filtering. As a reference for the scintillometer output, we also installed eddy-covariance (EC) equipment in the middle of the test-range to independently determine fluxes of heat, latent-heat and momentum as well as the structure parameters of temperature, C_T², humidity, C_q², and cross-term structure parameter, C_Tq.

Results are presented here showing the comparison of these variables, as measured by the different techniques, and the first assessment of the usefulness of the OMS combination to measure L_vE.