Use of commercial microwave links as scintillometers: potential and limitations towards evaporation estimation

Abstract. Scintillometers are used to measure path-integrated evaporation and sensible heat fluxes. They consist of a transmitter and a receiver separated along a line of sight of several hundreds of meters to a few kilometers. Turbulent eddies and the associated refractive index fluctuations along the path between transmitter and receiver cause diffraction of the transmitted beam, known as the scintillation effect. Optical and microwave scintillometers have been designed to measure the full spectral range of the signal intensity fluctuations caused by this phenomenon and quantitatively link these fluctuations to turbulent sensible- and latent heat fluxes. Commercial Microwave Links (CMLs), such as used in cellular telecommunication networks, are also line-of-sight instruments that measure signal intensity of microwave signals. However, CMLs are not designed to capture scintillation fluctuations. Here, we investigate if and under what conditions CMLs can be used to obtain the structure parameter of the refractive index, C_nn, which would be a first step in computing turbulent heat fluxes with CMLs using scintillation theory. We use data from three collocated microwave links installed over a 856 m path at the Ruisdael Observatory near Cabauw, the Netherlands. Two of these links are 38 GHz CMLs formerly employed in telecom networks in the Netherlands, a Nokia Flexihopper and an Ericsson MiniLink. We compare C_nn estimates obtained from the received signal intensity of these links, sampled at 20 Hz, with those obtained from measurements of a 160 GHz microwave scintillometer (RPG-MWSC) sampled at 1 kHz and of an eddy-covariance system. After comparison of the unprocessed C_nn, we rejected the Ericsson MiniLink, because its 0.5 dB power quantization (i.e., the discretization of the signal intensity) was found to be too coarse to be applied as a scintillometer. Based on power spectra of the Nokia Flexihopper and the microwave scintillometer, we propose two methods to correct for the white noise present in the signal of the Nokia Flexihopper: 1) we apply a high-pass filter and subtract the noise based on a comparison with the microwave scintillometer, and building on that 2) we select parts of the power spectra where the Nokia Flexihopper behaves in correspondence with scintillation theory, also considering different crosswind conditions, and correct for the underrepresented part of the scintillation spectrum based on theoretical scintillation spectra. The comparison and noise determination with the microwave scintillometer is provides the best possible C_nn estimates for the Nokia Flexihopper, although this is not feasible in operational settings for CMLs. Both of our proposed methods show an improvement of C_nn estimates in comparison to uncorrected estimates, albeit with a larger uncertainty than the reference instruments. Our study illustrates the potential of using CMLs as scintillometers, but also outlines some major drawbacks, most of which are related to unfavourable design choices made for CMLs. If these would be overcome, given their global coverage, the potential of CMLs for large scale evaporation monitoring would be unprecedented.