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

Abstract. Scintillometers are used to estimate path-integrated evaporation and sensible heat fluxes. Commercial microwave links (CMLs), such as are used in cellular telecommunication networks, are similar line-of-sight instruments that also measure signal intensity of microwave signals, just like microwave scintillometers do. 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, Cnn, 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 an 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 Cnn 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 with those of an eddy-covariance system. After comparison of the unprocessed Cnn, 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 the 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 a low quantile of the resulting variances of the Nokia Flexihopper and (2) we correct for the noise by comparing with a microwave scintillometer (MWS) and select the 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 provide the best-possible Cnn estimates for the Nokia Flexihopper, although this method is not feasible in operational settings for CMLs. Both of our proposed methods show an improvement in Cnn estimates in comparison to uncorrected estimates, albeit with larger uncertainty than when using the reference instruments. Our study illustrates the potential for using CMLs as scintillometers but also outlines some major drawbacks, most of which are related to unfavourable design choices made for CMLs. If these were overcome, given their global coverage, there is potential for CMLs to perform large-scale evaporation monitoring.