Using Commercial Microwave Links as Scintillometers to Estimate Evaporation: Potential and Limitations

Scintillometers are common instruments to estimate path-integrated latent 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 microwave beam (known as the scintillation effect). Scintillometers have been designed to measure the full spectral range of signal intensity fluctuations caused by this phenomenon and to quantitatively link these fluctuations to turbulent heat fluxes.

Commercial Microwave Links (CMLs), such as used in cellular telecommunication networks, also make use of microwave signals. However, CMLs have obviously not been designed to capture scintillation fluctuations. We investigate if and under what conditions CMLs can be used to obtain the structure parameter of the refractive index, C_nn, a first step in computing turbulent heat fluxes with CMLs. To do so, we use data from three collocated microwave links installed over a 856 m path at the Ruisdael Observatory near Cabauw, the Netherlands. We compare received signal intensity fluctuations sampled at 20 Hz from two 38 GHz CMLs formerly employed in telecom networks in the Netherlands, a Nokia Flexihopper and an Ericsson MiniLink, with measurements from a 160 GHz microwave scintillometer (RPG-MWSC) sampled at 1 kHz and an eddy-covariance system. After comparison of the unprocessed C_nn, we reject the Ericsson MiniLink, because its 0.5 dB power quantization was found to be too coarse.

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 calibration with the microwave scintillometer; 2) we select parts of the power spectra in which the Nokia Flexihopper behaves similar to the microwave scintillometer and correct for the lacking part of the scintillation spectrum based on scintillation theory. Calibration on the microwave scintillometer provides the best possible C_nn estimates for the Nokia Flexihopper, although this is usually not feasible for CMLs. Both proposed methods show an improvement of C_nn estimates in comparison to unprocessed estimates, though with a larger uncertainty than the reference instruments. Subsequently, we employ these C_nn values (together with widely available auxiliary data) to estimate the turbulent heat fluxes and we compare our estimates to those from the scintillometer setup and the eddy-covariance system.

Overall, our study illustrates the potential of using CMLs as scintillometers for evaporation estimation, especially due to their large global coverage, but also lays out major drawbacks, most of which are related to unfavorable design choices made for commercial microwave links.