TOFU: A Novel Doppler Unfolding Technique Using Optical Flow

Doppler radars measure Doppler velocity within the [-V_N, V_N] range, where V_N is the Nyquist velocity. Doppler velocities outside of this range are "folded" within this interval. All Doppler "unfolding" techniques use the folded velocities themselves. In this work, we investigate the potential of using velocities derived from optical flow techniques applied to the radar reflectivity field for that purpose. The analysis of wind speed errors using six months of dual-Doppler wind retrievals showed that 99.9% of all points are characterized by errors smaller than 26 ms^-1 below 5 km height, corresponding to a failure rate of less than 0.01% if optical flow winds were used to unfold Doppler velocities for V_N = 26 ms^-1. These errors largely increase above 5 km height, indicating that vertical continuity tests should be included to reduce failure rates at higher elevations. Following these results, we have developed the Two-step Optical Flow Unfolding (TOFU) technique, with the specific objective to accurately unfold Doppler velocities with V_N = 26 ms^-1. The TOFU performance was assessed using challenging case studies, comparisons with an advanced Doppler unfolding technique using higher Nyquist velocities, and six months of high V_N (47.2 ms^-1) data artificially folded to 26 ms^-1. TOFU failure rates were found to be very low. Three main situations contributed to these errors: high low-level wind shear, elevated cloud layers associated with high winds, and radar data artefacts. Our recommendation is to use these unfolded winds as the first step of advanced Doppler unfolding techniques.