We report nonlinear differential-resistance measurements in a high-mobility GaAs/AlGaAs two-dimensional electron system patterned in a Corbino geometry. At zero magnetic field, $R_{\mathrm{diff}}\left(I_{\mathrm{dc}}\right)$ exhibits a pronounced polarity-selective threshold peak on the negative-current branch, gradually suppressed by a perpendicular magnetic field. We interpret the Corbino anomaly as a local Cherenkov-like threshold for acoustic phonon emission, enabled by the radial current concentration $j_r\left(r\right)=I/\left(2\pi r\right)$, which drives the local electron velocity above the sound velocity in a region adjacent to the inner contact. The polarity selectivity is attributed to Peltier heating and cooling, which modify the local thermal and boundary conditions near the contact, rather than to the kinematic threshold itself. Our results identify the Corbino geometry as a sensitive platform for probing local nonequilibrium electron-phonon processes in a high-mobility electron system within an independently established hydrodynamic-crossover regime, and they suggest that the inner Corbino contact can act as a geometry-defined source for supersonic acoustic emission.

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• Classical transport
• Electrical conductivity
• Electrical properties
• Electron relaxation
• Mesoscopics
• Quantum transport
• Transport phenomena