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Delta ($\delta$) sunspots sometimes host fast photospheric flows along thecentral magnetic polarity inversion line (PIL). Here we study the strongDoppler shift signature in the central penumbral light bridge of solar activeregion NOAA 12673. Observations from the Helioseismic and Magnetic Imager (HMI)indicate highly sheared, strong magnetic fields. Large Doppler shifts up to 3.2km s$^{-1}$ appeared during the formation of the light bridge and persisted forabout 16 hours. A new velocity estimator, called DAVE4VMwDV, reveals fastconverging and shearing motion along the PIL from HMI vector magnetograms, andrecovers the observed Doppler signal much better than an old version of thealgorithm. The inferred velocity vectors are largely (anti-)parallel to theinclined magnetic fields, suggesting that the observed Doppler shift containssignificant contribution from the projected, field-aligned flows.High-resolution observations from the Hinode/Spectro-Polarimeter (SP) furtherexhibit a clear correlation between the Doppler velocity and the cosine of themagnetic inclination, which is in agreement with HMI results and consistentwith a field-aligned flow of about 9.6 km s$^{-1}$. The complex Stokes profilessuggest significant gradients of physical variables along the line of sight. Wediscuss the implications on the $\delta$-spot magnetic structure and theflow-driving mechanism.

Large Photospheric Doppler Shift in Solar Active Region 12673: I. Field-Aligned Flows