  We study the sensitivity of wave travel times to steady and spatially
homogeneous horizontal flows added to a realistic simulation of the solar
convection performed by Robert F. Stein, Ake Nordlund, Dali Georgobiani, and
David Benson. Three commonly used definitions of travel times are compared. We
show that the relationship between travel-time difference and flow amplitude
exhibits a non-linearity depending on the travel distance, the travel-time
definition considered, and the details of the time-distance analysis (in
particular, the impact of the phase-speed filter width). For times measured
using a Gabor wavelet fit, the travel-time differences become nonlinear in the
flow strength for flows of about 300 m/s, and this non-linearity reaches almost
60% at 1200 m/s (relative difference between actual travel time and expected
time for a linear behaviour). We show that for travel distances greater than
about 17 Mm, the ray approximation predicts the sensitivity of travel-time
shifts to uniform flows. For smaller distances, the ray approximation can be
inaccurate by more than a factor of three.
