MIN_WIND_SPEED defines the minimum observable wind speed, which is then used to correct the model forcing data. Anemometers typically have a minimum observable wind speed on the order of 0.5 m/s. When forcing the VIC model with hourly wind speeds, the model's surface flux estimates have been observed to "hiccup" (especially in dry climates), while this does not impact the long term energy balance, it will be very noticable when looking at daily fluxes. Therefore, VIC will use MIN_WIND_SPEED to restrict the minimum value of wind speed used to force the model.
Default value is set to 0 m/s, which works well when the model is forced with daily or monthly winds.