AbstractDoppler radars provide measurements of the radar radial wind component at high spatial and temporal resolution. The variational data‐assimilation framework enables direct use of these measurements in numerical weather prediction models. Bias estimation of Doppler‐radar radial winds requires special attention because of the azimuthal scanning strategy. Calculation of the bias statistic over all azimuthal directions results in a near‐zero value even in the presence of systematic differences between the measured and modelled wind speed and direction. This paper describes a method for bias estimation of Doppler‐radar radial‐wind observations. Intercomparison between the estimated biases of Doppler‐radar and radiosonde wind observations reveals that the wind speed and direction biases are of comparable magnitude. Copyright © 2007 Royal Meteorological Society