Using Doppler lidar observations to verify wind profile forecasts from the ECMWF IFS model

Long-range scanning Doppler lidars can provide the vertical profile of the horizontal wind within the boundary layer. We used Doppler lidar wind profiles from six locations around the globe to verify the wind profile forecasts generated by ECMWF from the operational IFS model. The six locations selected cover a variety of surface types (rural, marine, mountainous urban, coastal urban).

We first validated the Doppler lidar observations at four locations by comparison with collocated radiosonde profiles to ensure that the Doppler lidar observations were of sufficient quality. The two observation types agree well, with the mean absolute error (MAE) in wind speed almost always less than 1 m s^-1. Large deviations in the wind direction were usually seen only for low wind speeds. This is due to the ambiguity in wind direction at low wind speed and that the relative uncertainty in the wind direction measurement increases as the wind speeds decrease.

The Doppler lidar observations are at sufficient temporal resolution for us to generate time-height composites of the wind verification with one-hour resolution so that we can investigate the diurnal cycle. Verification of the model winds showed that the IFS model performs best over marine (ocean) locations. Larger errors were seen in locations where the surface was more complex, especially in the wind direction. For example, in Granada, which is near a high mountain range, the IFS model failed to capture a commonly occurring mountain breeze, a feature which is highly dependent on the orography at sub grid-scale.

At one location, we conditionally performed the wind verification based on the presence or absence of a low-level-jet diagnosed from the Doppler lidar observations. The IFS model was able to reproduce the presence of the low-level-jet but the wind speed maximum was about 2 m s^-1 lower than observed. This is attributed to the effective vertical resolution of the model being too coarse to create the strong gradients in wind speed observed.