Determining the Radar Error in a Tornadic Low-Level Wind Field

The low-level wind field was examined at two times during the Moore, Oklahoma tornado on May 20, 2013. The objective of this study is to quantify the error in radar measurement of low-level winds. The near-ground axisymmetric wind field from the Oklahoma City TDWR radar was retrieved by using the ground-based velocity track display (GBVTD) technique. The retrieved radial and tangential winds were used to calculate the horizontal winds at 100 meters above ground level, the lowest level available from the GBVTD analysis. The retrieved horizontal winds were plotted on the Moore tornado damage map every 75 meters from the centerline of the tornado track. Tree fall along the damage track is an indicator of the low-level tornadic wind field. The tree fall angles on the damage map were compared to the plotted angles of the horizontal winds to quantify the error in the radar-indicated low-level wind field. The direction of the radar-indicated horizontal velocity vector differs significantly from the tree fall direction, suggesting that the radar is not resolving the low-level wind field correctly. It is hypothesized that this difference is due to debris centrifuging. The magnitudes of the horizontal winds were also compared to the Enhanced Fujita scale (EF) contours on the damage map. Preliminary analysis suggests that the radar detected wind speeds are smaller in magnitude than the EF contours depict. Photogrammetry was used to calculate the low-level vertical velocities on the edges of the condensation funnel. These velocities are compared to the GBVTD retrieved low-level vertical velocities. More comprehensive findings from the damage map analysis and the photogrammetry vertical velocity comparison will be presented at the conference.