December 3, 2020 - 8:00 am
December 3, 2020 - 9:00 am
CategoriesSchool of Meteorology Colloquium
School of Meteorology Colloquium
Assessing Dual-Doppler Vertical Velocity Retrievals from Rapid-Scan Radar Data
Thursday, December 3rd
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Vertical velocity is the most difficult wind component to accurately retrieve from dual-Doppler observations. In recent, years it has been proposed to use a vertical vorticity equation constraint in addition to the mass conservation equation constraint in order to improve DDA vertical velocities. Prior OSSEs have shown that the vorticity equation constraint can improve vertical velocity retrievals in situations with missing low-level data and radar volume scan times that are sufficiently short to calculate the vorticity tendency accurately. However, up until now, the technique has never been tested with real observations from rapid-scan radars.
The vertical vorticity equation constrained DDA was tested using a dual-Doppler dataset collected by RaXPol and the AIR on 4 Sept 2018. The SMART-R3 was positioned under the storm and conducted near-vertical PPIs, which were used as a verification dataset. In general, the vorticity equation constrained DDA was able to improve vertical velocity retrievals, but the improvement was dependent on the time between volume scans and the technique used to calculate the vorticity tendency. One key difference between these results and those of the prior OSSE experiments is that the improvement in the retrieved vertical velocities occurred even though there was not a large data void between the lowest data level and the ground. Additionally, the vorticity equation constrained DDA was found to be more forgiving to radar data errors than the other DDA techniques. Considering that observation errors are a common occurrence in radar datasets, these results indicate that the vorticity equation constrained DDA could be more beneficial than what the original OSSE studies have shown.
EnKF analysis is another wind retrieval technique that has gained popularity in recent years. Unfortunately, in this study the EnKF analyses were hampered by catastrophic filter divergence. The adaptive inflation that was used to maintain ensemble spread due to the high number of observations caused large spread to develop in data sparse regions. This led to extremely large analysis increments through correlations with regions that had radial velocity observations. Despite the catastrophic filter divergence, EnKF analyses that used radar observations that were thinned to 150-s intervals produced vertical velocities that had better verification statistics than the best DDA. This suggests that if the filter divergence issue is controlled, EnKF analyses could have more accurate vertical velocities than those obtained by DDA.
Most importantly, the results shown in this work highlight that rapid-scan radar data is beneficial to vertical velocity retrievals when that data is paired with a vorticity equation constrained DDA. This DDA technique and the use of rapid-scan radars should be prioritized in future observational studies of convective storms.