Short-term (1-2 hour) forecasts of convective initiation rely, in part, upon the detection of convergence zones in the boundary layer. Detection of convergence requires accurate, high-resolution wind field estimates. In this study, we compare the performance of two wind field estimation techniques. TWINDS (Terminal Winds), an Optimal Estimation algorithm developed at Lincoln Laboratory that will be deployed operationally in the FAA's ITWS (the Integrated Terminal Weather System), and VDRAS (Variational Doppler Radar Analysis System), a 4DVAR algorithm developed and fielded by the RAP program at NCAR. These techniques differ markedly in their use of physical models: TWINDS applies no physical model constraints to its analysis, while VDRAS uses a 4DVar technique to fit the data with a boundary layer model as a strong constraint.

We studied a four-hour period on June 27, 2000 in the vicinity of DFW airport that produced several convective cells. Data were available from 3 Doppler radars (1 NEXRAD and 2 TDWRs), LLWAS and AWOS-ASOS ground stations. The data from the radars were most critical, and TWINDS and VDRAS wind fields were derived using both NEXRAD and TDWR (dual Doppler configuration), and then NEXRAD and TDWR separately (single Doppler configuration). We examined several characteristics of both techniques: ability to detect convergence zones in the boundary layer, noise in the convergence fields, sensitivity to data coverage, radar viewing geometry and input parameterizations, and computational load.

Both techniques successfully detected convergence zones at several outflow boundaries over the four-hour period of the study. Conversely, neither technique could generate convergence features in regions where no radar data were available or where the radar viewing angle was unfavorable (for the single Doppler analyses). Occasionally, convergence artifacts appeared at boundaries between data rich regions and data voids.

This case study clearly illustrates many of the similarities, capabilities and shortcomings of both techniques, through a full comparison will require study of additional cases. However, our results indicate that either technique may provide boundary layer convergence fields that can be useful to convective initiation forecast algorithms.

* This work was sponsored by the Federal Aviation Administration under Air Force Contract No. F19628-00-C-0002. The views expressed are those of the authors and do not reflect the official policy or position of the U.S. Government. Opinions, interpretations, conclusions, and recommendations are those of the authors and are not necessarily endorsed by the US Government.