ESTIMATING DIVERGENCE AND VORTICITY FROM THE WIND PROFILER NETWORK HOURLY WIND MEASUREMENTS

The large-scale horizontal divergences computed from the kinematic and vorticity methods based on a four-dimensional least squares fit (4-D LSF) to the hourly wind measurements from the Wind Profiler Demonstrat ion Network (WPDN) are compared to the divergence derived from a four- dimensional data assimilation (4-DDA) system. The kinematic method com putes divergence by adding the appropriate terms of the velocity deriv atives, while the vorticity method uses the dynamical balance of terms in the vorticity equation. The 4-D LSF method produces wind and vorti city analyses by approximating the analyzed wind field as a series of basis functions dependent on both space and time. The sensitivity of t he basis functions is studied with an analytic solution and with real data. It is shown that the LSF results are not sensitive to the choice of basis functions as long as they include adequate variation to reso lve the data. The high temporal resolution divergence and vorticity fi elds, which are obtained directly from hourly WPDN data, are used to i ndependently validate the 4-DDA derived vorticity and divergence, whic h depend both on observational data and on the dynamics and physics in data assimilation models. For the initial case study, the relative er rors between the 4-D LSF or the 4-DDA analysis and the WPDN measuremen ts are essentially the same, and the smooth parts of the vorticity der ived from these two analyses are similar. The large-scale divergence d erived from the vorticity method is also in good agreement with that d erived from the 4-DDA analysis. However, the divergence derived from t he kinematic method shows little resemblance to those derived from the vorticity method and 4-DDA analysis.