Retrieval of Model Initial Fields From Single-doppler Observations of A Supercell Thunderstorm. Part Ii: Thermodynamic Retrieval and Numerical Prediction

Abstract

In this two-part study, a single-Doppler parameter retrieval technique is developed and applied to a real-data case to provide model initial conditions for a short-range prediction of a supercell thunderstorm. The technique consists of the sequential application of a single-Doppler velocity retrieval (SDVR), followed by a variational velocity adjustment, a thermodynamic retrieval, and a moisture specification step. In Part I, the SDVR procedure is described and results from its application to a supercell thunderstorm are presented. In Part II, results from the thermodynamic retrieval and the numerical model prediction for this same case are presented. For comparison, results from parallel sets of experiments using dual-Doppler-derived winds and winds obtained from the simplified velocity retrieval described in Part I are also shown. Following the SDVR, the retrieved wind fields (available only within the storm volume) are blended with a base-state background field obtained from a proximity sounding. The blended fields are then variationally adjusted to preserve anelastic mass conservation and the observed radial velocity. A Gal-Chen type thermodynamic retrieval procedure is then applied to the adjusted wind fields. For all experiments (full retrieval, simplified retrieval, and dual Doppler), the resultant perturbation pressure and potential temperature fields agree qualitatively with expectations for a deep-convective storm. An analysis of the magnitude of the various terms in the vertical momentum equation for both the full retrieval and dual-Doppler experiments indicates a reasonable agreement with predictions from linear theory. In addition, the perturbation pressure and vorticity fields for both the full retrieval and dual-Doppler experiments are in reasonable agreement with linear theory predictions for deep convection in sheared flow. Following a simple moisture specification step, short-range numerical predictions are initiated for both retrieval experiments and the dual-Doppler experiment. In the full single-Doppler retrieval and dual-Doppler cases, the general storm evolution and deviant storm motion are reasonably well predicted for a period of about 35 minutes. In contrast, the storm initialized using the simplified wind retrieval decays too rapidly, indicating that the additional information obtained by the full wind retrieval (primarily low-level polar vorticity) is vital to the success of the numerical prediction. Sensitivity experiments using the initial fields from the full retrieval indicate that the predicted storm evolution is strongly dependent on the initial moisture fields. Overall, the numerical prediction results suggest at least some degree of short-term predictability for this storm and provide an impetus for continued development of single-Doppler retrieval procedures.