3DVAR versus Traditional Dual-Doppler Wind Retrievals of a Simulated Supercell Thunderstorm

Abstract

Use of the three-dimensional variational data assimilation (3DVAR) framework in dual-Doppler wind analysis (DDA) offers several advantages over traditional techniques. Perhaps the most important is that the errors that result from explicit integration of the mass continuity equation in traditional methods are avoided. In this study, observing system simulation experiments (OSSEs) are used to compare supercell thunderstorm wind retrievals from a 3DVAR DDA technique and three traditional DDA methods. The 3DVAR technique produces better wind retrievals near the top of the storm than the traditional methods in the experiments. This is largely attributed to the occurrence of severe errors aloft in the traditional retrievals whether the continuity equation integration proceeds upward (due to vertically accumulating errors), downward (due to severe boundary condition errors arising from uncertainty in the horizontal divergence field aloft), or in both directions. Smaller, but statistically significant, improvement occurs near the ground using the 3DVAR method. When lack of upper-level observations prevents application of a top boundary condition in the traditional DDA framework, the 3DVAR approach produces better analyses at all levels. These results strongly suggest the 3DVAR DDA framework is generally preferable to traditional formulations.

Vertical air motion retrievals in deep convective clouds using the ARM scanning radar network in Oklahoma during MC3E

“…This is the primary reason for capping our analysis domain at 10 km a.g.l. Although the upper level near the domain top can lack observation, the 3DVAR technique can produce better estimation compared with the iterative integration techniques (Potvin et al, 2012a).…”

“…A thorough sensitivity analysis could be used to determine this parameter space, but this is often ignored because studies typically consider theoretical wind retrieval performance by comparing it to a known truth field (e.g., model output in an OSSE). The weights optimized to minimize the residual error between the retrieved and truth wind fields are then adopted (e.g., Gao et al, 1999;Potvin et al, 2012a). For applications involving real radar datasets where no truth field is available, one must consider (i) determining the parameter space which produces physically sound wind fields and (ii) characterizing the solution spread within the parameter space determined by (i).…”

“…Iterative techniques solve the integral mass continuity equation throughout the column, contingent on known vertical velocity boundary conditions at the bottom level (i.e., upwards integration) and/or top level (i.e., downwards integration) (e.g., O'Brien, 1970;Ray et al, 1980;Protat and Zawadzki, 1999). This requires an estimate of horizontal wind divergence at each level made in a previous step, hence the non-simultaneity of iterative techniques (Dowell and Shapiro, 2003;Potvin et al, 2012a). By their nature, iterative upwards/downwards integration techniques propagate information in one direction, thus errors in horizontal wind divergence accumulate throughout the column, which in turn leads to larger errors in vertical velocity (e.g., Ray et al, 1980).…”

“…Since simultaneous techniques are by definition 3DVAR techniques, we will refer to them as such throughout the remainder of this study. The 3DVAR approach has previously been shown to provide more accurate dual-Doppler retrievals than traditional techniques in observing system simulation experiments (OSSEs; Gao et al, 1999;Potvin et al, 2012a).…”

Impact of Observation Operators on Low-Level Wind Speed Retrieved by Variational Multiple-Doppler Analysis

“…This method was first proposed by Armijo (1969) and various multiple-Doppler wind analysis procedures have been developed and evaluated subsequently (e.g., Ray et al 1975Ray et al , 1978Ray et al , 1980Doviak et al 1976;Bousquet and Chong 1998;Shapiro and Mewes 1999;Gao et al 1999Gao et al , 2004Liou and Chang 2009;Shapiro et al 2009;Liou et al 2012;Potvin et al 2012a, b, c). Potvin et al (2012c) reported that the variational multiple-Doppler wind analysis proposed by Gao et al (1999) is more accurate than the multiple-Doppler wind analysis that explicitly integrates the continuity equation vertically, such as that by Ray et al (1980). Therefore, the variational multiple-Doppler wind analysis has become the method commonly used for wind-field retrieval.…”