Reply

In the foregoing, we have addressed the criticisms expressed in S05 on the application of RKW theory to observed squall lines. Their first contention is that the results of a simple cold-pool-in-shear model are not relevant to observed density currents and squall lines. Our response is that S05's dimensionalization of the nondimensional results show in WR04's Fig. 5 was not done using "typical" values of cold pool temperature, and that by considering a truly typical range of observed cold pool strenghts, the simple model is indeed relevant to the range of commonly observed atmospheric conditions. The second contention addresses observations associated with one specific type of convective system, namely derechoes. Since derechoes represent a form of strong, long-lived convective system, and since the environments associated with derechoes appear suboptimal from the RKW perspective, they state the following: This leads us to question whether the value of C/ΔU has any relevance to the characteristics of observed squall lines. It certainly has no utility as a concept that can be used in the forecasting or nowcasting of squall lines when using standard observations that are available to forecasters. Our response is that RKW's theory addresses system structure and life cycle for the entire spectrum of squall lines, whether severe or not severe, and is supported by a multitude of case studies presented in the literature. On the other hand, the observational papers cited by S05 as not supporting RKW's concepts consider only a small subset of the observed squall-line spectrum (e.g., derechoes), and present virtually no information concerning system structure of life cycle, which is the essence of RKW's theory. However based on surface wind criteria (as is used to define derechoes), we point out that the observed environments are, in fact, consistent with the idealized modeling studies. Cold pool observations are especially difficult to come by in an operational setting. However, just because cold pools are not routinely observed, does not invalidate a theory that is based on their importance. Instead, we should be working to obtain better or more relevant observations (BAMEX is a good example of such an effort). A third contention addresses S05's perception that an insufficient range of cold pool strengths are presented in WR04 to establish the importance of the ratio C/ΔU to system structure. Here, we clarified that C does vary significantly over time within each simulation presented, and system structure responds as would be predicted. We also remind them that a much larger range of cold pool strengths were used in the original RKW and WKR studies, and the prescribed importance of C to system structure and life cycle has independently been established in other idealized simulation studies as well, for both midlatitude and tropical environments. Finally, S05 close with the following thought: Indeed, the question that operational forecasters have is not the academic question of whether or not the developing squall line is optimal, or even whether it could be stronger in some other environment, but the real-life question of whether the squall line will be severe or will sustain itself for another 300 km to threaten public safety in another part of the state or region. These are the questions that need serious attention, and the idea of optimality only appears to distract from the more important issues at hand. We agree that bow echoes and derechoes can occur for we aker magnitudes of shear than suggested in idealized simulations, or that, climatologically, shear layers are often not confined to 2.5 or 5.0 km alone. However, it is still important for forecasters to note when low-level shear is exceptionally strong, as all model results to date clearly suggest that such environments have the most potential for producing severe squall lines and bow echoes. Indeed, the Coniglio et al. (2004) observations clearly show a preponderance of shear in the lowest 3 km for derecho cases, with half of their cases displaying shear magnitudes greated than 20 m s^-1, consistent with all idealized modeling results. On the other hand, virtually none of Coniglio et al.'s (2004) cases show an absence of low-level shear. "Optimality" as defined by RKW is not the issue here; bow echoes have always been presented as "suboptimal" from the RKW perspective (i.e., cold pools must be able to strengthen to the point where C/ΔV > 1 for bow echoes to develop, even for strongly sheared environments; Weisman 1993; Weisman and Davis 1998). What is at issue here is the ability of forecasters to recognize the range of environments and associated physical processes that produce the full spectrum of convective-system types, whether severe or nonsevere. In this regard, RKW concepts and the related bow echo studies suggest that environments with strong low-level shear might be especially conductive to extreme, damaging wind events, and we submit that this does, indeed, address public safety forecast decision making. Basic science proceeds best through well thought out experiments (whether observational or numerical). We have tried to accomplish this with our idealized modelling studies. It is certainly appropriate and important to test such results against observations, and we have striven to do so when sufficient observations have been available (indeed, the studies of RKW and WKR were motivated originally to explain the "observed" range of squall-line characteristics). There are certainly other discrepencies between the modeling and observational studies that need to be explored further, such as microphysical sensitivities and the role of weaker or deeper shears. However, we feel that when one considers the full spectrum of observed squall-line structures and life cycles, RKW's concepts provide a solid foundation to build upon.