1. Introduction
In a recent paper, Rose et al. (2002) documented a severe-weather-producing squall line associated with a cyclone that developed east of the Rocky Mountains on 19–21 June 2000. It was found that the convection was associated with a cold front aloft (CFA; Hobbs et al. 1996; Locatelli et al. 1995, 1998, 2002a,b), and the authors argued that the accuracy of severe weather forecasts may be dependent on the successful detection and prediction of CFAs. The main conclusion reached by Rose et al. was that the CFA contributed significantly to the development of the squall line by providing lifting that helped to initiate the convection and to maintain the squall line (p. 772). The purpose of the present comment is to show that the convection initially developed along a well-defined surface trough and that the role of the CFA, with respect to convection initiation, and especially maintenance, is unclear.
2. Initiation of the squall line
The 19–21 June 2000 cyclone was associated with three distinct convective systems. Rose et al. examined the third convective system, which was the most extensive. The authors contended that the first two lines of convection were attributable to mesoscale processes rather than synoptic-scale processes and were, therefore, beyond the scope of the paper. The third convective system that developed, which was attributed to a synoptic-scale CFA, was initiated at approximately 2000 UTC 20 June. By 0300 UTC 21 June, the convection extended from south-central Oklahoma to Michigan (see their Fig. 5).
Figures 1–6 show surface conditions from 1800 to 2300 UTC 20 June in the region in which the convective system of interest was initiated. At 1800 UTC (Fig. 1), outflow from an earlier convective system and an associated mesohigh were present in the northern half of Missouri and central Illinois. A surface low pressure trough, as also analyzed by Rose et al., extended from north-central Iowa southwestward to central Kansas and the Oklahoma panhandle. A prominent wind shift was present along the portion of the surface trough in Oklahoma and southwestern Kansas (180° wind shift), but the wind shift was more subtle farther northeast in Iowa. Strong surface convergence was along the trough from central Kansas to southwestern Iowa.
By 1900 UTC (Fig. 2), the first echoes began appearing along the surface trough in northeastern Kansas, and by 2000 UTC (Fig. 3), echoes became more numerous along the trough. Additional echoes also developed approximately 75 km ahead of the surface trough in extreme eastern Kansas, where surface observations indicated a subtle wind shift and convergence (winds veered from approximately 220° to 240° across this wind shift). By 2100 UTC (Fig. 4), outflow associated with the developing convection was detectable in the surface observations. The organization of the convection continued to improve by 2200 UTC (Fig. 5), and by 2300 UTC (Fig. 6), a well-developed squall line was in progress from southeastern Kansas to northeastern Missouri.
It is surprising that no surface observations were presented by Rose et al. between 1200 UTC 20 June and 0300 UTC 21 June to substantiate the claim that “the forecasting methods used by the National Weather Service for this case, which focused on the position of the surface trough … poorly predicted the extent and location of the severe weather” (p. 755). Figures 1–6 quite clearly show that the squall line was initiated by processes associated with the surface low pressure trough. One could possibly speculate that the CFA also played a role in initiation, for example, perhaps the subtle wind shift in eastern Kansas ∼75 km ahead of the trough (Fig. 3) was associated with processes occurring above the boundary layer related to the CFA, but I feel that it is misleading to claim that a forecaster focusing on the surface trough and its strong convergence would have erred significantly in the anticipation of the location of convection initiation.
3. Mesoscale model simulation
The fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5; Grell et al. 1994) was run with a 30-km horizontal grid spacing by Rose et al. in order to explore the role of the CFA in the initiation and maintenance of the squall line. Aside from concerns as to whether the simulation was an accurate representation of the event (especially with regard to the severe weather forecasting parameters discussed on pp. 768–772 of their paper), it is not clear how a model having parameterized convection (Grell 1993) can be used to examine convection initiation and maintenance. Such a model likely could represent processes that enhance the likelihood of convection initiation, such as the deepening of the moist layer and reduction of convective inhibition due to cooling aloft; however, the details of the role of the CFA in the initiation process demand the explicit resolution of the initiation process (probably no coarser than 1–4-km horizontal grid spacing). The process by which a CFA might initiate surface-based convection, which inherently involves the convergence and ascent of boundary layer air, could be well worth studying with a higher-resolution simulation.
As for the role of the CFA in the maintenance of the squall line and production of severe weather, this is even less clear than the possible role of the CFA in initiating the convection. Again, it is not apparent how a mesoscale model having parameterized convection can be used to investigate the role of the CFA in the squall-line dynamics. The fact that the squall line outran the surface trough and appeared to remain collocated with the CFA for several hours following initiation (e.g., their Figs. 5 and 6) does not necessarily imply a dynamical role for the CFA in maintaining the squall line. Squall-line longevity and motion depend largely on the balance between low-level inflow and outflow (e.g., Rotunno et al. 1988, among others). Thus, convective storms frequently propagate at a forward speed exceeding the forward speed of the boundary by which the storms were initiated (Wilson and Megenhardt 1997; LaDue and Wood 2000), so that several hours after initiation, the convection may be a considerable distance ahead of the larger-scale “trigger.” (Convection also may move at a forward speed slower than that of the initiating boundary, so that the convection trails the surface boundary hours after initiation.)
4. Final remarks
Rose et al. are to be commended for their contributions to the conceptual model of Pacific cyclone evolution when such systems enter the Great Plains after traversing the mountainous western United States, which may include the development of a cold front aloft (CFA). Furthermore, the dynamical role of CFAs in promoting convection initiation along and in advance of mesoscale boundaries probably is a subject worthy of exploration. However, in Rose et al.'s recent article to which this comment pertains, I believe the conclusion that a CFA played a significant role in the initiation and maintenance of the 20 June 2000 squall line to be highly uncertain. At the very least, the claim that the location of the convection could not have been well predicted without the detection of the CFA is unwarranted. Using a numerical model to determine the precise role of the CFA, if any, in the initiation and maintenance of the convection requires explicit resolution of the convection, at the very least.
In addition to the above criticisms, even if the CFA did play a major role in the initiation and maintenance of the 20 June 2000 convection, there are unavoidable questions about the generality of the findings obtained from this single case. Such questions may limit the utility of CFA detection in an operational setting.
REFERENCES
Grell, G. A., 1993: Prognostic evaluation of assumptions used by cumulus parameterizations. Mon. Wea. Rev., 121 , 764–787.
Grell, G. A., Dudhia J. , and Stauffer D. R. , 1994: A description of the fifth-generation Penn State/NCAR Mesoscale Model (MM5). NCAR Tech. Note NCAR/TN-398+STR, 121 pp. [Available from National Center for Atmospheric Research, Boulder, CO 80301-3000.].
Hobbs, P. V., Locatelli J. D. , and Martin J. E. , 1996: A new conceptual model for cyclones generated in the lee of the Rocky Mountains. Bull. Amer. Meteor. Soc., 77 , 1169–1178.
LaDue, J. G., and Wood A. , 2000: Estimating boundary-relative flow in forecasting convection. Preprints, 20th Conf. on Severe Local Storms, Orlando, FL, Amer. Meteor. Soc., 66–69.
Stoelinga, M. T., and Hobbs P. V. , 1998: Structure and evolution of winter cyclones in the central United States and their effects on the distribution of precipitation. Part V: Thermodynamic and dual-Doppler radar analysis of a squall line associated with a cold front aloft. Mon. Wea. Rev., 126 , 860–875.
Stoelinga, M. T., Hobbs P. V. , and Hobbs P. V. , 2002a: Norwegian-type and cold front aloft–type cyclones east of the Rocky Mountains. Wea. Forecasting, 17 , 66–82.
Stoelinga, M. T., Hobbs P. V. , and Hobbs P. V. , 2002b: Organization and structure of clouds and precipitation on the mid-Atlantic coast of the United States. Part VII: Diagnosis of a nonconvective rainband associated with a cold front aloft. Mon. Wea. Rev., 130 , 278–297.
Stoelinga, M. T., Martin J. E. , Castle J. A. , and Hobbs P. V. , 1995: Structure and evolution of winter cyclones in the central United States and their effects on the distribution of precipitation. Part III: The development of a squall line associated with weak cold-frontogenesis aloft. Mon. Wea. Rev., 123 , 2641–2662.
Rose, S. F., Hobbs P. V. , Locatelli J. D. , and Stoelinga M. T. , 2002: Use of a mesoscale model to forecast severe weather associated with a cold front aloft. Wea. Forecasting, 17 , 755–773.
Rotunno, R., Klemp J. B. , and Weisman M. L. , 1988: A theory for strong, long-lived squall lines. J. Atmos. Sci., 45 , 463–485.
Wilson, J. W., and Megenhardt D. L. , 1997: Thunderstorm initiation, organization, and lifetime associated with Florida boundary layer convergence lines. Mon. Wea. Rev., 125 , 1507–1525.
Surface analysis at 1800 UTC 20 Jun 2000. Low pressure troughs, outflow boundaries, and station models have the same symbology appearing in Rose et al. (2002). Isobars are contoured at 1-hPa intervals. Radar echoes ≥30 dBZ but <45 dBZ are light gray. Radar echoes ≥45 dBZ are dark gray
Citation: Weather and Forecasting 18, 2; 10.1175/1520-0434(2003)018<0384:COUOAM>2.0.CO;2
Surface analysis at 1800 UTC 20 Jun 2000. Low pressure troughs, outflow boundaries, and station models have the same symbology appearing in Rose et al. (2002). Isobars are contoured at 1-hPa intervals. Radar echoes ≥30 dBZ but <45 dBZ are light gray. Radar echoes ≥45 dBZ are dark gray
Citation: Weather and Forecasting 18, 2; 10.1175/1520-0434(2003)018<0384:COUOAM>2.0.CO;2
Surface analysis at 1800 UTC 20 Jun 2000. Low pressure troughs, outflow boundaries, and station models have the same symbology appearing in Rose et al. (2002). Isobars are contoured at 1-hPa intervals. Radar echoes ≥30 dBZ but <45 dBZ are light gray. Radar echoes ≥45 dBZ are dark gray
Citation: Weather and Forecasting 18, 2; 10.1175/1520-0434(2003)018<0384:COUOAM>2.0.CO;2
As in Fig. 1 but at 1900 UTC 20 Jun 2000
Citation: Weather and Forecasting 18, 2; 10.1175/1520-0434(2003)018<0384:COUOAM>2.0.CO;2
As in Fig. 1 but at 1900 UTC 20 Jun 2000
Citation: Weather and Forecasting 18, 2; 10.1175/1520-0434(2003)018<0384:COUOAM>2.0.CO;2
As in Fig. 1 but at 1900 UTC 20 Jun 2000
Citation: Weather and Forecasting 18, 2; 10.1175/1520-0434(2003)018<0384:COUOAM>2.0.CO;2
As in Fig. 1 but at 2000 UTC 20 Jun 2000
Citation: Weather and Forecasting 18, 2; 10.1175/1520-0434(2003)018<0384:COUOAM>2.0.CO;2
As in Fig. 1 but at 2000 UTC 20 Jun 2000
Citation: Weather and Forecasting 18, 2; 10.1175/1520-0434(2003)018<0384:COUOAM>2.0.CO;2
As in Fig. 1 but at 2000 UTC 20 Jun 2000
Citation: Weather and Forecasting 18, 2; 10.1175/1520-0434(2003)018<0384:COUOAM>2.0.CO;2
As in Fig. 1 but at 2100 UTC 20 Jun 2000
Citation: Weather and Forecasting 18, 2; 10.1175/1520-0434(2003)018<0384:COUOAM>2.0.CO;2
As in Fig. 1 but at 2100 UTC 20 Jun 2000
Citation: Weather and Forecasting 18, 2; 10.1175/1520-0434(2003)018<0384:COUOAM>2.0.CO;2
As in Fig. 1 but at 2100 UTC 20 Jun 2000
Citation: Weather and Forecasting 18, 2; 10.1175/1520-0434(2003)018<0384:COUOAM>2.0.CO;2
As in Fig. 1 but at 2200 UTC 20 Jun 2000
Citation: Weather and Forecasting 18, 2; 10.1175/1520-0434(2003)018<0384:COUOAM>2.0.CO;2
As in Fig. 1 but at 2200 UTC 20 Jun 2000
Citation: Weather and Forecasting 18, 2; 10.1175/1520-0434(2003)018<0384:COUOAM>2.0.CO;2
As in Fig. 1 but at 2200 UTC 20 Jun 2000
Citation: Weather and Forecasting 18, 2; 10.1175/1520-0434(2003)018<0384:COUOAM>2.0.CO;2
As in Fig. 1 but at 2300 UTC 20 Jun 2000
Citation: Weather and Forecasting 18, 2; 10.1175/1520-0434(2003)018<0384:COUOAM>2.0.CO;2
As in Fig. 1 but at 2300 UTC 20 Jun 2000
Citation: Weather and Forecasting 18, 2; 10.1175/1520-0434(2003)018<0384:COUOAM>2.0.CO;2
As in Fig. 1 but at 2300 UTC 20 Jun 2000
Citation: Weather and Forecasting 18, 2; 10.1175/1520-0434(2003)018<0384:COUOAM>2.0.CO;2
