Denver Convergence Zone (DCVZ)

Thoughts from Albert Pietrycha and Ed Szoke (extracted from e-mail)


Albert Pietrycha:


I have been working with the Denver Cyclone and Denver Convergence Vorticity Zone (DCVZ) for better than 13 years and have documented over 55 cases. I am far from being an expert on the subject, but the materials I have online are for specific cases that involved mobile mesonet (MM) data collection, we are still investigating. In an attempt to best answer your questions on what these features are and available online data sources to locate and track these features, I offer the following. 

First, please note that that the DCVZ is NOT the same feature as the Denver Cyclone. The DCVZ can develop with or without the cyclone, and visa versa. Often forecasters and storm chasers interchange the two; a sloppy mistake. One is a gyre, the other a convergent shear zone. Where the confusion apparently lies is that often the DCVZ will develop along the eastern edge of the cyclone where southerly flow converges with the westerly return flow associated with the cyclone. _Usually_ the DCVZ will have a general north/south orientation, extending from south of Fort Morgan to Kiowa; it can develop west as I-25, and extend as far south as Colorado Springs. The environmental, mesoscale conditions favorable for the development of both the Denver Cyclone and DCVZ has been well document by the observational and modeling , but the two features are usually mis-forecasted operationally, given the lack of observational data. I won't go into the conditions that favor the development of each feature as there are numerous papers detailing this information. Here are a few I suggest reading. 

Brady, R. H., and E. J. Szoke, 1989: A case study of non-mesocyclone tornado development in northeast Colorado: Similarities to waterspout formation. Mon. Wea. Rev., 117, 843- 856. 

Smolarkiewicz, P. K., and R. Rotunno, 1989: Low Froude number flow past three-dimensional obstacles. Part I: Baroclinically generated lee vortices. J. Atmos. Sci., 46, 1154-1164. 

Szoke, E., and A. E. Pietrycha, 1998: The Landspout Life Cycle: Maybe Not as Simple as Previously Thought. Preprints, 19th Conf. On Severe Local Storms, Minneapolis, MN, Amer. Meteor. Soc., 68-71. 

________, and R. H. Brady, 1989: Forecasting implications of the 26 July 1985 northeastern Colorado tornadic thunderstorm case. Mon. Wea. Rev., 117, 1834-1860. 

________, and J. M. Brown, 1987: Observational characteristics of a mesoscale flow feature in northeast Colorado and its association with convective development and severe storms. Extended Abstracts, Third Conf. on Mesoscale Processes, Vancouver, Canada, Amer. Meteor. Soc., 116-117. 

________, M. L. Weisman, J. M. Brown, F. Caracena, and T. W. Schlatter, 1984: A subsynoptic analysis of the Denver tornadoes of 3 June 1981. Mon. Wea. Rev., 112, 790-808. 

Wakimoto, R. M., and B. E. Martner, 1992: Observations of a Colorado tornado. Part II: Combined photogrammetric and Doppler radar analysis. Mon. Wea. Rev., 120, 522-542. 

________, and J. W. Wilson, 1989: Non-supercell tornadoes. Mon. Wea. Rev., 117, 1113-1140. Weckworth, T. M., J. W. Wilson, and R. M. Wakimoto, 1996: Thermodynamic variability within the convective boundary layer due to horizontal convective rolls. Mon. Wea. Rev., 124, 769-784. 

Wilczak, J. M., and T. W. Christian, 1990: Case study of an orographically induced mesoscale vortex (Denver Cyclone). Mon. Wea. Rev., 118, 1082-1102. 

________, T. W. Christian, D. E. Wolfe, R. J. Zamora, and B. Stankov, 1992: Observations of a Colorado tornado. Part I: Mesoscale environment and tornadogenesis. Mon. Wea. Rev., 120, 497-520. 

________, J. W. Glendening, 1988: Observations and mixed-layer modeling of a terrain-induced mesoscale gyre: The Denver Cyclone. Mon. Wea. Rev., 116, 2688-2711. 

Wilson, J. W., G. B. Foote, N. A. Crook, J. C. Fankhauser, C. G. Wade, J. D. Tuttle, and C. K. Mueller, 1992: The role of boundary-layer convergence zones and horizontal roles in the initiation of thunderstorms: A case study. Mon. Wea. Rev., 120, 1785-1815. 

________, J. A. Moore, G. B. Foote, B. E. Martner, A. R. Rodi. T. Utall, and J. M. Wilczak, 1988: Convective initiation and downburst experiment (CINDE). Bull. Amer. Meteor. Soc., 69, 1328-1348. 

Zipser, E. J., and J. H. Golden, 1979: A summertime tornado outbreak in Colorado: Mesoscale environment 

I will add, however, the time frame most favorable for the development of the Denver Cyclone and the DCVZ is within one to two days after the passage of a Canadian cold front. Please see C. Doswell's 1980 BAMS article, "Synoptic-Scale Environments Associated with High Plains Severe Thunderstorms" for a detailed account of the post-frontal synoptic regime. BTW, based on work by Szoke, the DCVZ occurs most frequently in the month of July, and I certainly agree with his findings. 

The following is an example of a DCVZ with no Denver Cyclone. Note the two observations only reporting winds are the MMs denoting the location of the DCVZ.

http://stormeyes.org/pietrycha/vortex/dcvz/980723/98072322sfc.gif

For accompanying satellite, sounding, and WSR-88D data for this case see:

http://stormeyes.org/pietrycha/vortex/dcvz/980723/980723.html

The following is an example of a Denver Cyclone and a DCVZ. Note the cyclonic circulation:

http://stormeyes.org/pietrycha/vortex/dcvz/980716/98071621sfc.gif

For accompanying satellite, sounding, and WSR-88D data visit;

http://stormeyes.org/pietrycha/vortex/dcvz/980716/980716.html

Other cyclone cases are found under my "CASES" web page off my home page. 

http://stormeyes.org/pietrycha/vortex/dcvz/980716dcvz.html
 
For a brief manuscript detailing some of our MM data along the DCVZ go to: 

http://stormeyes.org/maps/sar.gif

Second, there are a variety of data sources on the World Wide Web that are helpful in locating and tracking either the Denver Cyclone or the DCVZ. Visible satellite data to track the DCVZ is rather difficult unless well-developed cumulus is present along the boundary. The best way to track the boundary is through the use of surface observations and FTG WSR-88D reflectivity and velocity data. The Platteville profiler data is helpful in determining the depth of a cyclone's circulation.  Regarding surface data, most of the surface plots on the web are poor in spatial resolution and limited in the amount of information provided, i.e., no MSLP, 3-h pressure tendencies, etc. An ideal surface map for Eastern Colorado in tracking a Denver Cyclone or DCVZ can be found here: 
http://stormeyes.org/maps/sar.gif

So what about storm chasing these two features? I have had huge success playing the DCVZ and Colorado tornadic supercells in general. The following are common myths that chasers and some forecasters have unfortunately come up with over the years. I answer back based on my chasing experience and my DCVZ climatology work. 

Myth #1: Storms that develop along the Front Range will move east, cross the DCVZ and intensify.  Actually, by far, the vast majority of the storms will move off the higher terrain of the Front Range foothills and dissipate, for apparent thermodynamic reasons (I can elaborate further why this happens in another email if interested). The small minority of storms that due survive moving off the higher terrain don't intensify when interacting with the boundary. However, _IF_ a storm develops along, and over the north slope of the Palmer Divide and moves northeast out onto the Plains while traversing the DCVZ, pick that storm and stay with it! Sometimes a storm will roll southeast off the Cheyenne Ridge and interact with the DCVZ near FT Morgan or eastern Weld County. This too is the storm of choice if you want to maximize the possibility to observe a tornado. The bottom line is you want a cell that moves along the vorticity source. Please note that the Palmer Divide and Cheyenne Ridge are not defined here as "higher terrain". The higher terrain = the foothills of the Colorado Rockies. 

Myth #2: Every surface boundary in the Denver area is the DCVZ.  More than half the boundaries I resolve in the data are actually related to 1) old frontal boundaries 2) lee troughs, 3) old outflow boundaries, 4) downslope induced "meso-gama drylines", or 5) unknown feature/origin and cause. The days I have chased 'unknown features', I usually return home disappointed. 

Myth #3: The Denver Cyclone and/or DCVZ will develop after the passage of a cold front.  The occurrence of the Denver Cyclone or DCVZ is common 1-3 days after the passage of a cold front but only if there is a relatively high degree of static stability present in the lower part of the atmosphere. Without the stability, the return southerly flow will flow freely over the Palmer Divide and no gyre or convergence boundary will develop. Unfortunately there is no magic # of stability to look for to help determine if a cyclone or convergence zone will form. What ever the 'right' amount of static stability is needed appears to be event dependant. I'm working on this issue presently. 

Myth #4: An isolated cell developing along the DCVZ or within the Denver Cyclone will propagate east, say towards Last Chance, and produce a tornado.  Not usually (once in a blue moon). Often if the only storms that develop are along the DCVZ then we know a few things. We can deduce that the atmosphere is strongly capped and needed some sort of deep boundary layer localized convergence to initiate the storms. Most times, once the cells move away from the DCVZ they loose their source of deep layer lift and dissipate due to the strong environmental convective inhibition. Sam, you've witnessed this same thing happen along the Caprock; an isolated storm goes up in the Palo Duro Canyon, moves east a bit, and dies. One final comment, sometimes DCVZ initiated storms will develop along the boundary, move east and become part of a squall. This usually is always in response to an approaching mid- or upper-level short wave trough. 

Ok, here are my "rules" for chasing in Northeast Colorado and the Denver Cyclone and/or DCVZ. 

1) If a true Denver Cyclone, the gyre should persist for more than 6 hrs (36 hrs is the longest document case in the literature). Wait for the cyclone to move east or northeast as it begins to shear out. As it does, play downstream and northeast of the eddy. Concurrent with shearing out, a DCVZ might develop. IF that happens play where the surface winds appear to have the greatest horizontal shear. 

2) If a true Denver Cyclone, KDIA will nearly always have the lowest reported MSLP in the area; low = 1 to 3 mb. Area=neighboring surface observations; COS-LIC-GLD-IML-CYS. This is one signal I have found that has always stood out in my climo work. Out of my 55+ Denver Cyclone/DCVZ cases, there were only two cases when DIA's reported MSLP was not the lowest pressure in the region. 

3) If chasing to observe non-supercell tornadoes, don't chase on a moving DCVZ. Based on my observations, the boundary needs to be nearly stationary during the time of convective initiation if a tornado is going to develop along it. Or, even if stationary, if the storms are expected to move rapidly off the DCVZ soon after initiation, kiss the tornado threat good-bye. The updrafts need the vorticity source. Once the storms leave the boundary, they leave the vorticity source behind. 

4) If chasing non-supercell tornadoes, you _MUST_ be on the backside of the updrafts (usually on the west side of the storm) if you want to see the tornadoes, AND be on the DCVZ prior to the development of rapidly growing towering CU. 

For supercells, all the usual severe conditions we normally look for apply, but I favor cells that move southeast off the Palmer Divide or Cheyenne Ridge, into deeper moisture (if moisture is present). If conditions look great for supercells and there is also a DCVZ present, I pick where conditions look best, usually ignoring the DCVZ; I favor areas of deeper moisture collocated with 700 mb flow greater than 15 kts. 

As for Al's (Moller) observations, I fully agree the lack of tornadoes (in the northeast corner of Colorado) are due to under reporting. And yes, southeast Colorado has a similar gyre, the sister to the Denver cyclone, the Raton Cyclone. I believe Ed Szoke has done research on this feature. I have started to look at this myself but the lack of good observations in that area make it difficult to resolve the eddy and/or convergence zone. I also have observed a "Cheyenne convergence zone" many times, north of the Cheyenne Ridge, orientated west-east. 

As a final comment to address your question about chasing and tornadoes in Colorado, the tornado max in Colorado is bimodal. Harold Brooks (NSSL) has shown that in the months of June and July, northeast Colorado is the hot bed, in the U.S. After living and chasing in Colorado, it appears to me that supercell tornadoes occur more frequently in June, and non-supercell in July. Other works (e.g. Grazulis, NWS, etc) have shown that eastern Colorado has a tornado max on June 11 followed by a secondary max in the first two weeks of July. 

Info from Ed Szoke:
 
Sam, I have much stuff, some in various articles (a kind of nice one is "Eye of the Denver Cyclone" in MWR, May 1991,which has nice satellite and sfc data. Otherwise, I presented a poster at the AMS conf in Aug in FLL, and have a web site with the cases on it at: 

http://laps.fsl.noaa.gov/szoke/mesoconf/cases_homepage.html

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Last revised: January 13, 2007