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The Denver Convergence-Vorticity Zone -
From A Storm Chasers Perspective
Preface
The Denver Convergence-Vorticity Zone has been studied since 1981 byvarious meteorologists, storm chasers and amateur weather enthusiasts.The numerous studies and analyses of each discipline have stemmed from afew different motivations. Meteorologists, especially those employed byNOAA, are most interested in protecting life and property in and nearthe Denver suburbs. Storm chasers wish to take advantage of film andphotographic opportunities, and share their catch with the public.Amateur weather enthusiasts primarily focus on the basics of thephenomena with a general interest in spotting and predictingnon-supercell tornadoes and severe weather events. The mostcomprehensive official study of the Denver Convergence-Vorticity Zonewas done by Meteorologist Edward J. Szoke of the NOAA Earth Systemsresearch Laboratory in Boulder, Colorado. Mr. Szoke has compiledextensive documentation of the wind flow patterns, overlain on terrainmaps, to support the general accepted theory of this importantwarm-season circulation. An additional study was completed on the 25thanniversary of the discovery of the Denver Convergence Vorticity Zone in2006 by Mr. Szoke, along with Dave Barjenbruch, Robert Glancy and RobertKleyla of the NOAA WSFO in Boulder, Colorado.
This paper will address the Denver Convergence-Vorticity Zone,(henceforth referred to as the DCVZ), from the perspective of stormchasers. The focus will be on the nomenclature, definition and theoriesof formation, and related severe weather. Finally, some strategies willbe put forth for storm chasers to consider when attempting to study andphotograph resultant non-supercell tornadoes and other circulationswhich can be a focal-point for the development of larger storms fartherout on the Colorado plains.
Discussion of Nomenclature and Terminology
The DCVZ has recently been otherwise referred to as the DenverCyclone. Its my personal preference to refer to the phenomena as azone of vorticity and convergence. The use of the term cyclone impliesthat the DCVZ is formed in a similar fashion as a mid-latitude cyclone.There are several key differences in the formation of the DCVZ comparedto a mid-latitude cyclone, although the main circulations of the two areboth cyclonic. A few of the basic differences are noted below.
First, cyclones which form in the mid-latitudes of North America areinitially formed by large-scale temperature differences, which generatepressure gradients and result in specific wind flow patterns, advection,convergence and synoptic scale dynamic lifting. This is known as abaroclinic environment, (fronts, and air masses of differing temperatureand moisture are present). A cyclone is a counter-clockwise, (northernhemisphere), circulation in association with baroclinic differentials,(temperature, moisture). The DCVZ is a zone of converging air resultingfrom wind shear as it relates to the convergence portion of the matrix.
The DCVZ is formed when low-level wind flow interacts with terrainfeatures in eastern Colorado. Often, the temperature field in easternColorado on a DCVZ day contains no synoptic-scale differentials,(baroclinicity), and the moisture field is consistent throughout, orshows only minor variation, with lower dewpoints usually the result ofhigher terrain rather than air mass characteristics. The key differencelies in the fact that a baroclinic environment causes the formation of atypical mid-latitude cyclone, while a baroclinically-induced wind fieldassists in the formation of the DCVZ. In the case of the former, it isthe baroclinicity itself that forms the cyclone, while in the case ofthe latter, the baroclinic environment is usually already formed whenthe DCVZ is initiated. So, a DCVZ could be formed as an after-product ofa passing mid-latitude cyclone or a cold front. However, it should benoted that a baroclinic environment is not always necessary to form aDCVZ because it is the result of the PBL wind field interacting withterrain, not temperature discontinuity. DCVZ can indeed form as theresult of a mid-latitude cyclone passing through the area with itscounter-clockwise circulation. This situation is more accurately knownas a Denver Cyclone.
Second, in the case of mid-latitude cyclones, the wind field is formeddue to pressure gradients, whereas the wind field in the DCVZ developsas a result of the overall synoptic-scale pattern or possibly due todiurnal wind flow. The difference being that the wind field is notnecessarily generated by differences in temperature or moisture across aboundary.
Terms used to describe the formation of the DCVZ include:
Mesoscale a meteorological scale referring to events and air massessmaller than synoptic scale. In the case of the DCVZ, mesoscale refersto the convergence boundary associated with the event, (20 to 200 km)and to the thunderstorms which sometimes form on the boundary, (20 to 30km).
Convergence a meeting of winds from opposing directions, sometimes ofdifferent intensities. The angle at which the winds meet in the DCVZ canhelp determine the intensity of the uplift and whether the resultantupdrafts will be successful in breaking any capping inversion.Convergence is one of the primary ways of achieving dynamic uplift,which is critical for condensation and precipitation.
Vorticity A spin or circulation of air present in the meso or synopticscales. As it relates to the DCVZ, the overall circulation iscounter-clockwise, generating positive vorticity. The mesoscalecirculations, or eddies, are on the order of 5 to 20 km.
Non-supercell Tornado A violently rotating column of air that occurswith a parent cumulus cloud in the growth stage, with vorticity thatoriginates in the boundary layer. The parent cloud does not contain apreexisting mid-level mesocyclone.
Growth Stage This refers to a cumulus cloud that is in the stage priorto the mature stage. The growth stage is marked by rapid verticaldevelopment. In the case of the DCVZ, the development is caused bydynamic lifting of air above the area experiencing convergence.
PBL (Planetary Boundary Layer) The layer of the atmosphere extendingfrom the surface to approximately 3 km above the surface. In relation tothe DCVZ, the PBL wind field is the prerequisite for the formation ofthe circulation.
Landspout A non-supercell tornado.
To conclude, a proper name for this phenomenon should be the DenverConvergence-Vorticity Zone, versus the Denver Cyclone, referring to theformation prerequisites of a mid-latitude cyclone. The other terms usedin descriptions of the DCVZ retain their proper meanings.
Definition and Formation
The DCVZ, as defined by the American Meteorological Society, is amesoscale flow feature of convergent winds, 50 to 100 km in length,usually oriented north to south, just east of the Denver, Colorado area.This is a general definition of the phenomenon, and its position, lengthand intensity can vary greatly depending on the synoptic conditions,temperature and moisture fields, interference from non-related synopticfeatures, and prevailing wind vectors.
The prevailing theory of the formation of the DCVZ is that a south orsoutheasterly wind flow interacts with the varied terrain in easternColorado. One of the prominent features is an outcrop called the PalmerDivide. This is a region of relatively high elevation, (approximately7,800 feet) which protrudes in an east-west fashion, somewhatperpendicular to the north-south line formed by the Rocky Mountains tothe west. The Denver metropolitan area sits in somewhat of a bowl oflower elevation, to the north and west of this location. Denver is lowerthan the foothills to the west, the slightly elevated terrain to thenortheast and the Palmer Divide to the south. All of these featurescontribute to the formation of the DCVZ. The Denver valley serves toenhance the deflected wind flow and concentrate it in a relatively smallarea.
The formation of a DCVZ requires a strong low-level wind flow from thesouth or most favorably the southeast. The geographical area involved inthe formation of a DCVZ extends from Cheyenne, WY in the north toColorado Springs, CO in the south. The western boundary is the foothillsof the Colorado Front Range to the west, and a line extending from Akronto Limon, CO in the east. The Palmer Divide or ridge is located in aneast-west orientation north of Colorado Springs and south of a line fromCastle Rock to Elizabeth to Kiowa, CO.
The DCVZ is thought to form as PBL air, moving from south-southeast tonorth-northwest, flows over the Palmer Divide in a general southeasterlyflow at the synoptic scale. The higher terrain serves to bend the windflow toward the west due to friction as it passes over the Palmer Ridge.A statically-stable lower tropospheric condition serves to concentratethe circulation in the PBL. The vorticity piece of the DCVZ forms as theair is deflected due to friction over the Palmer and bent toward thenorthwest. From here, some of the air can be deflected further westwardalong the Cheyenne ridge to the north in the case of a large-scale DCVZ.Otherwise, air flowing over the mountains from west to east moves downthe east slopes and meets the southeasterly flow somewhere east tosoutheast of Denver. This denotes the convergence piece of the DCVZ. The resulting severe weather forms along the convergence boundary wheredynamic uplift is most enhanced. There can also be a vast difference inthe moisture field of the mountain air and the air moving in from thesoutheast, which is usually more moist.
Nearly three decades of tornado reports have been gathered in easternColorado. While the levels of occurrence were down in the 1990s and2000s, the distribution of tornadic events remained fairly uniform.According to NOAA, greater numbers of tornadoes have occurred in thenon-mountainous counties just east of the Denver area since 1980.Coincidentally, the DCVZ frequently forms over these counties. WeldCounty has reported the highest number of tornadoes, partially becauseof its larger size. Second in ranking is Adams County, followed byArapahoe, then Douglas. Incidentally, the NOAA data have shown that asubstantial drop in the number of tornadoes in all of these counties hasoccurred in the current decade, coinciding with fewer DCVZ events.
The reasons for a southeasterly surface flow forming a DCVZ are usuallysynoptic in nature. It is common for a DCVZ to form one to three daysafter the passage of a Canadian cold front. These situations are morefrequent in the months of May and June, before the polar front moves toofar north. A few days after the front passes, the return flow around thefollowing high pressure ridge is normally from the southeast. Accordingto a study done by researcher and veteran storm chaser Albert Pietrycha,a high degree of static stability is required for a cyclonic circulationto form. That is, sinking air under the high pressure ridge. With staticstability present, the intensity of the southeasterly air flow over thePalmer Ridge can be kept in the PBL and the full momentum can be used inthe formation of the DCVZ.
Related Severe Weather
The most intense and dangerous weather resulting from a DCVZ is thenon-supercell tornado. However, the conditions on any given DCVZ day mayor may not generate NSC tornadoes. On some DCVZ days, the formation ofthe circulation and consequent convergence generates nothing more than afew cumulus clouds, or an invisible convergence boundary where theprevailing low-level southeast flow meets the north or northwest flowmoving south along the foothills. Referring to the 1981 Szoke study, ithas been found that on days where an ambient south or southeast surfacewind flow was present, a DCVZ formed about 80% of the time. A strongcorrelation with non-supercell tornadoes was also identified. In themonth of June, Szoke found that 40% of the DCVZ days had a tornado. Juneseems to be the most dominant month for the formation of a DCVZ, andtornadoes that formed in association with a DCVZ were not associatedwith any other type of severe weather. In other words, tornadoes havebeen forming in the vicinity of a DCVZ under conditions which formedordinary, or non-mesocyclone, thunderstorms. In the Szoke study,conclusions were drawn that June days with a DCVZ present had a 30%chance of being tornado days, with the chance rising to about 60% inthe presence of a strong DCVZ.
The figure above is a model of the formation of a non-supercelltornado, developed from a Doppler radar study of the DCVZ conducted bySzoke and Bradley in 1989. The importance of a boundary as a point ofinitiation was revealed in this, and subsequent, studies. It has beenwidely documented that non-supercell tornadoes can form in associationwith growth-stage cumulus clouds. These rapidly-developing cumulusclouds form along a convergence boundary such as the DCVZ, or aninteraction between low-level wind flow and an outflow boundary. Asillustrated at left, the convergence zone formation is the first phasein the development of clouds in the region. The elements of heat,moisture and convergence work together to form a circulation around avertical axis. In approximately 15 minutes, the visible cumulus cloudforms in the rising vertical column of air above the vort point, and thevorticity is vertically advected and imposed on the rising air, (centerframe). As the cloud develops pas the 30 minute mark, while still ingrowth stage, the vorticity is stretched as the air column risesrapidly. Air is pulled in at the base of the circulation to replace therising air, enhancing development. In this manner, a non-supercelltornado forms, essentially from the ground, up. Or, more accurately,involving the entire column of air as it rises from the ground to theLCL and beyond.
In addition, the position of the strongest convergence can have aprofound effect on what type of severe weather develops, and itsmorphology. Convergence zones that form farther west, usually in moredry air, may only generate a few clouds as visible clues. When the zoneforms farther east, in more abundant moisture and with more intenseheating, stronger updrafts can be the result. Generally, storms formingon the DCVZ are slow-moving compared to those forming due to othersituations. Most DCVZ storms sustain themselves in the same generalarea, rather than moving off onto the plains. When these storms doeventually get caught up in the mid-level flow, they sometimes loseintensity after moving off the convergence zone, because they no longerhave a source of uplift. Unless such a storm can sustain its updraft ina favorable environment, it may dissipate. A favorable environment wouldbe a region that has been experiencing strong diurnal heating and hashad good moisture advected into it. Another key ingredient is shear.Unfortunately, most DCVZ days lack vertical shear due to the nature ofthe preexisting conditions that formed the DCVZ in the first place. ADCVZ will usually form on days of limited shear, especially in the PBL.The only shear that is generated is due to the deflected wind flow,rather than baroclinic conditions. All of the shear is at thelower-levels of the atmosphere, in the PBL. Mid and upper-level windsare usually weak on DCVZ days.
DCVZ Chase Strategies
Non-supercell tornadoes can be fascinating to chase and photograph. Themain advantage in viewing/chasing these tornadoes over mesocyclonictornadoes is their slow movement. Supercell tornadoes can move as fastas 50 or 60 mph. Since DCVZ tornadoes form along a convergence boundarywhich sustains their growth and development, they tend to move at lessthan 25 mph and sometimes appear nearly stationary.
In anticipating a DCVZ chase day, the synoptic and mesoscale features tolook for include a stable southeasterly flow at the surface, strongdiurnal heating, a capping inversion and low values of mid-levelhorizontal and vertical shear.
Following frontal passage and the formation of a DCVZ, Al Pietrycharecommends an analysis of the persistence of the gyre. It should persistfor at least 6 hours to be considered a true DCVZ. According to hisresearch, the longest recorded DCVZ circulation was of a duration ofabout 36 hours. Surface data, satellite imagery and radar data are thethree critical elements that should be utilized when in chase mode ona non-supercell tornado event along the DCVZ. According to Mr. Pietrychaand other veteran storm chasers, the key strategy in interceptingnon-supercell DCVZ tornadoes is to locate on the DCVZ prior to thedevelopment of rapidly growing cumulus towers. Frequently, the time frominitial cloud formation to tornadogenesis is less than 30 minutes.
Surface data are essential for determining the location of theconvergence boundary. Since temperature and humidity differences may beminimal, use the surface wind observations to determine the strongestarea of convergence. Locating the boundary itself and the areas ofstrongest convergence can be difficult using only surface observations.This is due to the fact that there are very few official observationstations east of Denver, and they are widely scattered. Also, surfaceobservations are only issued once per hour and mesoscale conditions canchange much more rapidly than that. It is therefore necessary tosupplement surface data with WSR-88D data. The National Weather Serviceoffice in Boulder, CO maintains a Doppler radar facility northeast ofDenver, in perfect position for analysis of the DCVZ. As strong sheardevelops in the convergence zone, this will be evident in the colorsdisplayed on the WSR-88D data. Storm-relative motion mode will revealinbound winds as green and outbound winds as red. This will show theactual boundary, and eddies forming along it. These eddies can be afocal point for NSC tornadoes if a cumulus tower forms over them.High-resolution satellite data are also useful in locating the boundaryand any cumulus clouds forming on it. The high-res images are updatedevery 15 minutes and will show towers going up along the convergenceboundary. It should be noted that many DCVZ events generatenon-supercell tornadoes in the early afternoon hours, between noon and 2p.m. Therefore, shadows on the satellite images will be at a minimum.Careful analysis of satellite data on a large non-glossy display inlow-glare conditions is usually necessary to pick up small cumulus clouds.
Non-supercell tornadoes must be viewed from the west, looking east. Theprimary reason for this is their visual characteristics. Non-supercelltornadoes rarely reach above EF-1 intensity. Thus, they do not pull asmuch debris and dust from the surface up into their funnels.Additionally, there is no condensation funnel to speak of. This givesthe tornadoes a semi-transparent appearance. If they are viewed lookingwest into brighter skies, they will be nearly invisible. However,looking east from behind the developing towers, the sun will not be aninterference, and the dark color of the dust and dirt being sucked offthe ground will contrast nicely with either blue sky or white clouds inthe background.
Forecasting tornadoes on the DCVZ is a continued challenge. There are nodefinite parameters on which to base forecasts, and no obvious patternsto the formation of storms along the boundary which producenon-supercell tornadoes.
Conclusions
With populations growing rapidly in the Denver area, continued researchis necessary to better predict non-supercell tornadoes along the DCVZ.Non-supercell tornadoes can reach EF-1 intensity, causing substantialdamage to homes and businesses. The rapid development of DCVZ tornadoesis a serious challenge to their prediction. The goal of NOAA is to beable to more accurately predict DCVZ tornadoes, while limiting thenumber of false alarms, which damage credibility and undermine theconfidence of the public. Ill propose a plan here that may aid in theresearch of conditions leading to non-supercell tornadoes on the DCVZ.
With the development of GPS and mobile internet technology, it is nowpossible to coordinate a team of storm chasers positioned arounddeveloping cumulus towers on DCVZ days. My proposal would be to organizea volunteer group of chasers based in the Denver area who would beavailable to deploy on favorable days. In the chase vehicles would be alaptop computer with mobile internet. Interactive software can bedeveloped to that the positions of the chase vehicles can be constantlymonitored by a coordinator at NOAA in Boulder. The GR Level 3 radarprogram would be installed on each laptop in the chase vehicles. Also,NOAA could develop a standardized instrument package to measure windspeed and direction, temperature and humidity on each chase vehicle.This information would also be transmitted via mobile internet back toNOAA. In this way, a mobile mesonet of chasers could be positioned alongthe DCVZ to gather data, and then document any non-supercell tornadoeswith video and digital photography. The key to accurately researchingmesoscale events such as the DCVZ is to have a network of perhaps 50mobile observation platforms, capable of delivering a large data samplefor analysis. From this data, it may be possible to pinpoint enoughparameters on which to base a tornado forecast on DCVZ days. Thecost-benefit relationship is increasingly justifiable with the increasesin population in areas frequently affected by the DCVZ.
References
Observations Of The DCVZ Using Mobile Mesonet Data. Albert E. Pietrycha
and Erik N. Rasmussen. National Severe Storms Laboratory and Texas Tech
University. Cooperative Institute for Mesoscale Meteorological Studies,
NSSL and University of Oklahoma.
The Denver Cyclone And Tornadoes 25 Years Later: The Continued Challenge
Of Predicting Non-Supercell Tornadoes. Edward J. Szoke, NOAA Earth
System Research Laboratory/Global Systems Division, Boulder Colorado in
collaboration with the Cooperative Institute for Research in the
Atmosphere (CIRA). Dave Barjenbruch, Robert Glancy and Robert Kleyla,
NOAA National Weather Service Forecast Office, Boulder, Colorado.
