DCVZ, NST, & Tornadogenesis

Notes and Talks from Al Pietrycha, Drs. Bruce Lee and Paul Markowski DCVZ

• A mesoscale feature of convergent winds, 50 to 100 km in length, usually oriented north–south, just east of the Denver, Colorado area.

• Extending from south of Fort Morgan to Kiowa (~ 80 km)

• It can develop west as I-25, and extend as far south as Colorado Springs.

• The cause of the feature is an interacon of southeasterly low-level ﬂow with an east–west ridge known as the Palmer Divide extending onto the eastern Colorado plains to the south of Denver.

• In order for the DCVZ to form, the low-level must be relavely stacally stable across the Palmer Divide region.

• Several studies have documented the role of the DCVZ in tornado outbreaks across the Denver area. The presence of a strong June DCVZ has been associated with a 70% chance of NST tornado formaon. DCVZ

• Studies have revealed rotaonal instabilies as vercally oriented vorces 500 m to 4 km in diameter ﬂowing along the boundary.

• These microscale vorces have a wavelength of ~4 - 5 km.

• They exhibited assorted degrees of surface occlusion.

• Vercal vorcity associated with eddies was found to be on the order of 10-2 s-1.

• Strong moisture gradients existed across the boundary, the largest of which are collocated with a vortex. DCVZ

• Largest dew point diﬀerenals have been observed as much as 6°C across 500 m.

• The DCVZ is NOT the same feature as the Denver Cyclone.

• DCVZ can develop with or without the cyclone, and visa versa.

• One is a gyre (Denver Cyclone), the other a convergent shear zone (DCVZ).

• Oen the DCVZ will develop along the eastern edge of the cyclone where southerly ﬂow converges with the westerly return ﬂow associated with the cyclone.

• DCVZ has been shown to help iniate thunderstorms in the convecve season.

199807232200Z At 2205 UTC the largest shear was sampled associated with the velocity couplet (0.5° 44 kt gate-to gate shear). Image of reﬂecvity data recorded by the CSU-CHILL radar at 2208 UTC 16 July 1998. The Denver Convergence-Vorcity Zone is seen as a NNE-SSW band of enhanced reﬂecvies. White arrows denote the boundary locaon; white rectangle mobile mesonet transect locaons. Winds to the east of the zone were generally from the southeast; west of the zone, winds were generally lighter and from the northwest. Subjecve analysis of mobile mesonet dewpoint temperature (tenths °C) observaons across the DCVZ at 2057 UTC on 16 July 1998. Observaons are 18-sec averages (3 observaons) ploed every 6 sec using me-to-space conversion with a moon vector from 185 at 6.0 m s-1. Isodrosotherms shaded every 1°C. Winds in knots with one full barb, and one half barb equal to 10, and 5 knots, respecvely. As in previous Figure except for 2137 UTC 16 July 1998 Subjecve analysis of mobile mesonet dewpoint temperature as in Fig. 2 except for 2149 UTC 16 July 1998. As in previous Figure except for 2215 UTC 16 July 1998. 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. The small minority of storms that due survive moving off the higher terrain don't intensify when interacng 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! Somemes 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 boom line is you want a cell that moves along the vorcity 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 that are resolved in the data are actually related to 1) old frontal boundaries 2) lee troughs, 3) old oulow boundaries, 4) downslope induced "meso-gama drylines", or 5) unknown feature/origin and cause. Myth #3

• The Denver Cyclone and/or DCVZ will develop aer the passage of a cold front.

The occurrence of the Denver Cyclone or DCVZ is common 1-3 days aer the passage of a cold front but only if there is a relavely high degree of stac 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 stac stability is needed appears to be event dependant.

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). Oen 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 iniate the storms. Most mes, once the cells move away from the DCVZ they loose their sources of deep layer li and dissipate due to the strong environmental convecve inhibion. This same thing happens along the Caprock; an isolated storm goes up in the Palo Duro Canyon, moves east a bit, and dies. Somemes DCVZ iniated 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. Non Supercell Tornado

• What constutes an NST? – landspouts (common High Plains term)

– most waterspouts

– some tornadoes associated with bow echo storms and squall lines

– Note: many of the “ﬂanking line” tornadoes associated with supercells develop through the NST- genesis mechanism NST

• Why is it important? – Signiﬁcant poron of all tornadoes naonwide

– Intensity may reach F3

– NST forecasng skill is poor

– Predominant tornado type in certain parts of U.S.

– NST-genesis knowledge may help in understanding supercell tornadogenesis and hybrid genesis modes Non Supercell Tornado NST

• State of Forecasng/Nowcasng NSTs – Very lile skill in NST forecasng

– Recognion from SPC and local NWS forecasters of NST threat in very speciﬁc geographical regions

– Other NSTs associated with “bow echo” type storms - almost no forecast/nowcast skill Tornado Climatology (where are NSTs most prevalent?) NST

• Boundaries such, as DCVZ, can be proliﬁc misocyclone generaon sites

NST

• Recognion of shear signature along the line (implied misocyclone) • What does the boundary provide? – Horizontal shear zone (vercal vortex sheet) • Supports misocyclones

– Convergence zone • Strengthens vortex sheet • Provides deep moist convecve forcing

– Boundary-induced vercal circulaons may provide correct “matching” vercal shear with ambient environment to get deep upright updras Horizontal Shear Instability

Lee and Wilhelmson (1993) Lifecycle of NST Genesis

"Refined" Model of Non-Supercell Tornadogenesis

I & II V

III cold

old outflow boundary

cold

old outflow boundary NST

• NST Maintenance Mechanisms 1. Without vercal vorcity generaon, NST would only last a few min.

2. Primary maintenance is through vorcity advecon along boundary.

3. Tilng of ambient horizontal vorcity is a secondary mechanism.

4. Stretching of vorcity from (2) and (3) maintains NST intensity. Tornado Maintenance Mechanisms

z y

x NST

• Role of Parent Storm Downdra in NSTs – The requirement of vorcity in (or generated in) the parent storm downdra does not appear important for NST maintenance (unlike some of the current supercell tornadogenesis theories).

– The parent storm downdras for NST cases are important for providing peripheral convergence and stretching that may (1) “spin up” an exisng vorcity pool or (2) intensify an already tornadic strength circulaon.

Note: NSTs are likely strongest when the parent storm downdra inially wraps around the vortex! NST

• Nowcasng Guidelines – Environments of lile CAPE (<500 J/kg) possess lile NST risk

– Environments of moderate or greater CAPE (>1100 J/kg) may support intense NSTs

– A threshold likely exists between 5-10 m/s of velocity change across the boundary of the line-parallel wind, above which, the boundary is a viable NST iniaon site

NST

• Misocyclone merger events increase the possibility of NST occurrence NST • Single misocyclones along a shear zone are the excepon. – The occurrence of one NST should signal the nowcaster that other NSTs are possible along the common boundary. Tornadogenesis

• Storms intersecng preexisng mesoscale boundaries, especially boundaries that have had me for their cold sides to modify (i.e., the cold side has CAPE and does not have excessive CIN), are oen prime candidates for tornadogenesis.

• Tornadogenesis occurs possibly due to the low-level horizontal vorcity generated density gradients along the boundaries.

• Low-level horizontal vorcity or streamwise vorcity (vortex lines), needs to be vercal within a tornado. Tornadogenesis

• Vortex lines are ubiquitous.

• Convergence of vortex lines increases vorcity in the local area (i.e. wind shi).

• Preexisng vercal vorcity can get enhanced by a developing cumulus situated above the vercal vortex lines but the ancyclonic and cyclonic vorcity will cancel each other out.

• The RFD of a supercell only enhances the cyclonic vorcity.

• Tornadogenesis associated with the RFD is sensive to the slightness thermal variaon. Tornadogenesis

• Streamwise Vorcity or Horizontal Vorcity Tornadogenesis Tornadogenesis

• The RFDs of supercells that produce signiﬁcant tornadoes are not nearly as cold as the RFDs of supercells that are either nontornadic or produce only brief, weak vorces.

• Downdras that had large negave buoyancy (due to a large amount of rain and specifying dry environmental low- level condions), e.g., those with temperature deﬁcits exceeding 8 °C, were associated with only weak, short-lived tornadoes. The addion of more rain led to larger negave buoyancy as well as the ambient low-level relave humidity led to larger negave buoyancy which is not conducive for tornadogenesis. Tornadogenesis

• Cold, negave buoyant, air associated with the RFD will diverge from the axis of the developing tornado.

• Downdras that were not excessively cold (associated with only a lile rain water and specifying large low-level environmental relave humidity), e.g., those having temperature deficits of only a few degrees Celsius, led to the most intense and long-lived tornadoes. Tornadogenesis • Warm, posive buoyant, air associated with the RFD will converge from the axis of the developing tornado. • • The RFDs of supercells that produce significant tornadoes tend to have small temperature (and equivalent potenal temperature) deficits, usually less than 3°C, and occasionally less than 1°C. • • In contrast, the RFDs of non-tornadic supercells (or those that produce insignificant tornadoes) tend to have much larger temperature (and equivalent potenal temperature deficits), typically exceeding 5°C (and oen exceeding 8°C). Tornadogenesis

This ﬁgure shows how the streamwise vorcity (horizontal) becomes vercal and enhanced by the RFD (Markowski 2005). Tornadogenesis

• The differences in the figure are due to differences in the inward- directed pressure gradient force associated with the buoyancy pressure perturbaon field.

• Another way of looking at it is that cold downdra air is not easily "recycled" by the updra owing to its large convecve inhibion; thus, less vorcity stretching can occur along the Axisymmetric model results of (le) the experiment with a relavely warm downdra and (right) the experiment with a axis when the angular very cold downdra. Blue colors indicate virtual potenal momentum-bearing downdra temperature ﬂuctuaons, gray indicates cloud, solid contours are tangenal wind speed, and dashed contours enclose the parcels reach the surface. rain sha.

Tornadogenesis

• Conversely, warm downdra air is readily "recycled" by the updra, promong strong convergence of the angular momentum-bearing downdra parcels, and thus more intense vorces. – The cold (le) and warm (right) RFDs associated with the triggering of a tornado.