Leslie R. Lemon

Lockheed Martin Ocean, Radar, & Sensor Systems

Weather and ATC Programs

Syracuse, NY 13221

1.0 Introduction.

Supercell convective storms are defined by the presence and persistence of the essential and distinguishing characteristic, that of the deep mesocyclone (vorticity of 1 X 10 -2 S -1 or greater) within such a storm (Klemp, 1987, and others). These storms are of great importance due to their disproportional production of damage and nearly all strong or violent tornadoes. Here, the structure of that mesocyclone is considered, particularly its remarkable similarity to the Extratropical Cyclone (ETC). It is further suggested that the characteristic "clear slot" or "dry intrusion", by transporting dry air with high potential vorticity downward from aloft, may be responsible for both mesocyclone amplification and many of these associated strong and violent tornadoes.

2.0 Mesocyclone Structural Paradigms.

There are essentially two mesocyclone paradigms accepted and used today. The first and most widely accepted is that of the "rotating updraft". Its appeal is related to such dynamic concepts as the tilting of ambient horizontal vorticity into the vertical by the updraft, streamwise vorticity, helicity, and storm relative helicity (SRH) (Davies-Jones, 1984; Lilly, 1986; Klemp, 1987; Rasmussen and Straka, 1998). These concepts seem to explain the acquisition of the mid-level rotation (mesocyclone) within the updraft.

In fact, these variables can be estimated via examination and manipulation of the environmental soundings and used to predict supercell likelihood. Furthermore, there is even a rough correlation between the probability of tornadoes (once a storm motion is either assumed or measured) and the resulting environmental SRH.

The second mesocyclone paradigm is that of the ETC analog (Lemon and Doswell, 1979, hereafter, L&D). Its appeal is found when considering the typical "frontal structure" found in the low-levels of the supercell.

Corresponding author address: Leslie R. Lemon, 16416 Cogan Drive, Independence, MO 64055; e-mail: LRLEMON@compuserve.com.

and when the occlusion process of this frontal structure is observed. Furthermore, it is useful when attempting to explain the development of the low-level mesocyclone (and tornadogenesis) via solenoidal vorticity production, and even because of the so-called wrapping "clear slot", and "horseshoe shaped updraft", (e.g., Lemon, 1976; L&D, Burgess, 1982; Klemp, 1987).

Both paradigms have been successfully numerically modeled. See for example, Schlesinger (1978), Klemp and Wilhelmson (1978), Wicker and Wilhelmson (1995). Moreover, because of its initial association with the Bounded Weak Echo Region, L&D suggested that mid-level genesis of the mesocyclone was initially that of the rotating updraft but which soon transformed into a "divided structure" having the ETC form.

Which paradigm is considered often depends on the author and the discussion emphasis at the time. However, for the most part, and when considering all but the boundary layer flows, the rotating updraft concept is typically assumed. This is true even though some recent observations found that the mesocyclone gust front structures are not strictly low-level phenomena, but instead, extend through a considerable depth of the storm (Lemon and Burgess, 1992; Lemon and Parker, 1996).

Not surprisingly, this author draws attention back to the ETC paradigm of L&D and suggests that valuable insights into mesocyclone spin-up and tornadogenesis can be gleaned from it. Moreover, for reasons explained in L&D and with the added weight of the observed supercell deep frontal structures mentioned above and the concepts here, this author believes that the mesocyclone and supercell are, in reality, strongly baroclinic systems. The mesocyclone should not be viewed as a rotating updraft. Rather, when we see the mesocyclone through the ETC "conveyer belt" perspective of Harold (1973) and Browning and Roberts (1994), much can be learned.

3.0 Extratropical Cyclone and Mesocyclone Structure.




Figure 1. Structure of a developing extratropical cyclone. The cyclone center (L) is moving toward the upper left. Principal airflows, drawn relative to the system, are: the main warm conveyer belt (W1) (solid lines), the secondary warm conveyer belt (W2) (long-dash lines), the cold conveyer belt (CCB) (short dashed lines) and the dry intrusion (dotted lines). Fronts use conventional symbols. The cold front drawn with open symbols is the leading (east) edge of the upper dry intrusion. The cloud head (or comma head) is a hook shaped cloud feature with sharp boundary on the polward side of the clear dry slot (after Browning, 1997).

Here, due to space restrictions, we can only summarize the conveyer belt concept of the ETC. For more detail the reader is referred to Browning and Roberts (1994). This type of analysis stresses not the large air masses but the smaller scale concentrated airflows that determine the cloud and precipitation and the structure and evolution of the cyclone. It is carried out within isentropic surfaces in a coordinate frame traveling with the weather system.

Major ETC airflow features of interest here (from Browning and Roberts, 1994) are:

Warm conveyer belt - a strong, well defined flow of high q w that advances polward ahead of the cold front.

Cold conveyer belt - air ahead of and beneath the warm front which, relative to the advancing system, flows rapidly rearward around the polward side of the low center.

Dry Intrusion - a stream of air from the upper troposphere and lower stratosphere, which, after earlier decent, approaches the center of the cyclone as a well defined reascending dry flow with low q w .

Dry (clear) slot - a satellite detected, almost cloud-free, zone indicative of the dry intrusion approaching the center of a developing cyclone.

Cloud head (or "hook cloud" ) - a hook-shaped cloud feature, with sharp convex polward boundary, located on the poleward side of the dry slot.

Where are the similarities of these ETC airflows and the mesocyclone flow? The reader is referred to L&D and to Lemon (1976) for detail. First, it must be stressed that it is not the intent here to infer dynamic similarity of the ETC and mesocyclone in the strict sense of the term. An extratropical cyclone is quasi-geostrophic and hydrostatic, and vector winds at any level are predominantly two-dimensional. Mesoscale flows are substantially ageostrophic and nonhydrostatic and definitely three-dimensional. Nevertheless, the many resemblances suggest physical concepts which link these vortical flows. Here, we can only summarize the similarities due to space considerations.

At first glance, of course, the mesocyclone appears as a miniature ETC (Fig. 2). The warm conveyer belt analog in the mesocyclone is the convergent, warm and moderately humid flow of highest environmental q w along and ahead of the trailing outflow boundary and the often associated flanking line of overtaking towering cumulus merging with the primary cumulonimbus updraft (Fig. 2).

Figure 2. Schematic plan view of a supercell storm at the surface. Thick lines encompasses radar echo. The storm "gust front" structure and "occluded" wave are also depicted using a solid line and frontal symbols. Surface positions of updraft (UD) are finely stippled; forward flank downdraft (FFD) and rear flank downdraft (RFD) are coarsely stippled. Ground relative streamlines are also shown. Tornado location is encircled T. (After L&D).

The cold conveyer belt analog is the flow out of the FFD north of the warm front-like boundary and back into the updraft (UD) region (Fig. 2.). In Lemon (1976) this warm front-like boundary had its origins with the outflow boundary laid down by a previous storm. Flow north of this boundary is characterized by moderate q w (lower than that ahead of the trailing gust front but higher than that out of the RFD), cool temperatures, very high relative humidity, and is the source for the wall cloud as this air rises upward into the major storm updraft. Thus, in the mesocyclone and analogues to the ETC, two airstreams (that from the warm and cold conveyer belt analogs) combine to rise within the cumulonimbus updraft (which is the analog to the ETC cloud head).

Most importantly to this discussion is the mesocyclone analog to the ETC "dry intrusion" or "clear slot". Apparently descending from high-levels and on the storm rear flank, is the RFD. An extension of this forms the often rainy or hail filled but cloudless, and (during daylight) sunlight illuminated "clear slot" which wraps around and flows, to some extent, beneath the updraft base (L&D). The hook echo is largely, if not totally, within the wrapping clear slot. Surface conditions beneath the RFD are characterized by the lowest q w, coolest air, divergence, and higher pressure (Lemon, 1976). Further, the tornado is often located near the edge of the rain free cumulonimbus base and wall cloud positioned between the wrapping clear slot downdraft and the intense updraft feeding the wall cloud. (Interestingly, this is about the position of the "L" in Figure 1.).

Relative to the mesocyclone, and as pointed out by L&D and stated by the following from Rasmussen and Straka (1998), "visually the RFD is one of the most prominent features associated with tornadogenesis. It is usually manifest as a band or narrow, very deep, slot of cloud free air wrapping around a center of strong rotation and, eventually the tornado. Should the observer be close enough to the downdraft (on the order of 1-2 km from the tornado) they would notice rapidly sinking cloud fragments at the periphery of the clear slot and strong rising motion on the interior. Over 20 years of storm intercept observations show that typically the slot wraps around the region where the tornado forms about five to 10 minutes prior to tornado formation."

4.0 Mesocyclone "Dry Intrusion" Importance.
It is informative to examine the importance of the ETC "dry intrusion" and to draw a parallel to the mesocyclone clear slot/dry intrusion. The reader is referred to Browning (1990) , Browning and Roberts (1994), and Browning (1997) for more detail.

The cited references make it clear that much of the cold air that enters the cyclonic cloud system from the rear has had a history of descent from the upper troposphere and even some from the lower stratosphere (Fig 3.). Because this airflow is concentrated and belt-like, it is called a dry intrusion. It can be identified in several ways such as its dryness, its association with a tongue of high potential vorticity (PV) brought down from levels where the ambient PV is in close association with an upper tropospheric jet steak and tropopause fold. As it descends, it approaches and overtakes the cold front with its leading edge splayed out in a divergent, col-like fashion. When the dry high-PV air descends and encroaches on the baroclinic zone and cyclone center as a fast-moving dry intrusion it often produces rapid cyclogenesis. This is especially true

Figure 3 Three-dimensional representation of the ETC dry intrusion flow. Arrows are trajectories of air originating from a small region near the tropopause, drawn within a curved isentropic surface. After Danielsen, (1964).

when it overruns a high-q w WCB (Browning, 1977).

It also plays a major role in producing severe weather (strong winds, intense squall lines, and even tornadoes). In one study, Browning and Reynolds (1994) documented a dry intrusion filament of stratospheric air with high PV that plunged to the lower troposphere and caused severe wind gusts up to 70 kts. In another, Browning and Golding (1995) documented a dry intrusion that overtook a cold frontal location where there had been no precipitation. Quickly, convection developed, became intense and spawned two tornadoes over southeast England.

Here, it is proposed that the RFD and clear slot or dry intrusion of supercellular storms, like that associated with the ETC, have a high level origin (L&D). It is, at least in part, a filament of upper tropospheric or even stratospheric dry air, possibly originating from a tropopause fold, that descends in the clear slot, and approaches the mesocyclone center causing rapid deepening and tornadogenesis. (Note that neither the RFD nor clear slot is an occlusion downdraft. They are distinctly different and separate). As in the ETC, upon arrival at the mesocyclone circulation center, some of the air reascends within the mesocyclone core and developing tornado. Contributing to mesocyclogenesis and tornadogenesis, is the high PV of this air. How much of the PV associated with the static stability of this air that is retained during decent is questionable. However, the thermal wind contribution to the vertical wind shear across such a tropopause fold may contribute in such a way as to introduce large streamwise vorticity and helicity into the RFD and clear slot itself. Additionally, some vorticity may be added to this airstream due to evaporation of precipitation which falls into and descends with the dry intrusion from the back sheared, overhanging anvil,. (Note it is not inferred that this is the only vorticity source for tornadogenesis. Nor should it be inferred that all tornadoes develop in this manner.) However, the rather obvious importance of the clear slot and its similarity to the ETC clear slot, both in form and affect, suggest that the possible association of this feature with a filamentary tropopause fold and dry intrusion should be investigated. Airstream gas composition in the storm scale dry intrusion may help in this effort. Nesting a meso-gamma-scale model within an operational NWP model may also be an aid. Further, vorticity transport or development from aloft within the clear slot should be investigated. Does the supercell result from a pre-existing dry intrusion from aloft or does the supercell stimulate dry intrusion development? Examining the relationship of satellite observed water vapor dark zones to tornadic storms and their dry intrusions may help here. Radiosonde soundings properly placed near but to the rear of these storms may also help answer this and other questions.

It is proposed that before we can correctly understand and model tornadogenesis, we must first understand the true nature of mesocyclone structure. It is suggested that the context of tornadogenesis is, at least in most supercellular storms, that of a highly baroclinic system and not a barotropic "rotating updraft".

5.0 Acknowledgments.

The author wishes to thank Professor Keith Browning of Reading University, United Kingdom, for providing reprints of his work and for several very helpful discussions. The author also wishes to thank Dr. Robert Maddox for his very helpful discussions and suggestions.

6.0 References.

Browning, K. A.., 1990: Organization of clouds and precipitation in extratropical cyclones. Chapter 8. Extratropical Cyclones. C.W. Newton and E.O. Holopainen, Eds. Amer. Metero. Soc., 129-153.

_______, 1997: The dry intrusion perspective of extra-tropical cyclone development. Meteorol. Appl., 4, 317-324.

________, and B. W. Golding, 1995: Mesoscale aspects of a dry intrusion within a vigorous cyclone. Q. J. R. +Meteorol. Soc., 121, 463-493.

________, and N. M. Roberts, 1994: Structure of a frontal cyclone. Q. J. R. Meteorol. Soc., 120, 1535-1557.

Burgess, D. W., V. T. Wood and R. A. Brown, 1982: Mesocyclone evolution statistics, Preprints, 12th Conf. Severe Local Storms, San Antonio, Amer. Meteor. Soc., 422-424.

Danielsen, E. E., 1964: Project Springfield Report. Defense Atomic Support Agency, Washington D.C. 20306, DASA 1517 (NTIS # AD-607980), 97pp.

Davies-Jones, R. P., 1984: Streamwise vorticity: the origin of updraft rotation in supercell storms. J. Atmos. Sci., 41, 2991-3006.

Harold, T. W., 1973: Mechanisms influencing the distribution of precipitation within baroclinic disturbances. Q. J. R. Meteorol. Soc., 99, 232-251.

Klemp, J. B., 1987: Dynamics of tornadic thundertorms. Ann. Rev. Fluid Mech., 19, 369-402.

----------, and R. B. Wilhelmson, 1978: Simulations of right- and left-moving storms produced through storm splitting. J. Atmos. Sci., 35, 1097-1110.

Lemon, L. R., 1976: The flanking line, a severe thunderstorm intensification source. J. Atmos. Sci., 33, 686-694.

----------, and C. A. Doswell III, 1979: Severe thunderstorm evolution and mesocyclone structure as related to tornadogenesis. Mon. Wea. Rev., 107, 1184-1197.

-----------, and S. Parker, 1996: The Lahoma storm deep convergence zone: its characteristics and role in storm dynamics and severity. Preprints, 18th Conference on Severe Local Storms. San Francisco, CA.., Amer. Meteor. Soc., 70-75.

Lilly, D. K., 1986: The structure, energetics, and propogation of rotating convective storms. Part II: helicity and storm stabilization. J. Atmos. Sci., 43, 126-140.

Rasmussen, E. N. and J. M. Straka, 1998: Tornadogenesis: a review and a new conceptual model. Mon. Wea. Rev., in press.

Schlesinger, R. E., 1978: A three-dimensional numerical model of an isolated thunderstorm: Part I, comparative experiments for variable ambient wind shear. J. Atmos. Sci., 35, 690-713.

Wicker, L. J., and R. B. Wihelmson, 1995: Simulation and analysis of tornado development and decay within a three-dimensional supercell thunderstorm. J. Atmos. Sci., 52, 2675-2703.

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