SR-185

4-97

TECHNIQUES FOR ISSUING SEVERE THUNDERSTORM

AND TORNADO WARNINGS WITH THE

ABSTRACT

This paper reviews conventional radar warning guidelines and compiles techniques for the decision making process in issuing Doppler radar based severe thunderstorm and tornado warnings. The ideas presented in this paper are not meant to be the definitive study on the subject of radar techniques for warnings, but are only meant to be guidelines on the subject. Local warning techniques, such as vertically integrated liquid (VIL) techniques are only briefly mentioned, but the most widely used and accepted warning techniques are included in detail. The latest research on techniques for warning for non-supercell tornadoes and ordinary cell microbursts is also presented.

1. Introduction

A severe thunderstorm is defined by the National Weather Service as a thunderstorm that produces a tornado, and/or winds of at least 50 kt (58 mph), and/or hail at least in in diameter. Structural damage may imply the occurrence of a severe thunderstorm. A tornado is defined as a violently rotating column of air, pendant to a cumulonimbus, with circulation reaching the ground.

The purpose of this paper is to serve as a guideline for issuing severe thunderstorm and tornado warnings using information from the WSR-88D (Weather Surveillance Radar 1988 Doppler) Doppler radar. I have attempted to summarize most of the published and/or accepted techniques for issuing warnings based on Doppler radar. The paper is intended to be used as an easy reference by meteorologists tasked with issuing severe weather warnings. Another possible use of this paper is for training meteorologist interns on radar warning techniques. I cannot include all local office techniques that are available, such as local vertically integrated liquid (VIL) studies, but I hope I have included the most widely used and accepted techniques.

Section 2 discusses conceptual models of thunderstorms that will be referred to throughout this paper. Section 2 blends with Section 3, which relates environmental factors to thunderstorm types and expected severe weather. Even with Doppler radar, environmental factors should be taken into account before a decision is made on the type of warning issued.

With the development of Doppler radar, new techniques have been developed in addition to the criteria that were used for conventional (non-Doppler) radars. The Doppler radar criteria do not take the place of the Lemon criteria (Lemon 1980), but add many more features that can be used to issue severe thunderstorm and tornado warnings. The Lemon criteria and the additional Doppler radar techniques are discussed in Sections 4 and 5 of this paper respectively.

Recent research has produced a few techniques for warning for non-supercell tornadoes and for ordinary cell microbursts. These techniques are also incorporated into Section 5.

A summary of techniques is provided (just before the figures in the middle of the document) for quick reference.

2. Conceptual Models of Thunderstorms

Thunderstorms can be categorized according to certain characteristics they exhibit by various conceptual model, radar, or visual features. For the purposes of this paper the classifications of thunderstorms will follow the descriptions listed in this section. When the radar features or signatures of severe thunderstorms and tornadoes are discussed in later sections, references will be made to the conceptual models discussed in this section. Although these thunderstorm types are presented as distinctly different, in reality these features will usually blend into each other, or evolve from one type to another.

The first thunderstorm type is the ordinary (single cell) thunderstorm. This thunderstorm is characterized by the formation of a single thunderstorm updraft (as seen in Fig. 1), which is followed by a downdraft and dissipation of the thunderstorm within 1 hr (Doswell 1985). The ordinary thunderstorm can produce severe weather such as high winds or hail, but tornadoes are rare (although non-supercell tornadoes can occur). This thunderstorm type will be discussed later in the context of a severe thunderstorm, when the ordinary thunderstorm microburst radar signatures are presented (see Section 5.a.3). Forecasters can anticipate an ordinary thunderstorm microburst in a favorable environment (see Section 3.a).

b. Multicell thunderstorm

The multicell thunderstorm conceptual model is shown in Fig. 2. This thunderstorm type is characterized by multiple updrafts forming new cells as each downdraft (and precipitation) dissipates the previous cell (Ray 1986 [see Weisman and Klemp, Chapt. 15]). Cold air outflow from each dissipating cell triggers new cells along the leading edge of the outflow, generally in the direction of the storm motion. This thunderstorm type is more long-lived than an ordinary thunderstorm. The multicell thunderstorm is characterized by a strong radar reflectivity gradient on the leading edge of the thunderstorm, and possibly by a radar weak echo region (WER) just above the low level reflectivity gradient area. Squall lines usually contain several multicell thunderstorms, but can also contain embedded supercell storms (see below).

Multicell thunderstorms produce a variety of severe weather including large hail and damaging wind. Short lived tornadoes have been known to occur on the leading edge of the outflow. Generally the tornadoes that occur with multicell (or ordinary cell) thunderstorms are not as severe as the tornadoes that occur with the supercell thunderstorm.

c. Supercell thunderstorm

The supercell is generally defined as a thunderstorm with a persistent rotating updraft (mesocyclone). The supercell thunderstorm is a long-lived (over several hours time) thunderstorm with distinctive radar and visual features (Ray 1986 [see Weisman and Klemp, Chapt. 15]). The conceptual model of a supercell thunderstorm is presented in Figs. 3 and Fig.4. In Fig. 3, the radar signatures and wind fields of a supercell thunderstorm are shown. The familiar radar reflectivity signature commonly referred to as the "hook echo" is shown to be at the confluence of the thunderstorm updraft with the rear flank downdraft, and is the preferred region for strong tornado occurrence. The hook echo is also the region where the mesocyclone will be located at the surface, or aloft, in a supercell thunderstorm.

Figure 4c shows a mature supercell and depicts supercell thunderstorm radar features such as the bounded weak echo region (BWER), the hook echo, the strong leading edge reflectivity gradient, and the thunderstorm top overhanging the low level reflectivity gradient area. The BWER and the overhanging thunderstorm top are caused by the strong, nearly vertical, updraft of a supercell thunderstorm suspending precipitation particles aloft. The hook echo and mesocyclone are caused by the tilting of horizontal environmental vorticity into the vertical causing rotation of the updraft. This is also a factor in formation of a BWER.

The supercell thunderstorm typically moves to the right of the environmental storm motion vector, thus it will appear on the radar as moving to the right of other thunderstorms that may be present. Although anticyclonic supercells can occur (moving left of the storm motion vector), they have not been associated with tornadoes as the cyclonic right moving supercell thunderstorm. Supercell thunderstorms are capable of producing strong destructive tornadoes, large hail, and damaging wind.

Three sub-categories of the classic supercell have been identified. These are the low-precipitation (LP) supercell (Bluestein and Parks 1983), the high-precipitation (HP) supercell (Moller et al. 1990), and the mini supercell (Burgess et al. 1995). The LP supercell thunderstorm (Fig. 5) is found in the surface dryline environment just east of the Rocky Mountains over the western Great Plains (Moller et al. 1994). LP supercell environments are characterized low to moderate moisture values. Severe weather with LP supercells is limited to large hail and an occasional weak to moderate tornado.

The HP supercell (Fig. 6) occurs over the central and eastern United States and may be the dominant form of supercell nationwide. This type of thunderstorm has substantial precipitation in the mesocyclone (Moller et al. 1994). The HP supercell can produce tornadoes and/or high winds. Large hail is also possible with this type of supercell, but may not be as common as with the classic or LP types of supercells.

In addition, mini supercells as described by Burgess et al. (1995) can produce severe weather. Mini supercells are smaller than traditional supercells in both horizontal and vertical extent but still possess the same radar attributes, including hook echo, well defined weak echo region, bounded weak echo region, and mesocyclone. These are also referred to as low topped supercells because the tops of these storms are lower than 30,000 ft.

d. Bow echo thunderstorm

Another type of thunderstorm is the "bow echo" thunderstorm. The name "bow echo" is derived from a rather steady state radar signature in the shape of a bow as seen in Fig. 7 (Fujita 1978).

The persistence of the echo is caused by new updrafts forming on the leading edge of the bow echo. The bow echo is also characterized by bookend vortices. One bookend vortex occurs on the north side of the bow echo and contains cyclonic vorticity, while the other bookend vortex is located on the south side of the bow echo and has anticyclonic vorticity. Isolated tornadoes can occur in the cyclonic vortex of the bow echo.

The high winds that can occur with the bow echo usually are located in the rear of the center of the bow and are caused by horizontal buoyancy gradients along the rear edge of the buoyant plume aloft and cold pool near the surface generating horizontal vorticity, and accelerating the flow from rear to front at middle levels (Fig. 8, Weisman 1993).

The bow echo thunderstorm has been described as a type of HP supercell (Moller et al. 1990); however, in this paper I prefer to categorize the bow echo thunderstorm as an entity of its own.

e. Non-supercell tornadoes

A special case of severe event is the non-supercell tornado. These tornadoes occur in nonmesocyclone convection in an environment of weak shear over a boundary that is a source of vertical vorticity (Brady and Szoke 1989; Wakimoto and Wilson 1989). The tornado occurs in the development phase of the thunderstorm with the updraft of the thunderstorm stretching the environmental vertical vorticity.

A conceptual model of the non-supercell tornado is shown in Fig. 9. The figure depicts the source of vertical vorticity being stretched in the vertical by the convective updraft. The tornadoes caused by this mechanism are thought to be mostly F0 and F1 in intensity, but Wakimoto and Wilson (1989) have suggested that F3 damage is possible. The non-supercell tornado is generally short lived, only 5 to 10 min, but a few may last as long as 20 min.

3. Environmental factors versus storm type and severe weather

This section provides a brief discussion of some of the more commonly used environmental parameters and how they relate to storm type and severe weather. The first part discusses the ordinary cell microburst environment. Next is a section on tornado forecasting parameters, followed by a discussion on middle level winds and expected severe weather. A checklist of parameters correlated with derecho (and bow echo) formation is then presented. Lastly, results of numerical modeling of environment versus storm type are discussed.

Several environmental parameters have been correlated with severe thunderstorm and tornado occurrence. One such parameter is convectively available potential energy (CAPE), defined as the positive area on a sounding associated with the buoyant part of a lifted parcel ascent between the level of free convection and the equilibrium level.

Many meteorologists refer to a parameter called storm-relative environmental helicity (hereafter referred to as helicity). Helicity is usually determined in the lowest 3 km of the atmosphere and is defined as:

(1)

Helicity = w^. (V - V_s)dz

where w = k dV/dz, V is the wind velocity, and Vs is the storm velocity (Colquhoun and Riley, 1996). This parameter attempts to measure the amount of shear a thunderstorm experiences relative to storm motion.

Weisman (1996) presents an interesting discussion on the role of wind shear versus helicity in severe storms. He discusses the premise that the steadiness and propagation (right mover or left mover relative to mean flow) of supercells are a direct result of the development of rotation in the storm.

The steadiness and propagation of supercells are caused by an updraft that generates vertical vorticity in the middle levels of the storm due to tilting of environmental horizontal vorticity in a vertically sheared atmosphere. The vertical vorticity produces vertical pressure gradients within the storm that force the updraft to move toward a particular flank, which leads to a continuing updraft regeneration and propagation.

Helicity can only be applied to a particular storm motion, and thus storm motion must have already been observed or anticipated before helicity can be determined. In other words, helicity cannot be used directly to predict potential storm structure or motion from a given environment. Weisman contends that supercell propagation and dynamics are similar for both straight and curved hodographs. The important factors in predicting supercells are low level wind shear and depth of shear.

To quote from part of Weisman's article (emphasis added):

I believe that the vertical wind shear perspective is superior (to helicity), because it establishes the kind of physical cause and effect link between storm structure and the pre-storm environment that forecasters can readily apply when attempting to assess storm potential on any given day. To the degree that the pre-storm environment is known and that convection is going to occur, a forecaster can simply look at the hodograph and determine to a good approximation whether there is sufficient shear over a sufficient depth to promote supercell development (e.g. 20 ms-1 of wind variation over the lowest 4-6 km AGL), whether there will be symmetric splitting or preferred flank development, and even roughly what the range of storm motions will be for the ordinary cell versus supercells. As is depicted in even simple simulations, supercell genesis should be viewed as a process that includes not only the development of a single, right moving, cyclonically rotating updraft, but may also include an anticyclonic left-member in some cases, or multiple cell regeneration along the surface outflow in other cases. Only from this perspective can a forecaster properly interpret the complex evolutions apparent on the radar scope. (Weisman 1996)

Other parameters will be discussed below in the tornado environment section.

a. Ordinary cell microburst environment

A special environmental case is the atmospheric condition associated with the ordinary cell microburst. This type of microburst can be sub-categorized into dry-microbursts (Wakimoto 1985) and wet-microbursts (Atkins and Wakimoto 1991).

Dry-microbursts are usually seen over the High Plains and are defined as microbursts that have little or no rain during the period of high winds. The atmospheric conditions that support dry-microbursts can be best described by model soundings as seen in Fig. 10a (Atkins and Wakimoto 1991). This type of environment is characterized by very dry air in the sub-cloud layer.

A thunderstorm that develops in this environment can enhance its severity when downdraft strength is increased by the evaporative cooling that occurs as the rainy downdraft falls through the dry air just above the surface. Convective cells in this environment can produce damaging microbursts even though updraft strength may be weak. Generally this environment is characterized by lower values of CAPE and weak vertical wind shear (Ray 1986 [see chapter 15, Weisman and Klemp]).

Wet-microbursts are more common in the more humid regions, especially the southeast U.S. Wet-microburst atmospheric and radar characteristics differ from dry-microbursts by having shallow sub-cloud layers, higher radar reflectivities, and an environmental sounding that is more statically stable. The model soundings for wet-microbursts are shown in Fig. 10b (Atkins and Wakimoto 1991). One of the interesting environmental factors discussed in Atkins and Wakimoto (1991) is that on days favorable for wet-microbursts, the change in equivalent potential temperature (e = the difference between the maximum e found just above the surface and the minimum e aloft) must be at least 20 deg C (Fig. 10b).

b. Tornado forecasting

Recently, research has focused on a few parameters which seem to correlate better with the formation of supercells. One such parameter is the bulk Richardson number, BRN, which is defined to be:

B
BRN= ______
1/2 U²_s

                0 - 2 km average positive shear		13.6
		0 - 3 km average positive shear		10.7
		0 - 4 km average positive shear		9.1

_{Figure 11 shows the distribution of Us (0 - 6 km shear) for the Davies and Johns dataset. The majority of the cases (70 percent) were associated with Us values greater than 18 ms-1. Figure 12 shows the distribution of BRN for the same dataset (Johns et al. 1993). From the graph it is easy to see that BRN values under 20 were a significant supercell (possibly tornadic) threat. And in Fig. 13, tornado cases are plotted on a CAPE versus shear diagram. Most cases had CAPE under 3,200 and 0 - 2 km shear higher than 9 10-3 s-1. As previously discussed, Weisman (1996) states that in the lower 4 - 6 km of the atmosphere, vertical wind shear of 20 ms-1 or more is sufficient for supercell development (and possibly tornadoes).}

The authors suggest for very weak middle level storm relative environmental winds, any low level mesocyclones will occur early in the storm's life and be short lived, with the outflow dominating the storm (especially in a high CAPE environment). For very strong middle level storm relative winds, the low level mesocyclone would be very slow to develop or perhaps not develop at all. For moderate middle level storm relative winds, long-lived low level mesocyclones might result if the mesocyclone circulation and storm relative middle level winds are balanced in some sense. The results of their research is shown in _{Fig. 14, where q(max) is the maximum water vapor content in the boundary layer (g/kg), and H / v(min) is the ratio of 0-3 km helicity to the minimum storm relative wind (m/s) averaged over a depth of 1 km in the 6,000 to 28,000 ft layer. The reader is referred to the article for further information.}

Derechos have become more recognizable as another form of severe weather. Derechos are defined to be widespread, rapidly moving, convectively induced windstorms that produce significant damage and often casualties. Johns and Hirt (1987) provided a list of environmental factors favorable for the formation of derecho type damaging windstorms. The guidelines from Johns and Hirt have been put in a checklist format (_{Fig. 15) for easy reference. One type of thunderstorm that occurs often in derechos is the bow echo type storm, so this checklist will also aid the forecaster in determining if the environment is favorable for bow echoes.}

While the thermodynamics of the environment are important in determining updraft strength, vertical wind shear has a strong influence on thunderstorm structure. _{Figure 16 correlates the relationship between wind shear and storm type. These composite hodographs are for environments typical of ordinary cells, multicell storms, and supercell storms.}

In the case of increased shear of Fig. 16b, both multicell and supercell storms can occur. An explanation for this is presented by Rotunno et al. (1988). _{Figure 17 shows the theory that the essential factor of a long lived multicell thunderstorm (or line of thunderstorms) is the amount of low level shear that counteracts the circulation induced by the thunderstorm outflow. The outflow is produced by precipitation evaporating as it nears the surface. With shear and outflow (Fig. 17d), there is a balance of the circulation generated by the outflow with the environmental circulation generated by environmental shear. These circulations complement each other to enhance development of new thunderstorm cells. Rotunno et al. (1988) further state that this can cause either long-lived squall lines of multicell storms, or lines of nearly steady supercells (with the supercells separate from each other along the line). The squall line occurs when the low level environmental shear is directed perpendicular to the line, with weak shear aloft. The line of supercells occurs when strong deep environmental shear is directed at an angle to the line, which allows each cell to persist without interference from other supercell storms.}

Results were obtained from a typical severe weather type sounding shown in _{Fig. 18, which had a CAPE of about 2200 m2s-2. The study modeled thunderstorm structures which result from varying hodograph shapes and magnitudes. In Figs. 19A-F, "R" is the bulk Richardson number (BRN) described above. As the hodograph varied, the model produced:}

        Case A:	Short lived multicell
	Case B:	Supercell on the south end of a multicell line
	Case C:	Curved hodograph - Right flank supercell split from weaker left flank storm
		Straight hodograph - Right and left flank supercells
	Case D:	Right flank supercell
	Case E:	Weak squall line
	Case F:	Squall line: spearhead echo evolving into bow echo and comma echoes

Les Lemon (1980) provided a study of warning identification techniques and warning criteria for conventional radar. These techniques were generally accepted as the warning criteria used by the National Weather Service ( _{Fig. 20). Some meteorologists may have assumed with the installation of the WSR-88D Doppler radar the Lemon criteria for warnings using conventional radar were obsolete. This is not so! The Lemon criteria are still valid warning criteria and can be used with the additional warning guidelines for Doppler radar (Section 5). In fact, some of the WSR-88D algorithms were designed to incorporate features of the Lemon criteria.}

1. Lemon criteria - Fig. 20
2. Flare echo - _{Color Plate A1}
3. WSR-88D Doppler radar hail algorithm products
4. Storm top divergence - _{Fig. 21}
5. Moderate to strong mesocyclones with environmental factors that limit tornado development - _{Fig. 22}
6. VIL of the Day - _{Fig. 24}
7. VIL of the Day equation (Eq. 3)
8. VIL density (Eq. 4)

Another depiction of criteria for storm top divergence (hail size) and mesocyclones is shown in _{Fig. 23 (OSF 1995).}

1. Bow echo/derecho signatures - Fig. 7 and _{Color Plate A2}
2. Supercell signatures - Figs. 3, 4c, and _{Color Plate B1}
3. Mini supercell signatures - Tables 1 and 2
4. Base velocity magnitude

 (1)	Supercell Tornadoes
The techniques for supercell tornadoes are:
	(1)	Supercell signatures:
		a.	Bounded weak echo region - Fig. 4
		b.	Hook echo - Color Plate B1
		c.	Strong mesocyclone plus favorable tornadic environment - Fig. 22 and Color Plate B1
		d.	Tornado vortex signature
	(2)	Mini supercell signatures - Tables 1 and 2

  (2)	Non-Supercell Tornadoes
The techniques for non-supercell tornadoes are:
	(1)	A rapidly growing cell on a stationary or slow moving boundary, especially if there are kinks or bends on the boundary signifying cyclonic vorticity areas.
	(2)	A low level mesocyclone signature close to the radar (within 50 miles).