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14 Thunderstorm Fundamentals Copyright © 2017 by Roland Stull. Practical Meteorology: An Algebra-based Survey of Atmospheric Science v1.02 14 THUNDERSTORM FUNDAMENTALS Contents Thunderstorm characteristics, formation, and 14.1. Thunderstorm Characteristics 481 forecasting are covered in this chapter. The next 14.1.1. Appearance 481 chapter covers thunderstorm hazards including 14.1.2. Clouds Associated with Thunderstorms 482 hail, gust fronts, lightning, and tornadoes. 14.1.3. Cells & Evolution 484 14.1.4. Thunderstorm Types & Organization 486 14.1.4.1. Basic Storms 486 14.1.4.2. Mesoscale Convective Systems 488 14.1.4.3. Supercell Thunderstorms 492 14.1. THUNDERSTORM CHARACTERISTICS INFO • Derecho 494 14.2. Thunderstorm Formation 496 Thunderstorms are convective clouds with 14.2.1. Favorable Conditions 496 large vertical extent, often with tops near the tro- 14.2.2. Key Altitudes 496 popause and bases near the top of the boundary INFO • Cap vs. Capping Inversion 497 layer. Their official name iscumulonimbus (see 14.3. High Humidity in the ABL 499 the Clouds Chapter), for which the abbreviation is INFO • Median, Quartiles, Percentiles 502 Cb. On weather maps the symbol represents 14.4. Instability, CAPE & Updrafts 503 thunderstorms, with a dot •, asterisk *, or triangle 14.4.1. Convective Available Potential Energy 503 ∆ drawn just above the top of the symbol to indicate 14.4.2. Updraft Velocity 508 rain, snow, or hail, respectively. For severe thunder- 14.5. Wind Shear in the Environment 509 storms, the symbol is . 14.5.1. Hodograph Basics 510 14.5.2. Using Hodographs 514 14.5.2.1. Shear Across a Single Layer 514 14.1.1. Appearance 14.5.2.2. Mean Wind Shear Vector 514 A mature thunderstorm cloud looks like a mush- 14.5.2.3. Total Shear Magnitude 515 room or anvil with a relatively large-diameter flat 14.5.2.4. Mean Environmental Wind (Normal top. The simplest thunderstorm (see Figs. 14.1 & Storm Motion) 516 14.2) has a nearly vertical stem of diameter roughly 14.5.2.5. Supercell Storm Motion 518 equal to its depth (of order 10 to 15 km). The large 14.5.2.6. Bulk Richardson Number 521 top is called the anvil, anvil cloud, or thunder- 14.5.2.7. The Effective Layer 522 head, and has the official nameincus (Latin for 14.6. Triggering vs. Convective Inhibition 522 anvil). The anvil extends furthest in a direction as 14.6.1. Convective Inhibition (CIN) 523 blown by the upper-tropospheric winds. 14.6.2. Triggers 525 14.7. Thunderstorm Forecasting 527 14.7.1. Outlooks, Watches & Warnings 528 INFO • A Tornado Watch (WW) 529 14.7.2. Stability Indices for Thunderstorms 530 14.8. Storm Case Study 532 14.9. Review 533 14.10. Homework Exercises 533 14.10.1. Broaden Knowledge & Comprehension 533 14.10.2. Apply 534 14.10.3. Evaluate & Analyze 537 14.10.4. Synthesize 542 “Practical Meteorology: An Algebra-based Survey of Atmospheric Science” by Roland Stull is licensed under a Creative Commons Attribution-NonCom- mercial-ShareAlike 4.0 International License. View this license at © Gene Rhoden / weatherpix.com http://creativecommons.org/licenses/by-nc-sa/4.0/ . This work is Figure 14.1 available at https://www.eoas.ubc.ca/books/Practical_Meteorology/ Airmass thunderstorm having a single mature cell. 481 482 CHAPTER 14 • THUNDERSTORM FUNDAMENTALS If the thunderstorm top is just starting to spread (a) anvil out into an anvil and does not yet have a fibrous or updraft streaky appearance, then you identify the cloud as tower cumulonimbus calvus (see the Clouds Chapter). z rain & For a storm with a larger anvil that looks strongly downdraft glaciated (i.e., has a fibrous appearance associat- ed with ice-crystal clouds), then you would call the x cloud a cumulonimbus capillatus. tropopause Within the stem of a mature thunderstorm is the cloudy main updraft tower topped by an updraft upper-level updraft anvil bubble winds bubble (Fig. 14.2b). When this rising air hits the tropopause, it spreads to make the anvil. Also in (b) the stem is a downdraft with precipitation. When cloudy wake virga the downdraft air hits the ground it spreads out, the leading edge of which is called the gust front. z cloud When viewed from the ground under the storm, the base arc cloud arc cloud main updraft often has a darker cloud base, while gust front gust front the rainy region often looks not as dark and does not precipitation x have a well-defined cloud base. Not all cumulonimbus clouds have lightning y and thunder. Such storms are technically not thun- anvil derstorms. However, in this book we will use the word thunderstorm to mean any cumulonimbus (c) cloud, regardless of whether it has lightning. precip. More complex thunderstorms can have one or more updraft and downdraft regions. The most se- upper- vere, long-lasting, less-frequent thunderstorms are level gust front supercell thunderstorms (Figs. 14.3 & 14.4). winds at surface x 14.1.2. Clouds Associated with Thunderstorms Figure 14.2 Sometimes you can see other clouds attached to (a) Sketch of a basic (airmass) thunderstorm in its mature stage. thunderstorms, such as a funnel, wall, mammatus, (b) Vertical slice through the storm. Light shading indicates arc, shelf, flanking line, scud, pileus, dome, and bea- clouds, green and red shadings are moderate and heavy precip- ver tail (Fig. 14.4). Not all thunderstorms have all itation, and arrows show air motion. (c) Horizontal composite, these associated clouds. showing the anvil at storm top (as viewed from above by sat- Arc clouds (official name arcus, Fig. 14.2b) or ellite), the precipitation in the low-to-middle levels (as viewed shelf clouds form near the ground in boundary- by radar), and the gust front of spreading winds at the surface. layer air that is forced upward by undercutting cold air flowing out from the thunderstorm. These cloud bands mark the leading edge of gust-front outflow from the rear-flank downdraft (Fig. 14.4), usually associated with the flanking line. Often the un- dersides of arc clouds are dark and turbulent-look- ing, while their tops are smooth. Not all gust fronts have arc clouds, particularly if the displaced air is dry. See the Thunderstorm Hazards chapter. The beaver tail (Fig. 14.4) is a smooth, flat, narrow, low-altitude cloud that extends along the boundary between the inflow of warm moist air to the thunderstorm and the cold air from the rain-in- duced forward flank downdraft (FFD). A dome of overshooting clouds sometimes forms above the anvil top, above the region of stron- gest updraft. This is caused by the inertia of the up- © Gene Rhoden / weatherpix.com ward moving air in the main updraft, which over- Figure 14.3 shoots above its neutrally buoyant equilibrium Photo of supercell thunderstorm. R. STULL • PRACTICAL METEOROLOGY 483 level (EL). Storms that have overshooting tops Overshooting Top or Pileus are often more violent and turbulent. Dome EL The flanking line is a band of cumuliform Anvil clouds that increase from the medium-size cumu- Mammatus lus mediocris (Cu med) furthest from the storm Main to the taller cumulus congestus (Cu con) close Pileus Updraft z to the main updraft. Cumulus congestus are also Cu Virga Cu med informally called towering cumulus (TCu). The con Striations flanking line forms along and above the gust front, Flanking Line Beaver Tail LCL which marks the leading edge of colder outflow air from the rear-flank downdraft (RFD). Rain If humid layers of environmental air exist above SW Scud NE rapidly rising cumulus towers, then pileus clouds Wall Cloud Tail Cloud can form as the environmental air is pushed up and out of the way of the rising cumulus clouds. Pileus Funnel Cloud are often very short lived because the rising cloud or Tornado tower that caused it often keeps rising through the Figure 14.4a pileus and obliterates it. Sketch of a classic supercell thunderstorm (Cb) as might be The most violent thunderstorms are called viewed looking toward the northwest in central North America. supercell storms (Figs. 14.3 & 14.4), and usu- The storm would move from left to right in this view (i.e., to- ward the northeast). Many storms have only a subset of the ally have a quasi-steady rotating updraft (called a features cataloged here. Cu med = cumulus mediocris; Cu con mesocyclone). The main thunderstorm updraft = cumulus congestus; LCL = lifting condensation level; EL = in supercells sometimes has curved, helical cloud equilibrium level (often near the tropopause, 8 to 15 km above striations (grooves or ridges) on its outside similar ground); NE = northeast; SW = southwest. to threads of a screw (Fig. 14.4a). Supercells can pro- duce intense tornadoes (violently rotating columns of air), which can appear out of the bottom of an iso- lated cylindrical lowering of the cloud base called a wall cloud. The portion of the tornado made vis- Storm ible by cloud droplets is called the funnel cloud, Anvil edge Movement which is the name given to tornadoes not touching Forward Flank Downdraft the ground. Tornadoes are covered in the next chap- ter. Most thunderstorms are not supercell storms, Rain FFD Rain Beaver Tail and most supercell storms do not have tornadoes. Attached to the base of the wall cloud is some- times a short, horizontal cloud called a tail cloud, Rear Flank T Downdraft Main which points towards the forward flank precipi- RFD Updraft tation area. Ragged cloud fragments called scud Outflow T (cumulus fractus) often form near the tip of the tail Boundary- cloud and are drawn quickly into the tail and the Layer Line Winds Cu wall cloud by strong winds.
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