Copyright © 2015 by Roland Stull. Practical Meteorology: An Algebra-based Survey of Atmospheric Science. 14 THUNDERSTORM FUNDAMENTALS Contents Thunderstorm Characteristics 481 Thunderstorm characteristics, formation, and Appearance 482 forecasting are covered in this chapter. The next Clouds Associated with Thunderstorms 482 chapter covers thunderstorm hazards including Cells & Evolution 484 hail, gust fronts, lightning, and tornadoes. Thunderstorm Types & Organization 486 Basic Storms 486 Mesoscale Convective Systems 488 Supercell Thunderstorms 492 INFO • Derecho 494 Thunderstorm Formation 496 ThundersTorm CharacterisTiCs Favorable Conditions 496 Key Altitudes 496 Thunderstorms are convective clouds INFO • Cap vs. Capping Inversion 497 with large vertical extent, often with tops near the High Humidity in the ABL 499 tropopause and bases near the top of the boundary INFO • Median, Quartiles, Percentiles 502 layer. Their official name is cumulonimbus (see Instability, CAPE & Updrafts 503 the Clouds Chapter), for which the abbreviation is Convective Available Potential Energy 503 Cb. On weather maps the symbol represents Updraft Velocity 508 thunderstorms, with a dot •, asterisk *, or triangle Wind Shear in the Environment 509 ∆ drawn just above the top of the symbol to indicate Hodograph Basics 510 rain, snow, or hail, respectively. For severe thunder- Using Hodographs 514 storms, the symbol is . Shear Across a Single Layer 514 Mean Wind Shear Vector 514 Total Shear Magnitude 515 Mean Environmental Wind (Normal Storm Mo- tion) 516 Supercell Storm Motion 518 Bulk Richardson Number 521 Triggering vs. Convective Inhibition 522 Convective Inhibition (CIN) 523 Triggers 525 Thunderstorm Forecasting 527 Outlooks, Watches & Warnings 528 INFO • A Tornado Watch (WW) 529 Stability Indices for Thunderstorms 530 Review 533 Homework Exercises 533 Broaden Knowledge & Comprehension 533 © Gene Rhoden / weatherpix.com Apply 534 Figure 14.1 Evaluate & Analyze 537 Air-mass thunderstorm. 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 http://creativecommons.org/licenses/by-nc-sa/4.0/ . This work is available at http://www.eos.ubc.ca/books/Practical_Meteorology/ . 481 482 cHAPTer 14 • THUNDERSTORM FUNDAMENTALS B appearance BOWJM A mature thunderstorm cloud looks like a mush- VQESBGU room or anvil with a relatively large-diameter flat UPXFS top. The simplest thunderstorm (see Figs. 14.1 & [ SBJO 14.2) has a nearly vertical stem of diameter roughly EPXOESBGU equal to its depth (of order 10 to 15 km). The large top is called the anvil, anvil cloud, or thunder- Y head, and has the official name incus (Latin for USPQPQBVTF anvil). The anvil extends furthest in a direction as VQQFSMFWFM VQESBGU BOWJM blown by the upper-tropospheric winds. CVCCMF XJOET If the thunderstorm top is just starting to spread out into an anvil and does not yet have a fibrous or C DMPVEZ streaky appearance, then you identify the cloud as XBLF WJSHB cumulonimbus calvus (see the Clouds Chapter). For a storm with a larger anvil that looks strongly [ DMPVE CBTF BSDDMPVE glaciated (i.e., has a fibrous appearance associated BSDDMPVE HVTUGSPOU HVTUGSPOU with ice-crystal clouds), then you would call the QSFDJQJUBUJPO Y cloud a cumulonimbus capillatus. Within the stem of a mature thunderstorm is the cloudy main updraft tower topped by an updraft Z bubble (Fig. 14.2b). When this rising air hits the BOWJM tropopause, it spreads to make the anvil. Also in the stem is a downdraft with precipitation. When D the downdraft air hits the ground it spreads out, QSFDJQ the leading edge of which is called the gust front. When viewed from the ground under the storm, the VQQFS main updraft often has a darker cloud base, while MFWFM HVTUGSPOU the rainy region often looks not as dark and does not XJOET BUTVSGBDF have a well-defined cloud base. Y Not all cumulonimbus clouds have lightning and thunder. Such storms are technically not thun- Figure 14.2 derstorms. However, in this book we will use the (a) Sketch of a basic (airmass) thunderstorm in its mature stage. (b) Vertical slice through the storm. Light shading indicates word thunderstorm to mean any cumulonimbus clouds, medium and dark shadings are moderate and heavy pre- cloud, regardless of whether it has lightning. cipitation, and arrows show air motion. (c) Horizontal com- More complex thunderstorms can have one or posite, showing the anvil at storm top (as viewed from above by more updraft and downdraft regions. The most se- satellite), the precipitation in the low-to-middle levels (as viewed vere, long-lasting, less-frequent thunderstorms are by radar), and the gust front of spreading winds at the surface. supercell thunderstorms (Figs. 14.3 & 14.4). Clouds associated with Thunderstorms Sometimes you can see other clouds attached to thunderstorms, such as a funnel, wall, mammatus, arc, shelf, flanking line, scud, pileus, dome, and bea- ver tail (Fig. 14.4). Not all thunderstorms have all these associated clouds. Arc clouds (official name arcus, Fig. 14.2b) or shelf clouds form near the ground in boundary- 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 © Gene Rhoden / weatherpix.com Figure 14.3 have arc clouds, particularly if the displaced air is Photo of supercell thunderstorm. dry. See the Thunderstorm Hazards chapter. R. STULL • PRACTICAL METEOROLOGY 483 The beaver tail (Fig. 14.4) is a smooth, flat, 0WFSTIPPUJOH5PQPS 1JMFVT narrow, low-altitude cloud that extends along the %PNF &- boundary between the inflow of warm moist air to "OWJM the thunderstorm and the cold air from the rain-in- .BNNBUVT duced forward flank downdraft (FFD). .BJO A dome of overshooting clouds sometimes 1JMFVT 6QESBGU [ forms above the anvil top, above the region of stron- $V 7JSHB $VNFE gest updraft. This is caused by the inertia of the up- DPO 4USJBUJPOT ward moving air in the main updraft, which over- 'MBOLJOH-JOF #FBWFS5BJM -$- shoots above its neutrally buoyant equilibrium level (EL). Storms that have overshooting tops 3BJO are often more violent and turbulent. 48 4DVE /& The flanking line is a band of cumuliform 8BMM$MPVE 5BJM$MPVE clouds that increase from the medium-size cumu- lus mediocris (Cu med) furthest from the storm 'VOOFM$MPVE to the taller cumulus congestus (Cu con) close PS5PSOBEP to the main updraft. Cumulus congestus are also Figure 14.4a informally called towering cumulus (TCu). The Sketch of a classic supercell thunderstorm (Cb) as might be flanking line forms along and above the gust front, viewed looking toward the northwest in central North America. which marks the leading edge of colder outflow air The storm would move from left to right in this view (i.e., to- from the rear-flank downdraft (RFD). ward the northeast). Many storms have only a subset of the If humid layers of environmental air exist above features cataloged here. Cu med = cumulus mediocris; Cu con rapidly rising cumulus towers, then pileus clouds = cumulus congestus; LCL = lifting condensation level; EL = can form as the environmental air is pushed up and equilibrium level (often near the tropopause, 8 to 15 km above out of the way of the rising cumulus clouds. Pileus ground); NE = northeast; SW = southwest. are often very short lived because the rising cloud tower that caused it often keeps rising through the pileus and obliterates it. The most violent thunderstorms are called 4UPSN supercell storms (Figs. 14.3 & 14.4), and usu- "OWJMFEHF ally have a quasi-steady rotating updraft (called a .PWFNFOU mesocyclone). The main thunderstorm updraft 'PSXBSE'MBOL %PXOESBGU in supercells sometimes has curved, helical cloud striations (grooves or ridges) on its outside similar 3BJO ''% 3BJO #FBWFS5BJM to threads of a screw (Fig. 14.4a). Supercells can pro- duce intense tornadoes (violently rotating columns 3FBS'MBOL 5 of air), which can appear out of the bottom of an iso- %PXOESBGU .BJO lated cylindrical lowering of the cloud base called a 3'% 6QESBGU wall cloud. The portion of the tornado made vis- 0VUGMPX 5 ible by cloud droplets is called the funnel cloud, #PVOEBSZ which is the name given to tornadoes not touching -BZFS -JOF 8JOET $V the ground. Tornadoes are covered in the next chap- DPO *OGMPX #PVOEBSZ ter. Most thunderstorms are not supercell storms, 'MBOLJOH$V -BZFS and most supercell storms do not have tornadoes. NFE 8JOET Attached to the base of the wall cloud is some- (VTU'SPOU times a short, horizontal cloud called a tail cloud, /PSUI which points towards the forward flank precipi- tation area. Ragged cloud fragments called scud &BTU LN WJFXQPJOU (cumulus fractus) often form near the tip of the tail Figure 14.4b cloud and are drawn quickly into the tail and the Plan view (top down) sketch of a classic supercell thunder- wall cloud by strong winds. storm. T indicates possible tornado positions; RFD = Rear The wall cloud and tornado are usually near Flank Downdraft; FFD = Forward Flank Downdraft. Regions the boundary between cold downdraft air and the of updraft are hatched with grey; downdrafts are solid black; low-level warm moist inflow air, under a somewhat rain is solid light grey. Low surface pressure is found under the rain-free cloud base. In mesocyclone storms, you updrafts (especially near the “T” locations), and high pressure can see the wall cloud rotate by eye, while rotation under the downdrafts. Vectors are near-surface winds: cold are black, warm are grey. From the view point you see Fig. 14.4a. 484 cHAPTer 14 • THUNDERSTORM FUNDAMENTALS of the larger-diameter supercell is slower and is ap- parent only in time-lapse movies.