Chapter 15 Copyright © 2011, 2015 by Roland Stull. Meteorology for Scientists and Engineers, 3rd Ed. thunderstorm hazards Contents The basics of thunderstorms were cov- ered in the previous chapter. Here we Precipitation and Hail 545 cover the dangerous aspects of thun- Heavy Rain 545 15 derstorms, including: Hail 548 • hail and intense precipitation, Downbursts and Gust Fronts 554 • downbursts and gust fronts, Characteristics 554 • lightning and thunder, and Precipitation Drag 555 • mesocyclones and tornadoes. Evaporative Cooling 556 Two other hazards were covered in the previous Downdraft CAPE (DCAPE) 557 chapter: vigorous updrafts and turbulence. Pressure Perturbation 559 Outflow Winds & Gust Fronts 560 In spite of their danger, thunderstorms can also produce the large-diameter rain drops that enable Lightning And Thunder 563 beautiful rainbows (Fig. 15.1). Origin of Electric Charge 564 Lightning Behavior & Appearance 566 Lightning Detection 568 Lightning Hazards and Safety 569 Thunder 571 preCipitation and hail Shock Front 571 Sound 575 Tornadoes 577 heavy rain Tangential Velocity & Tornado Intensity 577 Thunderstorms are deep clouds that can create: Types of Tornadoes & Other Vortices 582 • large raindrops (2 - 8 mm diameter), in Evolution as Observed by Eye 583 • scattered showers (order of 5 to 10 km Tornado Outbreaks 583 diameter rain shafts moving across the Storm-relative Winds 584 ground, resulting in brief-duration rain Origin of Tornadic Rotation 586 [1 - 20 min] over any point), of Mesocyclones and Helicity 587 • heavy rainfall rate (10 to over Tornadoes and Swirl Ratio 592 1000 mm/h rainfall rates). Summary 593 The Precipitation chapter lists world-record rainfall Threads 593 rates, some of which were caused by thunderstorms. Exercises 594 Numerical Problems 594 Understanding & Critical Evaluation 597 Web-Enhanced Questions 600 Synthesis Questions 601 TVCTJEFODF VQ ESBGU TVO SBJO TVOCFBN [ SBJOCPX 8FTU &BTU “Meteorology for Scientists and Engineers, 3rd Edi- Figure 15.1 tion” by Roland Stull is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike Rainbow under an evening thunderstorm. Updraft in the thun- 4.0 International License. To view a copy of the license, visit derstorm is compensated by weak subsidence around it to con- http://creativecommons.org/licenses/by-nc-sa/4.0/ . This work is serve air mass, causing somewhat clear skies that allow rays of available at http://www.eos.ubc.ca/books/Practical_Meteorology/ . sunlight to strike the falling large raindrops. 545 546 chapter 15 thunderstorm HAzARDS Compare this to nimbostratus clouds, that create • Second, the vertical stacking of the deep cloud al- smaller-size drizzle drops (0.2 - 0.5 mm) and small lows precipitation forming in the top of the storm rain drops (0.5 - 2 mm diameter) in widespread to grow by collision and coalescence or accretion as regions (namely, regions hundreds by thousands it falls through the middle and lower parts of the of kilometers in size, ahead of warm and occluded cloud, as already mentioned, thus sweeping out a lot fronts) of light to moderate rainfall rate that can last of water in a short time. for many hours over any point on the ground. • Third, long lasting storms such as supercells or Why do thunderstorms have large-size drops? orographic storms can have continual inflow of hu- Thunderstorms are so tall that their tops are in very mid boundary-layer air to add moisture as fast as it cold air in the upper troposphere, allowing cold- rains out, thereby allowing the heavy rainfall to per- cloud microphysics even in mid summer. Once a sist. As was discussed in the previous chapter, the spectrum of different hydrometeor sizes exists, the heaviest precipitation often falls closest to the main heavier ice particles fall faster than the smaller ones updraft in supercells (see Fig. 15.5). and collide with them. If the heavier ice particles Rainbows are a by-product of having large are falling through regions of supercooled liquid numbers of large-diameter drops in a localized re- cloud droplets, they can grow by riming (as the liq- gion surrounded by clear air (Fig. 15.1). Because uid water instantly freezes on contact to the outside thunderstorms are more likely to form in late after- of ice crystals) to form dense, conical-shaped snow noon and early evening when the sun angle is rela- pellets called graupel (< 5 mm diameter). Alter- tively low in the western sky, the sunlight can shine nately, if smaller ice crystals fall below the 0°C level, under cloud base and reach the falling raindrops. their outer surface partially melts, causing them to In North America, where thunderstorms generally stick to other partially-melted ice crystals and grow move from the southwest toward the northeast, this into miniature fluffy snowballs by a process called means that rainbows are generally visible just after aggregation to sizes as large as 1 cm in diameter. the thundershowers have past, so you can find the The snow aggregates and graupel can reach the rainbow looking toward the east (i.e., look toward ground still frozen or partially frozen, even in sum- your shadow). Rainbow optics are explained in mer. This occurs if they are protected within the more detail in the last chapter. cool, saturated downdraft of air descending from Any rain that reached the ground is from wa- thunderstorms (downbursts will be discussed lat- ter vapor that condensed and did not re-evaporate. er). At other times, these large ice particles falling Thus, rainfall rate (RR) can be a surrogate measure through the warmer boundary layer will melt com- of the rate of latent-heat release: pletely into large raindrops just before reaching the ground. These rain drops can make a big splat on HLRR = ρL·· v RR (15.1) your car windshield or in puddles on the ground. Why scattered showers in thunderstorm? Of- where HRR = rate of energy release in the storm over –1 –2 ten large-size, cloud-free, rain-free subsidence re- unit area of the Earth’s surface (J·s ·m ), ρL is the gions form around and adjacent to thunderstorms density of pure liquid water, Lv is the latent heat of due to air-mass continuity. Namely, more air mass vaporization (assuming for simplicity all the pre- is pumped into the upper troposphere by thunder- cipitation falls out in liquid form), and RR = rainfall storm updrafts than can be removed by in-storm rate. Ignoring variations in the values of water den- precipitation-laden downdrafts. Much of the re- sity and latent heat of vaporization, this equation maining excess air descends more gently outside the reduces to: storm. This subsidence (Fig. 15.1) tends to suppress other incipient thunderstorms, resulting in the orig- HRR = a · RR •(15.2) inal cumulonimbus clouds that are either isolated (surrounded by relatively cloud-free air), or are in where a = 694 (J·s–1·m–2) / (mm·h–1) , for rainfall rates a thunderstorm line with subsidence ahead and be- in mm/h. hind the line. The corresponding warming rate averaged over Why do thunderstorms often have heavy rain- the tropospheric depth (assuming the thunderstorm fall? fills the troposphere) was shown in the Heat chapter • First, the upper portions of the cumulonimbus to be: cloud is so high that the rising air parcels become ∆T/∆t = b · RR (15.3) so cold (due to the moist-adiabatic cooling rate) that virtually all of the water vapor carried by the air is where b = 0.33 K/(mm of rain). forced to condense, deposit, or freeze out. From the Moisture chapter recall that precipi- table water, dw, is the depth of water in a rain gauge R. STULL • METEOROLOGy FOR SCIENTISTS AND ENGINEERS 547 USPQPQBVTF BOWJM Solved Example A thunderstorm near Holt, Missouri, dropped 305 mm of rain during 0.7 hour. How much net latent heat energy was released into the atmosphere over each square meter of Earth’s surface, and how much did it [ Z warm the air in the troposphere? Solution Given: RR = 305 mm / 0.7 h = 436 mm/h. CPVOEBSZMBZFS Duration ∆t = 0.7 h. Find: H ·∆t = ? (J·m–2) ; ∆T = ? (°C) Y RR First, use eq. (15.2): –1 –2 –1 Figure 15.2 HRR = [694 (J·s ·m )/(mm·h )]·[436 mm/h]·[0.7 h]· [3600s/h] = 762.5 MJ·m–2 The thunderstorm updraft draws in a larger area of warm, hu- mid boundary-layer air, which is fuel for the storm. Next, use eq. (15.3): ∆T = b·RR·∆t = (0.33 K/mm)·(305 mm) = 101 °C if all of the moisture in a column of air were to pre- Check: Units OK, but values seem too large??? cipitate out. As an extension of this concept, sup- Discussion: After the thunderstorm has finished raining itself out and dissipating, why don’t we ob- pose that pre-storm boundary-layer air of mixing serve air that is 101°C warmer where the storm used to ratio 20 g/kg was drawn up into a column filling be? One reason is that in order to get 305 mm of rain the troposphere by the action of convective updrafts out of the storm, there had to be a continual inflow of (Fig. 15.2). If cloud base was at a pressure altitude of humid air bringing in moisture. This same air then 90 kPa and cloud top was at 30 kPa, and if half of the carries away the heat as the air is exhausted out of the water in the cloudy domain were to condense and anvil of the storm. precipitate out, then eq. (4.33) says that the depth of Thus, the warming is spread over a much larger water in a rain gauge is expected to be dw = 61 mm.