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Forecasting Thunderstorms

Forecasting Thunderstorms

Coverstory

An understanding of provides Forecasting clues to these atmospheric monsters.

Thunderstorms BY ED BROTAK

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onvection remains a serious problem Forecasters then have to determine the inside air for the aviation community. Severe so that comparisons can be made. turbulence above the ground and strong Starting with the simple case of dry convec- with shear near the surface tion (no or ), we know that areC among the hazards caused by convective air expands as it rises, and the expansion re- activity, which plays a role in many aircraft sults in cooling. Using the basic laws of physics, Forecasting accidents each year. Moreover, the massive we can derive the rate at which dry air should hailstorm at Dallas–Fort Worth (Texas, U.S.) cool when lifted. This is called the dry adiabat- International Airport in April demonstrated ic (“adiabatic” refers to the expansion BY ED BROTAK how convection can seriously disrupt flight effect in this case), and the value is 5.5 degrees operations. Hundreds of flight delays and can- F per 1,000 ft (10 degrees C per 1,000 m). If cellations occurred, and damage to aircraft on the actual measured lapse rate is greater than the ground was extensive. this, then the parcel of air would be warmer Meteorologists must know how convec- than the environment and would continue to tion operates in order to forecast it. They must rise on its own. This is an unstable situation. make a model of the , and even of We find lapse rates like this fairly close to the the potential itself, to predict the ground, usually on days with abundant - that may be generated. The aviation in- shine. Columns of rising air, the that dustry would benefit from a better understand- glider pilots use, are common in this situation. ing of the workings of convection. But lapse rates of this magnitude are unusual at A simple key to understanding convection is higher , and this type of convection is to know that warm air rises and cold air sinks. not “deep” (i.e., not extensive). More precisely, warm air is less dense and there- fore buoyant (think of a hot air balloon). Cold air Dynamic and Dangerous is denser and sinks (e.g., cold air drainage into When water is added to the mix, the situation a valley at night). The terms warm and cold are becomes more dynamic and potentially danger- relative. A balloon with an inside air temperature ous. Convective , the family, of 32 degrees F (0 degrees C) still will rise if the always provide some turbulence, which can range outside air temperature is minus 40 degrees F from a few bumps in “fair weather cumulus” to the (minus 40 degrees C). Similarly, convection can potent updrafts and downdrafts in cumulonimbus occur with below freezing. thunderheads that can rip an airplane apart. On the plus side, the condensed water makes the air Lapse Rate currents visible as clouds. Imagine if a pilot could So, to determine if air is going to rise, sink not see currents of air rising and sinking at speeds or remain where it is, we need to know the that can exceed 100 mph (161 kph). temperature of the “inside air” (inside the bal- Besides making convective clouds and the loon or inside a cloud) and the temperature of various forms of associated with the air outside. We also need to know the lapse them, water plays a critical role in convective rate — that is, the change in temperature with development. When condenses, height. Outside air temperatures are measured heat is released. Technically, when water mol-

Liz Love-Brotak at least twice a day — typically at 0000 and 1200 ecules go from the energetic gas form (vapor) Greenwich Mean Time (GMT) — from dozens to the more confined liquid form (water) or of sites across the United States and hundreds solid form (ice), energy is released. This latent of other stations around the world. Balloon- heat release raises the temperature of the air borne instrument packs, called , within the cloud. If the parcel of air continues are launched to obtain data on temperature, to rise, it will cool at a slower rate — the moist moisture, and winds up to 100,000 ft. adiabatic lapse rate: 3 degrees F per 1,000 ft

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(5 degrees C per 1,000 m). With the few thousand feet thick, is all that is convection are the lapse rate and low- parcel cooling at a slower rate, it is still needed to support convection. Inter- level moisture. To quantify the fore- likely to be warmer than the surround- estingly, dry air aloft helps promote casts, meteorologists have developed ing air. Therefore, moist air is po- strong convection, whereas a deep a number of indices that incorporate tentially more unstable. This process moist layer aloft often produces heavy these two factors. The , does not require a lot of moisture. but less wind and turbulence. the Showalter Index, the Total-Totals Convective lifting is so strong, a moist So, the two primary factors that Index and the K Index can be calculated layer near the surface, perhaps only a meteorologists look at to forecast for each situation, and the numerical

Sounding at Springfield, Missouri, U.S., May 23, 2011

100 16,460 m Temperature Predicted temperature inside the cloud

255 268 283 304 335 386 13, 930 m

EL 200 12,115 m

10, 680 m

300 9,450 m

8,367 m CAPE 400 7,410 m

6,532 m 500 5,740 m

5,008 m 600 4,333 m 3,692 m 700 3,091 m 2,417 m 800 1,974 m 1,459 m LFC 900 965 m CL 490 m 1000 44 m mb −50 223 −40 233 −30 243 −20253 −10 263 0 273 10 283 20 293 30 303 40 313 50 0.1 0.2 0.6 1.0 2.0 3.0 5.0 10.0 20.0 40.0 g/kg

CL = condensation level LFC = EL = CAPE = convective available potential energy Source: Ed Brotak, from the Plymouth State Weather Center

Figure 1

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values determined from the calcula- required is called convective inhibi- cloud top was an impressive 17,000 m tions can be compared to standard tion (CINH). If this value is large (e.g., (57,000 ft). With the height of values for the occurrence of convection 200 or more) or there is nothing to 13,930 m (46,000 ft), this storm extended or severe convection. All of these indi- help the parcel rise, there will be no well into the . ces were developed prior to the advent convection. In this example, the CINH So far, we have discussed only the of computer technology. Although they is a minimal value of 3. updraft of a thunderstorm. In terms are still used today, computer-generated The yellow line is the predicted of development, it is the updraft that products are much better. temperature of the air inside the cloud. produces the storm. But turbulence The red and yellow lines intersect also consists of downdrafts, which can Sounding the Atmosphere initially at 1,300 m (4,200 ft). This produce strong winds and The main tool meteorologists use to is called the level of free convection at the ground. Initially, downdrafts are forecast convection is the sounding, a (LFC). Above this level, the air inside started as rain begins to fall from the vertical profile of the atmosphere. A the cloud is warmer than the air cloud, pulling some air down with it. standard plotted sounding consists of outside and will rise on its own. This Evaporative cooling lowers the temper- two lines showing temperature and dew becomes the updraft, the core of the ature of this descending air, accelerat- point, with wind data usually given on storm. The lines cross again up at ing the downdraft even more. Dry the side of the plot. Forecasters can use 13,000 m (42,000 ft), at what is known air aloft, which would intensify the actual morning soundings and allow as the equilibrium level (EL). Above cooling effect, is one thing meteo- for expected changes by afternoon or, this level, the air in the cloud is colder rologists look for in predicting strong with today’s sophisticated numerical than the environment. This often cor- downdrafts. Large thunderstorms and models, use computer-generated fore- responds with the cirrus anvil of the thunderstorm complexes often de- cast soundings for later in the day. thunderstorm cloud. velop complex circulations. Outside For an example, Figure 1 is the The updraft does not stop at the air can be pulled into this circulation 0000 GMT 23 May 2011 sounding for equilibrium level because the air in and produce a mid-level (10,000 ft or Springfield (Missouri, U.S.) Municipal the updraft has accumulated upward 3,000 m) inflow. This colder, drier air Airport. This sounding represents the momentum, or energy. This energy is can become a powerful downdraft. atmospheric conditions that produced proportional to the area on the sound- Also, this air brings with it momentum the thunderstorm that spawned the ing between the actual temperature trace gained from the winds aloft. These that devastated nearby Joplin, and the parcel temperature trace — that strong winds can be brought down to Missouri. The red line is the actual is, where the parcel is warmer than the the surface by the downdraft. temperature trace, and the black line environment between the level of free is the dew point from the surface to convection and the equilibrium level. Me- Convective Triggers 16,460 m (54,000 ft). From the surface teorologists call this the convective avail- Even if the environment is potentially temperature and dew point, we can able potential energy (CAPE). The CAPE unstable, something is needed to start calculate the condensation level (CL). indicates the potential strength of the or trigger the convection. Typically, For this calculation, we simulate the updraft. A CAPE of 500 usually would parcels of air need a boost to reach lifting of this surface air by using the support only weak convection, but the the condensation level — something dry adiabatic lapse rate to determine CAPE value here, 3,692, is indicative of to lift the unsaturated air upward, the height at which the air would be severe thunderstorms. This excess energy causing it eventually to cool to the dew cooled sufficiently that its temperature propels the actual top of the cloud well point. From there, the latent heat that equals the dew point. In this case, the above the anvil in what is referred to as is released can help the parcels utilize condensation level is 840 m (2,750 an . Viewed from above, the inherent instability. As mentioned ft). The condensation level typically the top of a thunderstorm looks like a above, strong heating of the surface by marks the base of the cloud. Below boiling cauldron. The air in the updraft the sun in the late spring or summer this level, where the parcel of air is surges upward and then sinks back down is a typical convective trigger. If the cooler, energy or lift must be provided in bursts. The actual height is a function temperature of the air near the sur- for condensation to occur. The energy of the CAPE. In this case, the predicted face warms sufficiently, the convective flightsafety.org | AeroSafetyWorld | May 2012 | 17 Coverstory

When synoptic forcing is very strong, convec- Convection Catalysts tion often organizes along lines parallel to the mean wind. These are the familiar squall lines.

Occasional Often, the convection itself is strong to severe. H convection Beside extreme turbulence aloft, strong winds at the surface are common, and is possible. Warm front L Strong convection possible Interestingly, moderate amounts of synoptic Cold front forcing and significant instability can combine

e to produce the strongest thunderstorms: the su- r a r y t l u e percells. This was the case with the Joplin storm. b k , i l le t s ib s o s o m p n Rotating Updrafts n io o t ti c c e e v Another factor that forecasters examine at low v n n o o c C g levels is wind shear. When winds veer (turn n H o tr Weak S convection clockwise) from the surface to several thousand possible feet, the updraft in a thunderstorm can convert this vertical wind shear into horizontal rota- tion. Rotating updrafts are associated with the strongest storms and produce the most severe H = high pressure area L= low pressure area Source: Ed Brotak weather, including strong straight-line winds, large hail and even tornadoes. To quantify this, Figure 2 meteorologists calculate the helicity, the dif- temperature can be reached, and parcels of air ference between the winds at different levels. will start to rise on their own. High helicity values (over 300) indicate greater Orographic lifting is another common cause potential for severe storms. of convection. Winds blowing upslope can lift On many days, the convection is shallow, parcels of air to their condensation level. This is resulting in only fair weather cumulus clouds why convection is more prevalent over moun- with little vertical development. The air may be tainous terrain. Convergence at low levels also too dry, and the clouds literally evaporate; or the can cause convection. When air converges near atmosphere may be too stable to allow much de- the ground, it is forced upward. This can happen velopment. In this situation, meteorologists of- ahead of a true front, along a gust front or the ten say the atmosphere is “capped.” Stable lapse outflow boundary from previous convection, or rates occur at levels above the effects of surface beneath various upper-level systems. heating. When the atmosphere is uncapped and The surface weather features shown in unstable, updrafts can soar tens of thousands of Figure 2 can cause typical “” showers feet, producing cumulus congestus, or towering and thunderstorms to develop in the warm, cumulus. When the updraft air finally reaches humid, southerly flow on the west side of a high its equilibrium level, it spreads out to pressure area, away from any fronts or lows. Air form the anvil characteristic of a cumulonimbus mass thunderstorms are the result of daytime cloud, the “thunderstorm cloud.” Regardless of heating. This convection is not organized and whether an anvil top has developed, cumulus usually is fairly weak. When convection occurs clouds of this magnitude pose the greatest risks closer to the low and fronts, but still in the warm to pilots.  air, it tends to be more organized and stronger. Edward Brotak, Ph.D., retired in 2007 after 25 years The convection is aided by divergence aloft as a professor and program director in the Department with upper-level troughs and the jet stream. of Atmospheric Sciences at the University of North This is what meteorologists call synoptic forcing. Carolina, Asheville.

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