Conventional Wisdom

Coverstory

n October 2006, following a series of a mesoscale convective system near the The study of convection deals with fatal crashes, the U.S. National Trans- equator. More recently, the fatal crash of vertical motions in the atmosphere portation Safety Board (NTSB) issued a medical helicopter in March 2010 in caused by temperature or, more pre- a safety alert describing procedures Brownsville, Tennessee, U.S., was related cisely, density differences. The adage pilotsI should follow when dealing with to a “mesoscale convective system with a “warm air rises” is well known. In “thunderstorm encounters.” Despite bow shape.”1 meteorological parlance, a parcel of air these instructions, incidents continued to To improve the warning capabilities will rise if it is less dense than air in the occur. One concern is that terminology of the various weather services, convec- surrounding environment. Warmer air often used by meteorologists is unfamil- tion has been studied extensively in is less dense and will rise. Conversely, iar to some in the aviation community. recent years, leading to many new dis- colder air, being denser, will sink. As For example, the fatal crash of the Hawk- coveries. Although breakthroughs in the pilots, especially glider pilots, know, you er 800A at Owatonna, Minnesota, U.S., science have increased our understanding don’t need moisture — that is, clouds — in June 2008 (ASW, 4/11, p. 16) involved and improved convection forecasts, the to have rising and sinking currents of a “mesoscale convective complex.” The problem of conveying the information to air. However, when air rises it expands crash of Air France Flight 447 in June those who need it remains, complicated and cools. If the air cools to its dew 2009, involving an Airbus A330 with a by the flood of new terminology which point, condensation occurs and a cloud loss of 228 lives, was believed to involve often accompanies scientific advances. forms if sufficient moisture is present. Convectional Wisdom Mesoscale convective systems must be understood to mitigate their threat to aviation.

By Ed Brotak U.S. National Oceanic and Atmospheric Administration U.S.

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The bowing section Cumulus clouds are the typical convective clouds. for hours. An MCS must, by definition, contain of a squall line (left) Convection, in operational meteorology vernacu- some convection but also may contain stratiform can be accompanied lar, refers to convective precipitation — showers precipitation — areas of rain — and areas of cloud by strong winds. and thunderstorms that are the end products of with no precipitation. Their size and duration convective activity. make them more of a hazard for aviation. Convective precipitation can be divided into The most recognizable and best-known two broad types — unorganized and organized. MCS configuration is the squall line. A squall Unorganized convection would be the typical “air line is a more or less continuous line of thunder- mass showers and thunderstorms” that develop in storms, at least initially. If conditions are favor- the warm season. They are the result of daytime able, the squall line can persist for hours and heating of humid air masses. The resulting convec- evolve into a much larger and complex system tion is usually haphazard, with no recognizable (Figure 1, p. 14). There may be several lines of pattern. Although all convection represents a prob- convection, with the strongest on the leading lem for aviation, these storms tend to be weak by edge of the system, usually the east or south side most standards. Occasionally, a pulse storm (ASW, in the northern hemisphere mid-latitudes, and 10/09, p. 12) will produce strong surface winds, progressively weaker behind it. but that’s about it. Individual convective cells, the A larger region of mainly stratiform rain with storms themselves, are fairly small — several miles possibly some embedded convection can develop across at most — and are rather short-lived, lasting behind the line or lines of stronger convective an hour or less. For aviation purposes, they usually cells, usually to the west or north. And trailing can be avoided or waited out. this, you can have a mesoscale low pressure area At other times, convection becomes organized. called the “wake low.” Squall lines may be sym- This is either the result of larger-scale atmospheric metrical, with the stratiform rain area just behind, forces at work or the interaction of various con- to the west of the convective line, or asymmetrical, vective elements independent of outside forces. with the convective cells more to the south and Organized convection takes the form of a meso- the stratiform precipitation more northward. scale convective system (MCS), the generic name In terms of aircraft operation, there are for a wide variety of systems. MCSs can be as large several areas to watch. Ahead of the main line

as several hundred miles across and can persist of thunderstorms, to the east or south, is the © Arnold Paul/Wikimedia

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(2 to 5 km) deep. MCVs occasionally have a life Squall Line System of their own, existing as long as 12 hours after the parent squall line has died out. Importantly, they can generate new convection or intensify L H existing convection as they move. On May 8, 2009, a particularly intense MCV ravaged parts of Kansas, Missouri and Illinois in the United States, with straight line winds over 100 mph (161 kph), large hail and dozens of tornadoes, L Stratiform rain area H some of them violent.

Thunderstorms To show what airport conditions are like during the passage of a mature MCS, consider the observations taken at Columbia, South L H Carolina, U.S., on the evening of June 3, 2011: At 19:56 local standard time (LST), winds were

Wake low Downdraft Out ow from the east-northeast at 7 kt, the visibility was high boundary 6.0 mi (9.7 km) in haze, the temperature was 91degrees F (33 degrees C), and thunder could Vertical Cross Section be heard with cumulonimbus clouds to the north. A wind shift was noted at 19:57 LST. This was with the passage of the outflow boundary or gust front. By 20:17 LST, the winds had picked up from the northeast at 12 kt, the temperature had dropped to 88 degrees F (31 degrees C), and barometric pressure was rising rapidly. At 20:31 Mesoscale convective vortex LST, the airport was under the leading convec- L tive cells. Winds were blowing from the north at 27 kt with gusts to 43 kt. The visibility had dropped to 1.5 mi (2.4 km) in a heavy thun- L H derstorm with rain and constant lightning. The Source: Ed Brotak temperature had dropped to 75 degrees F (24 degrees C). The heavy thunderstorms contin- Figure 1 ued for 25 minutes. At 21:08 LST, only a weak low-level outflow boundary also known as the thunderstorm was reported and barometric gust front. A rapid change in wind direction and pressure was falling rapidly. However, moderate increase in wind speed often follow its passage. to heavy rain continued for another 50 minutes. Obviously, the main line of storms should be At 21:56 LST, a gust of 26 kt accompanied the avoided due to the strong downdrafts and winds passage of the wake low. The rain ended at 22:01 at the surface and turbulence aloft. The trail- LST and the temperature was 70 degrees F (21 ing stratiform rain area is not as turbulent but degrees C). still may produce problems. The final wake low If a squall line or part of a squall line begins could be accompanied by strong, gusty winds. to curve or bow outward, it is referred to as a Another feature which may affect aircraft “bow echo.” Echo refers to a radar return, as is the mid-level “mesoscale convective vortex” these systems were first discovered and are (MCV). This cyclonic circulation can develop usually still identified on weather radar. The above the stratiform precipitation area. It can be bowing segment of the line can move very 30 to 60 mi (50 to 100 km) across and 1 to 3 mi quickly, occasionally in excess of 50 kt. Bow

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echoes are often associated with strong differs from other MCSs in shape; it is that ended in the fatal accident. Linear straight-line winds and occasionally rounded or at least elliptical. MCCs are squall lines and rounded MCCs are just weak tornadoes. On March 25, 2010, also large, covering thousands of square two types of MCSs. If a system doesn’t the pilot of the medical helicopter miles and can last six hours or more. fit into either of those categories, it is stationed in Brownsville referred to Primarily a summer phenomenon, simply referred to as an MCS. earlier decided he could beat a convec- MCCs develop in what appears to be a The movement of MCSs is affected tive line and make it safely back to fairly benign environment, often on the by two things — simple advection by Brownsville from Jackson. The line east side of an upper-level ridge, away the wind and propagation of the whole developed a bow, which shot ahead from any low pressure areas or fronts. system, which is the result of develop- of the main system with an estimated MCCs often start as unorganized, air ment and dissipation of individual forward speed of 60 kt. Radar indi- mass convection in the late afternoon convective elements — the storms cated that the helicopter was overtaken or early evening. The initially indepen- themselves. Advection by the wind is by strong convection before reach- dent storms start to interact and form a simple enough, with convective cells ing its destination and this may have cohesive, self-maintaining complex that being driven by the mean wind — the resulted in the crash, which killed the often lasts into or through the night. average wind in the layer of air contain- three people aboard. The strongest convection, similar to a ing them. Individual cells or storms can Another variation of the squall line squall line, is on the outside perimeter move rapidly at rates up to 60 kt. Prop- is the quasi-linear convective system of the system, with stratiform but often agation effects are more complicated. (QLCS). The QLCS has some linear parts heavy rain in the middle. Individual convective cells, which have but also other discrete elements. This In the case of the Hawker 800A a much shorter lifespan than the MCS, means that some storms are in a squall crash, the leading edge of the MCC with form and dissipate within the MCS. line but other nearby storms are separate. the strongest convection had affected the This affects the overall movement of QLCSs are often associated with strong Owatonna airport an hour earlier but the mesoscale system. Convective cells straight-line winds and tornadoes. had moved on by the time the aircraft and convective systems tend to propa- Another term which comes up in dis- arrived. However, the heavy stratiform gate in the direction from which warm, cussions about convection is “derecho.” rain region was still affecting the ter- moist air is being “fed” into them. This Not really a different type of MCS, a minal when the landing was attempted. is usually from the south, in the north- derecho adds a time element to the The wet runway complicated the pilot’s ern hemisphere. This causes a seeming description. It is a long-lived, often large efforts to stop the airplane, leading to his deflection to the right. Supercell thun- convective system which produces strong belated decision to attempt a go-around derstorms are notorious “right movers,” and often damaging winds for hours. The squall line leading this MCS is obvious in this photo from space. Squall lines usually develop where there is moderate to strong synoptic forcing — in other words, the line is the result of not just instability but other at- mospheric effects. Usually, squall lines occur in the warm sector of an extra- tropical cyclone or low, ahead of the associated cold front. There is often an upper-level trough just to the west with the jet stream. Other MCSs can develop on their own without much help. Before meteorologists came up with the generic MCS classification, they had already identified a very specific type of MCS which they named a mesoscale convective complex (MCC). An MCC National Oceanic and Atmospheric Administration U.S. www.flightsafety.org | AeroSafetyWorld | June 2011 | 15 Coverstory

moving well to the right of the mean MCS embedded in the inter-tropical wave lengths between 1,000 and 1,500 wind. For an MCS such as a squall line, convergence zone (ITCZ). The ITCZ mi (2,000–2,500 km) and continue for new cells tend to develop on the south is where the northeast trade winds about three days. Low-level conver- end of the line while older ones to the from the Northern Hemisphere collide gence is found just to the east of the north die out. This causes the whole with the southeast trade winds from trough axis and often generates con- system to move or propagate to the the Southern Hemisphere. The result- vection. The convection can organize right of the mean wind. For example, ing convergence produces lifting and, into tropical squall lines. Moving to it is common for a squall line to move with the very moist air, showers and the west in the tropical easterly winds, due east while individual storms within thunderstorms. Although the ITCZ these squall lines have the strongest it move rapidly northeast. is fairly continuous, there are areas of convection on their west side, with the MCSs are not confined to mid- enhanced lifting and convection. This stratiform rain area to their east. In the latitudes. There are tropical versions, was what Flight 447 flew into. Some of summer, these waves can impact loca- too. It is widely believed that a tropical the cloud bands were curved and there tions in the subtropics, such as Florida, MCS was involved in the Air France seemed to be a circulation center, both Texas or Mexico. Flight 447 crash in the tropical Atlantic indications of a well-developed MCS. If Although the name may not be not far north of the equator. Infor- conditions are favorable and the ITCZ widely known, mesoscale convective mation gathered from the recently is far enough away from the equator for systems are common in many parts recovered black boxes indicated that the Coriolis effect2 to enhance rotation, of the world. They produce much the airplane was cruising at 35,000 ft a tropical MCS produced along the more serious hazards to aviation than with no problems. But just ahead was ITCZ can become a full-fledged tropi- individual thunderstorms. Understand- an area of thunderstorms that infrared cal cyclone. Tropical cyclones up to and ing them is essential to treating them satellite imagery indicated had tops of including hurricanes and typhoons are with proper respect when they are 50,000 ft. The pilots were aware of this just larger versions of tropical MCSs. encountered. 

and warned the cabin crew of potential Besides the ITCZ, tropical waves Edward Brotak, Ph.D., retired in 2007 after 25 turbulence. Instruments showed that — also called easterly waves or African years as a professor and program director in the turbulence never became more than waves — can generate convection in the Department of Atmospheric Sciences at the moderate. However, the pitot tubes for the Atlantic basin. They are common University of North Carolina, Asheville. the airspeed sensors iced over when in the warm season, May to November. the plane encountered the high clouds, Tropical waves are inverted troughs of Notes triggering a series of events that ended low pressure, with the lowest pressure 1. NTSB Preliminary Report, ERA10MA188. with the aircraft stalling and falling into to the south. Low-level features move 2. Coriolis effect is the tendency for any the ocean. westward in tropical easterly winds moving body on or above the Earth’s The area of thunderstorms Flight or trade winds at an average speed of surface — for instance, winds — to be

© Jumbo0/Wikimedia 447 encountered was the result of an 15–20 mph (25–35 kph). They have deflected by the Earth’s rotation.

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