Conventional Wisdom
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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 Ipilots 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. 12 | FLIGHT SAFETY FOUNDATION | AEROSAFETYWORLD | JUNE 2011 COVERSTORY 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 WWW.FLIGHTSAFETY.ORG | AEROSAFETYWORLD | JUNE 2011 | 13 COVERSTORY (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 Outow 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 14 | FLIGHT SAFETY FOUNDATION | AEROSAFETYWORLD | JUNE 2011 COVERSTORY 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.