Professional Pilot, October 2016

the southern hemisphere But the greater proportion of ocean in the southern hemisphere ensures absorption of more heat and thus the ITCZ doesnt extend as far into the southern hemisphere as it does into the northern It is the migration of the ITCZ that produces the torrential and continuous monsoon rains in many parts of the world that straddle the edges of the tropics such as the storms found in India and southeast Asia It is also the reason for the lush jungles of Africa Indonesia and South America which rest near the equator and are within the reach of the ITCZ for most of the year The behavior and position of the ITCZ ultimately controls any weather a pilot might experience in the tropics and so it is a feature of the atmosphere with which all pilots should be familiar It is also important to understand that the ITCZ in many parts of the world can be disrupted by phenomena such as El Niño and La Niña and by movement over anomalously warm or cold ocean currents For example during an El Niño ocean waters warm in the eastern Pacific which heats the overlying atmosphere more than usual and results in significant thunderstorms along the western equatorial coast of South America where the cold Peru Current normally suppresses convection This change alters the flow patterns across the Pacific reducing rainfall and disrupting the Asian monsoons often producing drought from Australia to India Given the heat moisture and general atmospheric instability available across much of the tropics at all 36 PROFESSIONAL PILOT October 2016 levels of the troposphere cumulus decks and scattered airmass storms are common even outside the narrow zone of the ITCZ Unlike the middle latitudes where such storms often dont develop until later in the day tropical airmass storms can build at any time of the day or night though daytime storms are more common Along coastlines these storms may be drawn inshore as daytime heating of the land creates a smaller scale circulation called a sea breeze In general it takes a fair amount of land to heat up to generate a significant sea breeze so smaller islands may only occasionally experience it especially if they are normally influenced by steady ever present trade winds Tropical thunderstorms and cloud banks are also often found on the windward slopes of the various mountain ranges that pass through the low latitudes The Amazon River for example owes its existence to the Andes of Peru and Ecuador The easterly flow rises with the terrain producing clouds and copious rainfall When flying into complex terrain in the tropics pilots should always recognize the potential for mountain obscuration and maintain at least minimum safe altitudes Tropical cyclones The tropics are also home to tropical cyclones These mesoscale weather events develop when certain conditions are just right 1 They need exceptionally warm water to heat and humidify the atmosphere and generate thunderstorms 2 They need a strong lapse rate Hadley Cell ITCZ meaning the troposphere needs to cool rapidly with altitude to support deep convection 3 The troposphere is humid at all altitudes 4 Tropical cyclones need a lack of windshear aloft to keep the cyclones structure from disorganizing 5 They need some way of developing spin Spin normally requires the cluster of storms to be more than 5 latitude from the equator where there is sufficient Coriolis effect 6 There needs to be an upper air disturbance that can enhance instability and create a favorable region for thunderstorms to cluster This disturbance is normally provided by a wave in the easterly flow of the tropical jet stream that intermittently exists in the vicinity of the ITCZ Having all of these 6 factors in place does not guarantee formation of a tropical cyclone but it is rare that one will form without them all Fortunately because of the danger they present tropical cyclones are well tracked by satellite and aircraft and their tracks are forecast by no less than 12 models running on supercomputers Agencies such as NOAA and the Joint Typhoon Warning Center JTWC provide track forecast cones that give guidance about the likely trajectory of the storm system A cyclone may be only a few dozen kilometers in diameter or many hundreds but most will move past an area in a matter of a day or less The safest course of action for pilots is to stay well clear of the path of a tropical cyclone and if your aircraft is on the ground try to reposition it to an airport where it will be safer Tropics are dominated by an atmospheric circulation in each hemisphere known as the Hadley Cell Equatorial heating creates upflow from the surface to the tropopause about 60000 ft up in a region known as the ITCZ Aloft the air flows poleward before descending in the subtropical high Subtropical Jet Polar Jet Tropopause Ferrel Cell Polar Cell North Pole 60 N 30 N Equator