WX BRIEF

Intertropical Convergence Zone

This heat pump of the atmosphere can bring severe turbulence and danger to aircraft flying within it.

By Karsten Shein
Comm-Inst.
Climate Scientist


Average summer and winter positions of the ITCZ around the world. Extra heat generated by continents pushes the position of the ITCZ poleward relative to the position over open ocean.

Everything about the flight was routine. The crew of the transcontinental business jet had made this trip hundreds of times between the company's home office in FRA (Frankfurt, Germany) and its South American factory hub just outside GRU (São Paulo–Guarulhos, Brazil). Halfway along their route, at FL400, the pilots were relaxing with their coffee while the autopilot did the flying. Ahead they casually noted the occasional lightning flash indicating they were approaching the Intertropical Convergence Zone (ITCZ).

As they closed the gap, the crew saw that their course would take them through the tops of several of the towering cumuli ahead. The copilot informed the flight attendant in the aft cabin that they may experience a bit of light chop for the next 15 to 20 minutes, after which he re-secured his safety harness per company operations procedure. Within a minute, the airplane plunged into the upper reaches of the first cloud and the expected minor shaking brought on by convective turbulence. Another minute on, the autopilot suddenly disengaged, followed shortly by the autothrottle.

Composite satellite image of the ITCZ over the Pacific and East Asia in January. ITCZ is responsible for the monsoon climates throughout the region.

Before either pilot could even set their coffee aside, the bizjet began to roll as it continued to be rocked by the minor turbulence. The pilot, sensing an attitude change but getting conflicting readings from his instruments, then became momentarily confused and chose to ignore all of the instrument readings, instead relying on his physiological senses which directed him to roll back the other way and climb.

However, the control inputs were too great for the situation and as the aircraft rocked back and forth, they climbed to FL480 while the airspeed dropped rapidly. A final overcorrection put the aircraft in an unusual attitude.

Now descending rapidly, the pilot regained his senses, cross-checked his instruments and applied upset recovery procedures.

Discussing the incident over the remainder of the trip, the crew concluded they had failed to recognize the speed, height and severity of cell formation in the ITCZ. Although they had transited this zone many times, their recent experience flying in Europe during the winter had made them complacent. Generally, this type of weather didn't reach these altitudes in the northern hemisphere at this time of year.

Unfortunately, their company's sales director had been in the lavatory at the time–suffice it to say that he experienced a very unpleasant ride. The pilots' next meeting with the flight department manager would certainly not be a pleasant one.

Global circulation

Profile view of the atmosphere's circulation cells in a single hemisphere. ITCZ is the updraft region of the Hadley cell and may extend to 60,000 ft or higher.

The ITCZ is part of the global pattern of atmospheric circulation. This circulation is divided into 3 main cells in both the northern and southern hemispheres, and these cells are driven by the receipt of solar radiation, which in turn is dictated by the angle at which the sun's rays reach the earth's surface.

The most intense radiation is received by the tropics; the least is received by the poles. Therefore, the air heats the most near the equator and cools most above the arctic/antarctic circles. If the earth did not spin, that would mean that air would be heated and rise in the tropics, hit the top of the troposphere—above which the stratosphere's temperature inversion caps further ascension—and spread out toward either pole. As it transited along the top of the troposphere, it would cool and finally descend as cold, dense air near the poles.

At the surface, the rising tropical air would draw replacement flow from the higher latitudes, completing a single circulation cell in each hemisphere.
But because the earth also spins, the flow of air is redirected as it moves poleward or equatorward. This redirection helps to break the single circulation cell into 3 distinct cells.

On either side of the equator is the Hadley cell. But the air that rises in the Hadley circulation only makes it as far as the subtropics (around 30° N/S), where it meets upper air flowing equatorward in the midlatitude Ferrel cell. That convergence forces the air to descend, creating mostly clear skies and a lack of rainfall that supports the world's major hot desert regions.

At the surface, part of the airflow makes its way back to the equator as part of the Hadley cell while the remainder flows poleward, where it meets up with the cold flow moving in from the poles. The 2 flows meet along the polar front, which gives the majority of us in the middle latitudes our variable and seasonal weather. The polar cell completes the model, moving upper air flow back into polar latitudes, where it cools and descends. Combined, this 3-cell model of the atmosphere's circulation can help us understand and even predict weather and climate across much of the planet. It also helps us to understand the ITCZ.

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