WX BRIEF

Intertropical convergence

Pilots must keep this all-important global ring of convection in mind when flying between Northern and Southern Hemispheres.

By Karsten Shein
Comm-Inst, Climate Scientist


US Army Gulfstream C37A on the Odyssey FBO ramp at NAS (Nassau, Bahamas) in May 2007. Flying to tropical destinations often means crossing through the Intertropical Convergence Zone where towering cumuli and thunderstorms are a daily occurrence.

As student pilots we were taught to avoid thunderstorms at all costs. They are the wicked children of the sky who delight in pulling the wings from any aircraft that dares fly within their grasp.

Unfortunately, as we fly more, some pilots begin to lose this healthy fear of thunderstorms. This is especially true in professional aviation, where pilots may feel they do not have the luxury of deviating from their assigned schedule, or that the plethora of onboard weather instrumentation in aircraft capable of cruising above all but the most severe weather systems minimizes the danger posed by thunderstorms.

But nearly every year since the inception of powered flight, the final flights of a few professional pilots have culminated in a free fall from the base of a thunderstorm. In the middle latitudes, where thunderstorms are often parts of distinct squall lines stirred up by cold fronts, these lines normally have well defined ends around which aircraft can deviate with minimal disruption to the overall flight schedule.

Often these thunderstorms are very loosely connected and the line presents lots of clear air through which a pilot can pick a route. In addition, the relatively cool atmosphere at these latitudes limits the vertical development of most thunderstorms to under 35,000 ft or so.

Only a few larger storms will make it to 45,000 or 50,000 ft, and these storms are relatively easy to maneuver around. In rare instances, a squall line might be very long or solid with extremely strong cells, and a short set-down at a nearby airport may be the best course of action, especially if the system is moving rapidly.

But what if the line of storms has no end, grows well past your cruise altitude, doesn’t move much at all, and there are no airports to set down at? Welcome to the Intertropical Convergence Zone (ITCZ).

ITCZ explained

Atlantic region 12-hr high-level significant weather prognostic chart for Jun 27, 2009 shows the potential for isolated and occasional convection across the ITCZ region just north of the Equator.

Simply, the ITCZ is a region of the atmosphere in the tropics where the surface airflow from both Northern and Southern Hemisphere converges. Belts of high pressure exist over both hemispheres at around 20–30° latitude. We know these regions by the vast, hot deserts that tend to occur beneath them.

The surface outflow from descending air in these subtropical high-pressure cells forms the tropical trade winds—a relatively steady flow from the east in both hemispheres. In addition, this warm air often flows over large expanses of warm ocean water or moisture-rich land such as tropical jungles.

This means a great deal of evaporation and air that now has exceptionally high humidity. Eventually, the southeasterly trade winds of the Northern Hemisphere collide with the northeasterly trades of the Southern Hemisphere, forming a roughly stationary front—the ITCZ.

As commonly happens when surface air converges, it has nowhere to go but up. The constant trade wind flow from each hemisphere provides a continual push from behind to ensure that convergence is perpetual.

The byproduct of forcing air to rise is that, as it encounters decreased air pressure aloft, it is able to expand and cool. This cooling exponentially reduces its ability to hold water vapor, and much of the water entrained by the trade winds is condensed quickly into cloud droplets and rain.

As it happens, the surface convergence doesn’t lift the air to the stratosphere. On its own, the uplifted air would probably make it a few thousand feet aloft before density equalized and it would be pushed horizontally out of the way.

However, the high humidity of the rising air helps destabilize it and allows it to continue its ascent. As water condenses from the air, it releases latent heat that keeps the air warmer than the surrounding environment.

The warm air temperature and high humidity guarantee enough latent energy to help the air reach the stratosphere, which in the tropics normally begins between 50,000 and 60,000 ft.

Meteosat enhanced infrared (IR) image of the MCS through which Air France Flight 447 flew on Jun 1, 2009. Tops of the highest cells were at around FL 560. Approximate flightpath and intersections are overlaid on the image.

The ITCZ also does not move very much, although it does tend to migrate with the seasons and follows the Sun loosely.

This means that, in the Northern Hemisphere summer, the added surface heating draws the zone northward, while the opposite occurs during the Southern Hemisphere summer months.

This seasonal movement places the bulk of the ITCZ in the Northern Hemisphere between about 0 and 10° N from approximately June to December, and between about 10° S and 5° N the rest of the year.

The reason for its average position being slightly north of the Equator is the greater amount of surface heating in the Northern Hemisphere, due to the larger land areas there.

In addition, the moderating influence of nearly constant year-round water temperatures means that most of the north-south movement of the ITCZ is restricted to the land areas, where seasonal Equator/pole temperature differences fluctuate more.

Over the Atlantic, Pacific and Indian Oceans, the ITCZ maintains a relatively year-round position within about 5 degrees of the Equator.

Within the ITCZ

The trouble with planning for a flight across the ITCZ is that conditions tend to be right for convective activity anywhere along it. In addition, a weak easterly jet stream meanders on either side of the ITCZ.

Small waves in the tropical easterly jet can easily foster en­hanced thunderstorm development along the edge of the zone. These clusters of storms are known as mesoscale convective systems (MCSs). In rare cases, a wave in the tropical jet helps an MCS develop outside of the equatorial region between 5° N and 5° S.

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