WX BRIEF

Intertropical convergence

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


Enhanced IR satellite image of the Atlantic region at 1215Z on Jun 27, 2009. In colorized IR images, the coldest, high cloud tops are blue and white, while the warmer land and oceans are in reddish colors. Note the line of widespread convection at about 4° N, stretching from Africa westward into the Pacific. This line is the ITCZ.

Outside this area, the spin of the Earth provides a rotational component sufficient to get the MCS spinning, and it may develop into a hurricane. This method of hurricane formation is common in the tropical storms that sweep across the Atlantic Ocean from the Cape Verde Islands.

Most of the time, however, the wave in the upper air simply encourages development of a cluster of storms that grow and die in place. Unlike the individual towering cumuli cells in the ITCZ, which often exist for less than an hour and are generally easy to circumnavigate, a tropical MCS may cover a large area of airspace over open ocean and exist for several hours.

Unfortunately, the region of the ITCZ over the equatorial Atlantic Ocean is a place where formation of large and vigorous MCS activity is common. In fact, an Air France Airbus A330-200 crashed after encountering a particularly strong MCS on Jun 1, 2009 as it transited the tropical Atlantic enroute from GIG (Galeão, Rio de Janeiro RJ, Brazil) to CDG (Charles de Gaulle, Paris, France).

Although the cause of the accident remains under investigation and may not be weather related, the aircraft, which was flying at FL 330, did penetrate the MCS and flew near several cells with tops between approximately 47,000 and 56,000 ft msl. The aircraft very likely encountered graupel and strong turbulence as it flew 50 to 75 miles through the MCS.

ITCZ thunderstorms

ITCZ thunderstorms are slightly different from midlatitude storms. The differences are primarily due to the different atmospheric conditions in these 2 locations. The strong thunderstorms we endeavor to avoid in the midlatitudes are often due to 2 distinct air masses lying on top of each other.

In these situations, warm, humid air gets trapped beneath a middle to upper-level cold and dry pool of air. A small temperature inversion caps the surface air until it builds up enough energy to burst through the cap and rise explosively.

It is the excessive speed of the updraft and corresponding down drafts of cold, dry air that give these storms their immense danger to aviation.

Vertical air movement can easily exceed 50 kts and has been measured in excess of 100 kts. Furthermore, the boundary between a strong updraft and a strong downdraft means a violent shear zone with air flowing through it at a combined speed of potentially well over 100 kts.

Top of a tropical storm cell pokes into a stable layer in the upper troposphere around FL 300. Many ITCZ storms are able to rise to the stratosphere around FL 500, which places them well above the cruise altitudes of most aircraft.

The rapidly rising air also often encounters airflow from different directions as it rises, fostering the development of a rotating mesocyclone within the storm. In addition, strong electrostatic charges can build up in the cell, resulting in frequent lightning, which, while it can damage aircraft skins and sensitive electronics, can also alert a pilot to an approaching nocturnal storm.

Lastly, in the middle latitudes, the troposphere tends to cool rapidly with height, and the freezing line and zone of temperatures that support supercooled liquid rain drop­lets tend to occur somewhere in the lower to middle of many of these midlatitude storms, especially during the spring and autumn transition seasons.

Furthermore, while strong storms might poke into the stratosphere by a few thousand feet, the tops of thunderstorms are generally limited to the top of the troposphere. During spring or autumn, this occurs around 30,000 ft, and during the heat of summer, it may rise another 10,000 or 15,000 ft.

In the tropics, the near continuous heating from the Sun, coupled with the vast expanses of warm ocean water, means that the entire troposphere, from the surface to the base of the stratosphere, tends to be warm and humid.

On one hand, this means there’s an incredible amount of latent heat energy available to create monster storms—on the other hand, however, the warm and humid air aloft tends to dampen the otherwise explosive ascension of warm, humid air near the surface.

In fact, over most of the ITCZ the atmosphere is simply unstable throughout the troposphere, which means that there is little to let the surface air build up the excessive energy needed to create strong storms.

Instead, it is primarily the convergence of air into the ITCZ that kicks the surface air aloft. As the air rises and cools, the condensation of the moisture it holds releases enough latent heat energy to keep the air slightly warmer than the surrounding environment.

The air keeps rising, but the associated updrafts tend to be weaker than those usually found in a strong midlatitude storm cell. Updrafts in tropical storms are commonly around 10–30 kts, though strong updrafts in excess of 50 kts occur on occasion.

As the tropical air rises, it also has much further to go before it hits the base of the stratosphere. Tropical warmth expands the troposphere up to between FL 500 and 600. The tops of strong tropical storm cells can often ascend an additional 5000 or 6000 ft before the stable stratosphere overcomes the uplift.

Another difference between tropical and midlatitude storms is that tropical storms generally don’t have much lightning. In fact, while they may have an anvil top and dump excessive rain on the landscape below, some tropical storms may not fit the definition of a thunderstorm because they don’t exhibit any lightning.

The reason for this is not clear to researchers, but it is believed that the updrafts weaken sufficiently around the middle of the storm to inhibit the charge separation—where positive and negative ions move apart to different parts of the storm—that is necessary to create a lightning discharge.

Crossing the zone

Except for a few short hops, nearly all flights between the Northern and Southern hemispheres will need to cross the ITCZ. Fortunately, despite the perpetual convergence and lifting of warm, humid air, the majority of the ITCZ is clear air, albeit with reduced visibility due to the high humidity.

Low levels will commonly experience a wide­spread deck of cumulus clouds. By the afternoon and into evening, many of these will have grown into towering cumuli and then died out as the dry downdrafts disorganize the updrafts.

 

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