WX BRIEF

Intertropical convergence

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


Isolated tropical thunderstorm matures over the Caribbean in Apr 2005. Most storms in the ITCZ are isolated, but may reach heights over FL 500 and can contain severe conditions.

However, localized enhanced convergence, often due to a migratory upper air disturbance, can generate an MCS cluster of storm cells. While these cells generally do not reach the severity of midlatitude storms and may not reveal themselves with lightning, they can contain strong turbulence, heavy rain and even icing, and should be studiously avoided.

Unfortunately, real-time weather information about the ITCZ can be difficult to come by. Globally, there are only a few weather balloon stations within the ITCZ, and there are only a few more surface weather stations transmitting their observations.

Because most of the ITCZ is over the oceans, surface weather information is largely limited to periodic observations taken by passing ships. Also, there is almost no surface-based weather radar coverage, so pilots must rely on onboard radar to locate active storm cells.

Even strikefinders are of limited use due to the lack of lightning. Any timely upper air observations are likely to come either from Pireps filed by fellow aviators, or from the weather satellites orbiting high above the Earth.

Infrared satellite images will quickly reveal the location of the ITCZ as well as any active areas within it. Look for very bright white pixels on the satellite image—these are the high, cold cloud tops.

Current satellite images for most of the globe are available from sources such as ADDS (aviationweather.gov) or the University of Wisconsin Space Science and Engineering Center (ssec.wisc. edu/data/).

Global weather models are another good source of information about the ITCZ. These forecast models take all the available weather information and use what we know about the tropical atmosphere to process it at locations where actual observations may not exist.

On a broader scale and for short-term forecasting, these models can be fairly accurate. This information—along with available observations and satellite information—is what aviation forecast centers use to produce significant weather charts for the ITCZ regions, which can be accessed at ADDS.

Between satellite images, significant weather prognostic charts and a good professional weather briefing, flying the ITCZ should not bring any more surprises than flying anywhere else. Of course, given the data-sparse nature of the ITCZ, any Pireps you provide will help your fellow pilots as well as the forecasters who provide the briefings.

Karsten Shein is a climatologist with the National Climatic Data Center in Asheville NC. He formerly served as an assistant professor at Shippensburg University and was a scientist with NASA’s Global Change Master Directory. Shein holds a commercial license with instrument rating.

 

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