Quantcast Weather Along the ITCZ

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Weather Along the ITCZ

The degree and severity of the weather along the ITCZ varies, based on the degree of convergence. The zone of disturbed weather may be as little as 20 to 30 miles in width or as much as 300 miles. Under typical conditions, frequent rainstorms, cumulus and cumulonimbus-type clouds, and local thunderstorms occur. Violent turbulence may be associated with these storms, and cloud bases may lower to below 1,000 feet, or even be indistinguishable, in heavy showers. Their tops frequently exceed 40,000 feet. An extensive layer of altocumulus and altostratus usually occurs, due to the spreading out of the upper parts of the convective clouds. These layers vary in height and thickness with the currents of the air masses. At higher levels, a broad deck of cirrostratus spreads out on both sides of the zone. Icing in the heavy cloud masses is likely to reach serious proportions at altitudes above 15,000 feet, as this is roughly the average freezing level in equatorial regions.

Surface winds are generally squally in the areas of heavy showers. Usually, the winds in the squalls do not exceed 15 to 25 knots, but winds of 40 to 50 knots or higher have been reported. Visibility is usually good except when reduced by the heavy shower activity.

The intensity of the ITCZ varies inter-diurnally, from day to day, and to a lesser degree annually. Over ocean areas, precipitation reaches its maximum just before dawn, with a minimum occurring in the late morning or early afternoon. With the exception of coastal areas under the influence of onshore winds, the reverse is true over land—the daily maximum occurs in the late morning.

Seasonal Variation

Through the use of satellite data, the seasonal meridional displacement of the ITCZ band has been determined. See figure 9-3-6. Imagery has

Figure 9-3-5.—Satellite photograph of cloud band associated with ITCZ.

Figure 9-3-6.—Mean summertime position of the ITCZ. (A) Northern Hemisphere; (B) Southern Hemisphere.

shown the cloud band to have somewhat different characteristics in different parts of the world. In the Atlantic, it is centered about 3°N in the winter season and moves to about 8°N by late summer. In the Pacific, east of 150°W, there is no apparent seasonal fluctuation. Seasonal pressure changes over the North American continent may be responsible for the seasonal shift of the cloud band in the eastern part of the Pacific. There is some evidence of a second cloud band associated with the lTCZ in the eastern Pacific. Besides the main band north of the equator, a weak band appears at 5°S in the January through March time frame. The strength of this second band varies from year to year; in some instances it fails to develop at all. Over the Indian Ocean, during the Southern Hemisphere summer, the ITCZ cloud band is much broader than it is over either the Atlantic or eastern Pacific oceans.

Learning Objective: Describe tropical cyclone characteristics and categorize the development and life cycle of tropical cyclones.

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