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Using Channel 2 (3.9 µm) Imagery at Night, Part I

Routine examination of 3.9 µm imagery at night offers a good substitution for visible channel imagery. It can be used to track low-level cloud fields and thus infer near-surface wind circulation. This application is particularly useful in the tropics, where freezing levels are relatively high (~5 km) and conventional low-level wind data are sparse. This 30-minute interval loop, remapped to a Mercator projection, allows low-level winds to be easily inferred. The easterlies to the north of the tropical depression (TD-1, 1998) can be viewed in the stratus and stratocumulus cloud fields, as are the weak westerlies from the south near the imagery's lower edge. Note that the areas within this image that are enhanced in blue are colder than -40 degrees C and are indicative of frozen cloud particles.

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The ability to track low-level clouds at night using Channel 2 imagery can be particularly useful in the location of low-level circulation centers associated with strongly sheared tropical cyclones. In this case, the convection associated with the cyclonic circulation is displaced from its center, and low-level cloud motions are difficult to determine using Channel 4 imagery alone. However, these low-level cloud features, as shown above, are well resolved in the Channel 2 imagery. For night-time tropical weather analysis therefore, both Channels 2 AND 4 imagery should be analyzed routinely.

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