Regional And Mesoscale Meteorology Team
Daily Satellite Discussion
Friday July 25, 1997
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Today's featured images depict a 4-panel loop of average (upper images) and minimum
(lower images) radiances for 6.7 micrometers composited over 2-week periods in the
beginning of January (left images) and June (right images) for Central America. The
loop consists of bi-hourly images from 0015 GMT to 2215 GMT. Recall that in a normally
moist atmosphere, most of the radiation "seen" by the satellite comes from the 300-600
hPa layer. The rust color indicates warm regions, the progression from gray, white,
blue, green, red and finally to yellow signifies increasingly colder temperatures.
The minimum radiance composites find the coldest pixels at the specified time over
the 2-week period. It will therefore tend to highlight regions where deep convection
has occurred (or has not occurred). The average radiance composites combines warm and
cold imagery. Persistent deep convection will show up as cooler, whereas persistent
subsidence will tend to show up as warmer. In January, a strong, broad high pressure
sub-tropical ridge dominates the region bringing the "dry" season. The diurnal pattern
is fairly consistent as noted in the average radiance, and the minimum radiance
composite shows very little deep convection. By July, the ridge breaks down and
easterly tropical waves move through the area regularly, triggering deep convection.
This characterizes the "wet" season. Both the average and minimum radiance composites
show the diurnal pattern of deep convection over the region with maximum convection
occurring over most land masses by the end of the day (0015 GMT) and maximum convection
occurring over oceans in the early morning hours (1215 GMT).
Here's the loop !
Bernadette Connell