A
window on
water vapor
Four Doppler radars used refractivity data in 2006 and 2007 to track
low-level moisture across northeast Colorado in unprecedented detail.
REFRACTT, the Refractivity Experiment for H2O Research and Collaborative
Operational Technology Transfer, depicted the pools of water vapor
that slosh back and forth across the High Plains. Frédéric
Fabry (McGill University) hatched the scheme behind REFRACTT while
he was a postdoctoral researcher in NCAR’s Advanced Study Program
during the mid-1990s. Fabry’s idea was to measure how changes
in atmospheric moisture and density affect the speed of radar signals.
The changes in signal speed are subtle, so the technique requires a
fixed target—such as a silo or a power line—in order to
determine how the signal speed varies across different weather conditions.
The result is a map that shows water vapor (see illustrations) instead
of blobs of rainfall or bundles of wind. The idea is to help pin down
the locations where storms might start raging a few minutes to a few
hours later. The technique could benefit researchers as well as the
hundreds of forecasters who monitor thunderstorms and the public who
relies on their storm warnings.
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These images from northeast Colorado show how
radar-derived refractivity data reveal the difference between
moist and dry lower-level air (left vs. right), even when no
rain is falling
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