Transcript Strong Cold Front Hits the BAO Tower

Strong Cold Front Hits the BAO
Tower
7C in 3 hr criterion
Fig. 3. Frontal passages on (a) 4–6 and (b) 8–10 Mar 2003 from
the 1-min data at the Automated Surface Observing System
(ASOS) at OKC (UTC = CST + 6 h)
temperature decrease of 10C in 2 min
Using Simple First Order
Relationships to Explain Tilt of
Occluded Fronts
Classic Idea: Occlusion Type Determined By
Temperature Contrast Behind Cold Front and in
Front of Warm Front (“the temperature rule”)
But reality is very different
From Stoelinga et al 2002, BAMS
Literature Review
• Schultz and Mass (1993) examined all published cross
sections of occluded fronts. Found no relationship
between the relative temperatures on either side of
the occluded front and the resulting structure. Of 25
cross sections, only three were cold-type occlusions.
• Of these three, one was a schematic without any
actual data, one had a weak warm front, and one
could be reanalyzed as a warm-type occlusion
• Cold-type occlusions appear rare.
But what controls the slope?
• Virtually all fronts are first-order fronts (which
the horizontal temperature gradient changes
discontinuously with frontal passage) rather
than zero-order fronts (where temperature
varies discontinuously across the front)
• Historical note: in the original Norwegian
Cyclone Model they suggested all fronts were
zero-order fronts.
Basic Relationship
The relative value of the
vertical potential temperature
derivative will determine the
slope
• Occluded frontal surfaces generally mark a
maximum in potential temperature on a
horizontal surface, so the numerator on the
right side of (2) is always positive.
• Therefore, the sign of the slope of the
occluded front is determined only by the
denominator on the right-hand side of (2),
that is, only by the static stability contrast
across the front, and not by the contrast in
horizontal potential temperature gradient.
An Improved View: The Static
Stability Rule of Occluded Front Slope
• An occluded front slopes over the statically
more stable air, not the colder air.
– A cold occlusion results when the statically more
stable air is behind the cold front.
– When the statically more stable air lies ahead of
the warm front, a warm occlusion is formed.
– The is pretty much always the case.
An Example
Another Example