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