Split Fronts and Cold Fronts Aloft


Remember the katafront? Let's take this model one step further to understand Split Fronts and the closely affiliated Cold Fronts Aloft (CFA) phenomenon.

1. What does the katafront transverse circulation system suggest about where the major precipitation and the dry airstream are located?
APost-frontal precipitation and pre-frontal dryness.
BAlong-front precipitation and pre-frontal dryness.
CPre-frontal precipitation and post-frontal dryness.
DPre-frontal precipitation and descending dry air well ahead of the surface front.


Question One Hints



 

Your Answer
Q 1:
Correct Answer
Q 1: Question One Explanation


The leading edge of the overruning dry, low wet bulb potential air advancing ahead of the surface cold front often appears as a clearly identifiable upper cold front. Because of the separate existence of the upper cold front (UCF) ahead of the surface cold front (SCF), this configuration is referred to by Browning (1985) as "a split front", as shown below.


Broad red arrow denotes the warm conveyor belt. Green arrows show the introduction of dry, cool (low wet bulb potential temperature) air behind the split front. The CFA rainband is shown by the blue speckled band immediately ahead of the upper cold front (UCF) denoted by the open pips. The surface cold front (SCF) is well behind the UCF.

As the split front advects low wet bulb potential air over the moist air at low levels, potential instability is generated, and a line of convection is often the result. A variant on this same concept is the Cold Front Aloft (CFA), which Locatelli et al. (1995) describe as a cold-frontal zone whose base is situated above the surface in the lower or middle troposphere and which is located ahead of a surface pressure trough. The CFA reverts to a split cold front when the trough is an SCF, and to a warm occlusion when the trough is a warm occluded front.

For purposes of convenience in the schematic figures, we refer to all such fronts as "Upper Cold Fronts" (UCF), though recognizing that this does not necessarily mean that such fronts have linkages to tropopause folds as identified with true upper-level cold fronts as described by Keyser and Shapiro (1986). The "CFA rainband" is shown in the vertical cross section depicted below. It has been proposed since the time of Newton (1950) that upper cold fronts can explain at least some pre-frontal squall lines, but only recently has there been enough data and model capability to provide the evidence in support of this idea.


Vertical cross section along line A-B (see figure above) illustrating the vertical structure of split fronts. Surface cold front is located at the lower left. The upper-level cold front (UCF) is shown detached from and moving ahead of the surface front (or dryline or trough). The CFA/split front rainband is shown by the convective cloud with high rainrates at location #2. Gentler precipitation associated with warm frontal ascent is at location #1. Shallow convective clouds capable of producing light showers are shown in the warm air mass ahead of the surface front at location #3. The green arrow depicts the descending low wet bulb potential temperature air behind the split front.

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