4.2.1 Bow Echo Evolution

Shown for comparison to the previously discussed squall line examples, this conceptual model shows a weak echo area behind the core of the bow, referred to as a “rear-inflow notch” (RIN). This notch often signifies the location of a strong rear-inflow jet. Care needs to be taken when interpreting the radar imagery by using Doppler velocity data when possible to verify that there is strong approaching flow in the notch. In some convective cells the weakening in the reflectivity that appears to be a notch is really just caused by beam attenuation! Just like a shorter squall line, if a bow echo persists for more than three to four hours, the Coriolis force will intensify the northern vortex at the expense of the southern vortex creating the often-observed asymmetric evolution by the later stages.

4.2.2 Size and Intensity

Bow echoes follow the full symmetric-to-asymmetric squall line evolution, but on a smaller scale. Their extreme intensity is due in large part to their relatively small size. In particular, the smaller distance between the bookend vortices enhances the focusing effect on the mid-level flow between the vortices, which can significantly strengthen the rear-inflow jet. The descent of this enhanced rear-inflow jet to the surface produces the extreme surface winds associated with many bow echoes. It is important to remember that bow echoes are generally an indicator of intense winds, not a predictor. The echo has the bowed shape due to a focused intense flow into the back of the system.

4.2.3 Review Question: Bow Echo Stages and Severity

In a strong shear environment, which bow echo stage(s) are likely to produce large swaths of damaging winds at the surface?

(Choose the letter(s) corresponding to the image.)

a.

b.

c.

d.

e.

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The best answers are a) and e).

When both line-end vortices act in tandem (stage e) the rear-inflow jet becomes quite strong and strong winds are likely at the surface. Damaging winds may last into the later stages (stage a) as well, even when the system has become asymmetric, but the severe winds may be located primarily on the northern half of the system.

Note: Although not depicted in this simple conceptual model exercise, remember it is important to focus your attention on any small-scale bowing segments within the line during all stages of evolution in order to pinpoint the location of potential damaging winds.

End of Section 4.2: Bow Echoes - Evolution