4.3.1 Bow Echo: Cross Section View

Vertical cross sections through the core of a bow echo depict a strong vertically erect updraft at the leading edge of the system. There is also a strong elevated rear-inflow jet at midlevels impinging from the rear of the system to just behind the updraft. Then it abruptly descends and spreads out along the surface. Above the rear-inflow jet, the updraft current turns rearward feeding into the stratiform precipitation region. It should be noted that the preferred method is to assess rear-inflow motion in a storm-relative sense to verify the true strength of this feature. However in this example, storm-relative velocity was not available.

Compare the conceptual model to these radar-reflectivity and velocity cross sections through a bow echo. Note that in this case the radar is to the left of the storm, which is moving away from the radar. The reflectivity cross section shows a very strong deep cell right at the leading edge of the system with very little trailing stratiform precipitation. Also note the weak echo overhang on the front edge of the system (the downshear direction). In the velocity image the rear inflow (36-50 kt) remains quite elevated (10,000 ft or ~ 3 km) all the way to the leading line convection before diving toward the surface. Additionally, the system updraft is quite erect (depicted by the gray area with a hint of green, which is tilted only slightly rearward). These factors all indicate that the vertical wind shear was strong on this day.

4.3.2 Bow Echo: Cross Section Pressure Field

The pressure field is characterized by a strong mesohigh at the surface, associated with the cold pool, and a strong mesolow at midlevels, just above the mesohigh. Because they tend to occur when CAPE values are large, bow echoes have exceptionally warm updrafts above the cold pool. The strong warm pool aloft combined with the strong surface cold pool contributes to the development of an exceptionally strong rear-inflow jet toward the mid-level mesolow. Just like we saw in the Squall Line section, the strength of the RIJ is important because the stronger the storm-relative flow in the RIJ, the greater is the potential for damaging winds at the surface!

4.3.3 Radar-Observed Rear-Inflow Notch

As mentioned in the conceptual model discussion, there is a reflectivity feature created by the impinging RIJ called the rear inflow notch or RIN. Detecting a RIN can be useful in identifying the increased potential for strong winds within a bowing structure. (NOTE, the absence of a RIN does NOT mean there won’t be strong surface winds.) The RIN is a useful concept though because it is a reflectivity-only feature meaning it can be observed even when the radar-viewing angle through a bow echo is unfavorable for determining the actual wind velocity. Analysis of both these events showed that in both cases the rear-inflow jet descended and created damaging surface winds.

This next review question refers to the case scenario from the first section of this module. Test your skill at locating the bow echo and associated RINs in the data from this case.

4.3.4 Review Question: Locate Bow Echo

Click the image above to view animation.

In the ~ 45-minute long radar loop from 0113 UTC to 0204 UTC on 26 June 2003, a bow echo developed in the squall line. The bow echo had at least one rear-inflow notch (RIN).
Stop the loop on the last frame and draw the location of the bow and any associated RINs.

Review Question Feedback

Click the image above to view animation.

Expert Answer: The bow echo developed in west-central Illinois and moved northeastward or almost perpendicular to the line movement, in the direction of the mean low-level shear vector. Two RINs were evident at 0204 UTC, just south of the northern-end vortex. Given that the radar beam did not pass through excessively heavy precipitation before reaching the bow, it does not seem likely that these RINs are the result of attenuation. Luckily, Scott AFB was spared the worst of this event. The only official reports of severe winds occurred to the north, with the bow echo.

End of Section 4.3: Bow Echoes - Features