3.1 Profiler‐detected overshoot

Radar data from 2006 to 2016 were initially examined for PdO signatures occurring near‐simultaneously with tornadoes listed in the visual database. A PdO on all five beams of a radar was seen on almost all occasions when the radar was less than 100 km from any selected tornado. A tornado more distant than 100 km from any radar registered a PdO on the radar only once, and that was for a tornado 120 km away from the Markstay radar. All subsequent discussions will consider detections less than 120 km from the radar. However, additional criteria needed to be met before a tornado could be assured, as will be discussed shortly.

The process described in the last paragraph identified 31 distinct tornadoes having coincident PdO's on one or more radars, all of which were subsequently studied further. Another 26 tornadoes went undetected by radar because the nearest radars were not operational at the time, either because they were off for maintenance/repairs or, in several cases, because the tornado damaged power lines, terminating electrical supply.

Events showing storm‐caused power outages are nonetheless useful; an example is shown in Figure 3. It shows that the strongest backscattered signals occurred as electrical power to the radar was lost, confirming that the peak power corresponds with the most severe part of the storm, and most likely time for tornado formation. In this case the times of tornado occurrences were known; four tornadoes in the area around Harrow to Leamington were reported (https://en.wikipedia.org/wiki/Tornado_outbreak_of_June_5‐6,_2010). All occurred between 0655 and 0715 UTC; the radar signal peaked and then cut out due to power‐line damage at 0657 UTC. The radar signal began building up at least 30 min prior to the tornado touchdown(s), and would be evident to an observer 10–20 min before the storm peak, giving predictive capability. The white arrow on the NE beam panel indicates humid air spreading into the tropopause due to the stable stratospheric air above slowing vertical ascent; this is evident on all off‐vertical beams. Occurrence of another similar power cut‐out is shown in Figure S4.1 of Appendix S4.

Figure 3 Open in figure viewer PowerPoint Half‐hour averaged powers at Harrow, June 6, 2010. Missing data (black) before 0600 UTC occur because the radar was running in another mode at these times. Power was lost at 0657 UTC. The broken line in the upper curve denotes the radar‐determined tropopause. The white arrow on the NE beam indicates humid air spreading into the tropopause

This evidence that the humid air is being drawn into the tropopause also serves to further indicate that the radar is truly seeing structures in the main beam of the radar, and not suffering range‐aliased sidelobe contamination.

The occurrence of a PdO whenever a tornado is seen locally is of interest, but does every occurrence of PdO produce a tornado?

PdO's are relatively rare (0.07% occurrence), but not all occur in association with tornadoes. Visual observations of power contours over one full summer show that approximately 25% (±5%) of such signatures are associated with tornadoes in summer. They may also occur in winter, generally without tornadoes. As a point of comparison (but without inferring that PdOs are OT's), we note that percentages of OT containing simultaneous tornado detections vary between 14% in Europe up to 63% for the south‐central plains of the United States (cf Appendix S2.2).

An example of a 1‐month power‐height‐time plot is shown in Figure 4 as supporting evidence of their relative rarity. In this particular case the only PdO events evident both had associated tornadoes, but (as noted) in other periods such uniqueness was not always evident.

Figure 4 Open in figure viewer PowerPoint One month of three‐hourly averaged mean powers as a function of height and time for June/July 2014. Red blobs at 8.5–16 km are aircraft interference (cf text). Two episodes of PdO with associated tornadoes are indicated by black arrows. Grey arrows show events that might be confused with PdO but did not occur on other beams

Thus the detection of PdO alone is useful, but insufficient to firmly indicated tornado development. We therefore turn to additional parameters which may help to pinpoint tornadoc events.