Introduction
Based upon the references posted by Frank and Lee, long-established meteorological opinion holds that tropopause folds descend along the strong thermal gradients associated with a cold front. In addition, they tend to be rather transient with most persisting for 6 hours or less. Their relatively small-scale limits their influence to a region approximately 100 km to 200 km in length and 100 km to 300 km wide.
Pesky Meteogram
Before I proceed, I need to raise an issue regarding the KUNV meteogram that appeared in Lee’s previous post. I can’t provide an explanation, but this meteogram is not consistent with the hourly Metars or the meteogram that is currently available. The first image (below) presents the original meteogram (left) and current (right) one subsequently produced. Please note that the meteograms cover a different period, but this is not relevant to the error in the meteogram. The new (and correct) meteogram shows that the maximum gust of 40 knots on February 19th was observed at 12Z and not 4Z as originally reported.
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Meteograms for February 19th, 2011. The original version does not correlate with the hourly metars. The correct time of the 40 knot gust was 12Z. Click here for larger version. |
Synoptic Overview
As mentioned above, tropopause folds occur along the strong thermal gradients associated with a cold frontal boundary. A review of the HPC Surface Analyses (below) indicates that the cold front associated with the deep occluded low pressure system was already at the extreme southeastern corner of Pennsylvania at 0Z on the 19th. The relatively rapid southeasterly movement of this boundary continued and by 12Z it was draped across the southeastern US, several hundred miles to the south of KUNV. With the cold front so far to the south and in accordance with widely-accepted conventions, it seems very unlikely that a tropopause fold was responsible for the 40 knot gust at KUNV at 12Z.
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As shown on the 18Z HPC surface analysis, the cold font was even further away from the northeastern states. By 0Z on February 20, the cold front was well offshore and no longer appeared on the HPC surface analysis (not shown).
Regional Surface Observations
A regional analysis of wind gusts across the northeast on the 19th indicates that gusty conditions were a long-lived and wide-ranging phenomena. Using Google Earth, I plotted the time and value of the maximum wind gusts at a number of stations across a large area extending from Virginia to northern Vermont.
A review of the data presented on this graphic confirms that the times at which the maximum gust occurred can’t be correlated to the passage of cold front. Nor can a linear timeline be constructed from west to east which would suggest a progressive event as the cold front swept east. For example, the maximum gust of 41 knots at Johnstown, PA (KJST) occurred at 5Z. This observation is seven hour earlier than the maximum gust at KUNV, although KUNV is only 40 nm to the northeast.
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Regional analysis of maximum wind gust on February 19, 2011. Larger version. |
A 36 knots gust was observed at Wilkes Barre, PA (KAUP) at 2Z, 6Z, 7Z and 10Z – all were before the maximum gust at KUNV which is west of KAUP. The maximum gust of 37 knots at Burlington, VT (KBTV) occurred at 5Z, 7 hours before the maximum at KUNV. In addition, a maximum gust of 45 knots was observed at Dulles Airport (KIAD) at 13Z. KIAD is approximately 200 miles south of KUNV.
Regional Upper Air Observations
Wind speeds at 925 mb and 850 mb at 0Z and 12Z on February 19th and 0Z on the February 20th at 0Z are presented below. The 925 mb analysis at 0Z on the 19th shows the intrusion of very strong winds into the northeast. This analysis is confirmed by the February 19th 0Z sounding at Buffalo, NY (KBUF), where wind speeds were 68 and 65 knots respectively. The corresponding values at Albany, NY (KALB) were 43 and 48 knots. Winds approaching 40 knots at both 925 mb and 850 mb were observed as far south as Dulles Airport (KIAD).
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First row: 925 mb analysis at 2-19-2011 at 00Z, 2-19-2011 at 12Z and 2-20-2011 at 00Z. Second row: 850 mb analysis at 2-19-2011 at 00Z, 2-19-2011 at 12Z and 2-20-2011 at 00Z. Click on an individual image for a larger version. |
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The table below supports the 925 mb and 850 mb graphics above and confirms that strong winds in the lower troposphere were both geographically widespread and persisted throughout the day on February 19th.
Of course, the momentum from strong winds aloft can’t mix to the surface if the atmospheric profile is not sufficiently unstable. The series of observed soundings (below) occurred from February 19th at 0Z through 0Z on February 20th and correlate to the table of observations presented above.
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Observed soundings from KBUF, KPIT, KALB and KIAD at 2-19-11 at 00Z, 2-19-11 at 12Z and 2-20-11 at 00Z. Click on an invidual sounding for a larger version. |
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Allowances must be made for transitions in the soundings as time passes, but a comparison of the maximum observed wind gusts at a particular station to the nearest sounding site supports the conclusion that the lower troposphere was supportive of mixing to approximately 850 mb at all of the locations.
Summary
The observations from February 19th and the basic understanding of this feature do not support the hypothesis that a tropopause fold was responsible for the gusty conditions at State College, PA (KUNV) or elsewhere. The duration of high wind observations, the discontinuity in the times of the maximum gust and the distances between the selected stations all exceed the accepted temporal and spatial scales of a tropopause fold (100 km to 200 km in length and 100 km to 300 km wide). In addition, the majority of previously studied tropopause folds reached down to 500 mb and, in a few instances, to 850 mb. Tropopause folds reaching the surface are extremely rare.
An examination of the winds at 925 mb and 850 mb on the observed soundings support the conclusion that garden-variety mixing of momentum from speedy winds in lower troposphere to the surface (schematic below) were responsible for the prolonged and wide spread high wind observations.
It is not possible to prove with absolute certainty that a tropopause fold was not responsible for the gusty conditions on the 19th, but the evidence provided above suggests that it was extremely unlikely. Ockham’s (Occam) Razor as modified by Newton -- “we are to admit no more causes of natural things than such as are both true and sufficient to explain their appearance” -- is consistent with my conclusion that a very straightforward and common meteorological principle explains the conditions on February 19th.