Retired senior lecturer in the Department of Meteorology at Penn State, where he was lead faculty for PSU's online certificate in forecasting.
By: Lee Grenci , 12:40 PM GMT on March 09, 2013
An undular bore developed over a portion of the western Gulf States early Tuesday morning (March 5) and then headed toward the Gulf of Mexico. The 1415Z visible satellite image from GOES-13 (below; larger image) shows a classic series of undulating, low-level clouds that are the hallmark of undular bores.
The 1415Z visible satellite image on March 5, 2013, shows undulating clouds associated with an undular bore over portions of the western Gulf States and the coastal waters of the Gulf of Mexico. Larger image. Courtesy of Penn State.
Loosely defined, an undular bore is a disturbance (generally speaking, an area of unsettled weather) that's typically generated when a cold front collides with a stable air mass in the warm sector ahead of the front. There are more details to the story of undular bores, and I'll get to them in a moment. First, to get a better sense for the prevailing synoptic-scale pattern during the early morning of March 5, check out the 12Z (6 A.M. CST) surface analysis from the Hydrometeorological Prediction Center.
Now wait just a darn minute, Grenci...a stable air mass in the warm sector ahead of a cold front? Aren't you getting a bit senile? Perhaps, but keep in mind that FROPA (frontal passage) occurred over eastern Texas before sunrise on March 5. So a low-level temperature inversion associated with nocturnal cooling was likely present. Indeed, the 12Z Rapid-Refresh Model skew-T at Corpus Christi, TX, (below; larger image), for example, shows a temperature inversion extending from just above the ground to roughly 900 mb. For your convenience, I marked the top and bottom of the stable layer whose profile of temperature increased with increasing altitude.
The 12Z Rapid Refresh model skew-T at Corpus Christi, Texas, on March 5, 2013. Note the temperature inversion extending from just above the ground to roughly 900 mb. Larger image. Courtesy of Penn State.
I earlier promised you additional details about the meteorology of undular bores so you can better visualize and understand the genesis of these mesoscale systems. So let's look at an idealized cross section (below) of an undular bore that's generated by a cold front. A cross section, as you already know, is a vertical slice through the atmosphere (in this case, the lower troposphere). At any rate, the cross section depicts the shapes of the cold air mass following in the wake of the cold front (light gray) and the less stable air mass ahead of the cold front (dark gray). As the cold front intrudes on the less stable air mass in the warm sector, it's able to lift the less stable air (low-level convergence and the associated uplift). In the warm sector, the air overlying the stable air near the ground (white shading) can't be "too unstable" (it should be neutral or conditionally unstable). Otherwise, energy would be dispersed vertically and, perhaps, cumuliform clouds might form (instead of a train of waves in the lower troposphere).
Moreover, the front must move at an optimal speed for prevailing weather conditions. If the front intrudes too slowly into the warm sector (lifting is too weak), the undulating waves will not develop. If the front advances too quickly into the warm sector (lifting is too strong), waves will "break," and, thus, there will be two chances of organized cloud lines developing...slim and none.
So, as long as the air aloft in the warm sector isn't "too unstable" (again, it should be neutral or conditionally unstable), a series of waves forms and propagates downstream along the top of the now elevated temperature inversion (the top of the temperature inversion now lies at a "slightly" higher altitude). Assuming there's adequate low-level moisture available, net condensation occurs on the upswing of each of the waves, forming the pattern of undulating clouds that you observed on the 1415Z visible satellite images from GOES-13.
An idealized cross section showing dense, cold air associated with a cold front intruding and lifting less stable air ahead of it, setting the stage for undulating wave clouds (undular bore). Adapted from Hartung, et al.
There are even more details and mathematical limitations on undular bores, so hopefully you can appreciate why I initially described an undular bore as a "disturbance" that promotes a series of undulating waves (alternating currents of upward and downward motions). I'll add here that undular bores can also be initiated by pre-frontal troughs and other mesoscale boundaries such as a gust front associated with a group of thunderstorms.
Let's explore the "sensible weather" that typically accompanies the arrival of an undular bore (essentially the passage of the "head" wave in a series of waves...here's the unedited cross section from the Hartung, et al. paper; it will give you a more complete description of the terminology meteorologists sometimes use in the context of undular bores). At any rate, the pressure at the earth's surface rises rapidly in response to the cooling the air columns throughout the depth of the undular bore. Moreover, there's a dramatic shift in wind direction (toward the direction of movement of the undular bore). Surface winds can also be quite gusty as momentum from faster winds aloft gets mixed downward toward the ground.
To see what I mean, check out the METARS (weather observations) at Corpus Christi, Texas (below). It looks like the bore head passed around 1227Z (6:27 A.M.). Note the sudden gust to 38 knots and the observation of a squall (SQ). PRESRR means "pressure rising rapidly," so the observations fit our model of weather conditions that accompany the passage of a undular bore. The 1245Z image of base velocities captured some of the waves outbound from the radar at Corpus Christi.
The METARS (weather observations) at Corpus Christi, Texas (KCRP),from 08Z to 14Z on March 5, 2013. The undular bore passed KCRP at about 1227Z, Relevant observations are underlined in blue.
Over my many years as a weather forecaster, I have observed undular bores intersect with mesoscale boundaries and mountains, thereby helping to initiate thunderstorms. So, in my view, undular bores are worthy of study. Check out this paper, which was recently published in Weather and Forecasting...it was written by one of my great former students, Phil Lutzak, who graduated from Penn State's online program in weather forecasting.
P.S. Here's a movie of the Doppler velocities from the radar at Corpus Christi (from roughly 11Z to 14Z on March 5, 2013). Courtesy of Mark Thornton.
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