From the Lee Side

Undular Bore

Posted 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.

Lee

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.

Categories: Mesoscale Meteorology Physics

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1. shoreacres 12:57 PM GMT on March 09, 2013
It's going to take me a couple of read-throughs (or more) to get all of this, but it's really interesting. I live in League City, Texas, and I've worked outdoors varnishing boats for over 20 years, so I'm pretty sensitive even to weather events I don't understand. Here's part of a post I left on beell's blog on March 5: I'm not sure when the wind actually shifted, but it started blowing thrugh the north-facing windows at 4:30. When it starting ringing the wind chimes, I decided on coffee and forecasts. Sure enough - small craft warning. Wind warning. Fire warning... ADD: Looks like that north wind woke up the boyz, too. ;) Sent: 04:39 CST on 03-05-2013 Effective: 04:39 CST on 03-05-2013 Expires: 18:00 CST on 03-05-2013 Event: Wind Advisory I'm just delighted with this chance to better understand something I experienced, and I'm looking forward to to working my way through this later. (By the way - what pulled me in was your title - "Undular Bore". I thought, "I've known a lot of bores in my life. I wonder what this one's about?" Happy to have learned a new definition for the word.) Member Since: October 4, 2004 Posts: 195 Comments: 14790

2. Lee Grenci, Retired Senior Lecturer and Forecaster 1:02 PM GMT on March 09, 2013
Quoting shoreacres: It's going to take me a couple of read-throughs (or more) to get all of this, but it's really interesting. I live in League City, Texas, and I've worked outdoors varnishing boats for over 20 years, so I'm pretty sensitive even to weather events I don't understand. Here's part of a post I left on beell's blog on March 5: I'm not sure when the wind actually shifted, but it started blowing thrugh the north-facing windows at 4:30. When it starting ringing the wind chimes, I decided on coffee and forecasts. Sure enough - small craft warning. Wind warning. Fire warning... ADD: Looks like that north wind woke up the boyz, too. ;) Sent: 04:39 CST on 03-05-2013 Effective: 04:39 CST on 03-05-2013 Expires: 18:00 CST on 03-05-2013 Event: Wind Advisory I'm just delighted with this chance to better understand something I experienced, and I'm looking forward to to working my way through this later. (By the way - what pulled me in was your title - "Undular Bore". I thought, "I've known a lot of bores in my life. I wonder what this one's about?" Happy to have learned a new definition for the word.) Wonderful! Feel free to ask any questions! Thanks so much for taking the time to write. Much appreciated. Lee Member Since: October 24, 2012 Posts: 48 Comments: 392

3. georgevandenberghe 2:28 PM GMT on March 09, 2013
another interesting post about a topic that is not much brought up, at least in the last century when I was in school. On the other side of the front, the cold air sometimes behaves like a density current, another topic of exploration. East of the Appalachians we don't often get strong fairly shallow fronts like this; the cold air has to be fairly deep (or get fairly deep) before it makes it over the mountains. However backdoor fronts and sea breezes enhanced by a high to the east or northeast would produce this kind of front and I'll start looking for them. There is a Texas saying "There ain't nothin between here and the North Pole 'cept for a barbed wire fence" that comes to play with the northers that blast down the east side of the Rockies. The cross isobar flow is dramatic with these. Cold air can sneak into the DC area from the northeast. I've never seen it sneak in from the northwest, again because of the mountains. Sorry for going a bit off post topic but thanks for the post. How often do you see undular bores in PA from synoptic scale cold fronts? If you see a lot of them I've just missed them (until now) in my observations. Member Since: February 1, 2012 Posts: 3 Comments: 248

4. Lee Grenci, Retired Senior Lecturer and Forecaster 2:34 PM GMT on March 09, 2013
Quoting georgevandenberghe: another interesting post about a topic that is not much brought up, at least in the last century when I was in school. On the other side of the front, the cold air sometimes behaves like a density current, another topic of exploration. East of the Appalachians we don't often get strong fairly shallow fronts like this; the cold air has to be fairly deep (or get fairly deep) before it makes it over the mountains. However backdoor fronts and sea breezes enhanced by a high to the east or northeast would produce this kind of front and I'll start looking for them. There is a Texas saying "There ain't nothin between here and the North Pole 'cept for a barbed wire fence" that comes to play with the northers that blast down the east side of the Rockies. The cross isobar flow is dramatic with these. Cold air can sneak into the DC area from the northeast. I've never seen it sneak in from the northwest, again because of the mountains. Sorry for going a bit off post topic but thanks for the post. How often do you see undular bores in PA from synoptic scale cold fronts? If you see a lot of them I've just missed them (until now) in my observations. George, No need to apologize. I really enjoy reading your stuff. I wanted to use the term, "density current," but I refrained because the meteorology and terminology was already rather complicated. I haven't seen many undular bores over Pennsylvania. I see lots of mountain wave clouds, though, but they're two different phenomena. I suspect the mountains tend to disrupt any wave duct that might provide a nice, easy avenue for bores to travel. Member Since: October 24, 2012 Posts: 48 Comments: 392

5. AtHomeInTX 6:36 PM GMT on March 09, 2013
Like Shoreacres, this might take me awhile to understand all of it but I thought it was an interesting entry. Thanks for posting it. I'm a little farther up the coast on the Louisiana border. And it's nice to understand what caused those gusts that came through. Have to admit, the name grabbed my attention too. :) Member Since: August 24, 2010 Posts: 0 Comments: 3873

6. NumberWise 7:00 PM GMT on March 09, 2013
Like Shoreacres and AtHomeInTX, I enjoyed this interesting blog even if I didn't understand all of it. And I, too, paused when I saw the blog title, as it made me think of the tidal bore in the Bay of Fundy! Member Since: October 22, 2005 Posts: 0 Comments: 1566

7. bappit 7:35 PM GMT on March 09, 2013
Wikipedia has an interesting article for "Undular Bore". It also talks a bit about tidal bores but leaves the connection between the atmospheric and tidal ones a bit vague. Edit: The article might need improvement. Dunno. Says, "They normally occur within an area of the atmosphere which is stable in the low levels after an outflow boundary or a cold front moves through." Seems a bit off since that wording suggests the bore occurs at a location after the boundary passes. Member Since: May 18, 2006 Posts: 3 Comments: 4354

8. Lee Grenci, Retired Senior Lecturer and Forecaster 11:54 PM GMT on March 09, 2013
Quoting AtHomeInTX: Like Shoreacres, this might take me awhile to understand all of it but I thought it was an interesting entry. Thanks for posting it. I'm a little farther up the coast on the Louisiana border. And it's nice to understand what caused those gusts that came through. Have to admit, the name grabbed my attention too. :) Many thanks for your kind words. I confess that there weren't many choices for the title. Thanks again. Member Since: October 24, 2012 Posts: 48 Comments: 392

9. Lee Grenci, Retired Senior Lecturer and Forecaster 11:56 PM GMT on March 09, 2013
Quoting NumberWise: Like Shoreacres and AtHomeInTX, I enjoyed this interesting blog even if I didn't understand all of it. And I, too, paused when I saw the blog title, as it made me think of the tidal bore in the Bay of Fundy! Many thanks. Great point to make about the Bay of Fundy. Thanks so much for your contribution. Member Since: October 24, 2012 Posts: 48 Comments: 392

10. Lee Grenci, Retired Senior Lecturer and Forecaster 11:57 PM GMT on March 09, 2013
Quoting bappit: Wikipedia has an interesting article for "Undular Bore". It also talks a bit about tidal bores but leaves the connection between the atmospheric and tidal ones a bit vague. Edit: The article might need improvement. Dunno. Says, "They normally occur within an area of the atmosphere which is stable in the low levels after an outflow boundary or a cold front moves through." Seems a bit off since that wording suggests the bore occurs at a location after the boundary passes. Agreed. Very good scientific insight. Thanks so much for contributing. Member Since: October 24, 2012 Posts: 48 Comments: 392

11. Astrometeor 12:03 AM GMT on March 10, 2013
Never knew there was a name for this until I read your post Lee, thanks! Member Since: July 2, 2012 Posts: 27 Comments: 1171

12. Lee Grenci, Retired Senior Lecturer and Forecaster 12:09 PM GMT on March 10, 2013
Quoting Astrometeor: Never knew there was a name for this until I read your post Lee, thanks! You're quite welcome, and thanks for posting. Member Since: October 24, 2012 Posts: 48 Comments: 392

13. Lee Grenci, Retired Senior Lecturer and Forecaster 12:32 PM GMT on March 10, 2013
Folks, I added a short paragraph on the speed of the cold front (second graph below the Corpus Christi skew-T) to show you that there are limitations on the formation of an undular bore. Member Since: October 24, 2012 Posts: 48 Comments: 392

14. Lee Grenci, Retired Senior Lecturer and Forecaster 12:32 PM GMT on March 10, 2013
Quoting 24hourprof: Folks, I added a short paragraph on the speed of the cold front (second graph below the Corpus Christi skew-T) to show you that there are other limitations on the formation of an undular bore. Member Since: October 24, 2012 Posts: 48 Comments: 392

15. Lee Grenci, Retired Senior Lecturer and Forecaster 12:40 PM GMT on March 10, 2013
Folks, I posted a movie of Doppler velocities from the radar at Corpus Christi...it's at the very end of my blog. Member Since: October 24, 2012 Posts: 48 Comments: 392

16. shoreacres 1:23 PM GMT on March 10, 2013
I was happy that NumberWise mentioned tidal bores. Again, I knew the phenomenon but not the vocabulary. Some reading on tidal bores helped to make this post more understandable, and reminded me that "fluid dynamics" is a more expansive concept than I usually remember. A National Geographic article on tidal bores mentioned that " "Extreme meteorological conditions leading to a storm surge can also produce a tidal bore in a river that may under normal conditions not exhibit one." Interesting. I've never been able to really get my mind around the Skew-T. I finally went looking and found this, which seems like a good, basic explanation and helped considerably. I'm glad you included the graphic and sent me looking. Member Since: October 4, 2004 Posts: 195 Comments: 14790

17. Lee Grenci, Retired Senior Lecturer and Forecaster 1:39 PM GMT on March 10, 2013
Quoting shoreacres: I was happy that NumberWise mentioned tidal bores. Again, I knew the phenomenon but not the vocabulary. Some reading on tidal bores helped to make this post more understandable, and reminded me that "fluid dynamics" is a more expansive concept than I usually remember. A National Geographic article on tidal bores mentioned that " "Extreme meteorological conditions leading to a storm surge can also produce a tidal bore in a river that may under normal conditions not exhibit one." Interesting. I've never been able to really get my mind around the Skew-T. I finally went looking and found this, which seems like a good, basic explanation and helped considerably. I'm glad you included the graphic and sent me looking. I really enjoy exchanges like this between enthusiastic and knowledgeable weather watchers. With regard to skew-Ts, check out my earlier blog. Hope this helps. Lee Member Since: October 24, 2012 Posts: 48 Comments: 392

18. shoreacres 1:50 PM GMT on March 10, 2013
@Lee ~ Thanks so much for the link and the reminder that there are archives to be checked! I'm looking forward to the browse. Member Since: October 4, 2004 Posts: 195 Comments: 14790

19. Lee Grenci, Retired Senior Lecturer and Forecaster 1:58 PM GMT on March 10, 2013
Quoting shoreacres: @Lee ~ Thanks so much for the link and the reminder that there are archives to be checked! I'm looking forward to the browse. You're quite welcome. Feel free to ask any questions here, even if they're off topic. Cheers. Lee Member Since: October 24, 2012 Posts: 48 Comments: 392

20. georgevandenberghe 3:21 PM GMT on March 10, 2013
On the topic of bores and tides, if you ever have a chance to travel to Northern France, a visit to Mont St Michel in Normandy provides a fascinating chance to watch some of the highest tides in the world come pouring in. Tidal variation there is about forty feet and the sea bottom is very shallow for a long way out so the tide comes rushing in over a distance of several kilometers. You can see many different kinds of gravity waves in the channels surrounding the monastery when the tide comes in. Member Since: February 1, 2012 Posts: 3 Comments: 248

21. Lee Grenci, Retired Senior Lecturer and Forecaster 3:38 PM GMT on March 10, 2013
Quoting georgevandenberghe: On the topic of bores and tides, if you ever have a chance to travel to Northern France, a visit to Mont St Michel in Normandy provides a fascinating chance to watch some of the highest tides in the world come pouring in. Tidal variation there is about forty feet and the sea bottom is very shallow for a long way out so the tide comes rushing in over a distance of several kilometers. You can see many different kinds of gravity waves in the channels surrounding the monastery when the tide comes in. It's now on my list, George. Thanks. Member Since: October 24, 2012 Posts: 48 Comments: 392

22. beell 4:39 PM GMT on March 10, 2013
Wow. Mostly an uneventful winter here in Houston. A very cool feature passed right over my head and I missed it! Unlike shoreacres and other fellow Texans, I get an "F" as a weather observer. On Monday morning, HGX was advertising gusty winds and cold air advection with the Tuesday front. My work day usually starts pretty early. Never even glanced at the sat images on Tuesday. Just heard the winds outside and assumed the front was on track and on time. Thanks, Lee, for bringing the science and images to your blog to allow the opportunity to see what I missed. And a reminder for me, that sometimes, sticking one's head out the window and taking note of the clouds, winds, temps, dp's, etc, is a valuable compliment to understanding the charts and graphs and models that are sometimes erroneously referred to as "weather" . Member Since: September 11, 2007 Posts: 124 Comments: 12831

23. Lee Grenci, Retired Senior Lecturer and Forecaster 6:06 PM GMT on March 10, 2013
Quoting beell: Wow. Mostly an uneventful winter here in Houston. A very cool feature passed right over my head and I missed it! Unlike shoreacres and other fellow Texans, I get an "F" as a weather observer. On Monday morning, HGX was advertising gusty winds and cold air advection with the Tuesday front. My work day usually starts pretty early. Never even glanced at the sat images on Tuesday. Just heard the winds outside and assumed the front was on track and on time. Thanks, Lee, for bringing the science and images to your blog to allow the opportunity to see what I missed. And a reminder for me, that sometimes, sticking one's head out the window and taking note of the clouds, winds, temps, dp's, etc, is a valuable compliment to understanding the charts and graphs and models that are sometimes erroneously referred to as "weather" . That's good advice! Thanks! Member Since: October 24, 2012 Posts: 48 Comments: 392

24. WunderAlertBot (Admin) 1:40 PM GMT on March 13, 2013
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