Dryline bulges and storm initiation

Jul 23, 2004
Iowa City, IA
Yesterday (3/21/05) we saw a good example of a dryline bulge and subsequent storm initiation in the OKC area. Why do storms often initiate first at the bulge? Is it as simple as reasoning that the bulge results from an area of locally stronger mid-level UVV's and/or upper-divergence, which results in the BL mixing out faster at the bulge? bill
Nice question. The dryline - a separation of moisture-laden air in the east from dry air to the west, is an important feature in the plains states. During the day the dryline typically creeps eastward. While the dryline is not a front, it still a type of boundary that separates air masses and can promote lift. Upward forcing along the dryline often occurs along bulges. These areas contain enhanced moisture convergence, and that moisture provides fuel for thunderstorm development. The dryline alone does not trigger the storm, however ... storms usually initiate in tandem with an approaching upper air disturbance like we saw yesterday, where the dryline acted as a pre-frontal boundary ahead of the approaching cold front.

From one of Tim Marshall's articles on the dryline:

One of the key features I look for in forecasting dryline storms is a dry-line bulge. This forms when strong surface winds in the dry air accelerate a portion of the dryline eastward ahead of the rest of the boundary. (Such winds can cause intense dust storms that obscure the sun.) Sometimes a surface low-pressure center forms along the dryline bulge, enhancing moisture convergence and increasing the chances for storm development.

A dryline-frontal intersection is also an area of enhanced moisture--and surface wind--convergence. The front can be cold or warm, or a thunderstorm outflow boundary. Storms that form at such an intersection are literally located in a narrow canyon that channels moisture and wind into the storm. Stationary or slow-moving cold fronts create a "point" for storm development at the intersection. In contrast, a rapidly moving cold front merging with a dryline frequently creates a "line" of storms or a squall line.


Tim's article is a must-read ... it explains not only how to forecast dryline storms, but also how to read the sky as you are chasing on days with storms that initiate along the dryline. I'm sure there are many other helpful papers out there - and as I read this I'm not sure if it specifically addresses what you are looking for in your question - but maybe some of the mets out there can help out a bit -
Maybe the question is more why the dryline bulges - as in why it moves faster to the east some areas than others - and can the bulges be anticipated. This might be more informative than what a drline bulge is - since we could most likely all recognize one if we saw it on a surface map analysis.

I guess I was assuming from Tim's description that the bulge forms in response to surface lows that show up along the line, causing a push from local winds which extend portions of the dryline out a bit ... I would really like to know how those surface lows can be forecast, though, if anyone has an idea - the SPC certainly seems to know how ...
Maybe the question is more why the dryline bulges - as in why it moves faster to the east some areas than others - and can the bulges be anticipated. This might be more informative than what a drline bulge is - since we could most likely all recognize one if we saw it on a surface map analysis.


C'mon Guru Glen, you have the answer, so spit it out already :wink:. I am interested as well, since dryline bulges are something I have never really studied or looked into (given my geographical location)...
As mentioned, it has a lot to do with the winds above the top of the boundary layer which mix horizontal momentum and drier air downward into the boundary layer through mechanical turbulence. Any area where the winds are higher will consequently see greater turbulence..drier air mixes down and the dryline is displaced further to the east over a small location. This is your dryline bulge. The increased horizontal momentum also leads to increased convergence along the boundary.
Lot's of good discussion here, however no one has put their finger on exactly what it is I'm asking - that is, exactly what the mechanism is for increased convergence and storm initiation at the bulge?

The increased horizontal momentum also leads to increased convergence along the boundary.

Why? Is it because this region of stronger southwesterly or westerly winds just above the BL (low/mid level jet crossing the DL at the bulge location) is getting lifted over the top of the dryline surface, which results in a localized area of rising air rooted ABOVE the moist layer? Does that mean that the cap breaks, and the updraft initially is rooted just ABOVE the moist layer, only to quickly become rooted in the BL. OR, does convection result at a location of increased convergence in the moist layer, which causes storm initiation initially rooted in the BL?

Also, what is the mechanism for increased surface convergence along the bulge, if the bulge is the result of increased mixing out of the moist layer at this location? Does this get back to the point I made in my original post, where I suggested the bulge results from a mesoscale area of increased UVV's that result from an upper-level disturbance (the case yesterday) or other source of forcing - this same mechanism would cause increased convergence at the surface - and convective initiation - will also cause increased mixing out of the moist layer.

- bill
Bill, I'm not sure if you know how the dryline "moves" through the day, so I'll just quickly explain a bit... Drylines often set up with westerly and southwesterly flow aloft (downsloping causes lee-side troughing, which, given a surface high to the east, bring southerly / return flow to the Plains). Given the sloping terrain in the plains (higher elevations west, lower elevations east), the moist air is usually quite a bit more shallow to the west than it is in the east. For example, if there's a dryline near AMA-LBB line, the moist layer is usually much shallower in Borger, Clarendon, or Perryton, than it is in Dallas or Houston. So, when the sun comes up and insolation commenses, the air near the sfc near the dryline heats up and begins to mix with the air above. As this happens (mechanical mixing), the westerly flow aloft is transported down to the surface. This is what causes the winds to veer behind the dryline. At any rate, that is BASICALLY how the dryline propagates through the day. If there is an area of enhanced westerly flow aloft, there could be enhanced westerly flow transported to the surface. If you have southeasterly winds to the east of the dryline, and southwesterly winds behind the dryilne, you're going to get some degree of convergence at the dryilne. If you have the enhanced westerly momentum transfer, you can end up with enhanced convergence and dryline propagation.

It's worth noting that I believe that soil type gradients can play a signficnt role in dryline propagation. If you have plowed, dusty fields (which would likely result in more insolation being converted to sensible heating) north of some corn fields (in which more insolation would be converted to latent heating rather than as much sensible heating relative to the plowed fields), you will likely have greater vertical mixing to the north than the south. This will cause the dryline to propagate more quickly to the north (over the plowed land) than to the south. There are places on the plains where you have a signficant change in land use character / soil type, which would seem to be able to cause these dryline bulges. Granted, dryline bulges occur usually on the mesoscale, so we're talking about a large-scale soil type / land use change, not like five acres of corn fields next to five acres of plowed fields.
Just a thought but can anyone trace the path of the heat from the large grass fire in s.w. oklahoma ? Does it trail to the bulge area? Could the heat have affected anything? It was shown on oun at one time on their radar I believe