Sources of the EML

[Dan Robinson]

I watched Gabe Garfield’s excellent new videos on tornado forecasting and was interested to learn that I’ve apparently been mistaken for most of my chase career about where the EML (elevated mixed layer or colloquially, the cap) originates. In Gabe’s talk, he cites downsloping-driven warming/drying in the lee of the Rockies as a primary source of the EML. I was always under the impression that the EML was advected from the dry and warm higher-elevations in the desert Southwest. A common term in forecasting is “EML source region” which I’d always understood as the desert Southwest/Mexican mountains. Does that source region actually instead mean the lee side of the Rockies?

 

[gdlewen]

I watched Gabe Garfield’s excellent new videos on tornado forecasting and was interested to learn that I’ve apparently been mistaken for most of my chase career about where the EML (elevated mixed layer or colloquially, the cap) originates. In Gabe’s talk, he cites downsloping-driven warming/drying in the lee of the Rockies as a primary source of the EML. I was always under the impression that the EML was advected from the dry and warm higher-elevations in the desert Southwest. A common term in forecasting is “EML source region” which I’d always understood as the desert Southwest/Mexican mountains. Does that source region actually instead mean the lee side of the Rockies?

I'll watch the video, but there is a raft of literature supporting the theory that the source region for the air mass that forms the EML, as you have said, is the desert Southwest/Mexico.

This is a good starting place:

Elevated Mixed Layers in the Regional Severe Storm Environment: Conceptual Model and Case Studies

Abstract A conceptual model of the evolution of the severe local storm environment is discussed in conjunction with three case studies drawn from data obtained during the 1979 SESAME field program. The conceptual model describes how a particular configuration of topography and air flow can...

journals.ametsoc.org

There is a lot of stuff after this (and some preceding it, such as a Tellus paper by Carlson and Ludlum in 1968), but I think there's a lot of value in starting with the early papers and "reading through the history" of theory on subjects when you're a student but not in the classroom (essentially my situation.)

 

[gdlewen]

So here is how I view the process. (Obviously the literature does a better job of explaining this but we just went through this event so it's somewhat relevant and definitely useful as an exercise.)

On 3/14/2024, at 12Z, there was a strong inversion at the base of the EML. (I think the word used by meteorologists in place of strong is "stout", which always makes me think of beer or Friar Tuck.)

Here's the 12Z OUN sounding (it includes my analysis of Carlson's Lid Strength Index, which is preliminary as I work out the bugs, but it's a quick and easy graphic to use since I have it already on my HD.) The sounding is classified as a Lid-ERL because I haven't automated the detection of the top of the EML, so the static stability of the layer above the inversion just misses Carlson's threshold of 4.5˚C/100mb. (But it's an EML.)


The potential temperature just above the EML base is about 311K, and just below the EML base it is about 300K. So if we look at isentropic surfaces through those values, we should be able to demonstrate the homogeneity of the EML as well as it's source region.

	Along the 311K surface (in the EML), winds are generally SW and the humidity is very low. It​ seems obvious that the EML has its origin over the Desert SW/MX.
	For comparison: Just below the EML base the air mass contrast is very high. The base of the EML inversion ranges from about 900mb to 850mb ahead of the dryline.

Anyone who wants to chime in on the region of slightly-higher specific humidities over OK/TX (in the EML) that would be great. I get that, along the front, ascent represents a diabatic process than can push moisture across isentropic surfaces, so the ribbon of slightly-higher humidities along the position of the surface front makes sense. Not quite so clear about the area E of the dryline in OK/TX.