Southern Turbulence, And The Betrayal Of Nerds

Walt from South Carolina sends a gift - a spreadsheet with temperature data:

Hey Marc: I just got back from a business trip to San Diego. Beautiful city; I can see why everybody wants to live there. I got window seats on the flights. It is amazing how much one can recognize on the ground if you watch closely, and keep track of where you are. On the way out, I recognized Deming, Las Cruces, Carlsbad, and even the WIPP site! On the way back, I saw Mexicali, Yuma, Magdalena, and the VLA. Also recognized a limestone quarry in northern Alabama which Kathleen and I visited in May. I'll tell you, the desert around and south of Yuma is one inhospitable-looking place. My guess is that it's the most desolate tract of land in North America, except for perhaps Death Valley.

On the way back, the 757 had screens behind every seat. You could find a channel which listed altitude & temperature. So, in keeping with my nerd roots, I followed the temps as we descended from 41,000 feet in Mississippi, to landing in Atlanta. Check out the attached graph. I'd appreciate any insights you could give.

Hi Walt: I'm having trouble with these numbers. Particularly with the temperatures above 10,000 feet - that whole second inversion - they seem just way too high, maybe by 20 degrees F, or more, compared to temperature soundings for the day for Shelby, AL. Some of the temperatures at the highest levels loosely-follow wet adiabatic conditions when plotted on a Skew-T Log-p diagram, which makes me wonder whether the temperature thermistor was covered with some kind of water, or maybe ice. Hard to say, though.

The most interesting part of your sounding is that second inversion - what causes it? But if it's an instrument artifact, then it becomes less compelling.

The Shelby, AL soundings suggest that it was dry, with little convection, and on the whole, a nice day to fly. - Marc

Observations that day for Shelby AL, at 10/7 12Z (6 a.m. CST), and 10/8 0Z (6 p.m. CST).

Skew T - Log p diagram for Shelby AL, at 10/7 12Z (6 a.m. CST).
Skew T - Log p diagram for Shelby AL, at 10/8 00Z (6 p.m. CST).

Walt replies:

I knew it!!! Those bastards!!! When I flew over from Italy in June, at 39000 ft, the temperature was about -50 or -60 F the whole way. On this trip, they reported it at -30 to -35 F at 39K and 41K feet everywhere from CA to MS. If you can't trust data, what can you trust?

But that second inversion is interesting. I bet it could be real, because there were two periods of turbulence. Do two inversions happen often?

On most afternoons, that first inversion is almost always there - lower-level warmth comes from solar heating. The second inversion is a feature of high pressure systems, where air descends slowly from higher elevations, and warms up by adiabatic heating.

The weather maps for that afternoon (about 4 hours after your flight) show a rather striking upper-level, cold-core cutoff low centered over South Carolina. The low washes out at lower elevations (that's how one can tell it's cold-core), so at the surface, it's all but indistinguishable. At the surface, the entire eastern U.S., including the entire South, was under the influence of a high pressure system centered over New York.

What that means is that there was a significant wind speed gradient. Winds from the north over Alabama and vicinity were much faster at higher elevations than at lower elevations, meaning there was a lot of potential for turbulence.

It's plausible that there was descending air at high elevations over Alabama and vicinity, given that Alabama was on the western (upstream) side of the upper-level low, but it's not assured, since the wind speeds are not equal around the low. Also, the low was moving in a somewhat retrograde fashion (moving to the SSW rather than to the east). The temperature sounding shows a somewhat equivocal isothermal zone above the first inversion. Probably there were a lot of transient waves passing through this zone, so the situation could change noticeably from hour-to-hour, sometimes air rising, sometimes descending, and not always in synchrony from one level to the next, had one been around to measure it.

Turbulence acts in funny ways. Over mountain ranges, it can propagate forward and up, so flying over central or eastern Nevada, you can intercept Sierra Nevada-triggered turbulence, even though the mountains are many miles away. It can also propagate through atmospheric layers defined by inversion layers, sometimes long ways. The turbulence you noted was likely at the boundaries of different atmospheric layers defined by temperature inversions, or at least different temperature properties. There was certainly the potential for it, that day, given the presence of the upper-level low and the wind speed gradient, and despite the fact that elsewhere, it was A VERY NICE DAY.