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 , 1:20 PM GMT on July 24, 2013
One of the common explanations that I often hear on television for the development of severe thunderstorms along or just ahead of a cold front is the "clash of warm and cold air." This oversimplification has always annoyed me because such "clashes" often exist without a peep of thunder or a flash of lightning.
There's probably not a better example of the frequent "clash" between air masses along the West Coast of the United States, where cool Pacific air (a.k.a., the marine layer) "meets" hot air over the interior during summer. Consider, for example, the 23Z surface station models (below) over northern California on Friday afternoon, July 19, 2013 (read more about station models).
The 23Z surface station models (read more) over California on July 19, 2013 (23Z corresponds to 5 P.M. PDT). Courtesy of Penn State.
To get your time-zone bearings, 23Z is 5 P.M. Pacific Daylight Time. Generally speaking, temperatures along the northern California Coast were generally in the 50's and 60's while the mercury soared toward and above 100 degrees not all that far inland from the coast.
Although the coastal mountains of California act as an obstruction to the inland advance of cooler maritime air, the two contrasting air masses "rubbed shoulders" in the lowest few thousand feet above the earth's surface. To see the vertical juxtaposition of the two air masses, check out (below; unannotated image) the 12Z skew-T (review this topic) at Oakland, CA, on July 19.
The 12Z temperature (red) and dew-point (green) soundings at Oakland, California, on July 19, 2013 (12Z corresponds to 5 A.M. PDT). At the time, surface temperatures were in the 50s while 900-mb temperatures were close to 80 degrees (26 degrees Celsius equals 79 degrees Fahrenheit). For the record, the height of 925 mb over KOAK was 781 meters. Original image. Courtesy of Penn State.
Note the dramatic temperature inversion from 950 mb to 925 mb (roughly from altitudes between 500 and 800 meters; see the raw data from the 12Z radiosonde at KOAK). I also note that there was a deck of low clouds at altitudes between roughly 200 meters and 450 meters, the layer of air where the relative humidity was 100% (the temperature and dew point were essentially equal; revisit the raw radiosonde data for confirmation).
This inversion was so strong that temperatures were in the 50s near sea level, while, in the nearby hills around the Bay area, temperatures were just shy of 80 degrees. In a nutshell, temperatures increased almost 30 degrees Fahrenheit in roughly the first 1000 feet above the ground. In my view, this juxtaposition qualifies as a "clash" between air masses. If not, then just put me out to pasture and let me graze for the rest of my twilight years.
Yet this clash between air masses (cool Pacific air and hot air over the interior of California) didn't initiate any storms. For confirmation, check out the 23Z mosaic of composite radar reflectivity, which indicates that the entire state of California was bereft of showers or thunderstorms. As an interesting aside, note, on the 23Z radar image, the thunderstorms over Arizona, which developed in concert with the Southwest monsoon and the nomadic 500-mb low that I talked about in my last blog (to follow the path of this 500-mb low, go to this WPC's Web page and start clicking on "Previous Day").
Why weren't there any thunderstorms over California on Friday, July 19? Referring to the 12Z skew-T at Oakland, CA, the vertical juxtaposition of the air masses...cool near the ground and much warmer aloft...created a strong temperature inversion, thereby suppressing any surface-based convection. In other words, air parcels rising from the earth's surface and cooling during their ascent never could become warmer than their immediate surroundings, thus preventing parcels from becoming positively buoyant through a deep layer of the troposphere. In effect, air parcels originating from the earth's surface could not reach a level of free (and deep) convection. For the record, the very warm air at 925 mb was the footprint of an area of high pressure aloft...check out the 12Z NAM model analysis of 500-mb heights below.
The NAM model analysis of 500-mb heights at 12Z on July 19, 2013. Height contours are drawn every 30 meters in order to identify the closed warm area of high pressure centered over the West. The 500-mb low is the same low that cut-off over the Middle Atlantic States last week and drifted southwestward over Texas and then to the position indicated by the blue "L." Courtesy of Penn State.
The moral of my short story is that the "clashing of air masses" that many television weathercasters fall back on to "explain" the initiation of severe thunderstorms is not scientifically sound, in my opinion. Furthermore, this explanation suggests that severe thunderstorms cannot develop away from fronts (where the "clash" of air masses takes place), which, of course, is utter nonsense. Indeed, severe thunderstorms often occur in the warm sector of a mid-latitude low (example) or on the cool side of a warm or stationary front within 300 hundred kilometers of the fronts. This means that severe thunderstorms can erupt more than 150 miles away from a low's warm or cold front...no where near where the "clash" of air masses takes place.
I'm not done yet. The irony of using the word, "clash," in my opinion, suggests something violent must happen near the earth's surface in order to initiate tornadic thunderstorms (the expression "clash of air masses" really pervades the airways when big outbreaks of tornadoes are slated to occur). And yet, discrete or semi-discrete supercells with the capability of spawning tornadoes typically develop in areas where low-level lifting is relatively weak. Yes, I meant what I wrote..."relatively weak." Indeed, strong lifting along a cold front, for example, typically results in a line of thunderstorms whose primary threat is usually straight-line wind gusts (not tornadoes). Contrary to the impression lent by using "the clash of air masses," discrete supercells often form well ahead of a cold front (within the warm sector of a mid-latitude cyclone), where weak areas of low-level convergence can still get air parcels to the level of free convection (pre-frontal troughs, areas of subtle confluence of surface streamlines, etc.).
The bottom line here is that I never liked this explanation (gee, Lee, I really couldn't tell) :-). Yet I seem to hear this explanation more and more on television. Maybe it's just time for "old schoolers" like me to just grin and bear it.
I usually am ignored when I have these kinds of clashes with the media.
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