May 27, 2009

5/27/09: The Pattern and More

I learned a valuable lesson the last few days of my trip (I am sitting here at DIA waiting for my plane as I type this). I realize that most severe storm researchers here are looking ahead to a possible pattern shift, and what's happened in Colorado the last few days may not seem important.

But that Commerce City storm three days ago, and the storm southwest of Limon were eye openers for me. Those were not "landspout" storms (in the sense of a non-supercell thunderstorm just spinning up vorticity of the boundary layer). These were storms that formed initially in an environment that should not have supported supercells in the sense of the deep layer shear. Yes, yes, we are all used to pontificating about storm motions off the hodgraph and how you can get supercells in otherwise unfavorable environments. We all use the term "mesoscale accidents" to refer to storms that become supercellular out here without the obvious shear signatures in the morning's hodographs and wind profiles.

But even though I know that, and all severe storms researchers know that, I doubt anyone chasing Colorado those the Denver Convergence and Vorticity Zone (DCVZ) days over the weekend were thinking "...oh, I really think some mesoscale accidents will happen and, forget single cells spinning up a spout, we are really going to see some good structure..." Maybe someone did. I didn't. All that I know who were out here were looking for landspouts, no more, no less.

Frankly, those two days felt like "chase" days to me in the traditional sense. Thom and Scott and I were chasing supercells, with the updrafts in the correct locations with respect to the forward flank, with RFDs, etc. You know what, those two days allow me to leave the Plains feeling as if this trip was somewhat successful. Scott and I may write up something on the DIA storm on 24 May because of this.

But there's something odd about this pattern that we are in. Perhaps it relates to global warming, perhaps not.

I think the consensus from the linked ocean-atmosphere models used to project climate trends into the late 21st century show that most of the "warming" will take place in the upper middle and polar latitudes. If that's the case, the meridional temperature gradient will decrease a bit at the same time that the mean temperatures of the whole shabang go up.

Decreasing the pole-equator temperature gradient USUALLY means higher wave numbers (more long wave meanders). This suggests that whatever controls external to the atmosphere are at play (say SST patterns) might act to anchor these meanders. Suppose one "anchor" was a trough on the West Coast....that had a response a ridge over the Plains downstream. Then we'd have a season or a period like the one we've just experienced.

Suppose another time the anchor was in a different location, so that a mean trough sets up over the Rockies. That's what I meant really. I am not trying to relate mean temperatures to lapse rates or anything like that.

All other things being equal, the latter would result in a "synoptically-evident" pattern for severe weather in the Plains. And that could go on for weeks, or even months.

As far as lapse rates go, if the heating is equally distributed through the troposphere's depth, then the environmental lapse rate won't change much. However, the higher mixing ratios mean that when storms do go they would go with more CAPE. Hence, maybe not more storms, but storms, when they occur, that would be more violent. Who knows about how the lapse rates will really be affected, though.

There is much to ponder here for severe storms meteorologists and climate change atmospheric scientists.