Anthropogenic Global Warming to increase frequency and distribution of Severe Thunderstorms

[WXheights]

A new study supports increasing environmental conditions for the formation of more severe thunderstorms days over a wide area especially including The Northeast, New England, and adjacent Southeast Canada but also portions of the West coast and up into Canada.

 

This from what I can tell conflicts with other recent studies about less wind shear baroclinicity in a warming world and the questions of tornado frequency in the future.

 

Severe thunderstorms are one of the primary causes of catastrophic loss in the United States. However, the response of such storms to elevated greenhouse forcing has remained highly uncertain. We use an ensemble of global climate model experiments to probe the severe thunderstorm response. We
find that this ensemble exhibits robust increases in the occurrence of severe thunderstorm environments over the eastern United States. In addition, the simulated changes in the atmospheric environment indicate an increase in the number of days supportive of the spectrum of convective hazards, with
the suggestion of a possible increase in the number of days supportive of tornadic storms. Given current vulnerabilities, such increases imply increasing risk of thunderstorm-related damage if global warming continues.

 

http://www.pnas.org/content/early/2013/09/18/1307758110.full.pdf

 

 

[ORH_wxman]

It appears that the CMIP5 models pump up the surface moisture enough in their sims to offset the decrease in vertical wind shear...so we end up with a lot of high CAPE days enough to still produce severe thresholds even in the presence of decreasing wind shear.

[WXheights]

Less super cell and more pulse severe is how envision it but enter expanded tstm freq. out of season like late fall and winter early spring...New England storm chasing?

[CoastalWx]

I guess mid level warming is less irrelevant in these studies compared to one predicting less TC activity. Another in the list of possibilities I guess.

[ORH_wxman]

I guess mid level warming is less irrelevant in these studies compared to one predicting less TC activity. Another in the list of possibilities I guess.

 

 

The model has increased surface moisture to offset the decreased lapse rates, so it gives us higher CAPE in a warmer world.

[Jonger]

I love these statements.

 

Anyhow, would love to see this one pan out... T-storm activity is just a shell of its former self in my neck of the woods.

[f2tornado]

I guess mid level warming is less irrelevant in these studies compared to one predicting less TC activity. Another in the list of possibilities I guess.

 

The authors mention this through their convective inhibition (CIN) caveat and don't tackle it directly. The authors basically calculate the change in Craven SigSvr Parameter which is a product of CAPE and 0-6km shear and tell us the models say there will be more sweet spot days ignoring all else. The results are not a surprise since they ignored all else. The problem is we cannot ignore all else. The magnitude of change in CAPE is relatively small, on the order of 0-200 J/kg. That tidbit alone tells me whatever increase in severe events should be trivial. Further, we cannot ignore the cap which the quoted post mentions. Any bump in mid-level temps will act to suppress storms, potentially more than offsetting any additional CAPE. If you believe the desert southwest is gonna get real hot and even more dry in an AGW world then this air will form the EML (elevated mixed layer... the cap) in the Plains on east. Next, the authors only use 0-6km shear and ignore any change in the jet stream which is a big deal. A weaker jet removes some of the divergence aloft which is a lifting mechanism. The paper was a nice easy read and spoke my language but there is nothing really robust about it despite the title.

[CoastalWx]

The authors mention this through their convective inhibition (CIN) caveat and don't tackle it directly. The authors basically calculate the change in Craven SigSvr Parameter which is a product of CAPE and 0-6km shear and tell us the models say there will be more sweet spot days ignoring all else. The results are not a surprise since they ignored all else. The problem is we cannot ignore all else. The magnitude of change in CAPE is relatively small, on the order of 0-200 J/kg. That tidbit alone tells me whatever increase in severe events should be trivial. Further, we cannot ignore the cap which the quoted post mentions. Any bump in mid-level temps will act to suppress storms, potentially more than offsetting any additional CAPE. If you believe the desert southwest is gonna get real hot and even more dry in an AGW world then this air will form the EML (elevated mixed layer... the cap) in the Plains on east. Next, the authors only use 0-6km shear and ignore any change in the jet stream which is a big deal. A weaker jet removes some of the divergence aloft which is a lifting mechanism. The paper was a nice easy read and spoke my language but there is nothing really robust about it despite the title.

 

Yeah you hit on some of the factors I'm thinking of. Again, I'm no climo expert, but as a met...these are the things that I'm thinking of when factoring in warming. In any case, it's just another possible solution based off of GCMs. Perhaps the increase in CAPE overwhelms the cap, but I wonder how true this would be in real life.

[ORH_wxman]

Yeah you hit on some of the factors I'm thinking of. Again, I'm no climo expert, but as a met...these are the things that I'm thinking of when factoring in warming. In any case, it's just another possible solution based off of GCMs. Perhaps the increase in CAPE overwhelms the cap, but I wonder how true this would be in real life.

 

 

Relying on surface moisture increase to overcome shear decrease while ignoring mid-level capping seems a bit of a dubious set of variables to expect an increase in severe. Though if there was an increase, I envision it like WXheights does, more like an increase in pulse-type cells versus supercell outbreaks.

[f2tornado]

Relying on surface moisture increase to overcome shear decrease while ignoring mid-level capping seems a bit of a dubious set of variables to expect an increase in severe. Though if there was an increase, I envision it like WXheights does, more like an increase in pulse-type cells versus supercell outbreaks.

 

I agree. The authors did push the tornado threat however...

 

The first is that global warming results in a shift in the daily-scale CAPE/shear distribution toward a higher fraction of severe days that have high CAPE and strong low-level shear, with the increases in high CAPE being most concentrated in days with low convective inhibition. Our interpretation is of an increase in the number of days supportive of the spectrum of convective hazards (15), with the suggestion of a possible increase in the number of days supportive of tornadic storms (19).

 

It appears they rely on 0-1 km shear to substantiate this statement. While 0-1 km shear is certainly important for tornadogenesis, it's not gonna matter much for significant tornadoes in the absence of deeper layer shear usually needed to generate supercell thunderstorms. I won't completely dismiss the tornado connection though as an increase in 0-3 km CAPE with ample low level helicity should lead to more landspout type tornadoes.