Topography and tornadogenesis

[Wesley Carter]

First of all I would like to say I'm honored to be accepted into the Stormtrack community. I have been reading this forum for a couple of years and have learned a lot from you all and hope to contibute something positive over time.

In the past couple of years I have been studying severe weather and have come across articles in the AMS Journal and research papers discussing the effects of topography on storm development. The one I just read places a lot of importance on soil mositure for dryline storm development. It alludes to an "inland sea breeze" mechanism providing the needed third ingredient of lift. It made me think of tropical storm Erin last year, which if you recall made landfall in Texas and deteriorated until the remnants passed over rain-soaked Oklahoma. Erin then re-intensified to tropical storm stregnth again and even reformed an eyewall.

The question I pose is this: Do you all think that water impoundments and irrigation affect the development of thunderstorms and therefore by default the number of tornadoes? And more importantly, should these play any part in picking a chase target area with all other factors being similar?

Here are links to the article I am referring to in my question:

http://ams.allenpress.com/perlserv/?request=get-document&issn=1520-0493&volume=128&issue=07&page=2165&ct=1

[Blake Michaleski]

An area being heavily irrigated that would normally dry certainly does contribute to boundary layer moisture. The crops or whatever releases this moisture from the ground through transpiration. A rain soaked region contributes to boundary layer moisture through evaporation (the same way the GOM does). These two processes are collectively known as evapotranspiration. While difficult to measure evaporation and transpiration rates, dewpoint change is a good proxy. over the rain soaked region, under heavy incoming solar radiation, evaporation will be significant, leading to locally high dewpoints relative to the surrounding region. This can lead to a storm environment characterized by lower LCLs and effectively more surface based CAPE.

The correlation between LCL height and tornadogenesis has been researched and seeming well established. See Robert Prentice's Reply to a topic on this just a few days ago. He really does a great job explaining it so no need fo rme to repeat. http://stormtrack.org/forum/showthread.php?t=16547

Also something to note, along the borders of these irrigated regions there can likely be a vast difference in vegetation, this can cause the land to heat differently during the day. This typically creates what is known as a differential heating boundary which can further serve to increase surface convergence (possibly leading to initiation) and can enhance 0-1 storm-relative helicity. These boundaries also occur along the edge of a rain soaked region (like where a lone supercell tracked the day before). And just to throw it out there, cloud shields can cause differential heating as well. There is a good bit of research going on now at UAH on convective initiation concerning this type of boundary.


So yes... localized regions of higher soil moinsture can affect tornado potential.

[Nick Dawson]

How about a flooded area?

Do you think that all of the extra water around here would be enough to possibly raise the dewpoint noticeably? I'm in Norhtwest Missouri, where every river has at least minor flooding.

I'm thinking that Sunday night and into Monday that at least strong storms will rumble through the NW MO, SW IA, and SE NE area. With the ground being saturated, crops beginning to grow (transpiration), and the excess of flood water make a difference to storm development? Possibly initiating storms earlier in the day?

Now that I'm aware of the situation I'm ready to look at what happens throughout Sunday evening and overnight.

[Bobby Prentice]

Do you all think that water impoundments and irrigation affect the development of thunderstorms and therefore by default the number of tornadoes?

Thunderstorms...yes. In general, the larger/deeper the body of water the more significant the impact. For example, sea breeze (http://amsglossary.allenpress.com/glossary/search?id=sea-breeze1) and lake breeze (http://amsglossary.allenpress.com/glossary/search?p=1&query=lake+breeze&submit=Search) circulations are lifting mechanisms which help produce thunderstorms all the time. However, there aren't large bodies of water (e.g. Lake Michigan) over the central US. Mesoscale soil moisture/crop differences can play a part, but their impact is not well understood or easily forecastable.

Tornadoes...probably, but this is a complex, indirect, and poorly understood relationship. Heck, science still does not understand how supercell tornadogenesis works. That is why the VORTEX II experiment is scheduled for 2009 and 2010.

And more importantly, should these play any part in picking a chase target area with all other factors being similar?

This is problematic. There aren't any large bodies of water (e.g. Lake Michigan) over Tornado Alley (http://members.cox.net/rprentice/images/other/toralley.gif) / Chase Alley (http://members.cox.net/rprentice/images/other/Chasealley.gif). Small reservoirs are generally too small to make much of a difference. Mesoscale soil moisture/crop differences can play a part, but their impact is not well understood or easily forecastable.

[Wesley Carter]

Thanks for the replies. The topic of topography and tornadogenesis has intrigued me for a while because here in Middle TN tornadoes tend to prefer to touch down in certain areas more than others. One place they tend to touch down is the county in which I live. So I am trying to read all I can on the topic.

Nick, I am probably not qualified to give a definitive answer to your question but the paper I referenced in my original post addresses your question to a degree. It does appear that flooded areas and saturated ground can affect storm development. I'm not really sure that floodwaters downstream from where the precipitation occured would be enough to have a great impact though. I think the saturated ground plus all of the tributaries flooded in the area where the precipitation fell plays a bigger role. That is not based on anything I have read, I'm just deducing that so I definately stand to be corrected. Here is the link again: http://ams.allenpress.com/perlserv/?...page=2165&ct=1

If you find anything else online on the topic please feel free to reply to this or PM me, I am very interested in reading more. I'll do the same.

[David Wolfson]

I have somewhat of a contrarian view about evapotranspiration's effect on severe storms. At some level it seems "obvious" that more low-level moisture improves the environment for storms; however I think the factors involved make this a minor effect for the situations being discussed -- and even an inhibiting effect for storms triggered by diurnal heating.

Anecdotally it's pretty well accepted around these Arizona parts that significant summer monsoon precip one day tends to discourage storm development the following day. This appears to be so despite the fact that the intense sun heating and relatively dry (usually mid-60s dewpoints or less) air makes evaporation considerably more effective than in the Alley.

Over the course of a day evapotranspiration adds essentially no additional Theta-E potential storm energy to the atmosphere. The additional latent heat is balanced by diminished sensible heat. Only over many days is the air mass energy changed significantly by surface moisture.

The favorable profile for discrete severe storms is one with a "breakable" cap, i.e. where a temperature inversion inhibits clouds and convection until the combination of sufficient heating and dynamic forcing releases the potential storm energy. All things equal additional surface moisture should tend to increase low cloud coverage and decrease surface heating.

There is some statistical research that spring season precip positively affects both the frequency and locus (dryline location) of severe storms in the Alley. I think this is a different discussion; and one where the role of a sharpened dryline boundary plays a large part.

Now the role of cold-pool boundaries in storm development is pretty well established. Wide-scale precip or flooding almost certainly establishes boundaries that can be exploited by transient sources of dynamic forcing and forcing due to the boundary itself. Also, wide-scale evapotranspiration will increase the potential energy of an air mass over a multi-day time scale if the air mass hangs around over the same area. The typical severe storm situation I believe we're talking about here is highly advective; and the air mass is from a tropical source and doesn't linger very long over a given area.

So in summary I seriously doubt impoundments and irrigation (the question) play a significant role that can be exploited in picking a chase target. The existence of boundaries from wide-scale flooding, nocturnal MCSs, etc., should be noted and can be exploited. IMHO, FWIW.