Why has the vertical instability in the Caribbean been low

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Dean_175
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Why has the vertical instability in the Caribbean been low

#1 Postby Dean_175 » Wed Oct 19, 2011 7:28 pm

Why has the vertical instability in the Caribbean and tropical Atlantic been lower than average for the entire year?
http://www.ssd.noaa.gov/PS/TROP/TCFP/atlantic.html

Also, vertical instability is obviously an atmospheric phenomenon. So wouldn't one expect that it would change very rapidly because the weather changes rapidly? Its not like sea surface temperature anomalies, which one would expect to remain fairly unchanged over the span of weeks or months (the high amount of energy it takes to heat or cool water). But the graph clearly shows that the vertical instability over both the tropical atlantic and Caribbean has been below average for the whole year. Could it be due to positions and intensities of pressure systems such as Bermuda high?
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Re: Why has the vertical instability in the Caribbean been low

#2 Postby AJC3 » Wed Oct 19, 2011 8:45 pm

Dean_175 wrote:Why has the vertical instability in the Caribbean and tropical Atlantic been lower than average for the entire year?
http://www.ssd.noaa.gov/PS/TROP/TCFP/atlantic.html

Also, vertical instability is obviously an atmospheric phenomenon. So wouldn't one expect that it would change very rapidly because the weather changes rapidly? Its not like sea surface temperature anomalies, which one would expect to remain fairly unchanged over the span of weeks or months (the high amount of energy it takes to heat or cool water). But the graph clearly shows that the vertical instability over both the tropical atlantic and Caribbean has been below average for the whole year. Could it be due to positions and intensities of pressure systems such as Bermuda high?


First, let me point out that vertical instability was well below climatology all season throughout the entire Atlantic basin, including the subtropical Atlantic....
http://www.ssd.noaa.gov/PS/TROP/TCFP/atlantic.html

Next, let's look at how the parameter is calculated...
http://rammb.cira.colostate.edu/project ... enesis.asp

VERTICAL INSTABILITY: The vertical average temperature difference between the equivalent potential temperature of a parcel lifted from the surface to 200 hPa, and the saturation equivalent potential temperature of the environment, for each 5 by 5 degree area.

Essentially there are two causes that would reduce the vertically averaged temperature difference in the basin:

1) Warmer than normal mid level temperatures
2) Drier than normal mid level air

Either one would serve to cap instability. My money is one number 1...for the reasons cited in the following post.

viewtopic.php?f=54&t=111917&p=2198256&hilit=WVBT#p2198256

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Re: Why has the vertical instability in the Caribbean been low

#3 Postby Ptarmigan » Sat Oct 22, 2011 4:47 pm

I think it has to do with the air being drier than normal in the upper atmosphere.

viewtopic.php?f=59&t=112106&start=120

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#4 Postby brunota2003 » Sat Oct 22, 2011 10:10 pm

Why not a combination of both?

If the air is warmer than normal, it would mean that the air can hold more water vapor than normal...so by raising the temperature, but not the levels of moisture, wouldn't that also create drier than normal air? Thus causing even stronger of a cap.

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Re:

#5 Postby Ptarmigan » Sun Oct 23, 2011 12:26 pm

brunota2003 wrote:Why not a combination of both?

If the air is warmer than normal, it would mean that the air can hold more water vapor than normal...so by raising the temperature, but not the levels of moisture, wouldn't that also create drier than normal air? Thus causing even stronger of a cap.


Good point. I did note that the upper level is warmer even in active seasons, but the air is wetter than right now. A combination of drier and warmer air suppresses thunderstorms, which leads to a cap.

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#6 Postby AJC3 » Sun Oct 30, 2011 11:07 am

My contention remains that if the primary cause was drier than normal mid level tropospheric air, then the signal would have mainfested itself in the WVBT. Not only did this not occur, but the opposite actually held true for the most part - on average, WVBTs were colder than nomral, an indication of greater than normal mid level tropospheric moisture. This argues for warmer than normal mid tropospheric air being the root cause of vertical instability being below normal this year. It will be interesting to see if any rigorous post-mortem is done toward this end.

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Re: Why has the vertical instability in the Caribbean been low

#7 Postby Sanibel » Mon Oct 31, 2011 11:17 am

So, would that be a symptom of warming of the atmosphere on an increasing basis in line with Global Warming?

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#8 Postby brunota2003 » Mon Oct 31, 2011 8:33 pm

Or it could be the mid-level temps were the same, but surface temps were actually cooler...but I don't think temps were cooler than average over the entire world

Thinking about it, I think our instability "averages" are too high. If data only goes back to 2000, and we've been in the upside cycle of hurricanes, of course the "averages" will be too high, because it does not incorporate the downside cycle. It has been over a decade since we entered the "up" cycle, perhaps we are starting to swing back into the "down" cycle?

If the times from 2000 - 2010, the vertical instability was "100", and that is the only time period we have, then the "average" is "100". So of course getting a reading of "20" is going to look extremely odd. But if the 100 year average is "50", then that "20" doesn't look so odd in the long run.

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Re:

#9 Postby AJC3 » Mon Oct 31, 2011 10:10 pm

brunota2003 wrote:Or it could be the mid-level temps were the same, but surface temps were actually cooler...but I don't think temps were cooler than average over the entire world

Thinking about it, I think our instability "averages" are too high. If data only goes back to 2000, and we've been in the upside cycle of hurricanes, of course the "averages" will be too high, because it does not incorporate the downside cycle. It has been over a decade since we entered the "up" cycle, perhaps we are starting to swing back into the "down" cycle?

If the times from 2000 - 2010, the vertical instability was "100", and that is the only time period we have, then the "average" is "100". So of course getting a reading of "20" is going to look extremely odd. But if the 100 year average is "50", then that "20" doesn't look so odd in the long run.



Cooler surface temperatures being a factor is highly unlikely. Since we're talking about a basin comprised almost entirely of water, the low level temperature anomalies will invariably mimic SSTAs, which were largely positive.


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