Saturday, June 4, 2011

Sometimes it takes some effort to keep storms going...

In my last post I talked about how it looked like this weekend would be rather quiet weather-wise across the eastern half of the country due to hints at a building ridge across the region.  Well, this has turned out to be partially correct.  A surface high-pressure ridge has built across the southern US with a secondary ridge maximum further north in Canada.  There's week troughing in between these two pressure maxima, and we're seeing storms fire in that area tonight.

Fig 1 -- Sea-level pressure (contours) and composite radar at 330Z, June 5, 2011.
The strongest areas of storms is moving through the Indiana and Ohio area late this evening.  A glance at the 00Z sounding from southern Ohio shows good instability with moderately steep lapse rates and high CAPE, so the thermodynamic environment is certainly favorable for storms.
Fig 2 -- KILN sounding from 00Z, June 5, 2011.
However, like I said earlier, looking above the surface there is large-scale ridging that has built aloft across the eastern US, which we would usually think of as not conducive to storms.  The upper-level jet is rather weak, at least with how it's sampled at 300mb.  The color shading of winds on the image below shows a jet streak earlier this evening across Minnesota and the upper peninsula of Michigan at 00Z when the storms were moving through central Illinois.  Though the color shading shows a jet streak further north, I circled an area where you can see wind barbs indicating winds over 50 knots surrounded by lower-speed winds further south.  We could consider this area part of the jet streak, in which case the area where the storms were located in central Illinois would be under the right entrance region of the jet streak.  According to the four quadrant model, that would be an area of divergence aloft, supporting upward vertical motion.
Fig 3 -- Analysis of 300mb winds (colors and barbs) and heights (contours) at 00Z, June 5, 2011.
So we could construe a case for why we are seeing stronger storms there--there is weak upper-level support.  Very weak support.  But that jet has looked to be strengthening a bit, which has allowed storms to actually increase in coverage as the night has worn on.  What is causing this?

It goes back to those good old thermal wind arguments.  Remember that temperature gradients at the lower-levels influence the winds aloft through a mechanism called the thermal wind.  If we look at a surface map from 0300Z this evening, we can see that there is indeed a temperature gradient with warm air to the south and cooler air to the north between those two high pressure centers I talked about before.  In meteorologist-speak, a region like this where there are horizontal temperature gradients is called a baroclinic zone.
Fig 4 -- Surface analysis of sea-level pressure (contours), temperatures (colors) and winds (barbs)  at 0300Z, June 5, 2011.
I outlined the baroclinic zone with a red box in the map above.  You can see that in that region, temperature drops from warm in the south to cooler in the north.  Coincidentally, that's kind of where we're seeing our jet streak above--at least on the western end of the baroclinic zone.  Remember--the thermal wind relation means winds aloft are affected by temperature gradients below.

I mentioned that the jet aloft seemed to be strengthening, which continued to provide limited upper-air support (divergence aloft) and allowed storms to continue overnight.  Can we see a reason for this?  The wind barbs are kind of sparse on the surface map above, so below is a hybrid map I got from the College of DuPage website.  It has a water vapor satellite image in the background and overlaid on top in yellow are surface wind streamlines.  Think of a streamline as a continuous arrow that always follows the wind direction at a given moment in time.  They're useful to illustrate how wind is flowing.
Fig 5 -- GOES water vapor imagery at 0245Z overlaid by surface wind streamlines at 0300Z, June 5, 2011.
The red box roughly outlines the same "baroclinic zone" region in the surface map above (it's somewhat difficult to pin down just where the strongest temperature gradients are).  Notice what the streamlines are doing in that area.  In the northern half of the box, the winds are generally coming out of the north, but then they split and go east and west.  Similarly, in the southern half of the box, the winds come from the south before also splitting to go east and west.  This sort of wind pattern is known as a deformation pattern or confluent flow.

Now consider the effect of this wind pattern on the baroclinic zone.  The southerly winds in the southern half of the box are pushing the warm air to the south further north.  At the same time, the northerly winds in the northern half of the box are pushing the cooler air to the north further south.  Can you see how, with winds like that, it would work to strengthen the temperature gradient?

One fun feature of a confluent wind pattern like that is that there can be very little convergence of the winds--they don't just run straight into each other and stop.  If we had convergent winds at the surface, we know that where winds converge that tends to force air to rise--and if we had rising air we'd expect to see storms or lots of clouds or some evidence of that.  We can see that there is some wispy light white color on the water vapor image along and just north of that confluent zone where the winds are coming together out on the western end of the confluent region.  These somewhat light gray return indicating some enhanced water vapor being lifted into the upper troposphere.  So, there must be a little bit of convergence near the surface there forcing the rising motion.  But the fact that we don't see lots of storms popping up in that area indicates that the convergence is probably pretty weak (there's also a weak capping inversion on some soundings in that area which is working against rising motion).  The flow is probably mostly confluent.  To the east we can see that there's a very bright white area over Indiana and Ohio, indicating lots of water vapor in the upper troposphere.  That points to strong vertical motion lofting all that water vapor up there.  But that makes sense--that's where the storms are.  To support all that rising motion, there must be some good convergence at the surface, so the surface wind field is probably more convergent there.

Remember, confluent flow works to increase the temperature gradient.  This is a form of what is known as frontogenesis--the creation of a front.  And, because of the thermal wind relation, that increasing temperature gradient should lead to an increase in the strength of the jet above.  So--it looks like surface wind patterns are helping to keep the upper-level jet strong, which in turn is helping storms to sustain themselves over Ohio.  Pretty impressive for what otherwise would be a rather "boring" ridge...

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