Wednesday, December 14, 2011

A warm winter rain with isentropic lift

Today an upper-level shortwave moving across the Great Plains (as seen in this 500 mb map)...

 ...and associated lift with this feature is helping to bring lots of rainy weather to much of the central part of the country.  A surface low is trying to develop along the baroclinic zone (a zone of horizontal temperature gradients) that is interacting with the jet streak aloft.  Here's the radar and surface analysis from late this morning:
Areas of rain with embedded thunderstorms.  This is somewhat unique for this time of year (though not completely unheard of), particularly in the upper midwest.  Usually we'd be expecting snow by mid-December--not heavy rain.  But, temperatures are far from snow-producing with this weather event.  Here's the GFS forecast surface temperature map for 18Z this morning:
Notice that there is a large, expansive warm sector associated with this developing cyclone--temperatures in the 40s stretch as far north as northern Illinois and Indiana, and east all the way through Virginia and Maryland.  Even up into Minnesota we're still well into the 40s at the surface--much too warm for any snow.

The horizontal temperature gradients are not to strong with this cyclone as of yet, and that may be helping to slow its development.  It looks like there's a cold front trying to form back across Nebraska and western Kansas, but the air behind it really isn't that cold.  Furthermore, the winds behind the front have a strong westerly component, meaning they are coming off the high elevations of the Rockies and the high plains and down to lower elevations further east.  This sinking motion of the air will cause it to warm, further weakening any developing temperature gradients.  So, in short, this storm doesn't look like it's going much of anywhere on the cold air side.

The temperatures aloft also don't look very conducive for snow formation.  One parameter we often look at to get a quick first guess if it's going to snow or not is something called thickness charts, specifically with reference to a "critical thickness".  I wrote a blog post about this a while back, and you can read that here.  Basically, as the atmosphere gets warmer, air expands and the "thickness" between two pressure levels increases.  The opposite happens when the atmosphere gets colder--the thickness decreases.  Here's a map of the GFS 6-hour forecast of 1000mb-500mb thickness for 18Z today:
Thicknesses are shown in the blue and red dashed lines.  The blue solid line is the 5400 meter thickness line--often used as a "critical" thickness value.  North of this line where the thicknesses are lower, the lower atmosphere tends to be cold enough to support snow.  South of this line where thicknesses are higher, the lower atmosphere tends to be too warm to produce snow.  This doesn't always hold true, but it's often a good guess.  You can see here that the "critical" thickness line is well to the north of the areas of precipitation--back across the northern plains and up through Lake Superior.  This is just another way of seeing how expansive that warm sector of this wave has become.

With all this warm advection to the east of the shortwave aloft, it would make sense that we're getting lots of clouds and rain, if only we could prove that there was rising motion going on.  Lots of warmth and moisture aren't enough by themselves to cause clouds and precipitation--we need some mechanism to lift that air so it cools to its dewpoint and starts condensing into those clouds and precipitation.

One peculiar thing about air motions in the atmosphere is that often the air tends to follow "adiabatic" or "isentropic" surfaces.  As long as a parcel of air moves along an adiabatic surface, it does not gain or lose any energy.  This is one reason air tends to follow these surfaces--it takes very little effort to do so.  So, if the isentropic surface happens to be tilted upward and the winds are blowing air along this upward-tilted isentropic surface, the air will naturally want to rise along the surface.  This can be a source of widespread lifting motion and lead to lots of clouds and precipitation.

Here's a map of one particular isentropic surface this morning--the 295 Kelvin isentropic surface.  The blue contours are the height of the surface above the ground in terms of pressure level.  This means that the lower the numbers get, the higher the isentropic surface is above the ground.  Also shown are the winds along this surface and the theta-e (a way of looking at moisture and temperature) of the air at this level.
We see, following the blue contours, that this particular isentropic surface tilts upward to the north.  In south Texas, this isentropic surface is at the 950mb pressure level.  It's up at the 900mb pressure level by central Oklahoma, then rapidly rises through the central plains and upper midwest to 750 mb by central Wisconsin back towards central Colorado.  Furthermore, look at the winds on this level from Texas up through the central plains and into the upper midwest.  They're blowing generally from south to north.  Also, high theta-e values (the green shading) indicate very moist air.

So what does this mean?  We have southerly winds pushing moist air up along a surface that tilts upward as it goes further north.  So, as air parcels are pushed northward by these winds, they rise along with the isentropic surface (until they become saturated).  As this air rises, it will cool until the dewpoint is reached and clouds and precipitation start to form.  So, on this map, we're seeing a strong lifting mechanism from Texas up through the central plains and into the Mississippi River valley and the midwest.  We call this "isentropic lift".   And this explains a lot of why in the infrared satellite image...

...we see a big swath of clouds in that exact same region.  Air is being pushed northward along a surface that tilts upward in that direction.  So the air rises right along with it.  The result?  A widespread area of clouds and precipitation anywhere we're seeing strong isentropic lift.

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