The Truth about the "Baroclinic Leaf"

 The media has been going a bit crazy about a very common and usually benign meteorological phenomenon.   

A weather satellite feature called a baroclinic leaf--a thickening of the jet-stream clouds that is associated with the incipient development of midlatitude low-pressure centers.

This tempest in a teapot all started with a tweet by the National Weather Service on Tuesday that mentioned the baroclinic leaf (see below).  The purple arrow points to the feature, which is characterized by a curved mass of clouds.   

 Some folks think it looks like a leaf.  I am not convinced.

 

Below is an infrared satellite image showing the baroclinic leaf at a slightly different time (I put a red oval around it).

We see such features over the Pacific dozens of times each autumn and winter.  The term "baroclinic" indicates it is associated with a horizontal temperature gradient (or change with distance).   A well-known meteorological term.

As noted above, the media went silly about this run-of-the-mill feature.  The Seattle Times incorrectly called it a "rare cloud pattern"

KING-TV talked about a disruption in the blending of warm and cold fronts, whatever that means.

And the Seattle Stranger, never a place to get reliable weather information, said the  leaf was "uniquely moist" and "pretty."

Baroclinic Leaf 101

I teach weather satellite interpretation in several classes and often describe the details of the baroclinic leaf.  Let me tell you about it and what it tells us about the development of midlatitude storms from space.

Below is a schematic showing the development of a typical midlatitude cyclone. The shaded areas indicate clouds, the solid lines show heights (pressures) at mid-levels in the atmosphere (around 500 hPa, 18,000 ft).  The fronts are shown in blue, the surface low center with an "L" and the core of the jet stream (the current of strong westerly winds in the midlatitudes) by the dark solid lines.

As a weather system develops, we start with a rather linear cloud feature of near-uniform width.  As the system revs up, the clouds get curved and distorted, with the southern part being narrower.  This is the baroclinic leaf stage.

And as the low center strengthens, the cloud field transforms into a "comma"--lower right box.

Here is another rendition of the leaf (left panel) and comma (right) stages.

As I noted before, I could show you dozens of examples of baroclinic leaves in satellite imagery---they are not rare or unusual.   But if you are a meteorologist trying to find evidence of new mid-latitude cyclone, you should be on the lookout for the classic leaf signature.