Snowstorm Eye
Good day,
Believe it or not, extratropical storms can develop an "eye like" feature if intense enough and if the right conditions develop.
The picture (NOAA Satellite) above is a greyscale visible image of the same storm in question, the Blizzard of 2006 on Feb 12, 2006.
In the visible image, the elliptical "eye" feature is unmistakable (about 150 miles or so due east of the mouth of the Delaware Bay).
In this picture, for the Infrared channel, also from NASA, the higher (colder) clouds appear whiter and the lower clouds a darker scale of grey. The "eye" like feature can clearly be seen in the low deck of clouds (the ones producing heavy snow) in the same location (only not as dramatic as in the visible channel). The upper-air "dry slot" (jet stream level), where the high clouds are abscent, is actually right over the surface low center!
Anyway, such "eyes" have and do form in intense nor easters. The key here is the "cold core" convection, that is the low stratus and nimbostratus (steady heavy snow producing clouds), and possibly even some small low-topped cumulonimbus (thundersnow?) clouds as well.
This is undeniably caused by both the tremendous amount of lift in the atmosphere (cold air aloft, jet stream) but also very cold air moving over warmer water, causing cellular convection. Basically, we have an intense, and most likely occluded low at this point, and shallow cumulus type convection develops about the storm center.
In the center, the winds become lighter, despite its cold core characteristics, and lessens the spray kicked up off the sea surface (called the OAI or Ocean Air Interface) which causes evaporation and subsequent condensation to form the cellular clouds.
Remember, that such storms of this type are extratropical, and most energy "fueling" the storm is from differences in temperature (baroclinicity) and NOT convection ... However, cellular convection does exist, mainly when the storm is over water and colder air passes over the warmer water.
Examples of CELLULAR convection are clear in the image above. Note the "celllular" appearance of the clouds in the wake of the cold front (from the same storm) as it passes over the water, and warmer Gulf Stream current. Similar convection occurs over the Gulf of Mexico as well. Also, very interesting, note how the wind flow (from the NW) is DRY over land (such as FL) but quickly has cellular convection once offshore where evaporation occurs at the OAI as well as convection (known as CISK - Conditional Instability of a Second Kind) which is cold air over warm(er) water.
Now, getting back to the snowstorm (extratropical storm) with the "eye" ... The eye like feature is simply the center of low pressure where the winds become lighter, therefore CISK / OAI interaction becomes less, and clouds become thinner.
Also, there is much less forcing (convergence) at the exact center-point of such a large extratropical storm too, therefore less clouds. And finally, but a bit far-fetched, a slight and very shallow "warming" could also be occurring in the storm center, maybe only a degree or two ©, but enough to cause the cloud center to thin as well.
Above this entire storm, there may also be a vorticity maximum, which can indeed affect any layer below it, but in this case, is likely to be north and west of the surface low.
In a hurricane, core temperatures rise at least 10 degrees © or more from the surrounding environment, causing a real and true "eye" in the deep central convection. Inspired by this theory, shallower warm cored cyclones can develop over extreme northern areas, called POLAR LOWS. Like their distant tropical-bound "cousins", they can also possess an eye due to warming at the center of the cellular convection (see picture).
Above is a picture of a POLAR LOW - Note how similar it is to the "core" region of the Feb 12 blizzard above (if you removed the high clouds)!
In conclusion, Nor-easters (or any large ocean / near ocean) extratropical storm can have an eye. It is the interaction of the sea (OAI - Ocean Air Interface) and this cellular convection enhancing the core of the storm that causes this to happen.
Chris Collura - KG4PJN
