High Pressure Uncertainty

While the the Northeast U.S. is staggering under the onslaught of heavy snow and strong winds, Northwest meteorologists are dealing with one of our most difficult winter forecasting problems:  with high pressure over us, will low clouds and fog develop and remain over most of the day?    As we will see this is really a hard problem that plays to many of our weaknesses.

High pressure or ridging, as meteorologist often call it, should be be associated with sunny, fair weather, right?   Not over the NW lowlands in winter in many cases.

High pressure IS generally associated with a lack of heavy or moderate precipitation and the absence of storms, mainly because it is associated with sinking air.  Sinking air is poison to storms and is associated with warming aloft, sinking air is compressed as it descends (pressure increases towards the surface, of course).

High pressure in the winter often brings low level inversions, where temperature warms with height.  Why?   As shown by the diagram, the sinking associated with high pressure has to decrease near the surface for the simple reason that air can't move through the surface.  Sinking is stronger aloft and thus compressional heating is stronger aloft.   More heating aloft and less near the surface helps to build an inversion.

But there is another reason.   Sinking air aloft kills middle and upper clouds.  That allows the surface to radiate infrared heat to space, thus cooling it.   With a heater aloft and a cooling mechanism near the surface you get an even stronger inversion. 

What about heating of the surface by the sun during the day?   That would work against an inversion.  Unfortunately, our solar heating is very weak during midwinter and, of course, during our long mid-winter nights there is no solar radiation.   Inversions can thus easily form over night.

And then we have a further detail.   Fog and low clouds can form near the surface during our nights as the air cools to the dew point.  Clouds are highly reflective of solar radiation, but emit readily in the infrared.  Thus, they are cooling machines (reflect solar energy, but emit infrared radiation) and help to protect the low-level cool layer and thus the inversion. 

Inversions are very stable zones, meaning they work against vertical mixing.  Think of a a dense fluid beneath a lighter one, the dense fluid likes to stay on the bottom. Cold air is dense and warmer is lighter.

So high pressure helps produce fog and low clouds and inversions.  During the summer, nights are short enough and the sun's rays are strong enough that sufficient warming gets to the surface to heat the ground, evaporate the clouds, and destroy the inversion.  During the winter we can get stuck in inversion/low cloud conditions, sometimes for days.

Inversion of Seattle on Saturday over Seattle.  You can see the cold air layer (about 300 m thick) capped by a strong inversion.

The depth of the cool/cloudy layer is often relatively shallow:  few hundred meters is typical.    Numerical simulation of the existence and depth of such a layer is very difficult. Our models are too "mixy":  they tend to mix the layer out, and thus forecast that the weather will be less cloudy and warmer than reality.   Such a mistake was made last Saturday.  Here is a high resolution satellite image and the UW WRF model forecast for 4 PM on Saturday.  Look at central and southern Puget Sound.  Or eastern Washington.  Ooops.

WRF model cloud forecast for 4 PM

Visible Satellite Photo at 4 PM

 It is difficult to get the interplay of radiation, cloud physics, and near-surface meteorology (known as boundary-layer physics) correct in the model and it is hard to do this well subjectively.  My field has a lot of work to do to deal with this problem!