1b. Stability:

An unseasonable cold-air outbreak in the middle or late fall is an ideal setup for LES. Lake temperatures might remain in the 50's (degrees F) while it is not uncommon to see air as cold as 20F blow over the lake waters in mid and late-November. Converting to centigrade and meters, an unstable atmospheric layer occurs when the "lapse rate" of temperature change with height exceeds -1C/ (100 meters); in our example the lapse rate in the first 10's of meters above the lake might exceed -5C/(100 meters)! Needless to say, this is an extreme instance of surface heating from below and leads to rapid destabilization of air parcels and strong rising motions (although these large lapse rates are not uncommon in these circumstances). However, the destabilizing effect is quickly damped out as one moves upward from the lake surface so typical lake-effect snows occur in shallow layers of only a few thousand meters.

A number of computer "models" of lake effect snow have simplified the atmospheric structure within an LES system by defining two or three distinct layers. The first layer is right near the water surface, perhaps 100m where the environmental air is very unstable, dominated by turbulent air flow, and cools rapidly with height; this is called the surface layer. Above the surface layer is the friction or planetary boundary layer - that is, the layer of the atmosphere that is influenced by the surface characteristics of temperature and roughness or frictional elements; this layer is "neutrally" stratified (i.e. its lapse rate is always near -1C/(100m) and is called the "well-mixed" layer since air parcels that might start to rise will tend to continue ascent, but parcels which might fall will continue until a layer of different stability is encountered - the end result is a rapid and efficient overturning of the air in the lowest ~1 km of the atmosphere (approximately). Above the well-mixed layer is the "free atmosphere" which is unperturbed by surface conditions. During an LES the free atmosphere is cold, dry and often characterized by moderate to strong stability. Temperatures change little with height and may actually increase a bit with height (an inversion) - in such an environment air parcels have a tendency to sink and warm which is called subsidence.

In the Green Bay sounding shown below, a well-defined mixed layer with a good capping inversion is evidenced at about 760 mb; above which is the free atmosphere which exhibits weak static stability until the tropopause is reached at about 400 mb which exhibits strong static stability. This is a more typical looking sounding for an LES event.

The interface between the mixed layer and the free atmosphere is often characterized by an inversion; the well-mixed layer, through adiabatic motions and turbulence, has effectively redistributed the surface heat and moisture while the free atmosphere is inducing sinking and warming of air near the buffer between the two.

The discontinuity between these two layers results in a rapid spike of warming with height which gives a layer of strong stability and thus acts as an inhibitor or cap to rising air parcels from below. The height of this capping inversion is a measure of the competing strengths of the surface heating and convective mixing trying to weaken and elevate the cap, versus the large scale cold air flow (advection) and subsidence trying to strengthen and push the cap downwards. The models that have employed this scheme show the height of the cap will increase gradually as one moves over the lake (along the trajectory of the flow) and will increase sharply as one encounters the downwind shoreline.

If the cap defines the height of the cloud tops (which is consistent with observations), the greatest cloud tops will be found just inland on the leeward side of the lakes. In this visible satellite image, the formation of clouds is clearly a function of how far the airstream carries over the lake waters (this shows up well especially in lower Michigan).

Background

Terms for using data resources. CD-ROM available. Credits and Acknowledgments for WW2010. Department of Atmospheric Sciences (DAS) at the University of Illinois at Urbana-Champaign.

Wind Shear