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Objectives

Conceptual diagram of Lake-effect snow

a) cold air over warm water - latent/sensible heat fluxes mixed upward

b) mixed layer deepens

c) frictional convergence on lee shore

d) possible additional lift from topography

Ingredients for Lake-effect snow

a) Instability - Used to look at temperature difference between the lake and 850 mb temperature being 13 degrees Celsius or greater to develop LES bands. Now we look at paramaters like lake-induced CAPE. 

b) mixed layer depth - may be as or even more important than instability

c) wind direction and fetch - want to maximize fetch so that more fluxes get into the boundary layer

d) speed - calm winds will not allowing vertical mixing of fluxes.  shear - details in slide 19

e) microphysics - heavy snow events are favored with particular snow crystal types

f) upstream lakes - conditions downstream air mass

g) orography - forced ascent from topography results in heavier snow

h) synoptic influence - watch for cyclonic vorticity advection (raises the inversion height), secondary troughs (changes the wind direction) etc.

i) ice/snow cover - inhibits the fluxes from mixing upwards

Typical synoptic setup - Hudson Bay low, cold advection pattern at low levels, deep mixed layer in the sounding

Ideal LES sounding - illustrates deep PBL, little directional shear and strong cap. Concentrate on the sounding below 500 mb since that is where most of the "action" occurs.

Nakaya Diagram show snow crystal type as a function of temperature and supersaturation. Yellow area denotes most favored area for dendritic growth, graupel and riming. Graupel forms in regions of rapid ascent. Riming of frozen crystals occurs in supersaturated layers (most efficient on dendritic aggregates. Dendrites form near -15 degrees Celsius. Supersaturation (riming) occurs most easily when the initial relative humidity is high (see next slide)

For these lake-effect snow cases the pre-event 850 mb dewpoint was greater than 80% for all heavy snow events, and less than 80% for all trace cases. This illustartes the importance of starting with a relatively high RH in the PBL to achieve riming. 

John Quinlan (NWS Albany, NY) has trained the spotters in his CWA to take observations of snow crystal type. The data from this study confirm that dendrites, and aggregates (especially when rimed) produce the greatest snowfall totals.

The following 3 diagrams illustrate the importance of fetch (assume environmental conditions favor LES development and the only changing parameter in these 3 slides are PBL wind direction). Click the arrow button to go between the 3 frames in this slide

Fetch 1 - In southwesterly PBL flow - The angle of  the wind with respect to the major axis of the lake is less than 30 degrees and the fetch is maximized , therefore a single intense bands occurs off Lake Erie. For Lake Ontario the fetch is not maximized and the angle of the wind with respect to the major axis of the lake is greater than 30 degrees, therefore multiple bands occur.

Fetch 2 - Imagine a trough moves in from the west, changing the wind direction to westerlies starting in the west. The snowband on Lake Erie is pushed towards the shoreline towards the region which still favors a single band. The Lake Ontario band transitions to a single band due to the longer fetch and decreased angle the wind makes with the long axis of the lake.

Fetch 3 - The trough has crossed over the area, leaving westerly flow across the domain. The Lake Erie snowband develops multiple snowbands due to the decreased fetch and the increased angle the wind makes with the long axis of the lake (greater than 30 degrees). The Lake Ontario snowband develops a single intense band due to the wind flowing parralel to the long axis of the lake, maximizing the fetch.

Topography of the Great Lakes - note areas in your CWA where snowbands may be enhanced by upslope flow. Also note bays and peninsulas (see slide 22)

Types of lake-effect snowbands

Since lakes Erie and Ontario are elliptical a major axis exists. This means that the flow directions needs to be parallel to the major axis for the maxium fetch. Lake Erie usually freezes over in January (inhibiting the fluxes altogether), while Ontario remains mostly unfrozen.

PBL depth generally shallower over western Great Lakes because source region is from colder higher latitudes with no upstream large bodies of water.

Shorelines - Surface friction over land backs winds relative to those over water setting up convergence line along downwind shore.

Peninsulas - Surface friction over land also backs winds on peninsulas setting up convergence at the tip of the land mass.

showURL goes to an example of a Saginaw Bay snowband

To investigate the cloud microphysics note the height of the forecast -15 degree Celsius isotherm is where the relative humidity is relatively high (70%). This is close to 80% and remember this is only model guidance! For graupel considerations need to assess region of maxium ascent. One rule of thumb is to use the halfway point between the surface and lake induced equilibrium level as an estimate of the level of non-divergence. In this case this would be at about 1.3 km at 00:00 UTC (the beginning of our forecast period).