850 mb Discussion
destabilization of the lower atmosphere

The 850 mb pressure level analyses are quite valuable for analyzing all types of synoptic weather patterns. This level is often just above the "friction" layer - i.e. the surface will not, typically, have much influence on wind direction, wind speed or temperature at 850 mb (about 5,000 feet). Thus it is often part of the "free" atmosphere, but it is close to the top of the boundary layer (in many cases 850 mb falls within the boundary layer), and is often a level where clouds and precipitation occur. The 850 mb pressure surface, like the 500 mb pressure surface, is a "mandatory" level; this means that all radiosonde or upper air instruments record winds, temperature, and humidity precisely at 850 mb. There are a good deal of existing meteorological analyses and charts available for this level - so it a convenient surface to analyze our Veteran's Day LES storm.

In lake effect events, the 850 mb analyses are very important in deducing atmospheric stability, availability of cold air, cold air advection patterns, and wind flow. In the Veteran's Day storm, the 850 mb pattern is illuminating. One of the crucial questions prior to the storm was when the lower atmosphere would become cold enough to support widespread snow instead of rain or mixed precipitation. -10C is often used as a guideline for LES cases to deduce where and when it might become cold enough for efficient production of snowflakes within the LES clouds.

Note, that even with 850 temperatures warmer than -10C, given water surface temperatures near 10C, there would be a sufficiently large lake-to-850 mb temperature difference to categorize this event near the "moderate" category of lake effect snowfall potential. However, 850 mb temperatures of -6 to -8C are thought to be marginal for the production of snow in convective clouds which dominate in LES scenarios. Thus, the -10C isotherm at 850 mb is a reasonable index for predicting where the clouds are cold enough to support high concentrations of ice crystals and promote rapid growth of those crystals, while warm enough to still allow for a good deal of water vapor in the clouds (which is crucial to the growth and production of precipitation size showflakes).

During the storm, it appears as if the position and movement of the -10C isotherm at 850 mb coincided roughly with the onset of heavy lake effect snowbands. That is, the migration of the -10C isotherm south and east of a region was often coincident with the first appearance of snowfall in the favored snowbelt locations of that area.

The following images are 850mb analyses from WXP showing height contours (that is, the height above sea level at which 850mb of pressure is encountered above a given point), isotherms in degrees Centigrade, and the station models (providing temperature, height, dew point, wind direction, and wind speed) for each upper air station in the domain.

The first 850mb chart is analyzed for Nov 10, 1996 at 12z. The -10C isotherm is highlighted in red on each map to show its progress east and south during the first three days of the LES event.

By Monday Nov 11, 1996 the cold air aloft is seeping southward from Ontario, Canada. The 850mb upper low which corresponds to the center of coldest air is near -20C which is unusually cold for this time of year. The upper low stays roughly at the same latitude throughout the storm; however, the pool of cold air spreads in the horizontal - especially to the east making its way to the New England coast by Monday evening.

The destabilizing process in the lower atmosphere takes place rapidly in LES storms. Remember, the near-surface air over the lake waters is quite warm and moist at this juncture of the storm. The air is buoyant and unstable and is easily mixed upwards and "overturned". When this warm moist air reaches the 850mb level it will be mixed with this air as well. The properties of both layers will blend together; thus, 850mb air is "mixed down" to the surface as surface air is turbulently combined with the air from the lower "free" atmosphere. The net result is a deep layer of near-neutral air with temperatures falling off sharply with height, and large amounts of water vapor available for clouds and precipitation. Likewise, the higher winds (momentum) from the 850mb level are also mixed down to the surface which can help to steer and increase winds just above the lake waters; this helps to enhance evaporation into and heating of the over-water surface air.

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.

500 mb