A Compare and Contrast Study of Two Banded Snow Storms Part I – January 6 th, 2002.
Post on 19-Dec-2015
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Summary
• Banded snowstorms typically associated with major, deepening cyclones.
• Bands typically form in an area of mid-level deformation and frontogenesis.
• Storm-relative flows associated with the storm act to favor slantwise instability in the area of deformation.
Question: Did the earlier model run simulate the structure of the event, with bad placement the
only problem?
Or was the whole run just out to lunch?
Conclusion: The earlier runs forecast similar structures.
Errors were mainly in the placement of the key features.
Explanation 1) Inertial instability in the geostrophic wind field, resulted in horizontal (northward) ageostrophic accelerations, with individual parcels being forced upward along the sloping front zone. These accelerations act to remove the inertial instability in the real wind field.
Parcel becomes sub-geostrophic, PGF > Coriolis, parcel accelerates to the north.
N S
N
Another way of looking at it: The air parcel can’t make the sharp anticyclonic turn, so it accelerates down the height gradient. Key: the existence of the sharp, small-scale downstream ridge.
700 mb heights
L
Explanation 2) The real momentum surfaces of the real atmosphere are not sloping downward, like in the Eta forecast of geostrophic momentum, but are flatter than the Eta forecast of non-geostrophic momentum. Slantwise convection can occur.
Theta-e
Momentum
Summary - January 6th – “Classic” Aspects
• A rapidly deepening storm with strong, widespread forcing.
• Closed cyclonic circulation developed at mid-levels.
• Banding developed in an area of mid-level frontogenesis and lower to mid-level negative EPV.
SUMMARY – January 6th “Non-Classic” Aspects
• The system was associated with a compact short-wave located well downstream from the long-wave trough.
• The atmosphere was not close to gravitationally unstable above the frontal zone.
• Model forecasts indicated the presence of inertial instability in the geostrophic wind field.
Model forecast summary
• 12z Eta did an outstanding job forecasting the location of the deformation band and resultant qpf.
• The proceeding 00z Eta was too far southeast with the placement of most key features, resulting in forecast precipitation too far to the southeast.
• Eta short-range ensemble forecasts indicated some uncertainty during their 09z run, however the final solution was outside the range of solutions on the ensembles.
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