Transcript COLD AIR DAMMING

COLD AIR DAMMING
COLD AIR DAMMING
ROBERTATKINSON
ATKINSON
ROBERT
MATTHEWSTEFFEN
STEFFEN
MATTHEW
WHAT IS COLD AIR DAMMIMG?
 Low level cold air mass that is trapped
against mountains topographically
 East side of mountains in N. Hemisphere
 Influences the dynamic of overlying air mass
 Affects moisture, temperature, & stability
Characteristics of CAD
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Sloping inversion
Overrunning warm, moist air
May include a barrier jet
Precipitation aloft affected by cold air mass
– Snow
– Freezing Rain
– Rain
– Sleet
-Easterly Flow (Mid-level)
-Westerly Flow (Upper-level)
-Sloping Inversion
Possible Barrier jet
How weather is affected
-displaced precip maxima
-enhance LL cloudiness
-reduce surface temps.
-more precip
Barrier Jets
-forms near slopes
-significant low level winds parallel to mountains
-form if there is blocking with development of low pressure system to
south
What causes CAD to form
CAUSES
-inversion above cold air
-LL anti-cyclone
-force upslope flow
-terrain blocking
-abiabatic cooling
WHAT CAN FORM
-barrier jet
-upper level forcing
-affects sensible weather
Effects of terrain blocking
 Flow tends to be ageostrophic as winds turn
to low pressure
 Upslope flow decelerates air
– Cools adiabatically; cooler and denser
 Accounts for 30% of cooling
How to forecast CAD
850 mb heights
500 mb heights
-broad ridging over eastern US
-southern plains trough/Great lakes ridge
-southerly flow overrunning the cold air
-slow evolution of trough/ridge features
Climatology of CAD
Most common and
strongest in winter
-last longer (day and half)
Weakest in Summer
-shorter (day or less)
Types of CAD
 Classical: large strong surface high
 In-situ: diabatic process dominant
– Little or no CAA initially
– High pressure unfavorably located (east of US)
 Hybrid: combination of both types
-Surface high weak and centered off shore
-Diabatic cooling and advection important
Case study January 1999
Thickness
Satellite with Surface Obs.
1000 mb Winds
Predicted and Observed Soundings
Predicted
Observed
Model Difficulties
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Model resolution insufficient (horz/vert)
Terrain approximations inaccurate
Lapse rates not captured well
Does not portray terrain affects accurately
Numerical models often under forecast
amount of CAA
What ends CAD
 Warming at surface (southerly winds)
 Cold air depth decreases, pressure falls in
cold air
 Decrease in strength of overrunning flow or
subsidence aloft within overrunning flow