COLD AIR DAMMING ROBERT ATKINSON MATTHEW STEFFEN ROBERT ATKINSON MATTHEW STEFFEN COLD AIR DAMMING

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  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 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 How weather is affected -displaced precip maxima -enhance LL cloudiness -reduce surface temps. -more precip

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 -broad ridging over eastern US -southerly flow overrunning the cold air 500 mb heights -southern plains trough/Great lakes ridge -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 PredictedObserved

Model Difficulties  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