Cold Air Damming: An Introduction

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Transcript Cold Air Damming: An Introduction

Cold Air Damming:
An Introduction
Gail Hartfield
NWSFO Raleigh, North Carolina
By the end of this instruction, you
should be able to...
 List each damming type,
and describe the relative
roles of synoptic scale
forcing & diabatic processes
in each.
 Describe the major
influencing processes of
damming, both at the
surface and aloft, & explain
their effects.
 Discern between damming
and lookalike (nondamming) events.
Why Study Cold Air Damming?
It happens often & affects a wide area
Models don’t diagnose/forecast it well…
even mesoscale models have trouble
Occurrence has major implications on cloud
cover, temps, precip type, etc.
Not all ridges down the East Coast are
I live in Juneau. What do I care?
Cold air damming occurs often east of the
Rockies, too… not just the Appalachians
Many of the processes affecting damming
are noted in other phenomena as well
The need for a thorough understanding of
contributing processes is applicable to any
forecast problem
An event-specific forecast process can be
useful for many forecast problems
You aren’t glued to your current station!
Forecast Challenges of the Mid-Atlantic &
Southeast U.S. ...
 Often at southern
extent of cold air
 Highest mountains
in the Appalachians
 Gulf Stream &
Atlantic in close
 Extensive Piedmont
& Coastal Plain
The Damming Region (DR)
 Area under greatest
consideration for
 Damming dome
 Is by no means the
only area affected!
Spectrum of Cold Air Damming
and Lookalikes...
 …is a method of
classifying events based
on processes
 …was created to help
forecasters identify the
very different types of
damming events
 …helps with coordination
 …will continue to be
adjusted and improved as
more is learned
 Five types:
•3 damming
•2 lookalikes
 Damming=>
Froude Number
= (Brunt-Vaisala
H = height of mtn barrier
U = component of mean wind orthogonal to mtns
 = mean value through stable layer
h = height of stable layer
 Five types:
•3 damming
•2 lookalikes
 Damming=>
 Lookalikes=>
 All produce same
weather conditions
 Spectrum is a
“Classical” Cold Air Damming
 Strong forcing from
 Diabatic processes
unnecessary to initiate,
but can strengthen
 Note position and
strength of sfc high
Surface Processes of
Classical Cold Air
 “Parent” high is cold air
 E to NE flow is blocked &
deflected southward
 Adiabatic cooling=>
hydrostatic pressure rise=>
ageostrophic response
 CAA & low level stability
in DR are enhanced
However, in the
damming types…
• Synoptic-scale
forcing becomes
less important
• Diabatic processes
become more
important ...
Hybrid Damming
 Synoptic-scale forcing &
diabatic processes play
nearly equal roles
 Parent high may be:
 In good position but weak
 Progressive (limited CAA)
 Strong signatures aloft
often lacking
 Diabatic processes
enhance low-level stability
In Situ Damming
 Diabatic processes
 Little or no CAA initially;
cool dry air is deposited
 Sfc high is unfavorably
 Precip into this
pre-existing dry, stable air
instigates damming
In Situ Damming
6-7 Jan 1995
 Temperatures were in
the lower 60s in
Eastern NC & lower
30s in Central NC
 Boundaries can be
focus for severe
weather (more later)
 Millions of dollars in
damage in NC alone;
>120 kt gust at GSB
A Brief Look at the “Lookalikes”
 Weather conditions mimic
cold air damming
 Differs from damming…
  Flow is NOT
  Not connected to
a parent high
  Lacks signatures
above the boundary
 Two types: Cool air
pooling & upslope
Cool Air Pooling
 Pre-existing dry air
mass not connected to a
parent high
 No CAA into cool pool
 Precipitation induces
mesoscale high
 Mountains not required
 CAD events frequently
turn into cool air
Upslope Flow
Boundary Layer
 Adiabatic lift generates
considerable cloudiness
& cooler temperatures
 Resulting surface mesoSurface
00Z 10/14/95
high has no connection
to or support by a parent
 Low-levels too unstable
for damming
To recap the damming types…
Classical = support & forcing from synoptic-
scale features, surface & aloft; diabatic
processes not needed
Hybrid = support & forcing from both
synoptic-scale features & diabatic processes
In Situ = instigated by diabatic processes
with little or no support from synoptic-scale
Processes Aloft Contributing to
Cold Air Damming
Can effect near-surface environment
Notable mainly in classical and
sometimes hybrid CAD
Contributing processes evident at:
850 mb
500 mb
300-250 mb
CAD Processes & Signatures: 850 mb
 Anticyclone off SE
U.S. coast
 Light-moderate warm
moist flow atop cold
 Enhances CAD:
Generates clouds &
precip for increased
Strengthens inversion
CAD Processes & Signatures: 500 mb
 Split-flow regime
 Trough or low in Srn
 Trough or low over Ern
 Confluent flow over
NE U.S. anchors &
strengthens high
 Allows surface ridge
to be unimpeded by
CAD Processes & Signatures: 300 mb
 Jet entrance region is
over NE U.S.
 Ageostrophic
Produces subsidence
atop sfc high
Helps drive sfc cold air
Cold Air Damming Erosion
(or, When is this “dam” thing gonna end??)
One of the most difficult aspects of CAD, not
captured well by models
Incorporate model biases in forecast process
(e.g. NGM moves parent highs offshore too quickly)
Rules of thumb:
Strong events typically require strong
CFP to scour out wedge (esp. Oct-Mar)
Weak events with only low cloud cover
are susceptible to erosion by insolation &
mixing from above
Erosion & Breakdown: A Few
Questions to Ask
Is low level CAA ending? (e.g. parent high
moving offshore; being “pinched off”)
Are surface winds shifting out of damming
Is upper level support waning?
Is precipitation ending (influence of
diabiatic processes diminishing)?
Has dry air advection ended?
Could this event end as cool air pooling?
Cold Air Damming:
Forecast Operations
Tools for identifying an event & diagnosing
the influencing processes 
• Spectrum of Damming and Lookalike Events
• Glossary of Terms For CAD & Lookalikes
• Special AWIPS procedures
• Forecast Methodology for CAD
Tools for determining CAD onset and
erosion 
• Models (e.g. Eta, MASS, MM5) (longer term)
• Close monitoring of sfc/BL/UA features
• CAD Erosion Guidelines (in progress)
• Conceptual models
“Forecast Methodology for CAD”
Created to facilitate event identification and
the forecast process
Adapted for online use w/ MASS model (but
is also in questionnaire format)
Three parts:
Pre-Development (Is the stage set?)
Development (assessment/ID; is flow blocked?)
Breakdown & Erosion (identify possible
mechanisms of wedge erosion)
 Links to pertinent MASS
& Eta model fields
 Addresses:
 Sfc high initial
position, strength &
 Sfc temps/dewpoints
 Availability of dry air,
& dry air ridge (DAR)
 Links to MASS, NGM, &
Eta fields
 Addresses:
 Low level CAA
 Upper level support
(850/500/300 mb)
 Low level stability
“Breakdown & Erosion”
 In “yes/no” questionnaire
 Addresses:
 Cessation of diabatic
processes, low level
CAA, upper level
support, sfc high
 Presence of thermalmoisture boundaries
Thermal-Moisture Boundaries
(aka wedge fronts, piedmont fronts)
Delineate the southern and eastern edges of
the cold dome
Temp differences across TMB are often
20F or greater
Coastal front can “jump” inland into TMB 
Can act as a focus for severe weather 
Coastal Fronts
Development favored by:
 Very cold air over warm Gulf Stream
 Pre-existing synoptic frontal boundary
 Differential heating
 Convergence zone
Onshore movement indicated by:
 Offshore NE winds go SE (check buoy obs)
 Tight thermal gradient pushing westward
 Pressure falls & temp/dewpoint rises just inland
Coastal Fronts
Factors affecting inland movement or
 Strength of wedge
 Depth of cold dome on edges
 Offshore high pressure with sufficiently strong
southeasterly flow orthogonal to front
 Strong/strengthening TMB + weakening coastal
Will not likely move much farther west than
Raleigh/Burlington, NC
Severe Weather Along a TMB
Strong vertical shear along TMB enhances
severe threat
Type of damming can determine degree of
 Severe wx more likely with in situ damming
Cold front aloft (CFA) & accompanying dry
slot can enhance severe downdrafts
Check presence of low level jet streak
To wrap it up...
CAD mustn’t be oversimplified... the relative
roles of various processes differ in each event
Forecasters must understand the supporting
processes of each event & recognize the
Forecast methodologies targeting particular
weather problems (e.g. landfalling TCs, heavy
snow QPF) can make the entire forecast
process easier and more efficient
CAD boundaries can spawn severe weather