Cold Air Damming: An Introduction

Gail HartfieldNWSFO 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 (non-damming) 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 “damming”!

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 proximity

Extensive Piedmont & Coastal Plain

The Damming Region (DR)

Area under greatest consideration for “spectrum”

Damming dome deepest

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=> BLOCKED FLOW

Froude Number

H = height of mtn barrierU = component of mean wind orthogonal to mtns = mean value through stable layerh = height of stable layer

= (Brunt-Vaisala frequency)

Spectrum is a continuum

All produce same weather conditions

Lookalikes=> UNBLOCKED FLOW

Damming=> BLOCKED FLOW

Five types: •3 damming •2 lookalikes

“Classical” Cold Air Damming

Strong forcing from synoptic-scale features

Diabatic processes unnecessary to initiate, but can strengthen

Note position and strength of sfc high

Surface Processes of Classical Cold Air

Damming “Parent” high is cold air

source

E to NE flow is blocked & deflected southward

Adiabatic cooling=> hydrostatic pressure rise=> ageostrophic response

CAA & low level stability in DR are enhanced

• Diabatic processes become more important ...

• Synoptic-scale forcing becomes less important

However, in the non-classical damming types…

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

necessary

Little or no CAA initially; cool dry air is deposited

Sfc high is unfavorably located

Precip into this pre-existing dry, stable air instigates damming

In Situ Damming Event:

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 blocked

Not connected to a parent high

Lacks signatures above the boundary layer

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 pooling!

Upslope Flow

Adiabatic lift generates considerable cloudiness & cooler temperatures

Resulting surface meso-high has no connection to or support by a parent high

Low-levels too unstable for damming

00Z 10/14/95

Boundary Layer

Surface

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 features

Processes Aloft Contributing to Cold Air Damming

Can effect near-surface environment significantly

Notable mainly in classical and sometimes hybrid CAD

Contributing processes evident at:850 mb500 mb300-250 mb

CAD Processes & Signatures: 850 mb

Generates clouds & precip for increased stability

Anticyclone off SE U.S. coast

Strengthens inversion

Enhances CAD:

Light-moderate warm moist flow atop cold dome

CAD Processes & Signatures: 500 mb

Allows surface ridge to be unimpeded

by cyclogenesis

Split-flow regime

Confluent flow over NE U.S. anchors & strengthens high

Trough or low over Ern Canada

Trough or low in Srn Plains

Ageostrophic circulation…

CAD Processes & Signatures: 300 mb

Helps drive sfc cold air southward

Jet entrance region is over NE U.S.

Produces subsidence atop sfc high

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 configuration?

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)

“Pre-Development”

Links to pertinent MASS & Eta model fields

Addresses: Sfc high initial

position, strength & source

Sfc temps/dewpoints Availability of dry air,

& dry air ridge (DAR) development

“Development”

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

format

Addresses: Cessation of diabatic

processes, low level CAA, upper level support, sfc high support

Presence of thermal-moisture boundaries (TMBs)

Thermal-Moisture Boundaries(aka wedge fronts, piedmont fronts)

Delineate the southern and eastern edges of the cold dome

Temp differences across TMB are often 20F 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

“jump”: 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

front

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 threat 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 signatures

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