Cold Air Damming: An Introduction

1. Cold Air Damming: An Introduction Gail Hartfield NWSFO Raleigh, North Carolina

2. 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.

3. 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”!

4. 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!

5. 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

6. The Damming Region (DR) • Area under greatest consideration for “spectrum” • Damming dome deepest • Is by no means the only area affected!

7. 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

8. Five types: •3 damming •2 lookalikes Damming=> BLOCKED FLOW

9. Froude Number = (Brunt-Vaisala frequency) H = height of mtn barrier U = component of mean wind orthogonal to mtns  = mean value through stable layer h = height of stable layer

10. Spectrum is a continuum • Five types: •3 damming •2 lookalikes • Damming=> BLOCKED FLOW • Lookalikes=> UNBLOCKED FLOW • All produce same weather conditions

11. “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

12. 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

13. However, in the non-classical damming types… • Synoptic-scale forcing becomes less important • Diabatic processes become more important ...

14. 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

15. 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

16. 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

17. 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

18. 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!

19. Upslope Flow Boundary Layer • 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 Surface

20. 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

21. 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 mb • 500 mb • 300-250 mb

22. CAD Processes & Signatures: 850 mb • Generates clouds & precip for increased stability • Anticyclone off SE U.S. coast • Light-moderate warm moist flow atop cold dome • Enhances CAD: • Strengthens inversion

23. CAD Processes & Signatures: 500 mb • Split-flow regime • Allows surface ridge to be unimpeded by cyclogenesis • Trough or low in Srn Plains • Trough or low over Ern Canada • Confluent flow over NE U.S. anchors & strengthens high

24. CAD Processes & Signatures: 300 mb • Ageostrophic circulation… • Jet entrance region is over NE U.S. • Produces subsidence atop sfc high • Helps drive sfc cold air southward

25. 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

26. 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?

27. 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

28. “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)

29. “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

30. “Development” • Links to MASS, NGM, & Eta fields • Addresses: • Low level CAA • Upper level support (850/500/300 mb) • Low level stability

31. “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)

32. 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 

33. 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

34. 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

35. 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

36. 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