The 22 December 2013 Cold Air Damming Event across the Hudson River Valley in East-Central New York

Ian Lee, Dr. Dave FitzjarraldNOAA/NWS Albany, NYSUNY Atmospheric Sciences Research Center

NROW XVI4 November 2015

Overview• Cold air damming (CAD) occurred across portions of the Hudson River

Valley on 22 December 2013▫ Produced an extended period of dense fog, intervals of drizzle, and near-

freezing temperatures in valley areas

▫ Above the boundary layer capping inversion, the sensible weather was clear with overcast skies and warmer temperatures

▫ The strong inversion resulted in significant temperature differences exceeding 15°C amongst elevation-dependent sites spaced less than 10 km apart

▫ A Sudden Warming Event occurred during the early afternoon resulting in a period of enhanced turbulent mixing and fog dissipation

• So what made this event happen, and what clues can data mining available datasets offer us?...

Areas of Interest

Geographic Information Systems (GIS) map of Hudson Valley.

1. The Capital Region (including the cities of Albany, Schenectady, Troy, and Saratoga Springs)

2. The Helderberg Escarpment/northern Catskills

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Quick Review•Mechanisms that promote CAD events

▫Terrain-blocking (typically on the east side)▫Capping inversion▫Low-level anticyclone to induce terrain-forced

flow▫Adiabatic and/or diabatic cooling▫Cold air advection▫Barrier jets (if CAD event is long duration)▫Supportive boundary layer*

What can be gleaned from an operational perspective?

Setting the Stage For This CAD Event…

GIS map of observed snowfall from 14-15 December 2013. Many areas saw greater than 25.4 cm (10 in) of snow.

WPC surface analysis valid 1200 UTC 17 December 2013

Significant Nor’easter impacted much of the ALY CWA ~1 week prior to event, with strong

high pressure building across the region afterwards, locking in low-level cold air.

www.wpc.ncep.noaa.gov

Synoptic Overview – Upper Levels

www.spc.noaa.gov

• Deep upper-level trough over central U.S.• Ridging aloft across New England• Upper-level jet structure promoted a slow-moving synoptic pattern,

with highest winds well downstream of trough axis

SPC 500 hPa analysis, valid 1200 UTC 22 December 2013SPC 300 hPa analysis, valid 1200 UTC 22 December 2013

Synoptic Overview – Low-Levels and Surface

WPC surface analysis, valid 1200 UTC 22 December 2013www.spc.noaa.govwww.wpc.ncep.noaa.g

ov

SPC 925 hPa analysis, valid 1200 UTC 22 December 2013

• Strong low-level frontogentic forcing• Prominent high and low pressure centers at the surface• Low-level CAA favored adiabatic cooling prior to event• Overall synoptic setup promoted precipitation across upstate NY

Regional Radar Mosaicfffffffffffffffffffff

• Diabatic cooling effects from precipitation enhanced low-level cooling

• -RA, -FZRA, -FZDZ reported at KALB

Loop valid 0000-1600 UTC 22 December 2013

Visible Satellite Loop

www.aviationweather.gov, www.spc.noaa.gov

Valid 1500-1900 UTC 22 December 2013

• Mainly cloudy throughout most of day• Pockets of clearing just west of Capital Region

between 1545-1630 UTC and 1745-1815 UTC• Clearing likely caused by strong SW

downslope flow off northern Catskills• Helderberg Escarpment enhanced low-level

blocking flow with Froude Number (Fr) < 1

SPC 850 hPa analysis, valid 1200 UTC 22 December 2013

Infrared Satellite Loop

www.aviationweather.gov

Valid 1000-2030 UTC 22 December 2013

• Cloud top temperatures < -20°C for much of the morning• Temperatures warm to -20°C and -10°C by the late morning

hours (revealing lower cloud deck)• Temperatures warm to > 0°C (surface skin temperatures)

between 1600-1700 UTC and 1745-1815 UTC coinciding with pockets of clearing

• Cloud tops cool to < -15°C after 1830 UTC

KALB Hourly METARs

• Temperatures mainly in low/mid 30s most of day, with dense fog and VLIFR ceilings/visibility

• Temperatures spike into the low 50s, with period of VFR ceilings and improved visibility in early afternoon

• Spike coincided with clearing in visible satellite imagery and warming IR cloud tops

• Dense fog re-forms after cloud cover is reestablished

www.ncdc.noaa.gov

KALB Surface Time Series

• Sudden Warming Event during early afternoon 12/22/13, along with spike in moisture (1)

• Surface remained moist until finally mixing out ~00 UTC 12/24/13 (2)

• Spike in surface wind coinciding with sudden warming event, directional change from NW to SW (3)

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So what caused this sudden warming event and period of fog dissipation?...

The Role of The Boundary Layer…

12/21/13 0000 UTC 12/21/13 1200 UTC

Sounding Characteristics• Stable surface layer, with deep mixed layer above to ~1300 m• Moisture advection evident in specific humidity profile• Decoupling wind profile evident by 12 UTC 12/21/13 • Cloud deck between 1000-1500 m with RH profile ~100%• No noticeable change in wind profile

Prior to event, boundary layer stratification increasing with time, as

decoupling wind infers trapping of colder air near surface with lack of turbulence and WAA in mixed layer

12/22/13 0000 UTC

Sounding Characteristics• Continued WAA in mixed layer

• Moisture advection continues in specific humidity profile

• Decreasing slope in wind profile inferring decreasing momentum transport through boundary layer

• Cloud deck lowering with time with RH near saturation ~500 m

• Surface winds shift to northerly, allowing more cold air to filter in near the surface

• Onset of Early-Evening Surface Layer Transition (supportive for fog formation), with spike in specific humidity and decreasing RH profiles just off the surface

Stratus deck

12/22/13 1200 UTC

Sounding Characteristics• Continued WAA above 500 m with CAA

evident below 500 m

• Fog layer established between surface and 250 m (laminar, well-mixed convective surface layer)

• Strong stratification throughout rest of boundary layer – limited momentum transport

• Entrainment processes occurring near top of capping inversion ~1500 m, but unable to mix down to surface

• Inflection point in wind profile ~500 m infers area of turbulent mixing*

• What can be further gleaned regarding boundary layer mixing?Fog layer

12/22/13 1200 UTC – Turbulence Analysis

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Turbulence Features• Laminar flow in fog layer near the surface (1)

• Spike in turbulence above fog layer, however, only penetrated top of fog layer due to spike in gradient Richardson Number (2)

• Relative minimum in turbulence in mixed layer (3)

• Spike in turbulence through capping inversion entrainment zone (4)

• Lack of turbulence in 850-700 hPa layer* (5)• So what does additional thermal heating do to the

entrainment zone?...

balloon

Forces encountered by a rising balloon:• Upward force due to assumed

constant ascension rate over time• Downward force due to drag• Horizontal forces encountered while

ascending (ωb)• Averaging ωb through a layer can infer

turbulence encountered by the balloon

ωb

ωb = ωb + ωb’

Turbulence analysis reveals distinct stratification of

boundary layer, with layers of horizontally-oriented eddies limiting momentum transfer

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Role of Possible Kelvin-Helmholtz Instability

Generally favorable turbulence profile in ducting layer immediately above boundary layer capping inversion

1200 UTC 22 December 2013 Sounding characteristics:• Steep 850-700 hPa lapse rates ducted between two stable layers • Parallel-sheared flow coinciding with steep lapse rates

Low and mid-level winds favored downslope trajectories off the northern Catskills and Helderberg Escarpment. Combined with

adiabatic warming/drying and clearing pockets of sky cover, enough heating may have promoted a period of breaking waves

that mixed out capping inversion.

Critical Ri = 0.25

12/24/13 0000 UTC

By 12/24/13 00 UTC, boundary layer had fully

mixed out within dry, NW flow regime

Sounding Features• Deeper and well-mixed CBL to ~1500 m (1)

• Sharp slope in wind profile, indicative of strong momentum transport just above the surface (2)

• Vertically-oriented eddies allowed for drier air to mix down to surface (3)

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Surface Observations Valid 1500 UTC 22 December 2013

www.mesowest.utah.edu

For most of the day, temperatures within the

boundary layer (valley areas) were only in the 30s, but

quickly jumped to the low 60s across the higher terrain

above the capping inversion!

Rough approximation of Helderberg Escarpment

Possible meso-front

Photo courtesy of Dr. Jeff Freedman, SUNY Atmospheric Sciences Research Center

Photo overlooking the Hudson Valley from Thacher State Park. Thacher State Park is located along the Helderberg Escarpment in Albany County, with elevations ranging from ~90 m (300 ft) to ~550 m (1800 ft). It was noted that temperatures in the fog layer were ~1°C (33°F) and ~13°C (55°F) near the top of

the escarpment – a nearly 10-15°C (20-25°F) across a ~460 m (1500 ft) elevation change!

Notice the stratocumulus “lumpy-like” appearance at the top of the fog layer

Although difficult to see, presence of ducting waves in layer of hypothesized

Kelvin-Helmholtz Instability

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Conclusions• Cold air damming (CAD) across portions of the Hudson Valley

on 22 December 2013 resulted in a period of extended dense fog, along with extreme temperature differences exceeding 15°C amongst elevation-dependent sites less than 10 km apart

• The event was aided by a strongly stable boundary layer, a result in part of the local topography and blocking flow (Fr < 1)

• It is hypothesized that Kelvin-Helmholtz instability and increased thermal heating (pockets of clearing) promoted a period of breaking waves capable of mixing out the boundary layer capping inversion▫ Allowed for a period of enhanced turbulent mixing

• Data mining available datasets, such as the boundary layer, can aid in the forecasting/understanding of these events

Acknowledgements • SUNY Albany Atmospheric Sciences Research

Center▫ Dr. Jeff Freedman

• NOAA/NWS Eastern Region Scientific Services Division▫ Brian Miretzky

• NROW XVI Steering Committee▫ Warren Snyder▫ Tom Wasula▫ Neil Stuart▫ Vasil Koleci▫ Brian Frugis

This work is an unfunded component under CSTAR V, grant NA13NWS4680004.

Questions? Comments?

Ian.Lee@noaa.gov