Ian Lee Dr Dave Fitzjarrald NOAANWS Albany NY SUNY Atmospheric Sciences Research Center NROW XVI 4 November 2015 Overview Cold air damming CAD occurred across portions of the Hudson River ID: 472454
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Slide1
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 CenterNROW XVI4 November 2015Slide2
Overview
Cold air damming (CAD) occurred across portions of the Hudson River Valley on 22 December 2013Produced an extended period of dense fog, intervals of drizzle, and near-freezing temperatures in valley areasAbove 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 apartA 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?...Slide3
Areas of Interest
Geographic Information Systems (GIS) map of Hudson Valley.
The Capital Region (including the cities of Albany, Schenectady, Troy, and Saratoga Springs)
The Helderberg Escarpment/northern Catskills
1
2Slide4
Quick Review
Mechanisms that promote CAD eventsTerrain-blocking (typically on the east side)Capping inversionLow-level anticyclone to induce terrain-forced flowAdiabatic and/or diabatic coolingCold air advectionBarrier jets (if CAD event is long duration)
Supportive boundary layer*What can be gleaned from an operational perspective? Slide5
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.govSlide6
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
2013
SPC 300 hPa analysis, valid 1200 UTC 22 December 2013Slide7
Synoptic Overview – Low-Levels and Surface
W
PC surface analysis
, valid 1200 UTC 22 December
2013
www.spc.noaa.gov
www.wpc.ncep.noaa.gov
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 NYSlide8
Regional Radar Mosaic
fffffffffffffffffffff
Diabatic
cooling effects
from precipitation enhanced low-level cooling
-RA, -FZRA, -FZDZ reported at
KALB
Loop valid 0000-1600 UTC 22 December 2013Slide9
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
2013Slide10
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 UTCSlide11
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.govSlide12
KALB Surface Time Series
Sudden
Warming
E
vent
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
)
1
2
3
So what caused this sudden warming event and period of fog dissipation?...Slide13
The Role of The Boundary Layer…Slide14
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 profil
e ~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 layerSlide15
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 deckSlide16
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 layerSlide17
12/22/13 1200 UTC – Turbulence Analysis
5
4
3
2
1
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 (
ω
b
)
Averaging
ω
b
through a layer can infer turbulence encountered by the balloon
ω
b
ω
b
=
ω
b
+
ω
b
’
Turbulence analysis reveals distinct stratification of boundary layer, with layers of horizontally-oriented eddies limiting momentum transfer
2Slide18
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
= 0.25Slide19
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)
1
2
3Slide20
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-frontSlide21
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 InstabilitySlide22
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 apartThe 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 inversionAllowed for a period of enhanced turbulent mixingData mining available datasets, such as the boundary layer, can aid in the forecasting/understanding of these events
22Slide23
Acknowledgements
SUNY Albany Atmospheric Sciences Research CenterDr. Jeff Freedman NOAA/NWS Eastern Region Scientific Services DivisionBrian Miretzky NROW XVI Steering CommitteeWarren SnyderTom Wasula
Neil StuartVasil KoleciBrian Frugis
This work is an unfunded component under
CSTAR
V, grant NA13NWS4680004.Slide24
Questions? Comments?
Ian.Lee@noaa.gov