Cold Air Damming in the Appalachians Case Study Feb 13th 2008 Vanessa Przybylo

Where does the rain/snow line set up? Motivation

Virtual Potential Temp used CAD INDEX calculated hourly over nine years of data (2006-2014) following Miller et al. 2014   KCRW Charleston, WV Buoy 41036 KAHN Athens, GA DX1 = 293.3 km DX2 = 342.4 km DY = 387.3 km Center Greensboro station had to be colder than surrounding stations Index calculated by: 𝐴= Ѳ v KAHN − Ѳ v KGSO ∆Y 𝐵= Ѳ v KCRW − Ѳ v KGSO ∆𝑋1 𝐶= Ѳ v 41036 − Ѳ v KGSO ∆X2 CADINX=mean 𝐴, 𝐵, 𝐶 x 100 Averaged in all directions KGSO Greensboro, NC Virtual Potential Temp used Virtual ~ ignores moisture and density differences Potential ~ brought to 1000hPa (accounts for elevation)

Strongest event shown: CADINX = 5.77°C/100km Cold air becomes entrenched along the eastern slope of the mountain range Thermal trough present

Caused by H press in the Northeast (~>1030 hPa) Classical Case-strong CAA of stable air with minimal diabatic processes High press system strengthens and moves east More easterly onshore flow Air blocked by mountain and decelerates Coriolis decreases, PGF increases (balanced by friction) Parcels turn to the left (to the south in this case) Adiabatic cooling makes the air sink More damming due to larger press/temp gradient Lee trough amplifies Warm, moist, gulf air pulled westward due to low pressure to the west Increased cloudiness and precipitation results due to overrunning of warm air over cold dome

-Divergence in equatorward entrance region -Jet streak pushes high eastward 250 hPa, 0600Z 13 February 2008 Isotachs (shaded) and Geopotential Heights Surface low press system near Gulf States with deep upper level trough lagging behind Smaller upper level trough to the north (Great Lakes/Midwest region) Large-scale confluence over New England Surface cold front

Isallobaric Wind enhanced CAA Pressure falling over 6 mb in 3 hrs across North Carolina enhancing ageostrophic northerly wind

Shallow layer of cold air with inversion above NE flow at the sfc (barrier jet) WAA with veering winds strengthens the sfc inversion Easterly wind above barrier jet due to onshore flow W or SW flow at upper levels from trough swinging east Saturation from surface throughout the cloud layer

1800 UTC 500 hPa abs vorticity and geo heights ~ deep trough picks up moisture-lagging behind sfc Still have differential vorticity advection

Satellite at 2015UTC – near end of event Lingering stratus deck with sharp cutoff Most moisture advected through southern N.England Parent high offshore Weakening CAA Baroclinic leaf moving offshore

Conclusion Models usually exhibit a warm bias due to scouring the cold dome prematurely Need high resolution with a package that includes diabatic processes Along the coast, reverse bias where models do not forecast the warmth of the onshore flow

References Bailey, C. M., Hartfield, G., Lackmann, G. M., Keeter, K., Sharp, S., 2003: An objective Climatology, Classification Scheme, and Assessment of Sensible Weather Impacts for Appalachian Cold-Air Damming. Weather and Forecasting, 641-658, doi: http://dx.doi.org/10.1175/1520-0434(2003)018<0641:AOCCSA>2.0.CO;2 Bell, G. D., Bosart, L. F., 1988: Appalachian Cold-Air Damming. Monthly Weather Review, 137-161, doi:  http://dx.doi.org/10.1175/15200493(1988)116<0137:ACAD>2.0.CO;2 Miller, S., Hoffman, E., Kelsey, E., Cordeira, J, 2014: The New England Cold-Air Damming Experiment (CADEX). 1-3 Plymouth State University weather maps retrieved from http://vortex.plymouth.edu/u-make.html Surface Analyses retrieved from http://www.hpc.ncep.noaa.gov/archives/web_pages/sfc/sfc_archive_maps.php?arcdate=0 2/13/2008&selmap=2008021315&maptype=namussfc