Shafer

Cold Season Orographic Precipitation in Northern New England: 

Cold Season Orographic Precipitation in Northern New England Dr. Jason Shafer and Steve LaVoie Lyndon State College Meteorology Dept. 32nd Northeastern Storm Conference, March 11, 2007 Support provided by the Lyndon State College Board of Visitors

Why does it snow more in the mountains?: 

Why does it snow more in the mountains? White Mountains, NH Mt. Raymond, UT Oquirrh Mountains, UT Tuckerman Ravine, NH

Slide3: 

Map source: http://fermi.jhuapl.edu/states/states.html Green Mountains White Mountains

Mt. Washington: 

Mt. Washington Average Annual Snowfall: 311”, Max: 566”: (1968/69) Photo: http://www.mountwashington.org

Green Mountain Sites: 

Green Mountain Sites Mt. Mansfield Avg Annual Snowfall: 220”, Max: 330” Mt. Mansfield Mt. Mansfield Profile Jay Peak as per ski area: 345” (1981-2006), Max: 581” (2000/01) Data from: http://www.jaypeakresort.com/en/jay_peak/918/ Jay Peak Avg Annual Snowfall (3 km east of crest, 580m): 211” http://www.americasroof.com Photo: http://www.skiworldpress.com Mansfield Jay Peak

Slide6: 

R=0.90

Orographic Precipitation Focus: 

Orographic Precipitation Focus Not focusing on widespread heavy synoptic and mesoscale precipitation features and how the terrain modulates these structures Our focus is on events when there is significant orographic enhancement, when little precipitation occurs in nearby lowlands

Slide8: 

March 13-15 1984 Snowfall It’s not always about the terrain…rather mesoscale banding can be a key player

Slide9: 

Feb 13-15 2007 Snowfall

Slide10: 

Primary Orographic Precipitation Mechanisms Stable upslope 3. Seeder-feeder effect 5. Subcloud evaporation contrasts 2. Upslope release of potential instability (unstable upslope) 4. Terrain-driven convergence 6. Thermal convergence – thermally driven upslope

Slide11: 

Steenburgh 2003 1. Stable Upslope Neiman et al. 2002

Slide12: 

3. Seeder-Feeder Effect

Slide13: 

4. Terrain-driven convergence IOP3 Observed Precipitation (mm) Cox et al. 2005 Shafer et al. 2006

Slide14: 

Stable Upslope: Approaching Warm Front Aloft Franconia Ridge Altostratus Altostratus

Slide15: 

Hypothetical Seeder-Feeder Cloud Structure Seeder: Warm-frontal altostratus Feeder: Stratocumulus/stratus induced by orographic lift

Slide16: 

Unstable Upslope: Orographic ascent releases instability Cumulonimbi near the Wasatch Mountains, Utah

Strong Orographic Precipitation Event: 

Strong Orographic Precipitation Event Courtesy: Peter Banacos, NWS BTV

4 March 2006: A Unique Case: 

4 March 2006: A Unique Case Courtesy: Peter Banacos, NWS BTV Featured a backdoor warm front, stable environment

Slide19: 

Map source: http://fermi.jhuapl.edu/states/states.html Valley Stations Jay Peak Newport Burlington Mt. Mansfield Berlin Mt. Washington Mountain Stations 220m (722’) 61m (200’) 311m (1020’)

Slide20: 

Data and Methods Study period begins in 1988, when we have consistent data from all locations - 18 years of data. Event identification criteria: 1. Mountain stations with over 0.5 inches of daily precipitation. 2. Located corresponding valley precipitation. 3. Determined our precipitation-enhancement ratio (Mtn./Valley). 4. Compared ratio to climatological ratio. 5. Eliminated cases that were “wetter” than climatological ratio.

Slide21: 

Mt. Washington-Berlin: 3.10 Mt. Mansfield-Burlington: 2.62

Slide22: 

Preliminary Results 372 matching mountain days (2/3 Mountain stations). Mt. Washington-Berlin: 166 Days met criteria. Mt. Mansfield-Burlington: 237 Days. Jay Peak-Newport: 182 Days. Only 27 days matched at all 3 Mtn locations. Green Mountain (VT) sites: 145 Days. On 19 March 1999, we had the following LPE values: Newport: 0.02 Jay Peak: 1.95 Burlington: 0.02 Mansfield: 1.66

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The Composites In all, five different composites were produced. These were made using the NCEP-NCAR Reanalysis: http://www.cdc.noaa.gov/Composites/Hour/ The mean 0 hour was either 12z or 18z the day of the event. However, it is likely that the event started up to 12 hours before this hour. There is some variability between the composite members. The following slides show selected times from the Jay Peak-Newport composite.

Slide24: 

L HR: -24 500hPa Geopotential Heights 850hPa RH Sea Level Pressure 850hPa Heights

Slide25: 

L HR: -12 L 500hPa Geopotential Heights 850hPa RH Sea Level Pressure 850hPa Heights

Slide26: 

L L HR: 00 L L 500hPa Geopotential Heights 850hPa RH Sea Level Pressure 850hPa Heights

Slide27: 

L H L L L L L HR:+12 500hPa Geopotential Heights 850hPa RH Sea Level Pressure 850hPa Heights

How does this compare to St. Jean and Sisson 2004?: 

How does this compare to St. Jean and Sisson 2004? 7 Critical Ingredients for Orographic Precipitation: 1. Cutoff upper-level low to the east/northeast of the region- resulting in strong northwesterly flow. No cutoff low was present in our top case. A cutoff low was present for 40% (8 of 20) of our composite members. 2. Nearly saturated low-level environment. Composite postfrontal environment is nearly saturated (RH>80%) with values over 60% during the event. There was a nearly saturated environment for 80% (16 of 20) of our cases. 3. Strong low-level winds (greater than 20kts at 925mb, greater than 30kts at 850, cross-barrier flow  wind direction from 270 to 320). The mean 850hPa wind direction for our top 20 cases is ~310 degrees or a northwest wind. Composite members ranged from 225 to 360 degrees (850hPa).

Slide29: 

Critical Ingredients for Orographic Precipitation (Con’t) 5. Convective instability: steep low-level lapse rates, theta-e decreasing with height in the low-levels. Preliminary evaluation of soundings show that most of these cases match this criteria. 6. Favorable dendritic snow growth region (temperatures from -12 to -18C) 7. Event duration of at least 12 hrs. Could not be determined because we worked exclusively with daily data. 4. Cold advection in the low-levels. Cold Air Advection is present for 75% (15 of 20) of our cases. This appears favorable for orographic precipitation, but 3 March 2006 was associated with a backdoor warm front.

Questions?: 

Questions? March 13-15 1984 Total Snowfall