Orographic effects on coastal cyclogenesis in New England

Abstract for Mesoscale Conference ID: 155184  PW:  519918

Orographic Effects on Coastal Cyclogenesis in New England

The East Coast of the United States is a unique region for winter storms (Nor'easters).  The presence of the Gulf Stream and the Appalachian Mountains contribute to cyclogenesis, the development of a coastal front, the track of the resulting cyclone, and the type of precipitation that subsequently falls.  Cold air damming (Bell and Bosart, 1988, Kocin and Uccellini, 2004) is usually considered an important ingredient for the development of a Nor'easter, and the Appalachian Mountains are thought to be required for cold air damming to occur (Miller, 1946).   With high pressure to the north and northwest, cold air drains southward along the New England coast.  The mountains prevent this flow from turning towards the west, resulting in ageostrophic flow and allowing the cold air to penetrate far to the south.  This results in a wedge of cold, stable air between the mountains to the west and the coastline to the east. 

As a Nor'easter develops along the coast, relatively warmer air from over the water flows up and over this cold wedge, producing snow.   As the storm strengthens, a coastal front usually develops to the north and east of the cyclone, along the coast.  With the cold air dammed, the coastal front remains along the coast, and the Nor'easter tends to move to the northeast, over the water.  Without the cold air damming, frictional effects would still create a coastal trough and wind shift.   

In this experiment, we have used the fifth generation of the Penn State University - National Center of Atmospheric Research Mesoscale Model, version 3 (MM5) to examine the effects of the Appalachian Mountains on the behavior of two Nor'easters:  January 19 – 21, 1978, and February 5-7, 1978.  Both of these storms set 24 hour snowfall records in Boston, MA.  The January storm was a type A storm, coming from the Gulf of Mexico, while the February storm was a type B (Miller, 1946), redeveloping along the coast, as the primary storm from the Midwest stayed to the west of the Appalachian Mountains.

For each case, the MM5 was run for 96 hours, starting 48 hours before the storms reached coastal New England. North American Regional Reanalysis data was used for initialization and boundary conditions.  An outer domain had 60 km grid spacing, while an inner domain had 20 km grid spacing. Each case was run once with the normal terrain in place, and then a second time with the terrain in the model reduced to near sea level.  In the runs without the mountains present, the initial conditions were essentially an interpolation of the near-sea-level data at the same latitude. 

The results show that without cold air damming, coastal temperatures were significantly warmer, and the cyclones moved faster towards the north.  The tracks were essentially unchanged, suggesting that the combination of surface fluxes from the ocean and the location of upper-level forcing determined the location of the cyclones.  The ageostrophic low –level flow present in the cold wedge disappeared, as the physical barrier of the mountains was gone.  Finally, the coastal front in the January case (there wasn't a strong coastal front in the February case) remained present without the mountains, indicating that the differential friction was more important than orography, as suggested by Bosart et al. (1972). 

The figure below shows the near-surface temperature difference between the model runs with and without the mountains for the February case, when the storm is located off the coast of Delaware.  The warmer coastal temperatures north of the storm are evident, being as much as 7 ^oC warmer in the run without mountains.  Also notice the influx of colder air north and west of the coastline.  Here the removal of the mountains allowed the cold air from the northwest to advect in easily.  We will present details of these simulations, including surface and upper-level flow, storm tracks, and sea-level pressure patterns.

References:

Bell, Gerald D. and Bosart, Lance F.  1988.  Appalachian Cold-Air Damming.  Mon. Wea. Rev.  116:137-161.

Bosart, Lance F., Vaudo, Cosmo J., and Helsdon Jr., John H.  1972.  Coastal Frontogenesis.  J. Appl. Meteorol. 11: 1236–1258

Kocin, Paul J. and Uccellini, Louis W.  2004a.  Northeast Snowstorms Volume I: Overview.  Meteorological Monographs.  American Metrological Society, Boston.  Vol. 32 Number 54, 296pp.

Miller, James E. 1946. Cyclogenesis in the Atlantic Coastal Region of the United States.  J. Atmos. Sci  4:29-44.