Mesoscale structure in a coastal front: a case study

Between 31 March 2004 and 03 April 2004, more than three inches of rain fell in southeastern New England, with some sections receiving more than seven inches. Throughout the region daily rainfall records were broken, including 6.03” on 01 April 2004 at Boston, MA, breaking the old record of 4.29”. The rainfall came primarily from a coastal low pressure center, and was augmented by the presence of a coastal front and topographical enhancement.

Cold air damming was initially present along the New England coast, and extending southward toward the Delmarva Peninsula. As time passed, the damming became less intense, but the cold air remained in place along the coastal plain of Massachusetts, New Hampshire, and Maine. As the coastal low pressure center advanced towards the northeast, the warmer air from over the ocean was unable to intrude into the interior of Massachusetts and New Hampshire, and a weak coastal front formed on 01 April. Over the next 24 hours, heavy rain fell along the coast, and just inland, suggesting that the presence of the strong temperature gradient at low levels contributed to the upward motion, and hence, to the precipitation rate.

Preliminary examination of the radar data suggests that banding occurred along the east-facing hills of Massachusetts and New Hampshire where the upward vertical motion and precipitation were the greatest, extending along a strong baroclinic zone just above the lower 50 hPa. Additionally, synoptic scale calculations have already shown that on the large scale, differential vorticity advection dominated the production of upward vertical motion, but this forcing was concentrated in the 850 – 1000 hPa layer, rather than above 500 hPa. Low level temperature advection was also strongest at low levels. Altogether there were five separate banding episodes visible in the radar data from Boston/Taunton, Massachusetts, with a variety of widths and orientations. (An example appears in the figure showing the radar reflectivity at 0.5 degrees elevation for 0829 UTC on April 1, 2004.)

We will use conventional surface and upper-air data, Level II Doppler radar data (including both reflectivity as well as Doppler winds), National Centers for Environmental Prediction (NCEP) operational forecast model output, and output from a triply nested run of the fifth version of the Penn State/NCAR Mesoscale Model (MM5) to detail the structure of the atmosphere along the New England coastline during this precipitation event. We will address the interaction between synoptic-scale forcing and the mesoscale bands of precipitation using both areal plots and vertical cross-sections.