Coastal Crossings of Mid-Atlantic Mesoscale Convective Systems

This work is part of an ongoing investigation of the modification of mesoscale convective systems (MCSs) by coastal gradients. Recent work numerically simulating idealized MCSs in a coastal environment has provided some insight into the physical mechanisms governing MCS coastal crossing success. In a RKW type thermodynamic and kinematic environment, all MCSs successfully crossed the coastline using sea surface temperature values representative of the western Mid-Atlantic between January and August, though storm-scale forcing for ascent varied between simulations. Perpetual crossing success was, in part, due to the nature of the RKW environment, designed to support long lived squall lines. To advance this work, bridge these numerical experiments to real cases, and continue to isolate the storm-scale physical processes contributing to coastal crossing success and failure, idealized experiments are performed using a thermodynamic and kinematic profile in which warm season Mid-Atlantic MCSs typically develop.

The current work uses the Cloud Model 1 (CM1; Bryan and Fritsch 2002) to systematically study the interaction between MCSs and offshore marine atmospheric boundary layers (MABL) in a Mid-Atlantic environment. At present, idealized simulations are run in 2-dimensions, with a 250 m horizontal resolution and a vertical resolution ranging from 100 m in the lowest 3000 m stretched to 250 m at the top of the 15 km domain. Simulations use an average ambient vertical profile, which supports mid-Atlantic MCSs from Letkewicz and Parker (2010). The left half of the 800 km domain is configured to represent a land surface, while the right half is assigned as water. Sensitivity experiments are conducted to quantify the influence of varying depths and strengths of the offshore MABL on MCS evolution, with values representative of boundary layers observed over the Mid-Atlantic coastal waters during the warm season. Convection is initialized at t=0 over land by horizontally forced convergence through the lowest 5 km, applied during the first 30 minutes. The MCS develops to maturity prior to encountering the coast.

Preliminary results indicate that the presence of a relatively shallow, weak MABL (2 K over 250 m depth) extends the lifetime and reduces the speed of the MCS, compared to an MCS that continues to propagate over a land surface. A convective system that moves over a cooler MABL (4K over a 250 m depth) weakens at the coastal boundary and subsequently decays. Causes for the differences in MCS evolutions will be addressed, with comparisons to previous work simulating coastal MCSs in an RKW type environment.