The Interaction between QLCSs and Marine Atmospheric Boundary Layers: A Process Study

Quasi-linear convective systems (QLCSs) can pose a substantial threat to coastal cities and communities, such as the New York City – Long Island region and the North Carolina outer banks. For example, the 10 September 2010 QLCS that moved over New York City was responsible for 2 tornadoes (EF0; EF1), a microburst with peak winds of 123 mph resulting in 1 fatality [National Climatic Data Center (NCDC) Storm Data], and substantial property damage through New York City. Regardless of the relatively low intensity of the tornadoes, this was a high impact event due to its location. Prior to this event, no severe thunderstorm or tornado watch was issued, and a tornado warning was issued only a few minutes prior to the development of the first tornado, giving local residents limited time to react.

Predicting the evolution of QLCSs crossing coastal boundaries is a challenge. As these organized convective storms move from inland regions to over the coastal oceans, their evolution is impacted by spatial gradients in temperature, moisture, and wind as they interact with coastal sea breeze boundaries and the associated marine atmospheric boundary layers (MABL). There have been a limited number of studies exploring the interaction between organized convective lines and the coastal environment (i.e. Lericos et al. 2007; Lombardo and Colle 2012; Lombardo and Colle 2013). Previous research (Lombardo and Colle 2012; Lombardo and Colle 2013) has shown that the evolution of a QLCS interacting with the northwestern Atlantic MABL may be independent of offshore atmospheric instability. On average, QLCSs that decay moving offshore are associated with larger offshore instability than events that maintain their intensity over the coastal waters. This contradicts the assumption that QLCSs decay offshore due to reduced or a lack of instability over the relatively cool coastal waters. Exploring 2 contrasting case studies of QLCSs crossing the northeastern U.S. coastline, Lombardo and Colle (2013) showed that the decaying QLCS remained surface based, while the QLCS that sustained its intensity more than 100 km from the coastline became elevated forced by a cold pool-bore hybrid. These 2 events highlight the need for additional analyses to quantify the ambient conditions and physical processes associated with the different QLCS evolutions.

This work aims to use the Cloud Model 1 (CM1; Bryan and Fritsch 2002) to systematically explore the physical processes controlling the interaction between QLCSs and sea breeze boundaries/MABLs under an array of environmental conditions in an idealized setting. Fundamental questions include (1) Under what ambient land and MABL conditions does a QLCS decay or sustain its intensity upon crossing a coastal boundary? (2) What forcing mechanisms are associated with each evolution (i.e. cold-pool, gravity wave, bore)? (3) How do the characteristics (i.e. speed, intensity) of the QLCS vary upon encountering the MABL?