Coastal Mid-Atlantic Squall Line Behavior

Squall lines that develop upstream of a large water body (e.g., ocean, sea, large lake) and move toward the coastline respond to the relatively stable marine atmospheric boundary layer (MABL), which can propagate tens of kilometers inland as a sea breeze. Previous research quantified storm characteristics and physical forcing mechanisms following the collision with a stable marine layer in an idealized 2D framework, using an analytic thermodynamic and kinematic environment that favored long-lived storms. This work builds on these findings, exploring 3D storm-scale processes for systems in a less favorable environment, more representative of the eastern US mid-latitude coastal region.

Idealized numerical simulations are performed in 3D with the Cloud Model 1 (CM1; Bryan and Fritsch 2002), at 200-m horizontal resolution and 50-m vertical resolution in the lowest 3 km with 95 vertical levels. The environment is based on vertical profiles observed during MCS events in the mid-Atlantic (Letkewicz and Parker 2010). Storms that develop within this geographic region have the potential to impact coastal and offshore entities. Two vertical wind profiles are used to quantify the sensitivity of coastal squall line evolution to wind shear, (1) 15 m s^-1in the lowest 3.5 km (shallow shear), (2) including an additional 4.5 m s^-1of shear from 3.5—6 km (deep shear). CAPE is approximately 1500 J kg^-1. Storms are initiated using momentum forcing (Morrison et al. 2015) with random thermal perturbations to develop 3D circulations. The marine layer is inserted as a cold blob at time 0, with a parameter space including two depth (750 m, 1.5 km) and thermal perturbation (-3 K, -6 K) values.

Within the mid-Atlantic, the distance between the Appalachian terrain and the coastline increases moving from north to south. Given that the Appalachians are a common region for convective initiation, storms will encounter the coast at different phases in their lifecycle (ceteris paribus). Additional sensitivity experiments explore the impact of storm maturity on its evolution surrounding and following the storm-MABL interaction by varying the collision timing.

Preliminary analyses show that storms are more linearly coherent, robust, and produce more rain with decreasing MABL temperature; the control experiment produces the weakest storm. This contradicts the storm-MABL relationship from our previous 2D simulations, which used a more favorable squall line environment. Rain increases and is displaced ahead of the main convective system, as cells develop (tens of mins) prior to the cold pool-MABL collision. For those that interact with the MABL later in their lifecycle, storm lifetime is extended with a longer period of enhanced rain following the cold pool-MABL collision.