Mid-latitude Squall Lines Interacting with Offshore Stable Boundary Layers

Organized mesoscale convective systems, specifically quasi-linear systems or squall lines, that develop upstream of a large water body (ocean, sea, large lake) and move toward the coastline respond to the 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 favors long-lived convection. 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 plains.

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 thermodynamic-kinematic profile was based on Letkewicz and Parker (2010), an average environment within the Atlantic coastal plain downstream of the Appalachian Mountains that supports organized convection that successfully crossed the terrain. Storms that develop within this ambient and geographic environment have the potential to move offshore, and at a minimum are representative of systems observed in the region. The vertical profile has 1500 J kg^-1 of CAPE, with either 15 m s^-1 of vertical wind shear in the lowest 3.5 km (shallow shear) or with an additional 4.5 m s^-1 of shear from 3.5—6 km (deep shear). Storms are initiated using momentum forcing (Morrison et al. 2015) with random thermal perturbations to develop 3D circulations. The marine layer included in the sensitivity experiments is 1.5 km deep with a potential temperature perturbation of -6 K. Choices of marine layer depth and buoyancy are intentionally on the upper end of the observed range, and are likely some of the most extreme conditions a Mid-Atlantic squall line may encounter in the warm season. Marine layer depth is also chosen in part to reduce the low level potential instability within the vertical profile, which has been shown to support convection over stable layers. The marine layer is inserted one hour prior to the desired collision time to ensure control over the collision conditions.

Within the Mid-Atlantic, the distance between the Appalachian terrain and the coastline increases moving from north to south. Both the elevated terrain and the associated lee trough are common locations for convective initiation (CI) within the region. For terrain induced CI, the distance between the CI location and the coastline varies. All else being equal, the interaction between the storm and the marine layer will occur at different points in the storm’s lifecycle depending on latitude. Preliminary numerical simulation results indicate that the ability of a storm to survive post-collision is sensitive to the time of interaction during the storm lifetime. Storms that encounter a marine layer earlier in their lifetime are prone to complete decay within 2 hours of collision, though storms that mature for an additional 1-2 hrs continue successfully over the stable layer. Interaction with a marine layer can also impact storm intensity, structural characteristics, and storm mode. For organized linear systems (shallow shear simulations), the more intense convective regions weaken post-collision. For a broken line of cells (deep shear simulations), the storms weaken and evolve into more linear structures post-collision. Ongoing work aims to quantify the storm scale processes associated with the various storm evolutions post-collision.