Wednesday, 9 November 2016
Broadway Rooms (Hilton Portland )
Quasi-linear convective storms (QLCS) produce nearly half of the severe weather along the densely-populated coast of the northeastern United States. These storms typically form over land and propagate towards the ocean to the east. The change in surface properties in transitioning from a terrestrial to marine environment and its impact on boundary layer structure and wind shear profile makes QLCS prediction particularly difficult in this region. The ambient thermodynamic and kinematic regimes and storm scale physical processes that support QLCS propagation through the coastal transition zone are not well understood. The overall goal of this work is to advance the prediction of coastal squall lines by identifying offshore marine boundary layer (MABL) conditions and storm propagation mechanisms that contribute to coastal crossing success. This presentation focuses on the influence of changes in the thermodynamic properties of the marine boundary layer on QLCS coastal crossing success within the well-studied mechanical context of the Rotunno-Klemp-Weisman (RKW) shear profile. The interaction of an idealized squall line with a cool marine boundary layer is examined using the CM1 numerical model (Bryan and Fritsch, 2002). The Weisman-Klemp (WK) temperature and moisture profile are used as initial conditions. Surface temperature and moisture content are decreased and increased, respectively, across the two-dimensional 800 x 20 km domain to simulate the change from land to sea. The sea surface temperature (SST) values used in these experiments are based on buoy observations of temperature range in the western North Atlantic during the warm season. The model is initiated with an MABL of variable height, temperature, and moisture content based on the SST being tested and observations of typical MABL temperature and moisture structure. We hypothesize that the changes in downstream low level equivalent potential temperature and CAPE caused by insertion of the MABL will change the squall line strength and structure. Preliminary results indicate that the QLCS weakens in the presence of a 0.5 km MABL over a 292 K ocean surface, as inferred from the decrease in maximum vertical velocity and increase in minimum surface pressure values observed in the entire domain when the storm crosses the coast. Coincident with the observed decreases in these proxies for convective strength, the top of the cold pool evolves into an elevated wave above the MABL. Further experimentation with variations in the boundary layer horizontal temperature gradient, MABL moisture content, and MABL height will be presented, including discussion of passive tracer analyses.
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