The sensitivity of squall line motion to environmental changes in 3D idealized WRF forecasts

The forecasting of mesoscale convective systems (MCSs) by numerical weather prediction (NWP) models has long represented a major challenge to both the research and operational forecasting communities. The movement of these high-impact weather systems is often influenced by both synoptic- and mesoscale processes -- this merging of scales is known to pose a significant challenge to NWP models. Yet despite the importance of the interaction of processes on different temporal and spatial scales, most early modeling investigations of MCSs consider these two scales separately by using 2D idealized models at higher (mesoscale) resolution, or by using 3D full-physics models that looked at larger-scale features. A shortcoming of the former is that the omission of the Coriolis force precludes a realistically-evolving synoptic environment, and of the latter, that some mesoscale processes are not adequately resolved. The interplay that may occur by combining these two approaches remains comparatively uninvestigated.

In this study, a “blended” modeling approach has been utilized to learn more about how synoptic-scale processes may influence the formation and movement of midlatitude MCSs in a 3D idealized model setting. Specifically, the following hypothesis is investigated: Varying parameters such as wind shear and moisture on the synoptic scale will impact the main processes by which mid-latitude squall lines move, via one or more of the following mechanisms: (i) cold pool formation, (ii) gravity wave generation, (iii) pre-line isentropic ascent, or (iv) the downward transport of high-momentum air from aloft. Idealized initial conditions designed to yield a squall line associated with a midlatitude baroclinic jet stream have been constructed, and this environment has been used to initialize the full-physics 3D WRF model. The advantage of this idealized environment is the ease with which parameters such as shear, stability, and convective triggering may be controlled in a dynamically consistent fashion; unlike in most 2D idealized studies, these values are changed in a way that maintains thermal wind balance in the 3D synoptic environment.

In this initial inquiry into the realm of full-physics, 3D, idealized WRF modeling of midlatitude MCSs, sensitivity experiments are conducted in which the moisture and wind shear profiles are altered in order to mimic changes in the large-scale environment. Results from these experiments are helpful in diagnosing the ways in which MCS movement may differ as a result of these changes in the environment, as well as a result of varying model physics packages. These experiments allow isolation of the influence of these environmental factors on squall line movement, as well as allow for these effects to be uniquely examined in a 3D, thermal-wind-balanced, full-physics framework.