Evaluating the Effects of Terrain-Channeled, Low-Level Flow on Convective Organization

Forecasting severe weather in regions of complex terrain is often difficult due to convective environment inhomogeneities induced by terrain. In the northeastern United States, one common inhomogeneity arises where terrain channeling of low-level flow occurs in valleys, where localized backed flow, and a subsequent increase of low-level wind shear, may develop ahead of convection. This study uses idealized Weather Research and Forecasting (WRF) experiments to investigate how convective organization changes with representative low-level wind profiles.

A number of idealized WRF simulations were conducted with different initial wind profiles in the 0–1.5-km layer, while keeping the thermodynamic profile the same across simulations. This layer was chosen to reflect the depth over which terrain-channeling effects occur in northeastern U.S. valleys. The wind profiles are based on in-valley and out-of-valley locations from parent numerical simulations of past severe weather events in the northeastern U.S. The parent simulations had 0­–1-km storm-relative helicity perturbations of approximately 40 m^2/s^2 within valley locations compared to out-of-valley locations. The importance of these in-valley perturbations to convective organization were evaluated using the aforementioned idealized simulations. Circular and linear warm bubbles were initialized to initiate discrete and linear convective modes. Analysis of maximum vertical velocity, vorticity tendency, and pressure perturbation fields will be presented. The components of the pressure perturbation, the (linear and nonlinear) dynamic and buoyancy components, will be evaluated to diagnose their contributions to vertical accelerations and the evolution of the various convective modes. The goal is to develop thorough dynamical reasoning for why convection may intensify in terrain-channeled regions in the northeastern U.S., where low-level kinematic forcing is more favorable, which can lead to an increased risk of damage in valley locations during real cases.