Convective initiation ahead of squall lines involving small hills

On 21 June 2003, Doppler radar imagery from Oklahoma revealed a strong correlation between new convective cells forming in advance of a squall line and small-scale terrain features such as river valleys lined by small hills. A number of previous observational and modeling studies have shown that terrain can play an important role in triggering convection. These studies have mostly focused on large-scale terrain features or were concerned with radiative heating during daytime. This case, however, is associated with very small-scale orographic features and occurred at night, so the processes involved are probably rather subtle.

Our analysis suggests that the new convection in this case results from a combination of gravity waves excited by the squall line and by flow over the small hills. High frequency gravity waves generated by storm unsteadiness can propagate through the storm's inflow environment when trapped from above by the forward anvil. It is also known that a “top heavy” heating profile resulting from convective latent heat release excites low-frequency gravity waves that can more than temporarily but beneficially modify the lower tropospheric inflow environment, by cooling and moistening as well as inducing flow towards the storm. This can modify the winds passing over small topographic features. When the initial environmental winds near the surface are weak, the induced inflow can produce a critical level at which the ground-relative horizontal wind is zero. Below the critical level gravity wave energy is confined instead of upward propagating; this can result in downdrafts on the hill's upwind side moisture convergence above the hill.

We will demonstrate this interaction using a variety of idealized numerical simulations involving two- and three-dimensional models and hills of various shapes and sizes, along with both explicitly simulated squall lines and specified heat sources. Our goal is to understand how mesoscale convective systems can alter its environment over complex terrain and also how the evolution of MCSs themselves can be influenced by even small scale topographic features.