Synoptic-flow interaction with valley cold-air pools and effects on cold-air pool persistence: Influence of valley size and atmospheric stability

Idealized two-dimensional simulations have been used to examine the dependence of valley cold-air pool (CAP) behaviour, morphology and duration (persistence) on terrain scale. The simulations use terrain of constant aspect ratio but ranging from modest to Alpine scale, under midlatitude winter conditions with a finite background flow and incorporating a diurnal cycle. CAPs in smaller valleys are found to be more morphologically constrained, whereas larger valleys provide scope for more complex flow dynamics and more dramatic interplay of the external flow with the valley, including gravity wave breaking. Simulations of a valley within a plateau show that, as valley depth exceeds the depth-scale of the nocturnal stable boundary layer, the CAP becomes increasingly confined and sheltered from the background flow (reduced mixing), becoming harder to remove by processes related to daytime insolation, and it can eventually become persistent (never removed). Both the surrounding terrain and gravity wave dynamics are found to play a part in the enhancement of sheltering. Therefore background atmospheric stability and wind are also crucial, summarized in a non-dimensional valley depth parameter which largely determines CAP duration. More stable conditions, representative of (for instance) large-scale subsidence or warm advection aloft, result in longer durations and a greater likelihood of persistence. Stronger nocturnal radiative cooling results in stronger CAPs, which are harder again to remove. Several mechanisms appear to act in concert to give rise to these behaviours and the dependence on non-dimensional valley depth. Replacing the valley-in-plateau with a double hill enhances wave breaking, which complicates the relationship to non-dimensional valley depth, reducing CAP duration significantly when subsidence or warm advection-like effects are absent.