Simulations of sheltering in valleys: the formation of night-time cold-air pools

Numerical simulations of flow over two-dimensional valleys have been performed in order to study the occurrence of pools of cold air which form at the bottom of valleys during stable night time conditions. The results show that during strong surface radiative cooling and light-wind events, the near-surface potential temperature at the bottom of a valley can be several kelvin (K) below the environmental mean. This is true for quite shallow valleys with depths and widths of 50 m and 1 km, respectively, and is a result of in situ sheltering at the valley bottom. For windier conditions or less rapid cooling, the cold-pool temperature contrasts are reduced.

For shallow valleys the magnitude of the difference between the potential temperature at the bottom of the valley and the mean value increases with increasing valley depth. However, results indicate that there is a critical valley depth, beyond which the valley flow becomes decoupled from that aloft and there are no further increases in the potential temperature difference. This critical valley depth depends on the wind speed and radiative cooling rate and the results indicate it is a function of a non-dimensional valley depth (or inverse Froude number), which is itself a property of the undisturbed profiles of wind and stability.