Numerical analysis of a thermotopographically-induced mesoscale circulation in a mountain basin using a non-hydrostatic model

Summary The boundary-layer wind field during weak synoptic conditions is largely controlled by the nature of the landscape. Mesoscale (sub-synoptic) circulations result from horizontal gradients of sensible heat flux due to variation in local topography, variation in surface-cover, and discontinuities such as land-sea contrasts. Such flows are usually referred to as thermally-driven circulations, and are diurnal in nature and often predictable. In this paper we use a state-of-the-art non-hydrostatic computer model to shed light on the physical mechanisms that drive a persistent easterly wind that develops in the afternoon in the Mackenzie Basin, New Zealand. The easterly – Canterbury Plains Breeze (CPB) – is observed early in the afternoon and is often intense, with mean wind speeds reaching up to 12 m s⁻¹. Although computer modelling in mountainous terrain is extremely challenging, the model is able to simulate this circulation satisfactorily. To further investigate the mechanisms that generate the Canterbury Plains Breeze, two additional idealized model experiments are performed. With each experiment, the effects of the synoptic scale wind and the ocean around the South Island, New Zealand were successively removed. The results show that contrary to previous suggestions, the Canterbury Plains Breeze is not an intrusion of the coastal sea breeze or the Canterbury north-easterly, but can be generated by heating of the basin alone. This conclusion highlights the importance of mountain basins and saddles in controlling near-surface wind regimes in complex terrain.