Nocturnal Low Level Jets in a Time-Varying Geostrophic Flow

In the West African monsoon, nocturnal low level jets (NLLJs) commonly occur due to the strong diurnal cycle. They are important in transporting heat, moisture and aerosols overnight. The mixing due to enhanced shear below the jet has been linked to cloud formation, and downwards mixing of momentum from the jet in the morning is an important mechanism for erosion of the nocturnal boundary layer and for dust uplift. Classical models for NLLJs assume a constant geostrophic wind, however in West Africa there is a significant diurnal cycle in the geostrophic wind resulting from the diurnal cycle in the monsoon flow. The impact of this time-varying geostrophic wind is investigated using simple analytical models and more detailed numerical simulations. The system exhibits behaviour similar to a coupled oscillator. Comparison of these models with observations of NLLJ development from radiosondes launched during AMMA (African Monsoon Multidisciplinary Analysis) suggest than this coupling may explain why the rotation rate of the wind vector in the NLLJs is different from that expected using classic theories.