Release of potential shearing instability in warm frontal zones

Potential shearing instability (PSHI) exists in the atmosphere when the Richardson number in a stably stratified shear layer would be less than 4 if the layer were lifted to saturation. Sometimes the air entering a warm frontal zone near the position of the surface warm front is initially unsaturated, its Richardson number is greater than a, but PSHI exists over a deep layer. This paper presents a study of a warm frontal zone using observations from the high-resolution Doppler radar at Chilbolton, supported by radiosondes and output from the mesoscale version of the operational United Kingdom Meteorological Office Unified Model. Very large-amplitude Kelvin-Helmholtz billows were observed to form where the PSHI was realised following the onset of saturation. The billows were probably intense enough to be a hazard for air traffic and an important source of momentum transfer. Rough estimates from the radar of vertical momentum fluxes due to the billows are shown to be broadly consistent with measurements by the same radar of the change in velocity profile across the frontal zone from start to end of the billow train. The present version of the Unified Model does not parametrize vertical mixing due to shearing instability in the free atmosphere; this may be an important omission.