Cloud‐top striations above ana‐cold frontal circulations

Satellite images occasionally show regular arrays of cloud‐top striations within mesoscale cloud features associated with vigorous ana‐cold frontal circulations. Observational case‐studies with a variety of types of radar are presented to show that the striations are due to bands of upright upper‐level convective cells. These convective cells form at the top of the rearward‐sloping‐ascent part of a transverse frontal circulation which, in one of the cases studied, had a multi‐layered structure fed in turn by upright line convection lifting air from the boundary layer. This coexistence and interdependence of upright and slantwise forms of convection may not in itself be uncommon; what is less usual in these results is the way in which the upper‐level convection is manifested as bands, or striations, orientated roughly parallel to the wind shear at cloud‐top level but almost perpendicular to the strong thermal‐wind shear in the underlying frontal zone—a behaviour that may be specific to vigorous ana‐cold frontal systems. The striations have a large wavelength (30 to 55 km) but we are unaware of any simple dynamical mechanism that can account for the convection being organized on this scale. The striations are regarded as an important indicator of accompanying adverse weather. Earlier studies have suggested that such striations are associated with rapid cyclogenesis and surface gusts. One of the cases presented here falls into this category; the other, more detailed, case‐study is notable for the rainfall associated with the vigorous frontal circulations.