The role of dynamic and diabatic processes in the generation of cut-off lows over Northwest Africa

Abstract

The present observational study addresses the role of dynamic and diabatic processes leading to the generation of four deep upper-level troughs/cut-offs, involved in two extreme precipitation episodes over West Africa during the cool season. The elongated potential vorticity (PV) streamers associated with the observed troughs form as a result of an equatorward transport of high-PV air downstream of a large ridge over the central North Atlantic. Strong deformation along the eastern side of the ridge leads to a thinning of the PV streamers. In some situations the tips of the streamers break up and form distinct and long-lived stationary cut-offs near West Africa, in particular if the presence of another PV ridge downstream allows a complete isolation from the midlatitude westerlies. In other situations a prior anticyclonic wave-breaking event over Europe leads to an advection of high-PV towards the Iberian Peninsula that merges with the streamer and impedes a complete cut-off. The observations presented here suggest that the rapid amplification of the PV ridges over the North Atlantic and thus the subsequent streamer formation are related to upstream latent heating through non-conservative diabatic reduction of upper-level PV and through the strong divergent outflow near the tropopause that support large negative isentropic PV advection. The intense latent heat release is promoted by cyclo- and frontogenesis, and the transport of warm, moist air by a low-level jet ahead of the surface cold front (often called a warm conveyor belt; WCB). Diabatic PV tendencies are highest where the WCB rises over the surface warm front to the northeast or east of the cyclone centre. In most cases the distinct heating occurs in connection with a strong upper-level jet and a rapid deepening of the involved surface cyclone. More quantitative dynamical and statistical studies of the suggested relation are needed to better understand the relative contributions of single factors to the large and synoptic scale evolution that leads to PV streamers/cut-offs near West Africa.