Diagnostic study of a HyMeX heavy precipitation event over Spain by investigation of moisture trajectories

Abstract

The western Mediterranean area is often affected by heavy precipitation events (HPEs). To understand these events, the research programme ‘Hydrological Cycle in the Mediterranean Experiment’ (HyMeX) was established. We investigated a HyMeX HPE over Spain that took place between 28 and 29 September 2012. Due to extreme local precipitation that occurred over the Spanish Mediterranean coast and amounted to over 200 mm rain in a few hours, this event not only is of scientific interest but also had a large social impact. A detailed process study of the scale dependence of the HPE's triggering is performed with the numerical weather prediction (NWP) model Consortium for Small‐scale Modeling (COSMO) and the Lagrangian Analysis Tool (LAGRANTO). Large‐scale lifting is reasonably well captured by global‐scale models and local‐scale convection is tackled by convection‐permitting models. Between these phenomena, there is still a scale where uplift takes place and convective clouds can lead to partly severe precipitation. For the investigated case study, mesoscale horizontal moisture convergence and orographic enhancement over land are identified as the two triggering mechanisms leading to severe precipitation. Vertical profiles of equivalent potential temperature are used to explain the evolution of stability at two selected sites over land and sea. Moisture trajectories indicate the pathways of air masses involved in the HPE. On the one hand, the synoptic flow and the mesoscale convergence line are well represented by the trajectories; on the other hand, additional information about moisture sources was found. Two moisture uptake regions could be identified for this convective HPE: the early part of the event was fed from the eastern Atlantic Ocean, the latter part from the Mediterranean Sea itself. Compared with other cases with strong dynamic forcing, this event is governed mainly by regional diabatic processes.