Mesoscale numerical study of two cases of long-lived quasi-stationary convective systems over eastern Spain

A set of mesoscale numerical simulations using the Pennsylvania State Unive rsity-National Center for Atmospheric Research model is used to investigate two cases of extreme precipitation over eastern Spain. Both cases (3-4 Nov ember 1987 and 20 October 1982) were characterized by quasi-stationary meso scale convective systems that developed over the Valencia region and lasted more than 30 and 12 h, respectively. Rainfall totals in 24 h exceeded 800 mm on 3-4 November and 400 mm on 20 October at some localities of that regi on. The first event occurred within a weak and very stagnant synoptic patte rn under a persistent easterly/northeasterly low-level jet stream impinging on the Valencian orography. In contrast, the second case involved a westwa rd-moving surface low driven by an upper-level jet streak, which evolved al ong the northern edge of an upper-level cutoff low over North Africa. In bo th cases, the mesoscale model forecast spatial details of the precipitation field reasonably accurately, as well as capturing its long duration, but u nderestimated the storm total precipitation. Model output fields suggest th at the development of a surface mesolow by latent heat release, as well as lee cyclogenesis induced by the Atlas Mountains, could have played an impor tant role in both events by providing low-level convergence and enhanced up slope winds. Thus, a factor separation technique is used to explore this is sue. For the event of 3-4. November 1987, latent heat is decisive for expla ining the precipitation maximum over central Valencia, and the Atlas orogra phy induces rainfall enhancement over the same zone. For the event of 20 Oc tober 1982, me latent heat release is again shown to be important, whereas the Atlas Mountains orography factor appears to inhibit rainfall. This is t he first time that it has been documented that the Atlas-induced modulation of the surface pressure field is not a positive factor for heavy precipita tions over eastern Spain. This exceptional case may be due to a negative in teraction between the Atlas Mountains and the upper-level dynamics and fron tal forcing for this event.