Effects of the Alps and Apennines on forecasts for Po Valley convection in two HyMeX cases

Two Intensive Observation Periods (IOPs) of the Hydrological cycle in the Mediterranean eXperiment (HyMeX) are examined in this study. IOP6 and IOP13 were characterized by troughs with associated cold fronts entering the western Mediterranean and, in both cases, organized frontal convection in the Po Valley was observed. These similarities notwithstanding, predictability of the Po Valley convection was limited for IOP6 for most of the forecast models available during HyMeX, but not for IOP13. Using the Weather Research and Forecast (WRF) model in research mode, the present study confirms the relatively good forecast for frontal convection in IOP13 is not very sensitive to modelling assumptions. In contrast, it is found that only a two-way-nested simulation, initialized close to the event, was able to produce a realistic representation of the squall-line in the Po Valley for IOP6. A comparison between a ‘successful’ and an ‘unsuccessful’ simulation of the Po Valley convection in IOP6 suggests the sensitivity lies with the orographic flow modification which was at the threshold between ‘flow-over’ and ‘flow-around’ responses to the Maritime Alps. In particular, in the ‘flow-over’ regime, downslope winds from the Maritime Alps and Apennines suppress the convergence/uplift where the front encounters the barrier wind in the Po Valley, while in the ‘flow-around’ regime it is unimpeded. A delicate balance between the competing mechanisms of orographically induced subsidence on the lee side of the Apennines and frontal uplift in the Po Valley is found to be crucial for squall-line survival.