Formation and maintenance mechanisms of the stable layer over the Po Valley during MAP IOP-8

During Mesoscale Alpine Program (MAP) IOP-8, a strong stable layer formed over the Po Valley and northern Ligurian Sea. Based on observations, reanalysis data and prior studies, we hypothesize that differential advection (Lin et al., 2005) led to the formation of the stable layer and differential advection along with blocking of cool easterly flow by the western flank of the Alps over the Po Valley played significant roles in the maintenance of the stable layer. Numerical sensitivity tests with the MM5 model were performed to examine these possible formation and maintenance mechanisms of the IOP-8 stable layer. When the western flank of the Alps was removed, the stable layer still formed, but eroded more quickly and became much shallower and narrower at the later stage of IOP-8, which is consistent with the hypothesis. It was also found that the Dinaric Alps and evaporative cooling did not play significant roles in forming and maintaining the stable layer.