Orographic effects on rainfall in MAP cases IOP 2b and IOP 8

Although the large-scale flow was similar, important differences in mesoscale atmospheric structure made the difference between moderately intense rain in Intensive Observing Period (IOP) 2b and relatively light rain in IOP 8 in the Lago Maggiore Target Area (LMTA). This conclusion is supported here through analysis of data from, and numerical simulations of, these two cases. Analysis of the large-scale data shows that there was in both cases a moist tongue of southerly flow moving from the west to the east of the Alpine south side. Mesoscale data analysis and numerical simulations suggest that the most important difference between the two cases was the presence of a cold stable air mass in the Po Valley in IOP 8 which persisted through the period in which the large-scale moist tongue was progressing eastward and prevented the most humid air from reaching the LMTA. Another difference contributing to the greater rainfall in the LMTA in IOP 2b was the development of conditional instability (with associated convective rain) due to the effect of the Alps on the eastward passage of the moist tongue; in IOP 8 the atmosphere remained locally stable throughout the period.