The orographic precipitation associated with the MAP IOP-8 is investigated by using the PSU/NCAR MM5 model. During IOP-8, a deep trough approached the Alps from west, which induced low-level jets and then produced orographic precipitation over the Southern Alpine slopes and extended further upstream. Significant rainfall was also observed over the Ligurian Apennines. A quasi-pressure ridge to the east helped to slow down the precipitation system triggered by the orography as well as that associated with the front (eastern) flank of the propagating trough. An upper-level diffluent region advanced to the location over the Alps at about the time the orographic precipitation occurred. The synoptic and mesoscale situations for IOP-8 were similar to those for IOP-2B, except that: (a) the low-level flow was more from the southeast; (b) there was a cold dome formed over the southern slopes of the Alps; (c) the upstream airstream was stable; and (d) a mesoscale vortex was produced in the concave (Lago Maggiore) region of the Alps. These factors added together and resulted a lighter orographic rainfall for IOP-8. In addition, the rainfall region in IOP-8 extended farther upstream (south) of the Lago Maggiore region.

Two types of rainfall were identified: (1) orographic rain and (2) rainfall associated with the trough. The model reasonably simulated the orographic rainfall pattern. However, the quantitative precipitation forecast of the orographic rain of IOP-8 is quite challenging. Several sensitivity experiments have been made to investigate the sensitivity of the predicted rainfall to different popular cumulus parameterization (CP) schemes as well as two popular microphysics parameterization (MP) schemes. The combination of Betts-Miller (CP; 45 km), Grell (CP; 15/5 km), and Goddard (MP; 45/15/5 km) schemes produced the best results (control run), compared with observed rainfall. This is consistent with earlier studies; the Grell scheme is more suitable for simulating more stable precipitation systems. We found that the grid-explicit (MP) rainfall is more sensitive to orographic lifting, while the subgrid (CP) rainfall is more sensitive to moisture convergence. The simulated radar reflectivity and vertical velocities indicate that the precipitation system is in stratiform over the areas around first two mountain peaks, which are consistent with observations over a southeast to northwest cross section. However, the model simulates a strong updraft over the upslope of the third mountain peak and graupel over the first and second mountain peaks. Another discrepancy is the maximum rainfall over the coast of Gulf of Genoa simulated by the control run is too high, compared with rain gauge observations. Analysis of the observed and model soundings of IOP-8 at both Gagliari and Genoa implies that although the simulated lower layer is stable, the air is quite moist, thus orographic lifting alone would force the flow to saturation easily and produced more rainfall than the actual case. This will help dry out the flow after passing across the Ligurian Apennines, and reduce the rainfall over the southern Alpine slopes. Opposite to IOP-8, rainfall over the Ligurian Apennines during IOP-2B is lighter than that over the Alps. Effects of Ligurian Apennines and grid resolutions on the precipitation system of IOP-8 over the Alps will be investigated.