Origins of the reversed flow over the windward Alpine foothills during MAP IOP3 and IOP8

The synoptic‐scale situations during the two Intensive Observing Periods (IOPs) of the Mesoscale Alpine Programme (MAP) characterized by persisting precipitation and down‐valley flow as observed by the Doppler on the wheel radar (DOW) over the two Alpine valleys (Toce and Ticino) were investigated. The first period (IOP3, 24–27 September 1999) was characterized by a south‐westerly flow and convective precipitation, whereas the second period (IOP8, 20–21 October 1999) by a down‐valley flow and stratiform precipitation. During IOP3, the down‐valley northerly flow over the north‐western part of the Po valley was confined within a thin layer of 200–300 m depth. During IOP8, the northerly flow extended from the Alps foothills to the Ligurian sea in a deep layer of 1–2 km height. The Meso‐NH numerical model with a 2.5 km horizontal mesh grid reproduced the characteristics of both cases from the valley scale to the meso‐β scale. Sensitivity experiments were, furthermore, performed in order to highlight the relationships between wet or dry drainage flow channelled by the Alpine orography, mesoscale upstream flow blocked by the Alps, and a regional cold pool at the foothills inside the Po valley. During IOP8, the diabatic processes were dominant at local scales in confirmation of Steiner results based on the DOW observations and they were significant at regional scales upstream of the Alps by maintaining a cold pool over the Po valley. The IOP3 low‐level flow reversal occurred as the flow began to cross over the Alps instead of turning around. A downslope flow is associated with the blocked‐regime period upstream of the Alps. Copyright © 2006 Royal Meteorological Society