Effects of diabatic cooling on the formation of convective systems upstream of the Apennines during MAP IOP-8

In this research we have examined the effects of the Apennines and a stable layer in the Po Valley on orographic convection over the Alps and surrounding areas through a series of 2 dimensional, idealized simulations. In the 1MTN simulation (a single mountain simulation representing the Alps), convection was very localized over the peak of the Alps. In the 2MTN simulation, which included a smaller mountain representing the Apennines upstream of the Alps, there was little difference in the convective regime, the location and amount of the precipitation maximum, and the scale and location of the convective cells. However, in the case of two mountains with prescribed cooling over the idealized Po Valley (referred to as STBL), the convective systems were modified dramatically with downstream propagating cellular convection developing above the Po Valley and upstream of the Apennines. As the convection upstream of the Apennines became more vigorous, the supply of warm moist air to the the region above the Po Valley was cut off thereby reducing the strength of the convection above the Po Valley.

The induced stable layer in the STBL simulation is believed to behave as effective terrain connecting the Apennines and Alps. Therefore, the effective slope of the mountain in the STBL simulation is much gentler than in the other simulations. It is hypothesized that since the lifting associated with convergence along this gentler slope occurs over a larger area, a larger region is destabilized leading to the development of cellular convection. To test this notion, a simulation called EMTN was performed where the terrain was modified such that it occupied the same area as the stable layer in the STBL simulation. In this simulation, the nature of the convection was very similar to that in the STBL simulation, with cellular convection existing along the gentle portion of the slope. From these simulations we can conclude that (1) the introduction of the Apennines alone does not significantly alter the nature of the convection; (2) the introduction of a stable layer in the Po Valley does modify the nature of the lifting leading to the development of cellular convection over the Po Valley; (3) a lack of convection along the slopes of the Alps in blocked flow events (case STBL) may not be entirely due to low Froude number flows in the Po Valley, but could also be due to modification of the incoming airstream by the convection induced by the effective terrain upstream of the Apennines, and (4) the stable layer between the Apennines and Alps behaved as an effective mountain.