Formation and Maintenance Mechanisms of the Stable Layer over the Po Valley during MAP IOP-8

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Abstract During intensive observation period 8 (IOP-8) of the Mesoscale Alpine Program, a strong stable layer formed over Italy’s Po Valley and the northern Ligurian Sea. This stable layer has been shown in previous research to be important for the formation of convection over the Ligurian Sea and the lack thereof over the Po Valley and southern slopes of the Alps. The purpose of this study is to investigate the mechanisms that acted to form and maintain the stable layer during IOP-8. This aim is accomplished through inspection of observed data as well as numerical simulations and sensitivity experiments. Observations and reanalysis data show that starting on 17 October 1999, a relatively cool, stable air mass was advected around the eastern side of the Alps into the lower atmosphere of the Po Valley. Both observational data and model output show this air mass as being blocked as it encountered the western Alps, thus resulting in an accumulation of cool, stable air at low levels in the Po Valley during the ensuing 60 h. When southerly flow approached northern Italy beginning on 20 October 1999, both the western Alps and the northern Alps appeared to help retain the low-level, cool, stable air over the Po Valley. A trajectory and sounding analysis shows that warmer, less stable air originating from over the southern Mediterranean Sea was advected atop the low-lying stable layer within the Po Valley. It is hypothesized that this differential advection, as well as blocking by the western and northern flanks of the Alps, were responsible for the longevity of the stable layer. A series of numerical simulations and sensitivity experiments were performed to test the above hypotheses. These tests support the hypotheses. Other mechanisms were also considered, including blocking of solar radiation by clouds, friction, and evaporative cooling. These simulations revealed that all three processes were critical for the longevity of the stable layer and point to the importance of accurate model representation of subgrid-scale processes.