Thursday, 27 October 2005
Alvarado F and Atria (Hotel Albuquerque at Old Town)
During IOP-8 of the Mesoscale Alpine Program (MAP), a strong stable layer formed over the Po Valley and 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. Herein, we investigate the mechanisms that acted to form and maintain the stable layer during IOP-8 through inspection of observed data as well as numerical simulations and sensitivity experiments. Observations and reanalysis data show two mechanisms that likely acted in concert to form and maintain the stable layer. Starting on 17 October 1999, a relatively cool, stable air mass was advected around the eastern side of the Alps into low levels of the Po Valley. Analyses of observational data and model output show this air mass as being blocked as it encountered the western flank of the Alps, thus resulting in an accumulation of cool, stable air at low levels in the Po Valley during the ensuing 60 h. A trajectory and sounding analysis shows that there was a continued advection of relatively less stable air originating over the Ligurian Sea atop the low lying stable layer in the Po Valley. It appears that this superpositioning of a less stable air mass atop a more stable air mass as well as blocking by the western flank of the Alps was responsible for the longevity of the stable layer. A series of numerical simulations and sensitivity experiments were performed to test the above hypothesis. These tests support our suspicions. In addition, these tests show that one commonly assumed mechanism that acts to maintain stable layers in cases of cold air damming, evaporative cooling, did little to alter the development or life cycle of the stable layer in this case study.
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