1. A comparison of the down valley flow for the MAP IOP8 and IOP3 with the numerical laboratory Mesonh model. Nicole Asencio, Joël Stein and Michel Chong CNRM Météo-France and Laboratoire d’Aérologie.

2. Presentation Introduction and numerical set-up. IOP3 and IOP8 rain and wind validations. Mesonh numerical laboratory to compare IOP8 and IOP3 reversal flow origins. conclusion

3. Numerical Set-up 10km model 2.5km model Non-hydrostatic MESONH model Two-way interactive grid-nesting models Microphysics with ice. Turbulence 1D BL89. Arpege analyses for the initialization and the coupling every 6 hours. 10km model: convection scheme KAFR 2.5km model: explicit cold microphysics 20/10/99 25/09/99 22/10/99 for IOP8 27/09/99 for IOP3 54 hours

4. Synoptic presentation 500hPa Surface D D L L A H H 20 October 1999 1200UTC

5. IOP3 Daily Rain validation 25 September26 September Rain gauges 2.5km model Correlation coefficient first day second day IOP3 0.63 0.81

6. IOP8 Daily Rain validation 20 October21 October Correlation coefficient first day second day IOP8 0.75 0.58 Rain gauges 2.5km model

7. Wind vectors and Potential temperature at 450m ASL Wind vectors and Potential temperature at 450m ASL Cold pool K

8. zoom: Wind vectors and Potential temperature at 450m ASL zoom: Wind vectors and Potential temperature at 450m ASL DOW K

9. IOP8 and IOP3 comparison: meridional wind at the valleys outflow IOP8IOP3 W WEE Toce+Ticino Valleys Toce+Ticino valleys Como Valley Deep layer maximum 1km-2km Thin layer maximum 200-400 meters

10. Presentation Introduction and numerical set-up. IOP3 and IOP8 rain and wind validations. Mesonh numerical laboratory to compare IOP8 and IOP3 reversal flow origins. conclusion

11. IOP3 diagnostics RAIN 0-450m WIND DIRECTION 2.8-6km WIND INTENSITY Upper-level wind intesity rain Outflow

12. IOP8 diagnostics RAIN 0-450m WIND DIRECTION 2.8-6km WIND INTENSITY Upper-level wind intesity rain Outflow

13. Sensitivity experiments Inside a small area associated to accumulated rain location: Nev = no evaporation cooling Nmt = no melting and no evaporation cooling Slope= no melting and no evaporation cooling above relief 10km model 2.5km model during a short period associated to reversal flow duration: 30 hours for IOP3 24 hours for IOP8

14. IOP3 Sensitivity experiments NEV EXPERIMENT NMT EXPERIMENT

15. IOP3 Main Origin

16. IOP8 Sensitivity experiments RAIN REF 0-450m WIND DIRECTION 2.8-6km WIND INTENSITY NEV NMT SLOPE

17. conclusion IOP8 The reversal flow within the Toce valley mainly originates from the subsidence induced by the melting over the orography. The diabatic effects over the plain contribute to maintain the cold low-level airmass which spreads over the western part of the Po valley. The reverse flow extension over the plain is driven by the cold pool intensity. IOP8 The reversal flow at the regional scale is not sensitive to the diabatic effects, the down-valley flow inside the Toce valley is linked to diabatic mechanisms above the relief. The upstream flow intensity inducing the transition from blocked to unblocked regime drives the duration of the reverse flow. IOP3