Nicole Asencio, Joël Stein and Michel Chong

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 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. 2.5km model 10km model 10km model: convection scheme KAFR 2.5km model: explicit cold microphysics 54 hours 22/10/99 for IOP8 27/09/99for IOP3 20/10/99 25/09/99

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

5. IOP3 Daily Rain validation 25 September 26 September Rain gauges 2.5km model Correlation coefficient first day second day IOP3 0.63 0.81

6. IOP8 Daily Rain validation 20 October 21 October Rain gauges 2.5km model Correlation coefficient first day second day IOP8 0.75 0.58

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

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

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

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

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

13. Sensitivity experiments 10km model during a short period associated to reversal flow duration: 30 hours for IOP3 24 hours for IOP8 2.5km model 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

14. IOP3 Sensitivity experiments NEV EXPERIMENT NMT EXPERIMENT

15. IOP3 Main Origin

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

17. IOP8 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. • 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