1/15 Orographic forcing and Doppler winds, the key for nowcasting heavy precipitation in the mountains Luca Panziera, Urs Germann MeteoSwiss, Locarno-Monti, Switzerland Federal Office of Meteorology and Climatology MeteoSwiss

2/15 ITALY SWITZERLAND Milan Turin Zürich Verona E u r o p e a n A l p s Lago Maggiore region

3/15 Sketch: Kappenberger cold front cold air warm air Jet N get estimate from Doppler Orographic forcing of heavy rainfall Gulf of Genova Venezia Vienna get estimate from radio- sounding + mesonet Temporal + spatial distribution of orographic rainfall is related to mesoscale wind flow and thermo- dynamic upstream conditions. Houze et al 2001 Medina+Houze 2003 Ferretti et al 2000 Hypothesis: If mesoscale wind and thermodynamic conditions explain space-time patterns of heavy orographic rainfall we can use Doppler and radiosounding for nowcasting. (large-scale persistent precipitation, not isolated convection)

4/15 Sketch: Kappenberger cold front cold air warm air Jet Steps Gulf of Genova Venezia Vienna Test hypothesis (92 days of pcp) 1 technique to estimate mesoscale wind from Doppler thermodynamic conditions from radiosoundings Superposed epoch analysis 92 days of precipitation Usage for nowcasting 2 look at individual events and validate idea to use mesoscale wind and thermodynamic conditions as predictors of orographic pcp Hypothesis: If mesoscale wind and thermodynamic conditions explain space-time patterns of heavy orographic rainfall we can use Doppler and radiosounding for nowcasting. (large-scale persistent precipitation, not isolated convection)

5/15 6km 0km Radar Lema100km NW100km SE Low Level Flow Low Level Flow (LLF): km height, lowest sweeps: Low-Level inflow Upper Level Flow (ULF): km height, large ring around radar: Synoptic flow Mesoscale winds: LLF+ULF Upper Level Flow

6/15 6km 0km Radar Lema 100km NW100km SE Mid Level Flow Cross Barrier Flow Mid Level Flow (MLF): km height, narrow circle around radar: Flow ascending over terrain Cross Barrier Flow (CBF): km height, just south of the Alpine crest: Detect cold front crossing the Alps Mesoscale wind: CBF + MLF

7/15 Sine fitting: v r (az i ) = a 1 cos (az i ) + a 2 sin (az i ) Solve multiple regression using normal equations and Singular Value Decomposition. Automatic multi- parameter quality control of the wind estimates. Estimate wind flow Elevation angles: orange 5.5° black 4.5° yellow 3.5° green 2.5° 30m/s towards 30m/s away Direction of flow Intensity of flow Dealiased Doppler wind 1kmX1°,5 min.

8/15 Lago Maggiore area (~8000 km 2 ) 92 days of orographic precipitation For the period Jan 2004-May 2008 we found 49 heavy precipitation events: totally 92 days Large-scale long- lasting orographic precipitation. Not isolated convection

9/15 Precipitation field (QPE) QPE, 1km, 5min 150km mm/h Automatic hardware calibration (since 1993), automatic system monitoring (since 1993), strategy to avoid clutter (since 1993), 9-step clutter elimination (since 1993/1999), beam-shielding correction (since 1997), profile correction (since 2001), bright-band correction (since 2001), long-term gauge adjustment (since 2003). Germann + Joss, Springer, Germann et al, QJRMS, 2006.

10/15 Superposed epoch analysis of 92 days: flow mm/h # of hours 120 km Rain rate # of events White line: 800 m orographic level <5 hours

11/15 Superposed epoch analysis of 92 days: flow+stability Froude number U Alps H U: mean layer perpendicular wind (from S-SE Low Level Flow) N m : mean layer moist Brunt-Väisälä frequency (from stations and radiosoundings) H: barrier height (3000 m)

12/15 Froude number for Low Level Flow from South and South-East Fr not def.(N 2 <0)0<Fr<1Fr>1 N 2 from stations (LOM/CIM) N 2 from radiosoundings N 2 from stations (SBO/GEN) mm/hRain rate

13/15 3 October 3 October October 500 hPa geopot. height Looking at individual events surface analysis Locarno Monti 78mm in 1h Brissago 200mm in 24h 31 hours of precipitation in Lago Maggiore region

14/15 A decrease of intensities of the flows seem to anticipate the end of the persistent heavy orographic rain. The change of direction of the flows also is noticed just before the end of the precipitation South East West North Flow intensity m/s Rainfall amount mm (4 catchments) Flow direction 21UTC9UTC

15/15 Superposed epoch analysis shows that: Wind speed Wind direction Intensity of the rain Location of the maxima accumulations Stability of the lower atmosphere Intensity of the rain Conclusions What next? Include time information in superposed epoch analysis Introduce orographic forcing into probabilistic extrapolation nowcasting

16/ mm/h Thank you for your attention! 120km