P5.25 NUMERICAL SIMULATIONS OF HEAVY RAINFALL DURING THE MESOSCALE ALPINE PROGRAM (MAP)

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1 P5.25 NUMERICAL SIMULATIONS OF HEAVY RAINFALL DURING THE MESOSCALE ALPINE PROGRAM (MAP) Jmes A. Thurmn, Y.-L Lin*, nd J.J Chrney North Crolin Stte University, Rleigh, North Crolin 1. Introduction To ssess the chrcteristics of hevy precipittion over the Lgo Mggiore region of northern Itly, we selected two IOP s from the MAP Field Study of Fll The IOP-2B (19 Septemer) nd IOP-8 (19-21 Octoer) events were selected for intensive study ecuse they represent two precipittion events tht exhiit similr low-level flow ptterns, yet differ in the mount of oserved precipittion (Houze et l., 2000). The precipittion in IOP-2B developed in ssocition with cold frontl pssge cross the southern Alps s trough moved from western Europe into northern Itly. Hevy rinfll developed on the western side of Lgo Mggiore s moist inflow from the south nd southwest over the Mediterrnen Se impinged on the steeply sloping terrin of the southern Alps. This southerly low level flow hs een seen in other Alpine events (see Sénési et l., 1996; Msscnd et l., 1998; Buzzi et l., 1998; Buzzi nd Foschini, 2000; Fehlmnn nd Qudri, 2000; Tripoli et l., 2000) Precipittion in IOP-8, in contrst to the convective environment of IOP-2B, developed in more stle environment hed of front s 850 hp southesterly flow overrn surfce cool lyer. In order to understnd these events etter, numericl simultions using the PSU/NCAR MM5 model were performed. Ech IOP event ws simulted using triple nested grid of 45, 15, nd 5 km resolution. 2. Results Following re results of the simultions for IOP-2B nd IOP IOP-2B Numericl simultions for IOP-2B cptured the low pressure system off the west cost of Irelnd (Fig. 1) nd deep upper level trough over the estern Atlntic (Fig. 2). A rod 300 hp jet ws locted over Spin with diffluence over the Alps. Throughout the simultion, the precipittion ws locted with the Corresponding Author Address: Yuh-Lng Lin Deprtment of Mrine, Erth, nd Atmospheric Sciences, Box 8208, North Crolin Stte University, Rleigh, NC emil: yl_lin@ncsu.edu confluent southwest, south inflow towrd the western Alps (Fig. 3). The southern re of the rinfll ws closely ligned with the inflow while the northern re of the rinfll ner the concve region of the Alps ws ssocited with the esterly flow perpendiculr to the mountins. A contriuting fctor to the instility ssocited with IOP-2B ws the northwrd trnsport of tongue of high θ e 850 hp ir from northern Afric over the Lgo Mggiore region etween 0000 UTC 20 Septemer nd 1200 UTC 20 Septemer 1999 (Fig. 4). The southern re of the rinfll ws just west of this high θ e flow. Also during this time, the Lgo Mggiore region ws under 300 hp diffluence, which ws collocted with low-level confluence nd orogrphic lifting, leding to scent nd rinfll. 2.2 IOP-8 In contrst to IOP-2B, the dominnt moist flow for IOP-8 ws from the southest off the Adritic Se. At the time of the simultion initiliztion, 1200 UTC 19 Octoer 1999, low pressure system ws west of the Alps over the By of Biscy. Twenty-four hours lter (1200 UTC 20 Octoer), closed low ws over the By of Biscy (Fig. 5). Initilly t upper levels, rod zonl 300 hp jet extended from Spin into Russi with trough west of Frnce over the Atlntic. By 0000 UTC 21 Octoer, jet strek hd formed long the Mediterrnen cost of Frnce with diffluence over the Lgo Mggiore region (Fig. 6). Twelve hours lter, the right entrnce region ws just northwest of Lgo Mggiore, possily providing lifting vi geostrophic circultions or upper level divergence(fig. 6). Rinfll totls were not hevy until 36 hours into the simultion (0000 UTC 21 Octoer). As with IOP-2B, the rinfll then propgted estwrd (Fig. 7). Exmintion of the 5 km output shows the low-level esterly flow turning to southest flow over the Adritic nd flowing into northest Itly. A confluence zone cn lso e seen in northest Itly s northest, esterly, nd lter southesterly winds cme together inlnd from the Adritic (Fig. 7). As previously noted, the rinfll for IOP-8 developed s southesterly flow overrn surfce cool lyer. A model cross section (Fig. 8) shows the surfce cool lyer over the sme period s the rinfll (36-48 hours) (See Fig. 7 for cross section loction). The isentropes show the loction of the surfce cool ir with

2 the flow coming off the Adritic nd rising over the terrin. 2.3 Rinfll Totls The 48 nd 60 hour ccumulted precipittion (mm) for IOP-2B nd IOP-8 respectively re shown in Figure 9. The model results were consistent with oservtions in tht the ccumulted rinfll for IOP-2B ws lrger thn for IOP-8. For IOP-2B, the oserved totls over Lgo Mggiore rnged from mm (Houze et l., 2000). Model ccumulted precipittion rnged from mm over Lgo Mggiore. For IOP-8, oserved totls over Lgo Mggiore rnged from mm (Houze et l., 2000) with model ccumultions of mm. It ppers tht the model did dequtely in simulting the rinfll mounts. 3. Summry in MAP. Preprints, Ninth Conf. on Mountin Meteorology, Aspen, CO, Amer. Meteor. Soc., 1-5. Msscnd, A., H. Wernli, nd H.C. Dvies, 1998: Hevy precipittion on the Alpine south-side: An upper-level precursor. Geophys. Res. Lett., 25, Sénési, S., P. Bougeult, J.-L. Chèze, P. Cosentino, nd R.-M. Thepenier, 1996: The Vison-L-Romine flsh flood: Mesoscle nlysis nd predictility issues. We. nd Forecsting, 11, Tripoli, G.J., G. Pnegrossi, A. Mugni, S. Dietrich, nd E. Smith, 2000: Orogrphiclly induced flsh floods on the northern Itlin cost. Preprints, Ninth Conf. on Mountin Meteorology, Aspen, CO, Amer. Meteor. Soc., Simultions using the MM5 model showed the evolution of rinfll over the Lgo Mggiore region of northern Itly for two MAP events, IOP-2B nd IOP-8. IOP-2B ws mrked y south/southwest low-level flow impinging ginst the mountins. Also, low level flow of high θ e ir into the Lgo Mggiore region from North Afric dded to the instility of the tmosphere leding to rinfll production. The results for IOP-8 showed the southesterly flow off the Adritic flowing into the Alps with the 850 hp flow overrunning surfce cool lyer nd rising over the terrin. Acknowledgements This reserch is supported y NSF Grnt ATM Prt of the computtions performed t North Crolin Supercomputer Center 4. References Buzzi, Andre, Nziori Trtglione, nd Piero Mlguzzi, 1998: Numericl simultions of the 1994 Piedmont Flood: Role of orogrphy nd moist processes. Mon. We. Rev., 126, , nd L. Foschini, 2000: Mesoscle meteorologicl fetures ssocited with hevy precipittion in the southern Alpine region. Meteorol. Atmos. Phys., 72, Fehlmnn, R. And C. Qudri, 2000: Predictility issues of hevy Alpine south-side precipittion. Meteorol. Atmos. Phys., 72, Houze, R., S. Medin, nd M. Steiner, 2000: Two cses of hevy rin on the Mediterrnen side of the Alps Figure km resolution se level pressure (every 4 hp) nd winds (1 full r equls 10 ms -1 ) for ) 0000 UTC 19 Septemer nd ) 0000 UTC 20 Septemer 1999.

3 Figure km grid 300 hp heights (every 120 m), isotchs (every 20 ms -1 ), nd winds (1 full r equls 10ms -1 ) for 0000 UTC 19 Septemer Figure km grid 6 hour ccumulted rinfll (mm) nd winds (1 full r equls 10ms-1) for 0300 UTC 20 Septemer Bold line is 1000 m terrin elevtion contour. Dot represents pproximte loction of Lgo Mggiore. Figure km grid 850 hp θ e nd wind vectors (convention s in Figure 3) for ) 0000 UTC 20 Septemer nd ) 1200 UTC 20 Septemer Bold line is 1000 m terrin elevtion contour.

4 Figure 5. As for Figure 1 except for ) 1200 UTC 19 Octoer nd ) 1200 UTC 20 Octoer Figure 6. As for Figure 2 except for ) 0000 UTC 21 Octoer nd ) 1200 UTC 21 Octoer Figure 7. 6 hour ccumulted rinfll (mm) for 0000 UTC 21 Octoer for ) 15 km grid nd ) 5 km grid. Bold line is 1000 m terrin elevtion contour. Stright line running northwest to southest shows the loction of the cross section in Figure 8. Dot represents Lgo Mggiore.

5 Figure km grid cross section of Potentil temperture (every 2 K) nd geostrophic circultion vectors for ) 0000 UTC 21 Octoer nd ) 0600 UTC 21 Octoer. Loction of cross section is shown in Figure 7. Northest is into pge. Figure 9. ) 48 hour ccumulted rinfll (mm) for IOP-2B for the period 0000 UTC 19 Septemer to 0000 UTC 21 Septemer nd ) 60 hour ccumulted rinfll (mm) for IOP-8 for the period 1200 UTC 19 Octoer to 0000 UTC 22 Octoer Bold line is 1000 m terrin elevtion contour nd the dot represents Lgo Mggiore.