SNOW AND AVALANCHES FORECASTING OVER THE ANDES MOUNTAlNS. Jose A. Vergara* Departamento de Geoflsica, Universidad de Chile

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1 SNOW AND AVALANCHES FORECASTNG OVER THE ANDES MOUNTAlNS Jse A. Vergara* Departament de Geflsica, Universidad de Chile Rene len, Camins y Nieve, ANDNA-CODElCO, CHLE ABSTRACT: During the last three years (1998-0), the University f Chile and COOElCO-ANONA has directed significant research twards the implementatin and evaluatin f an peratinal reginal weather frecast ver the Andes Muntains (Vergara et. A, 1999). The implementatin is designed t prvide traditinal synptic frecasts at high-reslutin grid spacing and lcal weather frecasts fr prviding warning f avalanches during winter time. The current analysis uses data based n daily snw precipitatin in lagunitas (2670 m abve msl), lcated in the central part f Chile f the Andes Muntains, as well as daily mdel frecasts. Fr this statin the perid study cvers the last years. The 1997 and 0 winter was ne f the wettest winters during the last 30 years. This paper explres the perfrmance f snw and avalanches frecasts during several cases f strng weather cnditins verthe Andes Muntains (Figure 1) during E NiJi year f 1997 and cmpares the mdel results with snw bservatins. The results shw that the 24 hur frecasts explain abut 80% f the variance in snw depth data. The study demnstrates the peratinal capability f the mdel t frecasts ttal snw depth during the strm. Finally, the mdel is shwn t be an effective tl fr supprt f avalanches frecasts ver the Andes Muntains due t the strng relatinship between snwfall and avalanches. KEYWORDS: Snw, avalanches, meterlgy, Andes Muntains, Chile 1. NTRODUCTON The Andes muntains, the lngest chain f muntains in the suth hemisphere, shws a strng and cmplex variatins in the snw and meterlgical cnditins as ne mve frm east t west, because the Andes is the narrws and lngest muntains in the wrld. The annual mean snwfah in the Andes central part Chile is clse t 10 meters, with a strng interannual variability frm 2.2 meters n 1a Nina year f 1998 t meters n the el Nin years f 1972 (Figures 2 and 3), and the snw may fau in the any time n the winter frm April t September with a maximum in June/July (Figures 3 and 4). During the winters the means minimum temperatures is clse t -10C and the maximum mean temperature is clse 12C (Figure 5). The high temperatures n the muntains usually take place immediately fllwing the strms assciate t anticyclne subsidence, and the avalanches frmatin n the study area (Figure 1) is prprtinal t the number f hurs f clsed rads due t avalanches (Figure 6), them the maximum avalanches frmatin are maximum in June-July (Figure 7). But the rati f the number f avalanches t the ttal snwfall f the strm increase during the secnd part f the winter and spring time because the temperature increase dramaticahy and them melting (Figure 7). Because the strng relatinships frm the since f the strm and the number f avalanches (Figures 6 and 8). The weather bservatin a frecasts are the primary tls fr predicting the ttal snw precipitatin n the strm, the avalanches ptential and hurs f rads are clse due t the avalanches. Crrespnding authr address: Jse Vergara, Departament de Gefisica Universidad de Chile, Santiag-Chile; jvergara@dgf.uchile.cl. 566

2 Figure 1 Snw depth stake at Lagunitas statins (2670 m abve ms/); Frm September f 1998 at the end f the winter (right), ne f the driest winters f the last ne hundred years and September f 0 (left) ne fwettest last thirty years. 567

3 Fjgure 2: Piuquenes t Haulage Rads thugh the avalanches area, during f year 1998 winter (tp), and 0 (bttm). 568

$. ""'\ \ Days Figure 3: Time evlutin fsnwpack depth during the EJ Nin year 1997 (circle) and La Nina year 1998 (squared) and 0 year (triangle) at Lagunitas statin central part f Chile (Figure 2).$

4 2. RESULTS 75O H&-ti-..-t.H-+--t--t LlO 'l"'-fq'"! 1SO *i-O't Jj, !600 j S1 1S1 2" J ""'\ \ Days Figure 3: Time evlutin fsnwpack depth during the EJ Nin year 1997 (circle) and La Nina year 1998 (squared) and 0 year (triangle) at Lagunitas statin central part f Chile (Figure 2). This paper explres the perfrmance f the NCEP Reginal Spectral Mdel (Vergara, et.al, 1998) during winters f 1997 (E Nifi) and 1998 (La Nina). The winter f 1997 ne f the wettest n recrd is imprtant in terms f precipitatin. The 1998 winter was ne f driest winters f the last ne hundred years (Figure 2) and n imprtant rainfall events ccurred. During bth years the RSM was run peratinally and several cases fextreme weather events ccurred during these years (in particular during the 1997 winter). J 1\ \.. 1/ 1\,/. " 1\ j Y..... r-... LJ.., ",, JAN MAR MAY JUL SEP NOV Mnth n rder t evaluate the mdel perfrmance, mdel utputs fr the tw-years are crrelated with bserved precipitatin data. The RSM precipitatin frecasts ver the Andes Muntains in the central part f Chile explain abut 70% f variance in the bserved daily rain data, with explained variance f 78% fr the 24 hur frecasts and 66% fr the 48 hur frecasts with a mean errr clse t 25 mm (Vergara, 0). Snwfall frecasts at Lagunitas lcal statin explain abut 81 % (24 hur frecasts, Figure 9a) and 78% (48 hur frecasts, Figure 9b) f the snwdepth, with a mean errr clse t 20cm (24 hurs) and 3Ocm( 48 hurs). The RSM prvides accurate frecasts f the heaviest rainfall and snwfall events ver Chile and the Andes Muntains. These frecasts f heavy events are f particular interest since they can prvide better avalanches frecasts and warning due t the strng relatinship between snwfall and avalanches (Figures 6 and 8). The high reslutins weather frecast, is able t prvide better frecasts fr lcal weather cnditins because f its ability t reslve small scale weather features and the cmplex and strng slpe ver the Andes tpgraphy D Hl -15!,...., 1""-,- 1'- '". -, A:r- 4A JUL " 1M SEP J'm.... L.!... J Figure 5: Annual cycle f maximum (cntinues line) and mean (dts line) temperature in the Lagunitas statin. '.. i9ure 4: Annual cycle f maximum (cntinues hne) and mean (dts line) snw precipitatin in the Lagunitas statin. 569

5 1 -,-----,------,--,---...,.----,----; '" '---"'-q'--"--r---+--j---1 :; :; e _------b-----j '" Q +--_j '-a---h-+-_1 ' : j ::: +--_j y,-::--+---, Ttal Annual Snw (em) Figure 6: The annual number f hurs f clsed rads due t avalanches as a functin f ttal annual snw at Lagunitas statin (2670 msnm) ,-----,------,-----, t----- fn :s '_t_---l _j ---j Q) g' _j ---j "S 120 -j 'f, , '" , Q) ii f-:;;;:-f _j ---j C'" ,, _j ---j 24 -tiii;;-::i...---t----r---t------j OF-= l Ttal Snw (em) Figure 8: The number f hurs f clsed rads due t avalanches as a functin f ttal snw during 1997 at Lagunitas statin {2670 msnm). f "''\, J J \ /.. "'".. "- \.... JAN MAR MAY JUL SEP NOV Figure 7: Annual cycle f the number f maximun (cntinues line) and mean (dts line) hurs f clsed rads due t 1997 at Lagunitas statin. 3. CONCLUSONS Perfrmance f the peratinal frecast system ver the Andes Muntains in the central part f Chile was presented. The bjective f the system is t prvide better snwfall frecasts fr synptic and messcale phenmena. The mdel demnstrated an peratinal capability t run successfully ver Suth America (in particular Chile) and successfully simulated strng rainfall events, especially during the El Nin event f 1997/1998. The mdel prved t be an effective tl fr the supprt f muntains weather snwfall and rain frecasts ver the chilean muntains. 4. ACKNOWLEDGEMENTS This study was supprted by grants FONDECYT , COOELCO ANONA

6 .,- 2 S 150 n w 100 (c m ) 50 Lagunitas Vergara, J., 0: Snw and Rain frecasting ver the Andes Muntains, AWRA"s Spring Specialty Cnference "Water Resurces n Extreme Envirnments", FORECAST 24 Hurs (mm) r Lagunitas SN W (c 100 m) ,-ifl f-----ir FORECAST 48 Hurs (mm) Figure ga, b: Regressin f 24 (tp) and 48 (bttm) h<:>ur snw frecasts n cntrl statin bservatins. The slid line represents the linear regressin. 5. Referen ces Vergara. J., Juang, H.-M and S-Y Hng, 1998: An evaluatin f the reginal frecasting fr Chile-Suth America, AMS 12 th Cnference n Numerical Weather Predictin, Vergara, J A. Ellena and R. Len, 1999: La meterlgia en el mantenimient de camins de alta mntana, V Cngres Previal Chile,

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