Operatinal Use f the Mdel Crcus by French Avalanche Frecast Services E.Brun Meterlgie Natinale Centre d'etudes de la Neige BP 44 Dmaine Universitaire 3842 St-Martin d 'eres France ntrductin Since 1971 the peratinal avalanche frecasting is perated by the french natinal meterlgical service in the Alpes and in the Pyrenees. n the Alps abut 1 snw weather statins prvide twice a day with infrmatins n weather and n snw surface state and nce a week with snw pit infrmatins t describe the internal state f the snw cver. These infrmatins are at the basis f snwcver stability analysis. Even ver a small muntain area snwcver is characterized by a great variability due t different altitudes slpes and rientatin and 1 snw pits in the french Alps are nt enugh t describe the whle natural diversity. Furthermre snwcver state may vary significantly during a week and smetimes the avalanche frecaster waits impatiently fr the next snw prfiles. That is t slve this lack f infrmatins that we have develped a numerical mdel f snw cver which simulates snw prfiles frm the bserved r frecasted weather cnditins. Rapid descriptin f the mdel The numerical mdel named Crcus cmputes the internal and external heat and mass exchanges in rder t simulate snw temperature density and liquid water cntent prfiles and als snw cver stratigraphy. t uses unidimentinallayers f variable thickness. eat cnductin is slved by a Cranck and Nichlsn scheme. Phase changes due t melting r freezing are directly cmputed thugh phase changes between the slid r liquid and the gazeus phases are taken int accunt thrugh an effective thermal cnductivty cefficient. Water transmissin is derived using free water cntent as a maximum value. Snw settlement f each numerical layer is derived accrding t its grains type and size. 31
At the snw atmsphere interface radiative heat exchanges sensible and latent heat fluxes and precipitatins are cmputed using a time centered scheme which avids large errrs in heat exchanges invlving snw surface temperature (i.e sensible and latent heat exchanges and als lngwave radiative utput). The riginal feature f this mdel is t simulate snw metamrphism f each layer and s building up the snw cver stratigraphy. t allws t let match the natural and numerical layers f the snwpack and als t simulate with accuracy snw cver albed accrding t snw surface grains size and shape. Metamrphism is derived using laws that were determined experimentally in cld lab. Experiments were cnducted n the all knwn metamrphism n varius snw types. Mdel validatin The mdel Crcus was tested n the measurements field f Le Cl de Prte lcated in the nrth french Alps (Massif de la Chartreuse 132m a.s.l) in a lightly windy glade. During the winter 88/89 the parameters necessary as input data fr the mdel were measured autmatically nce an hur: air temperature and humidity wind velcity snw and rain precipitatins incming lng wave and shrt wave radiatin. T cntrl the mdel results snw depth snw surface temperature and bttm water run-ff were measured autmatically and nce an hur t. Once a week a snw pit prvided with temperature liquid water cntent and density prfiles. Snw grains were taken ut frm each snw layer and put in sub-freezing is-ctane t stp fr a lng time all metatmrphism. They were later bserved under a micrcscpe t get a perfect descriptin f them. Crcus validatin was cnducted cmparing snw internal and surface bservatins with the results f the simulatin. N reinitializatin appeared t be necessary during the whle winter. A first prfile was deduced frm a pit at december the 17th. and the simulatin stpped at snw melting at may the 7th. Snw depth snw surface temperature and bttm water run-ff are cmpared n figure 1 t 3. After this test the mdel was cnsidered efficient enugh t be used by the french peratinal avalanche frecast services. The riginal feature f this mdel cmes frm its ability t simulate snwpack stratigraphy with accuracy. Operatinal use f the mdel Crcus n the Alps 5 weather statins perate avalanche frecast n the different muntainus massifs lcated in their regin. These weather statins get meterlgical mdels utputs 32
satellite Metesat and AVRR pictures precipitatin radars pictures and all ther stuff necessary fr weather frecast. Fr avalanche frecast they use a sftware n a PC t stre and display snw and weather infrmatins they cllect frm abut 2 snw-weather statins in each regin. These snw-weather statins are mainly hld by patrl services f ski resrts. The sftware n their PC enables t display snw prfiles the evlutin f snw surface and f weather since the last three weeks maps f bserved parameters etc.. The mdel Crcus was adapted t run n their PC and t use as input data the meterlgical data measured by the different snw-weather statins.a run sessin is rganized accrding t the fllwing scheme: - the frecaster chses a gegraphical lcatin and the simulatin perid (usually the last 24 hurs). e chses it frm a file that stres up t 15 predetermined lcatins defined by their altitude latitude slpe rientatin lngitude and smetimes their masks related t slar radiatin. - he chses an initial snw prfile that can be a new prfile deduced frm a snw pit r a prfile derived by a previus simulatin. - he chses t use recrded weather data if he wants t update the derived snw cver r he intrduces frecasted weather data if he wants t simulate snw cver evlutin within the next 24 hurs. - the mdel Crcus is autmatically activated - the frecaster may lk at the hurly simulated prfiles the simulated snw surface temperature and the simulated bttm water run-ff. e may stre n the cmputer sme f these prfiles t use them after as initial prfiles fr a next run which is usually the next day. At a given lcatin the interest f simulating the snw cver evlutin depends naturally n the situatin. The mdel Crcus is very useful t get infrmatins n the fllwing events that affect significantly snw cver stability: - liquid water appearing at snw surface - wetting f internal deep layers - depth f refreezing during the night - ccurence f temperature gradient metamrphism - fresh snw settlement 33
- crust frmatin. Future develpements After using the mdel Crcus during a first winter seasn the avalanche frecasters f the different weather statins judged the mdel very useful but they have regretted t have n time enugh t let run the mdel mre than ver ne r tw lcatins a day especially under wrrying avalanche situatins when they had much wrk. T prgress in the way t use the mdel we have begun an imprtant applicatin t prvide autmatically the mdel with input meterlgical data. t will use meterlgical mdels utputs autmatic and human snw weather bservatins satellite and radar pictures. A meterlgical mdel f limited area has been develped t determine the wind and the air temperature at an hrizntal step f 1 km n a real relief f the french Alps. Optimal analysis methds will prvide with cludiness and precipitatins. Once meterlgical parameters are determined by this applicatin Crcus will run n a central cmputer t simulate the snw cver evlutin n a great number f gegraphical lcatins. The frecasters f the different weather statins will get directly Crcus utputs (snw prfiles) withut t spend any time t let it run. Such a develpment will enable the expert system MEPRA (presented during SSW88) t extract simulatin results fr an autmatic analysis f snwpack stability. 34
Snw Depth Cmparisn 125 -.------------------------ W U1... E () '-../...c -+-'... C (f) 1 75 5 25 f'j' : snw pit : simulatin... depth sensr :: ;\'.' :=:d t:.. \\l'\ i j:i ':i ;: h ::. :': \t\.! : \ - \ 1 '1\'f\ i 17/12 31/12 14/1 28/1 11/2 25/2 11/3 25/3 8/4 22/4 Figure 1 Date
u -.-/ -+-J cd.. S w m -+-J U cd 4--i rj) 'U rj) cd -5-1 -15 Figure 2... -2-2 Snw surface temperature cmparsn.. '..... '.........'.....'....c-:........... '-.. -15-1. \.........'Y........ _-.41. -5.....-... :..:w.....'.......... :!n'.'.'..:..-!f'.'1.. : '.' 1;;... - l!. 4:.i..... '. t... :...s::....... =-... : f..... -... ''..' :...!f'''..... ( f :..' : '. '.'...;.- i >'.. 'f... ' _..'.'..'...;.s :'...:..':(. ;...'. \\..... y.; ;..:.. :. '..W'..''.......... '....1!-......... e..'_'.'....- _... M...'....' :.'...:......-....\:........'..t:. - -. \.&' '... ::...i:co :.' 1..;.....i.. t;t='..... j..i.. '......sa'............ Simulated snw surface temperature (Oc)
Bttm Water Run-ff Cmparisn 7 6 E 5 E...... 5 m2 lysimeter ---- simulatin 5 -+ 4... 3 4 \ dl 1 / / -+- 3 3: w c... ::l 2 >- 2 1 2 3 4 5 ' \ ' 1 + 1 A' ' v. ' \ \ \ ; \\..\.-\J U \ - \ 1'1. f \\ '. 17/12 31/12 14/1 28/1 11/2 25/2 11/3 25/3 8/4 22/4 6/5 Figure 3 Date