Operational Use of the Model Crocus

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1 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

2 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

3 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

4 - 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

5 Snw Depth Cmparisn W U1... E () '-../...c -+-'... C (f) 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

6 u -.-/ -+-J cd.. S w m -+-J U cd 4--i rj) 'U rj) cd Figure Snw surface temperature cmparsn.. '..... ' '.....'....c-: ' \ 'Y _ :..: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)

7 Bttm Water Run-ff Cmparisn 7 6 E 5 E m2 lysimeter ---- simulatin \ dl 1 / / : w c... ::l 2 > ' \ ' 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

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