Ten years of operational numerical simulations of snow and by eld18221

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									    Ten years of operational numerical simulations of snow and mountain
       weather conditions and recent developments at Météo-France.
    Y. Durand, G. Guyomarc'h, L. Mérindol, J.-G. Corripio, G. Giraud, E. Brun, E. Martin.
  Météo-France, Centre d'Études de la Neige (CNRM), 1441 rue de la Piscine, F 38406 Saint Martin d'Hères.



Introduction: The development of snow                    CROCUS is thus forced by the SAFRAN
evolution numerical modelling in the                     objective analysis which provides hourly
beginning of the nineties at the Snow                    values of temperature, wind, cloudiness,
Study Centre (CEN) of Météo-France has                   humidity, precipitation conditions and
induced the necessity of building a full                 incoming radiations. Except for the
suite of software including an objective                 precipitation where only a daily analysis
analysis program suited for mountainous                  by Optimal Interpolation (OI) is performed
conditions and able to determine relevant                and then split at hourly time steps, all the
meteorological near-surface conditions.                  main quantities are hourly analysed
The purpose was thus to implement an                     through a mixed method (OI and
operational survey of the snow pack                      variational) due to the multi-purpose
(including its internal characteristics and              software design. Several algorithms, such
stratigraphy) in mountainous areas where                 as a precise determination of the snow/rain
no routine snow observation is available.                limit altitude and the use of satellite
Since the winter season 1991-92, the first               information are also incorporated.
version of this chain has been used                      The spatial working grid of the whole
operationally by the French forecasters in               chain is irregular and corresponds to the
the Alps (1995-96 in the Pyrenees).                      massif scale (~400 km2). The models are
Several improvements and developments                    run for seven slope aspects at different
have been then carried out by automatic                  elevations with 300m vertical steps. It is
monitoring and contacts with the users.                  worth noticing that the full system is never
The chain is also being used for researches              re-initialised along the year with snow
in hydrology and in climate change impact                observations; the snow evolution at every
assessment.                                              location being only forced by the
                                                         meteorological inputs.
                                                                 An avalanche hazard diagnosis is
                                                         then provided at each study point by the
                                                         MEPRA model. Results are deducted from
                                                         the estimated mechanical properties of
                                                         each snow layers. The risk of spontaneous
                                                         avalanche is thus based on the comparison
                                                         between the shear strength and the shear
                                                         stress applied to each layers. In case of wet
                                                         snow, the risk can be increased by melting
                                                         destabilisation or decreased by refreezing.
                                                         Concerning the accidental risks, the
Flow chart of the full chain.                            predicted shear stress is increased by the
                                                         effect of overloading due to a skier and
The present models: The suite is                         also typical structures of slabs and weak
organised around the CROCUS snow                         layers are also searched into the snow
model which calculates the energy and                    profile. The risk estimation is completed
mass evolution of the snow cover with a                  with a classification of the avalanche type.
vertical discretization up to 50 levels in               The use of such indications is presented in
including snow metamorphisms and a                       the following figure.
representation of each snow crystals types.
Figure illustrating the underlying CROCUS-
MEPRA processes (vertical structure, used
variables, type of crystals, diagnostic of an
accidental risk on the Belledonne massif, 1 January
96 at 12 UTC)

A forecast version of this chain is also
available and runs at a range of 2 days. It is
based on downscaling operators treating                Figure showing 10 years of comparisons (81-91)
                                                       between measured snow depth (dotted line) and
the information from larger scales NWP                 simulated (solid line) at the Tignes ski resort
models (Arpège or Aladin) and on the use               (Vanoise massif)
of observations of analogous weather
situations of the past.                                The current developments: The present
                                                       line of research aims at a better spatial
                                                       resolution of the provided results,
                                                       especially for the avalanche diagnoses, at a
                                                       finer spatial scale of about 1 km, what we
                                                       call "local scale". This requires to better
                                                       take into account the fine scale forcing of
                                                       the local topography, the effects of which
                                                       are less important at the massif scale and
                                                       different other related phenomena. An
                                                       important point is thus to insert the
                                                       complex effects of the snowdrift which
                                                       transported amounts cannot be ignored so
                                                       as its implications on the snow stability.
Figure showing the Alpine massifs irregular grid       The treatment of such small but important
and four 24h precipitation fields at level 1800 m on   phenomena is presently a challenge due to
a flat aspect. From the left, you find: the SAFRAN     the permanent interaction between snow
analysed field on 11 January 99, the final
                                                       and weather conditions and to their very
forecasted field at this date and the Arpège
downscaled field and the analogous past situation.     fine scale resolution and high complexity
                                                       which often exceeds the present state of the
The operational validation is obtained by              art in current modelling. The used
simulating precise locations where regular             solutions are fine mesh modelling and
manual or automatic snow and weather                   several downscaling operators. Among
observations are performed without                     these operators are the local simulation of
incorporating these data in the analysis               the snow profiles, the modifications of the
scheme or by comparison with different                 velocity field, the snow drift occurrence
independent fields as on the previous                  and the precipitation redistribution. We are
figure                                                 presently working on these different
                                                       subjects as well as on the validation of the
                                                       results.
The SAFRAN wind field is adapted to the              Related applications: SAFRAN and
local topography by a small numerical                CROCUS have been widely used for more
model of potential vorticity, SAMVER,                specific research applications. We shall
and through statistical relationships with           principally quote climate change with
the main features of the local topography.           modifications of the SAFRAN outputs
These last results have been obtained by             according to several climate change
the use of comparisons between SAMVER                scenarii.
and MESO-NH on common runs. The
snow drift effects are simulated on a test
area (2x3 km) on a grid of 45m mesh
(model SYTRON2) and are presently
under evaluation.




                                                     Figure showing the impact of a scenario of a
                                                     temperature increase of 1.8 °C on the snow on
                                                     ground duration in the different alpine massifs at
                                                     level 1500m asl. The duration is reduced from 5 to
                                                     4 months in the Northern Alps and from 3 to 2
                                                     months in Southern massifs.

                                                     Several other studies, especially for
                                                     hydrology have also successfully been
                                                     done. An operational suite of automatic
                                                     estimation of hydrological discharges
                                                     based on the SAFRAN analysis is being
                                                     implemented over the whole French
                                                     territory.
Figure showing the SYTRON2 6hrs modelled
snowdepth difference on 22 March 2002 due to         References:
snow drift phenomena. The scene is located in the    Brun, E., P. David, M. Sudul and G. Brugnot 1992. A
Grandes Rousses massif (orography near 2700m            numerical model to simulate snow cover stratigraphy
asl plotted in bold lines) with eroded areas in         for operational avalanche forecasting. J. Glaciol.,
brown colours and accumulation in green colours.        38(128), 13-22.
The mesh size is about 45m and the plotted wind is   Corripio, J. G., 2003. Snow surface albedo estimation
                                                        using terrestrial photography, Int. Journal of Remote
obtained from SAMVER.
                                                        Sensing . Submitted.
                                                     Durand, Y., G. Giraud, E. Brun, L. Mérindol and E.
The validation of such experiment is very               Martin, 1999. A computer-based system simulating
                                                        swowpack structures as a tool for regional avalanche
difficult. The present used methodology is              forecast. J. Glaciol., 45(151), 466-484.
the comparison of the observed surface               Guyomarc'h G , L. Mérindol, 1998. Validation of an
snow albedo (by the use of digital                      application for forecasting blowing snow. Ann.
                                                        Glaciol., 26, 138-143.
photographs) and the modelled resulting              Lafore, J. P., J. Stein, N. Asencio, P. Bougeault, V.
albedo when the snow crystal features have              Ducrocq, J. Duron, C. Fischer, P. Hereil, P. Mascart,
been modified by the transport effects and              J. P. Pinty, J. L. Redelsperger, E. Richard, and J.
                                                        Vila-Guerau de Arellano, 1998: The Meso-NH
the deposition.                                         Atmospheric Simulation System. Part I: Adiabatic
Other developments are also in progress in              formulation and control simulations. Annales
the framework of the chain as new stability             Geophysicae , 16, 90-10.
                                                     Martin E., E. Brun, Y. Durand, 1994. Sensitivity of the
diagnostics linked to micro mechanical                  French Alps snow cover to the variation of climatic
studies or as new estimations of the liquid             variables. Ann. Geophy. 12, 469-474.
water fluxes.                                        Mérindol L., Y. Durand and G. Guyomarc’h. 2000:
                                                        “Simulation of Snowdrift over Complex Terrain”.
                                                        ICAM 2000, Innsbrûck, 11-15/9/2000.

								
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