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| Titel |
Defining predictand areas with homogeneous predictors for spatially coherent precipitation downscaling |
| VerfasserIn |
Sabine Radanovics, Jean-Philippe Vidal, Eric Sauquet, Aurélien Ben Daoud, Guillaume Bontron |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250076304
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| Zusammenfassung |
| Statistical downscaling aims at finding relationships between local precipitation (predictand)
and large-scale predictor fields, in various contexts, from medium-term forecasting to climate
change impact studies. For distributed hydrological modelling the downscaled precipitation
spatial fields have furthermore to be coherent over possibly large river basins. This study
addresses this issue by grouping coherent predictand areas in terms of optimised predictor
domains over the whole of France, for an analogue downscaling method developed by Ben
Daoud et al. (2011).
This downscaling method is based on analogies on different variables: temperature,
relative humidity, vertical velocity and geopotentials. These predictor variables are taken
from ERA40 at 2.5 degree resolution and local precipitation over 608 climatologically
homogeneous zones in France are taken from the Safran near-surface atmospheric reanalysis
(Vidal et al., 2010). The predictor domains for each zone consist of the nearest grid cell for all
variables except geopotentials for which the optimum domain is sensitive to the predictand
location. For large catchments with diverse meteorological influences it is thus beneficial to
optimise the predictor domains individually for areas with different influences (e.g. Timbal et
al., 2003). The drawback is that different predictor domains may provide inconsistent values
between elementary zones. This study therefore aims at reducing the number of different
predictor domains by grouping the predictand areas that may use the same predictor
domain.
The geopotential predictor domains were first optimised for each of the 608 zones in the
Safran data separately. The predictive skill of different predictor domains is evaluated with
the Continuous Ranked Probability Skill Score (CRPSS) for the 25 best analogue days
found with the statistical downscaling method averaged over 20 years. Rectangular
predictor domains of different sizes, shapes and locations are tested, and the 5 ones that
lead to the highest CRPSS for the zone in question are retained. The 5 retained
domains were found to be equally skillfull with a maximum difference of around
1% of CRPSS on average, and are thus all candidates for clustering predictand
zones.
An objective procedure has then been implemented for clustering zones together, based
on their sharing a common predictor domain inside their 5 near-optimal domain ensemble.
For zones sharing several near-optimal predictor domains, the aim was to minimise the
number of disjoint predictand areas. Furthermore solutions that lead to more similar sized
areas were preferred. This procedure defines areas with natural spatial coherence and reduces
the number of different predictor domains using a procedure based on objective rules, unlike
most of studies where this is done either subjectively or arbitrarily. It allowed to reduce
significantly the number of independent zones and to identify large homogeneous areas
encompassing relatively large river basins. Further developments will address the issue of
spatial coherent downscaling for predictand areas that do not share any near-optimal predictor
domains.
Ben Daoud, A., Sauquet, E., Lang, M., Bontron, G., and Obled, C. (2011).
Precipitation forecasting through an analog sorting technique: a comparative
study. Advances in Geosciences, 29:103-107. doi: 10.5194/adgeo-29-103-2011
Timbal, B., Dufour, A., and McAvaney, B. (2003). An estimate of future climate
change for western France using a statistical downscaling technique. Climate
Dynamics, 20(7-8):807–823. doi: 10.1007/s00382-002-0298-9
Vidal, J.-P., Martin, E., Franchistéguy, L., Baillon, M., and Soubeyroux, J.-M.
(2010) A 50-year high-resolution atmospheric reanalysis over France with
the Safran system. International Journal of Climatology, 30:1627-1644. doi:
10.1002/joc.2003 |
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