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| Titel |
Probabilistic precipitation and temperature downscaling of the Twentieth Century Reanalysis over France |
| VerfasserIn |
Laurie Caillouet, Jean-Philippe Vidal, Eric Sauquet, Benjamin Graff |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250105513
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| Publikation (Nr.) |
 EGU/EGU2015-5044.pdf |
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| Zusammenfassung |
| This work proposes a daily high-resolution probabilistic reconstruction of precipitation and
temperature fields in France over the last century built on the NOAA 20th century global
extended atmospheric reanalysis (20CR, Compo et al., 2011). It aims at delivering
appropriate meteorological forcings for continuous distributed hydrological modelling over
the last 140 years. The longer term objective is to improve our knowledge of major historical
hydrometeorological events having occurred outside of the last 50-year period, over which
comprehensive reconstructions and observations are available. It would constitute a perfect
framework for assessing the recent observed events but also future events projected by
climate change impact studies.
The Sandhy (Stepwise ANalogue Downscaling method for Hydrology) statistical
downscaling method (Radanovics et al., 2013), initially developed for quantitative
precipitation forecast, is used here to bridge the scale gap between 20CR predictors –
temperature, geopotential shape, vertical velocity and relative humidity – and local
predictands – precipitation and temperature – relevant for catchment-scale hydrology.
Multiple predictor domains for geopotential shape are retained from a local optimisation over
France using the Safran near-surface reanalysis (Vidal et al., 2010). Sandhy gives an
ensemble of 125 equally plausible gridded precipitation and temperature time series over the
whole 1871-2012 period. Previous studies showed that Sandhy precipitation outputs are very
slightly biased at the annual time scale. Nevertheless, the seasonal precipitation
signal for areas with a high interannual variability is not well simulated. Moreover,
winter and summer temperatures are respectively over- and underestimated. Reliable
seasonal precipitation and temperature signals are however necessary for hydrological
modelling, especially for evapotranspiration and snow accumulation/snowmelt
processes.
Two different post-processing methods are considered to correct monthly precipitation and
temperature time series. The first one applies two new analogy steps, using the sea surface
temperature (SST) and the large-scale two-meter temperature. The second method is a
calendar selection that keeps the closest analogue dates in the year for each target date. A
sensitivity study has been performed to assess the final number of analogues dates to retain
for each method. A comparison to Safran over 1958-2010 shows that biases on the
interannual cycle of precipitation and temperature are strongly reduced with both methods.
Using two supplementary analogy levels moreover leads to a large improvement
of correlation in seasonal temperature time series. These two methods have also
been validated before 1958 thanks to both raw observations and homogenized time
series.
The two post-processing methods come with some advantages and drawbacks. The calendar
selection allows to slightly better correct for seasonal biases in precipitation and is therefore
adapted in a forecasting context. The selection with two supplementary analogy levels would
allow for possible season shifts and SST trends and is therefore better suited for climate
reconstruction and climate change studies.
ÂÂÂCompo, G. P. et al. (2011). The Twentieth Century Reanalysis Project. Quarterly
Journal of the Royal Meteorological Society, 137:1–28. doi: 10.1002/qj.776
ÂÂÂRadanovics, S., Vidal, J.-P., Sauquet, E., Ben Daoud, A., and Bontron, G. (2013).
Optimising predictor domains for
spatially coherent precipitation downscaling. Hydrology and Earth System Sciences,
17:4189-4208. doi:10.5194/hess-17-4189-2013
ÂÂÂVidal, 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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