 |
| Titel |
Retrospective analysis of a nonforecasted rain-on-snow flood in the Alps – a matter of model limitations or unpredictable nature? |
| VerfasserIn |
O. Rössler, P. Froidevaux, U. Börst, R. Rickli, O. Martius, R. Weingartner |
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| ISSN |
1027-5606
|
| Digitales Dokument |
URL |
| Erschienen |
In: Hydrology and Earth System Sciences ; 18, no. 6 ; Nr. 18, no. 6 (2014-06-19), S.2265-2285 |
| Datensatznummer |
250120389
|
| Publikation (Nr.) |
 copernicus.org/hess-18-2265-2014.pdf |
|
|
|
|
|
| Zusammenfassung |
A rain-on-snow flood occurred in the Bernese Alps, Switzerland, on 10 October
2011, and caused significant damage. As the flood peak was unpredicted by the
flood forecast system, questions were raised concerning the causes and the
predictability of the event. Here, we aimed to reconstruct the anatomy of
this rain-on-snow flood in the Lötschen Valley (160 km2) by
analyzing meteorological data from the synoptic to the local scale and by
reproducing the flood peak with the hydrological model WaSiM-ETH (Water Flow
and Balance Simulation Model). This in order to gain process understanding
and to evaluate the predictability.
The atmospheric drivers of this rain-on-snow flood were (i) sustained
snowfall followed by (ii) the passage of an atmospheric river bringing warm
and moist air towards the Alps. As a result, intensive rainfall (average of
100 mm day-1) was accompanied by a temperature increase that
shifted the 0° line from 1500 to 3200 m a.s.l. (meters above
sea level) in 24 h with a maximum increase of 9 K in
9 h. The south-facing slope of the valley received significantly more
precipitation than the north-facing slope, leading to flooding only in
tributaries along the south-facing slope. We hypothesized that the reason for
this very local rainfall distribution was a cavity circulation combined with
a seeder-feeder-cloud system enhancing local rainfall and snowmelt along the
south-facing slope.
By applying and considerably recalibrating the standard hydrological model
setup, we proved that both latent and sensible heat fluxes were needed to
reconstruct the snow cover dynamic, and that locally high-precipitation sums
(160 mm in 12 h) were required to produce the estimated flood
peak. However, to reproduce the rapid runoff responses during the event, we
conceptually represent likely lateral flow dynamics within the snow cover
causing the model to react "oversensitively" to meltwater.
Driving the optimized model with COSMO (Consortium for Small-scale
Modeling)-2 forecast data, we still failed to simulate the flood because
COSMO-2 forecast data underestimated both the local precipitation peak and
the temperature increase. Thus we conclude that this rain-on-snow flood was,
in general, predictable, but requires a special hydrological model setup and
extensive and locally precise meteorological input data. Although, this data
quality may not be achieved with forecast data, an additional model with a
specific rain-on-snow configuration can provide useful information when
rain-on-snow events are likely to occur. |
| |
|
| Teil von |
|
|
|
|
|
|
|
|