K. Liechti and M. Zappa (WSL) L. Panziera and U. Germann

YEAR 3: Flash-flood early warning using weather
radar data: from nowcasting to forecasting
K. Liechti and M. Zappa (WSL)
L. Panziera and U. Germann (MeteoSwiss)
manuscript in preparation
NORA: The NORA Ensemble originates from meteorological analogs of the current meteorological situation. An archive is searched
for past situations with the most similar mesoscale winds, air mass stability and Weather radar quantitative precipitation estimates
(QPE). The radar QPEs following these past analogs are the basis of the NORA ensemble.
REAL: REAL provides an ensemble of 25 weather radar QPE, each of which results from the sum of the current radar image and a
stochastic perturbation field. REAL has been produced ever since April 2007 in hourly time steps and with a spatial resolution of 2x2
km for the Southern Swiss Alps and has been coupled with the hydrological model PREVAH in operational mode since then.
Hydrological Model
PREVAH (Precipitation Runoff Evapotranspiration HRU related model):
Spatial information on elevation, land cover and soil properties is aggregated to hydrological response units HRUs (Viviroli et al. 2009).
Selected areas
Advanced weather radar products
(NORA and REAL) were avaialble
for a limited region of the the
southern Swiss Alps. Three catchments have been selected
The Calancasca catchment with
120 km2 is very rural, has steep
slopes and it ranges from 740 m
asl to 3200 m asl in altitude.
Waters from the headwaters are
diverged to a neighboring catchment
2
The Verzasca catchment with an area of 186 km is very little influenced by human activity and not affected by water management at all
down to the gauge. Elevations range from 490 to 2900 m asl.
2
The Ticino catchment with its 1515 km is highly influenced by human
activities. Altitudes range from 220 m to 3400 m asl. The influence of
water management is substantial. Redistribution of water for hydropower is accounted with simple conceptual approaches.
BRIER SKILL SCORE
Experimental setup
Five flash flood forecasting chains with lead time of 8 hours and different
configurations of rainfall nowcasting and forecasting products have been
examined. 1) NORA initialized after a real-time PREVAH run driven by deterministic radar (RAD), 2) the persistence (PERS) of the current radar
QPE of time t0 (i.e. taking the signal of t0 for the next eight hours) forces
PREVAH after a real-time run forced by RAD, 3) COSMO-2 (a high resolution numerical weather prediction model) forecast initialized after PREVAH
realized probabilistic initial conditions as forced by REAL, 4) COSMO-2 initialized after PREVAH realized a deterministic initial condition using RAD as
a forcing, and 5) COSMO-2 initialized after PREVAH realized a deterministic initial condition using
initialiswitch to
interpolated pluviometer
sation
forecast
data (PLU) as a forcing.
initial states
realtime
forecast
All configurations have
been evaluated with
probabilisitc measures of
agreement (BSS, FAR and
POD) for a total sample of
over 1300 flash-flood
forecasts within the period
June 2007 to November
2010
initial
states
from
PLUVIO
nowcast
of day -5
-5d
FALSE ALARM RATIO AND PROBABILITY OF DETECTION
RADAR
NORA
RADAR
PERS
REAL
COSMO-2
RADAR
COSMO-2
PLUVIO
COSMO-2
-4d
t0
8h
SAMPLE FORECAST
Results
Anticipation of Flash-Flood events with up to 8 hours lead time take largest
profit from having an ensemble of initial conditions, as realized here by forcing PREVAH with REAL.
The use of NORA shows added value as compared to forecasts with COSMO-2 having the same initial conditions as
provied by forcing PREVAH with RAD. Furthermore NORA beats also the forecasts relyiing on the persistence of the
latest radar QPE.
Previous work showed that initial conditions generated by forcing PREVAH with pluviometer information yield better
runoff nowcasts. This skill is also propagated in the forecast period forced by COSMO-2. This is mostly obious in the
Verzasca catchment.
Conclusions
A first effort in verification of QPE-forecasts as applied to hydroloigical forecasting has been realized. A combination of the five presenteted options for chaining nowcasting and forcasting products needs
to be evaluated.
FP7-ENV-2008-1 IMPRINTS 226555