1. Sub-daily temporal reconstruction of historical extreme precipitation events using NWP model simulations
Vojtěch Bližňák1 Miloslav Müller1,2, Petr Zacharov1, Marek Kašpar1, Róbert Kvak1,2
1 Institute of Atmospheric Physics, CAS, Prague, Czech Republic
2 Charles University in Prague, Faculty of Science, Prague, Czech Republic
Motivation
Data
NWP model COSMO
Horizontal resolution: 2.8 km
Frequency of forecasts: 10 min
Purpose
Enables to evaluate historical extreme precipitation events (EPEs) in terms of their sub-daily extremity
Enables a quantitative assessment of the relationship among properties of EPEs and causal atmospheric (thermo) dynamic conditions
Current status:
Observations of historical EPEs in a high temporal step with a sufficient level of accuracy are missing
Possible solution:
Combination of rain gauge and NWP model data using an appropriate adjustment method
Temporal reconstruction
Rain gauge data
rain gauges
Temporal resolution: 24 hours
Radar reflectivity data
2 Czech C-band Doppler radars - Brdy, Skalky
Constant Altitude Plan Position Indicator (CAPPI 2 km)
Temporal resolution:
10 minutes (up to May 2009)
5 minutes (since June 2009 onwards)
Horizontal resolution: 1 km
Verification
Temporal reconstruction
Selected EPEs
NWP MODEL
10-min precipitation totals
Stratiform precipitation (large area)
Prevailing N-NW flow
High precipitation totals in the west part of the Czech Republic
Precipitation caused immense floods on Vltava and Labe rivers
Long-lasting precipitation (several days)
Convective precipitation (local)
Prevailing NE-E flow
High precipitation totals in the east part of the Czech Republic
Precipitation caused flash floods on Jičínka river
Time of the highest precipitation totals: UTC
Correlation of the fields with a shift between -70 and +70 pixels in a direction [xi, yi]
Direction [x, y] with highest correlation
NWP MODEL
10-min shifted precipitation totals
NWP MODEL
24-h precipitation sum
RAIN GAUGE RECORDS
24-h precipitation sum
24-h shifted precipitation sum
Ratio
24-h adjusted precipitation sum
Adjustment (Sokol, 2003)
Results
Observation
Non adjusted model
In general good forecast
Adjusted model
Minor adjustment in respect to good original forecast
Observation
Non adjusted model
Mismatch of the forecast and observation
Adjusted model
Very good match with observation
Verification
Averaged Fraction Skill Score (FSS) of 10-min precipitation totals from raw and adjusted model
Improved FSS values for adjusted model
Comparison of 24-h precipitation sums
Comparison of 24-h precipitation sums
Improved accuracy of 24-h sums
Significantly improved accuracy of 24-h sums
FSS for 10-min precipitation totals from raw and adjusted model
Similar FSS values for raw and adjusted model
Similar correlations of 10-min re-forecasts
Significantly improved FSS values for adjusted model in the time of the storm occurrence
Significantly higher correlations of 10-min re-forecasts in the time of the storm occurrence
Conclusions and outlook
NWP model COSMO produces 10-min re-forecasts of stratiform precipitation with a good level of accuracy => small differences in the accuracy of raw and adjusted model
NWP model COSMO adjusted by 24-h rain gauge observations including a shift based on the highest correlation between modelled and observed precipitation sums significantly improved the accuracy of 10-min re-forecasts of convective precipitation in space and time
Suggested adjustment method will be then applied to more historical EPEs to demonstrate its sub-daily extremity
Each EPE will be also evaluated by return periods of respective maximum precipitation totals utilizing return levels of given time intervals.
Acknowledgement
The research was supported by the Czech Science Foundation under the S project, and by the Czech Ministry of Agriculture under the QJ project. The acknowledgement belongs to the Czech Hydrometeorological Institute for provided data and Z. Sokol for provision of adjustment procedure.
References
[1] Sokol, Z., The use of radar and gauge measurements to estimate areal precipitation for several Czech river basins. Stud. Geophys. Geod., 47, 587–604.
[2] Ebert, E.E., Neighborhood verification: a strategy for rewarding close forecasts. Weather Forecast., 24, 1498–1510.
9th EMS European Conference on Severe Storms –
ECSS 2017, September 2017, Pula, Croatia