1. New Method for Bias Correcting GCM data
Development of climate change impact assessment technology (BA14Y9005)
New Method for Bias Correcting GCM data
Priyantha Hunukumbura, Daikichi Ogawada & Akiko Matsumura
応用水理G

2. Climate change Impact assessment in Brantas Basin …
GCM Scale
Downscaling
Statistical
Meteorological Variables in GCM scale should be downscaled to point scale for basin scale hydrological modeling. Statistical downscaling is one way of doing it.
Dynamical
GCM Grid
Point Scale

3. The method that we used…
quantile mapping method
GCM Grid
Compare & Mapped
into Point scale
Point Scale

4. Comparison of Before and after Bias Correction
Monthly　Rainfall
Monthly　Rainfall
Top 20
Top 20

5. Problem… Observed GCM-1 Will look closer year 2000 flood
When applying bias corrected downscaled data into a hydrological model
Observed
GCM-1
Will look closer year 2000 flood

6. Problem…
Observed
GCM-1
Year2000 flood

7. Identified weaknesses of the method
Not Keeping the Spatial Coherence – (this is very important in hydrological applications where the runoff is not only determine by the magnitude but also its spatial distribution)
Trying to Match the large scale process directly into point scale process is not reasonable
Due to scale difference
Due to scale difference

8. Proposed Method
Assume the same observed spatial distribution (Spatial Weight Matrix)
Basin Average OBS
Basin AverageGCM
Spatial Weight Matrix
Basin Average
(mm)
ST1 …
ST_n
5.5
0.2
1.2
0.8 .
100
1.1
0.5
2.2 
GCM RF
Rainfall(mm)
Time

9. Basin Scale Comparison
Results
Basin Scale Comparison
Rainfall in January

10. Basin Scale Comparison …
Results …
Basin Scale Comparison …
May in January

11. Point Scale Comparison …
Results …
Point Scale Comparison …
GCMs
cccma_cgcm3_1
cccma_cgcm3_1_t63
cnrm_cm3
csiro_mk3_0
csiro_mk3_5
gfdl_cm2_0
gfdl_cm2_1
giss_aom
iap_fgoals1_0_g
inmcm3_0
ingv_echam4
ipsl_cm4
miroc3_2_hires
miroc3_2_medres
miub_echo_g
mpi_echam5
mri_cgcm2_3_2a

12. Results … Spatial Coherence Raw GCM Observed Traditional Method
New Method

13. Discussion
New BC method was developed to address the issues of keeping spatial coherence in bias corrected data
Strengths
Simple
Easy to implement and straightforward
Spatial coherence is maintained
Scale differences is considered
Weaknesses
Necessary to assume the observed spatial rainfall distribution
Further study
Applicability of Weather Generators keeping spatial and temporal coherence
How to address the scale issues when applying weather generators

14. Thank You !!!!

15. Thank You !!!!

16. Why there is a big difference ???
GCM- GRID
Rain Gauge 1
FOBS
FGCM
GCM RF
2000/01/05
Rainfall(mm)

17. Analyzing the Results - Brantas River Basin, Indonesia
1. Check for Spatial Coherence
Observed
GCM-1
Log[Rainfall]

18. 2. Check for Temporal Coherence
Indices used to analyze the temporal characteristics
Day
Rainfall
Consecutive Total Rainfall
Rain Spell Size
Rain Spell Amount
1999/01/01 5 1
1999/01/02
1999/01/03 3
1999/01/04 2
1999/01/05 10 15
1999/01/06
1999/01/07
1999/01/08
1999/01/09 25

19. 2. Check for Temporal Coherence
OBS
Will look closer

20. 1994-1995 2. Check for Temporal Coherence OBS
Need to find a way to quantify the temporal similarities
Then we can select better GCMs
OBS

21. Histogram of Rain Spell Total Rainfall

22. 2. Check for Temporal Coherence
Annual Maximum Discharge
As the basin is small, we can expect that this difference is mainly due to the mismatch of temporal characteristics of the bias corrected GCM rainfall

23. Findings
Though the quintile based downscaling is used for analyzing the effect of climate change on meteorological variables, it can mislead when use them for analyzing the basin scale hydrological impacts.
Alternative methods (such as Dynamic downscaling, Numerical weather generators, etc) may be necessary to overcome the mismatch in the Spatial signatures.
Mismatch in the Temporal signatures can be used to reject some GCMs .
Uncertainty in the projected stream flow is a combination of different sources such as GCM uncertainty, Bias correction method, hydrological model, Observations etc..

24. Analyzing the Results - Brantas River Basin, Indonesia
1. Check for Spatial Coherence
Observed
GCM-2

25. Analyzing the Results - Brantas River Basin, Indonesia
1. Check for Spatial Coherence
Observed
GCM-2

26. 1. Check for Spatial Coherence
Observed
GCM-2

27. 2. Check for Temporal Coherence

28. Combined Effect on Annual Maximum Discharge