Transcript Slide 1 - Divecha Centre for Climate Change

Sensitivity of the Hydrological Cycle to
the Regional Model domain
B. Bhaskaran
Asian Climate Change and Variability: Trends and Policy
DCCC, Bangalore, India, 20-22 July 2011
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Background
Adaptation strategies require high quality
climate information…
1) Accurate forecast information
depends on the quality of forecast models and predictability etc
2) At relevant temporal and spatial scales
monthly-seasonal-decadal-centennial time scales
50km,25km and even 15km horizontal spatial scales
3) Current global climate models are typically 200km in
horizontal resolution
downscale this using either statistical or dynamical methods
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Downscaling: fine scale physical variables
. . . from large-scale circulation features
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Downscaling Strategies
Dynamical Downscaling
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Downscaling Strategies
Regional
Dynamical Downscaling
Global
Input
Global
RCM
Input
GCM
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Output
Met Office Regional Climate Model
PRECIS is a version of the Hadley
Centre Nested Regional Model
Regional Domain (~5000 sq kms)
Finer Resolution (50 to 25 kms)
Driven by a Global Climate Model
at the lateral boundaries
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Present-day simulation
Monsoon Precipitation (mm/day)
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Unidata IDV was used for the plots
For climate change simulations . . .
“The one-way nesting approach requires the large scale circulation of the
driving model over the regional model domain to be similar to that of the
regional model over the same domain.” – Bhaskaran et al (1996)
However,
1) More than one domain could satisfy this condition – does it matter?
2) Largest of these domains may be desired by the stakeholders for
climate change simulations – capture as many countries as possible!
Sub-regions of the domain may require climate change information for
decision making – these are too small to satisfy the above condition
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Domain and Experiments
Two Domains at 25 and 50km resolutions
Driven by ERA 40 Reanalysis for 1979-1990
Four 12-year long simulations
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River systems in the HadGEM2-ES
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Mean Sea Level Pressure
small
25km
50km
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large
Geopotential Height Anomalies (200hPa)
small
25km
50km
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large
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Seasonal Mean Rainfall
8
4
6
Precip simulation is more
sensitive to domain size
0
2
Precip Response (%)
10
12
Increases in small domain
R1
Increases in 25km version
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R2
R25
R50
(a)
0.06
0.00
0.02
0.04
pdf
0.08
0.10
0.12
Daily Rainfall Characteristics
0
5
10
15
20
25
30
Precip (mm/day)
Increased number of moderate rainfall events
in the R1 (small) domain
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(b)
600
1000
800
Pressure (hPa)
400
200
Dynamic Precipitation Index
0.5
0.6
0.7
0.8
0.9
1.0
Correlation
DPI    q
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where

dp
dt
30
10
15
20
(b) Cor = 0.903
0
0
5
10
15
20
Precip (mm/day)
25
(a) Cor = 0.908
5
Precip (mm/day)
25
30
DPI at 500hPa is a measure of the local convection
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0.0
0.6
0.8
1.0
1.2
30
25
15
20
(d) Cor = 0.902
0
0
5
10
Precip (mm/day)
10
15
20
25
(c) Cor = 0.895
5
Precip (mm/day)
0.4
DPI500 for R225 (g/kg*Pa/s)
30
DPI500 for R125 (g/kg*Pa/s)
0.2
0.0
0.2
0.4
0.6
0.8
1.0
DPI500 for R150 (g/kg*Pa/s)
1.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
DPI500 for R250 (g/kg*Pa/s)
The moderate precipitation range of 6-12 mm/day
corresponds to DPI 500 range of 0.12-0.40 g/kg Pa/S
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pdf
0.08
0.0
0.00
1.0
0.04
pdf
0
5
10
15
20
Precip (mm/day)
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(b)
3.0
(a)
2.0
0.12
PDFs of precipitation and DPI at 500 hPa
25
30
0.0
0.2
0.4
0.6
0.8
1.0
DPI at 500hPa (g/kg*Pa/s)
1.2
Conclusions
Large-scale driving circulations are not sensitive to domain size
But the resulting precipitation characteristics are highly sensitive
to domain size on seasonal as well as daily timescales
In R1 (small) domain the number of moderate rainfall
events (6-12 mm/day) are higher than in the R2 (larger) domain
This increase is directly related to the increased convective activity
in the small domain
These results are robust and remains valid irrespective of the resolution
This suggests that the conventional way of selecting an optimum domain for
assessing climate change impacts over the sub-region (River Basins) may
Introduce one more level of uncertainty in the hydrological cycle
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Questions/Suggestions/Comments
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Finer Scale Precip Details
Observed
10km
50km
Regional
Model
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300km
Global
Model
25km
Regional
Model
Uncertainty in Model Predictions
- Perturbed Physics Ensemble
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