Ensemble Simulation of Indian Summer Monsoon Rainfall by an Atmospheric General Circulation Model
IR@C-MMACS: CSIR-Centre for Mathematical Modelling and Computer Simulation, Bangalore
View Archive Info| Field | Value | |
| Title |
Ensemble Simulation of Indian Summer Monsoon Rainfall by an Atmospheric General Circulation Model
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| Creator |
S, Sajani
K, Rajendran |
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| Subject |
Climate Modelling
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| Description |
This study examines the fidelity of the Meteorological Research Institute (MRI) atmospheric general
circulation model (AGCM), ensemble runs forced with observed sea surface temperature (SST), in simu-
lating Indian summer monsoon rainfall (ISMR), and its interannual variation. Despite the simple en-
semble mean (SEM) capturing essential features of climatological ISMR pattern and its extreme ISMR
anomalies, it still shows certain systematic bias in simulating mean seasonal variation of rainfall over
the Asia-Pacific region. Concurrently, the ISMR interannual variability throughout the analysis period
is not adequately represented.
A bias-correction is applied to remove this bias by deriving weights for the member simulations for
each Julian day separately at every grid point through multiple linear regression of their daily rainfall,
against corresponding observation over a 23-year ‘training phase’ (out of the total 24-year analysis pe-
riod). Thereafter, at every grid point, for each Julian day of the remaining 1-year ‘forecast phase’, the
bias-removed ensemble mean (BREM) is computed as an optimal linear combination of weighted mem-
ber simulations. In cross validation, each year in the analysis period is treated successively as the ‘fore-
cast phase’, with the remaining 23 years included in the corresponding training phase. The methodology
minimises the systematic bias in mean seasonal variation of rainfall, and BREM consequently improves
upon SEM in simulating the mean ISMR pattern, and its interannual variability over the entire analysis
period.
The skill of BREM in forecasting the ISMR, and its intraseasonal variability is validated, for the
severe monsoon drought of 2002. The effective removal of climatological bias brings out the realistic pre-
cipitation response in BREM to fluctuations in SST boundary forcing. As a result, BREM captures the
seasonal rainfall anomaly pattern of 2002, and markedly improves its intraseasonal evolution. Apart
from the basic skill of the AGCM ensemble system, pronounced equatorial SST impact in modulating
the monsoon circulation during 2002, played a seminal role in the success of the methodology. The anal-
ysis also underlines the importance of mean seasonal variation, not only for capturing ISMR climatology,
and its interannual variation but for improving its intraseasonal variability as well. With the aid of real-
istic SST forecasts, this methodology has the application potential for dynamical prediction of ISMR.
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| Publisher |
Meteorological Society of Japan
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| Date |
2007
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| Type |
Article
PeerReviewed |
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| Format |
application/pdf
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| Identifier |
http://cir.cmmacs.ernet.in/104/1/rajendran_07b.pdf
S, Sajani and K, Rajendran (2007) Ensemble Simulation of Indian Summer Monsoon Rainfall by an Atmospheric General Circulation Model. Journal of the Meteorological Society of Japan, 85 (3). pp. 213-231. ISSN 0026-1165 |
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| Relation |
http://www.jstage.jst.go.jp
http://cir.cmmacs.ernet.in/104/ |
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