Dynamical Downscaling of Seasonal Climate Prediction Over East Asia for March-Septemb

er 2003 at CWB

 

Chih-Hui Shiao, Ying-Jui Chuang

Central Weather Bureau, Taipei

October 2003

The plan for the CWB dynamical downscaling forecast systems

System I :

NCEP-RSM (60-km) + IRI-ECHAM4.5 (T42L18)

System II :

CWB-RSM (60-km) + CWB-GCM (T42L18)

2 system X 5 members

Model integration : 7 months

Domain : 100 。 E-135 。 E, 10 。 N-35 。 N

The current status

Forecast period : 4 months

Domain : 100 。 E-135 。 E, 10 。 N-35 。 N

System I :

NCEP-RSM (60-km) + IRI-ECHAM4.5 (T42L18)

Two member (ASST and PSST)

System II :

In past studies, CWB-RSM was applied to simulate the 1998 and 2002 summer monsoon over East Asia has successfully captured not only the regional circulation but also the precipitation patterns.

CWB-RSM (60-km) + CWB-GCM (T42L18)

CWB-RSM (120-km) + ECHAM4.5 (T42L18)

Base field : ECHAM4.5 (T42L18)

support by International Research Institute for Climate Prediction (IRI)

20 members of 7 month-long forecast for each month

a set of AMIP simulation

To examine the performance of NCEP RSM and CWB RSM in the dynamical downscaling precipitation forecast, based on the forecast results on the past half year.

Integration length : 4 month-long forecast for each month

Experimental period : March-September 2003

Domain : East Asia

The evolution of precipitation on March-September 2003 by CAMS_OPI

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mm

/day

CAMS_OPI

CAMS_OPI precipitation on 2003

(average over 115 。 E-125 。 E, 15 。 N-30 。 N)

NCEP-RSM (60-km)

The precipitation anomaly

The anomaly of analysis is the CAMS_OPI analysis in 2003 compare with the average of 1979-1995.

The forecast precipitation anomaly of NCEP RSM is the forecast in 2003 compare with the simulation of NCEP RSM forced by the ECHAM4.5 AMIP simulation averaged in 1971-1986.

Precipitation Anomaly of M2+M3+M4 with ECHAM4.5 / NCEP RSM

NCEP RSM

ECHAM

Regional downscaling forecasts is under control of GCM strictly except those mesoscale phenomena result from high-resolution topographic and coastal effects.

NCEP RSM

CAMS_OPI

The major difference of precipitation anomaly between forecast and analysis are the forecast is too wet along the coastline and too dry in midlatitude

One possible source of the precipitation anomaly bias may due to the difference between the AMIP simulation and the analysis.

AMJ JJAMJJ JAS

The difference of ECHAM4.5 AMIP run to GPCP in 1979-1986 mean

NCEP RSM

CAMS_OPI

AMJ JJAMJJ JAS

AMIP run bias

Summary I

Anomaly forecast is possible if the model climatology is

‘realistic’.

The precipitation difference between the GPCP analysis and ECHAM4.5/NCEP RSM downscaling AMIP simulation are used to modify the ECHAM4.5/NCEP RSM currently forecasts.

Bias Correction

Forecast precipitation by NCEP RSM without and with bias correction (average over 115 。 E-125 。 E, 15 。 N-30 。 N)

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mm

/day CAMS_OPI

ECHAM_M1RSM_M1

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mm

/day CAMS_OPI

ECHAM_M1NRSM_M1N

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Mar Apr May Jun Jul Aug Sep

mm

/day CAMS_OPI

ECHAM_M2NRSM_M2N

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mm

/day CAMS_OPI

ECHAM_M2RSM_M2

without bias correction with bias correction

(M3 and M4)

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Mar Apr May Jun Jul Aug Sep

mm

/day CAMS_OPI

ECHAM_M3RSM_M3

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mm

/day CAMS_OPI

ECHAM_M4

RSM_M4

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mm

/day CAMS_OPI

ECHAM_M3NRSM_M3N

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mm

/day CAMS_OPI

ECHAM_M4NRSM_M4N

without bias correction with bias correction

Downscaling forecasts by NCEP RSM with bias correction

(average over M2+M3+M4 )

AMJ MJJ JJA JAS

Downscaling forecasts with CWB RSM

(average over M2+M3)

AM ASMJ JJ JA

R.M.S.E. of precipitation with different forecast start month (average over M1,M2,M3,M4)

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Mar Apr May Jun Jul Aug Sep

mm

/day ECHAM

NCEPRSM_NCWBRSM

R.M.S.E. of precipitation to different forecast length

0

2

4

6

8

10

M1 M2 M3 M4

mm

/day

ECHAMNCEPRSMNCEPRSM_NCWBRSM

Summary II

Bias correction is necessary for single model forecast.

How to eliminate the large-scale bias but keep local phenomena is still known.