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The role of ocean and atmosphere biases on land precipitation-biases during Indian Summer Monsoon in

CFS forecastsBala Narapusetty1,2, Raghu Murtugudde1, Arun

Kumar3, Hui Wang3

2015-Feb18-22

1 – ESSIC, UMD2 – Hydrological Sciences Laboratory, NASA/GSFC3 – NOAA Climate Prediction Center

Outline

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• Motivation: Systematic dry-precipitation bias over

land and wet-bias over Eastern-Equatorial Indian Ocean• Analysis

• Impacts on local Hadley circulation• Biases in equatorial crossing of ITCZ in April/May• Biases in Findlater jet and Monsoon precipitation• Spatial structure of March-initialized forecast biases in

precipitation, SST and zonal wind.• Precipitation pattern correlations over the land and in

BoB

• Results and conclusions• Overall-picture: Schematic-view of bias propagation• Summary

Details of datasets used in this study

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• Forecast data:• CFSv2 monthly-mean forecasts (up to 11 lead-

months) obtained by initializing around the beginning of each month from 1982-January to 2011-March.

• 24 ensemble forecasts each month.

• Observations• Precipitation: APHRODITE

Gauge-based and 0.25o spatial-resolution (http://www.chikyu.ac.jp/precip)CPC and GPCP : As needed

• SST : NOAA Optimum Interpolated (OISST; Reynolds et al., 2002)

• 10-m zonal and meridional winds : CCMP 3.5

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Precipitation averaged over Central India

Obs. Precip.: APHRODITE(1982-2007)http://www.chikyu.ac.jp/precip/

The x-axis shows forecast month as target

CI: (16.5o-26.5oN; 74.5o-86.5oE)

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Precipitation averaged over Eastern equatorial

Indian Ocean

Obs. Precip: CPC(1982-2011)

The x-axis shows forecast month as target

Averaged over: (5oS-5oN; 90o-110oE)

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Precipitation averaged over Western equatorial

Indian Ocean

Obs. Precip: CPC(1982-2011)

The x-axis shows forecast month as target

Averaged over: (5oS-5oN; 50o-70oE)

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Mass flux stream function (averaged over 60E-110E)

CFSR

CFSTG#1

CFSTG#3

CFSTG#2

The Y-axis shows pressure in hPa

The Equatorial crossing of ITCZ in the month of

Apr/May is crucial for Monsoon rainfall

(shading units in mm day-1)

Averaged over 70o-95oE

N. Eq. : Eq-5oN; 70o-95oECI : 16.5o-26.5oN; 74.5o-86.5oE

In the March-initialized April forecast, the ITCZ is mostly stuckaround Equator and results in excess precipitation during May.The land-biases grew into JJA.

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(shading units in mm day-1)

Averaged over 50o-70oE

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Averaged over 90o-110oE

The ITCZ averaged averaged 50o-70oE and 90o-110oE

Negative biases in Findlater jet grew with longer-lead

forecasts and so the dry-land biases

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mm day-1

NWEIO: Findlater Jet: 5o-20oN; 50o-70oECI : 16.5o-26.5oN; 74.5o-86.5oE

Biases (CFS-OBS) in precipitation, SST, Zonal Wind

in March-initialized forecasts

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Precip bias (mm day-1)in shading

SST bias(oC)

in shading

Biases in zonal wind grew with long-lead forecasts

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Signature of off-equatorial Rossby-wave and equatorial Kelvin wave in D20's annual-

harmonic

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Mar initialized d20 climatologies show deepening

of thermocline in the SWEIO in MAM

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And Kelvin-wave signature in EIO in early summer

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GPCP correlations in BoBSimilar to Meehl et al. (2012)

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Same, but for April-initialized forecasts

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Unlike in observations, the ENSO years' JJA forecasts are wetter over CI in CFS long-lead

forecasts

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CPC (ENSO-NONENSO)

CFS (ENSO-NONENSO)

Precip difference (mm day-1)in shading

Precip difference (mm day-1)in shading

Differences in JJA forecast biasesENSO biases-Non-ENSO biases

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Precip bias mm day-1 in shading

SST increase due to d20 increasein SWEIO + WEIO (late spring)

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Negative Precip. Bias inCentral India

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High-level view of bias propagation

Summary

Negative biases in Findlater jet + weakupwelling on West Arabian-sea (late spring + early summer)

Easterly wind biases + Ekman pumping south of the Equator (spring)

Biases in ITCZEq-crossing (Mid and late spring)

Enhanced Kelvin wave propagates warmer SSTs to Tropical EEIO(summer)

development of Large-scale cyclonic systemin the mid-Arabia sea(Summer)