The Atlantic ITCZ and related atmospheric features: a model-data comparison
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The Atlantic ITCZ and related atmospheric features: a model-data comparison
Yochanan Kushnir, LDEO, Columbia University
John Chiang, UC Berkeley
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Data and Models• Observations:
– SST climatology: Reynolds and Smith V2
– Surface wind stress and SST anomalies: NCEP/NCAR CDAS1(1979-2001)
– Precipitation: GPCP 1979-2001
• CCM3:
– “AMIP” runs: CCM3.10 (T42/L18) forced with HadISST 1.1
• ECHAM:– “AMIP” runs: ECHAM 4.5 (T42/L19) forced with NCEP SST
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Climatology of SST & PrecipProjection on SST PC 1 Projection on SST PC 2
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Climatology: Observations & CCM3Projection on SST PC 1 Projection SST PC 2
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Climatology: Observations & CCM3-OBSProjection on SST PC 1 Projection SST PC 2
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Atlantic ITCZ Rainfall: Mean & VariabilityMiddle Basin 35°-15°W Eastern Basin 15°W-10°E
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Climatological annual cycle and 1st EOF of interannual variability (1979-2001)
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SWCF-precip relationship, SST > 27C ERBE/CMAP and CCM3
(following C. Bretherton)
ERBE/CMAP CCM3
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Total CF-precip relationship, SST > 27CERBE/CMAP and CCM3
ERBE/CMAP CCM3
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A SWCF ‘fix’ for CCM3
• Ad-hoc change to CCM3 ice and liquid water scattering particle sizes from 20S-20N
• Resulting SWCF-precip and Total CF-precip relationship resembles ERBE-CMAP
• TOA global radiative balance approximately preserved
Precip vs. SWCF
Precip vs. total CF
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CCM3 ‘fix’ - control: projection on the SST mean
Control CCM3 - ObsCCM3 ‘fix’ - control
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CCM3‘fix’ - control: projection on the SST first harmonic
Control CCM3 - ObsCCM3 ‘fix’ - control
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Climatology: OBS & ECHAM4.5-OBSProjection on SST PC 1 Projection SST PC 2
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Atlantic ITCZ Rainfall: Mean & VariabilityMiddle Basin 35°-15°W Eastern Basin 15°W-10°E
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Climatological annual cycle and 1st EOF of interannual variability (1979-2001)
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CF-precip relationships for ECHAM 4.5 (SST > 27C)
Pcp vs SWCF Pcp vs LWCF Pcp vs total CF
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Summary
• CCM3 exhibits substantial biases in precipitation over the Atlantic ITCZ and neighboring land regions in both the annual mean and seasonal cycle
• An ad-hoc adjustment to the CCM3 SWCF-precipitation relationship to match the observed appears to decrease the precipitation biases
• ECHAM 4.5 biases appear generally smaller than CCM3, and it exhibits better CF-precip relationship
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The following are extra slides
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SWCF-precip relationship - ERBE/CMAP and CCM3, land regions 20S-20N
ERBE/CMAP CCM3
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Total CF-precip relationship - ERBE/CMAP and CCM3, land regions 20S-20N
ERBE/CMAP CCM3