Vavrus AMWG2007

Arctic Cloud Biases in CCSM3: 

Arctic Cloud Biases in CCSM3 Steve Vavrus Center for Climatic Research University of Wisconsin

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Observed Annual Cycle of Low Cloud Fraction over Arctic Ocean * * * * * * * * * * * * CAM3 (T42)

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T42 CAM3 Fixed SSTs

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DJF Low Cloud Bias DJF 2 m Air Temperature Bias CCSM3 (T85) CCSM3 (T85)

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Extratropical atmosphere too moist in CAM3

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Correlation between monthly biases of PW flux and Arctic low cloud = 0.7

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1. CAM3 formula: f = [(RH-RHmin)/(1-RHmin)]2 Possible Formulas for Stratiform Cloud Fraction (f)

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1. CAM3 formula: f = [(RH-RHmin)/(1-RHmin)]2 2. Randall et al. (1996): f = RHp[1-exp(-aqc/(1-RH))] where qc = cloud water content Possible Formulas for Stratiform Cloud Fraction (f)

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1. CAM3 formula: f = [(RH-RHmin)/(1-RHmin)]2 2. Randall et al. (1996): f = RHp[1-exp(-aqc/(1-RH))] where qc = cloud water content GENESIS1: f = f * [max(0.15,min(1.0,q/3.0))] where q = specific humidity (g/kg) Possible Formulas for Stratiform Cloud Fraction (f)

Jones et al. (2004, Ambio): Rossby Centre Arctic RCM1. Physical justification:a) Limited number of CCN during polar winterb) Very stable boundary layer --> small sub-gridscale variability: 

Jones et al. (2004, Ambio): Rossby Centre Arctic RCM 1. Physical justification: a) Limited number of CCN during polar winter b) Very stable boundary layer --> small sub-gridscale variability

Jones et al. (2004, Ambio): Rossby Centre Arctic RCM1. Physical justification:a) Limited number of CCN during polar winterb) Very stable boundary layer --> small sub-gridscale variability2. Improvements relative to observations (SHEBA):: 

Jones et al. (2004, Ambio): Rossby Centre Arctic RCM 1. Physical justification: a) Limited number of CCN during polar winter b) Very stable boundary layer --> small sub-gridscale variability 2. Improvements relative to observations (SHEBA): Downwelling Longwave Radiation Biases Simulated and Observed Cloud Fraction Slingo Xu/Randall

Jones et al. (2004, Ambio): Rossby Centre Arctic RCM1. Physical justification:a) Limited number of CCN during polar winterb) Very stable boundary layer --> small sub-gridscale variability2. Improvements relative to observations (SHEBA):: 

Jones et al. (2004, Ambio): Rossby Centre Arctic RCM 1. Physical justification: a) Limited number of CCN during polar winter b) Very stable boundary layer --> small sub-gridscale variability 2. Improvements relative to observations (SHEBA): Downwelling Longwave Radiation Biases Simulated and Observed Cloud Fraction Slingo Xu/Randall

Jones et al. (2004, Ambio): Rossby Centre Arctic RCM1. Physical justification:a) Limited number of CCN during polar winterb) Very stable boundary layer --> small sub-gridscale variability2. Improvements relative to observations (SHEBA):: 

Jones et al. (2004, Ambio): Rossby Centre Arctic RCM 1. Physical justification: a) Limited number of CCN during polar winter b) Very stable boundary layer --> small sub-gridscale variability 2. Improvements relative to observations (SHEBA): Downwelling Longwave Radiation Biases Simulated and Observed Cloud Fraction 3. Rossby model “produced the most accurate cloud scheme” in an Arctic RCM intercomparison (Inoue et al., 2006)

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1. CAM3 formula: f = [(RH-RHmin)/(1-RHmin)]2 2. Randall et al. (1996): f = RHp[1-exp(-aqc/(1-RH))] where qc = cloud water content GENESIS1: f = f * [max(0.15,min(1.0,q/3.0))] I applied Equation 3 (“Freezedry”) to low-level, stratiform clouds in: CAM3 with fixed SSTs CAM3 FV with fixed SSTs CAM3 with slab ocean CCSM3 [CAM3 with UW PBL scheme (Sungsu), CAM3 with new microphysics (Andrew)] Possible Formulas for Stratiform Cloud Fraction (f)

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Effect of Freezedry in CCSM3: DJF Surface Temperature DJF Low Cloud Fraction

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Effect of Freezedry in CCSM3: JJA Surface Temperature JJA Low Cloud Fraction

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CAM3 with UW PBL: DJF Low Cloud Fraction DJF Temperature Bias

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Effect of Freezedry in CAM3 with UW PBL: DJF Surface Temperature DJF Low Cloud Fraction

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Effect of Freezedry in CAM3 with UW PBL: JJA Surface Temperature JJA Low Cloud Fraction

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DJF Low Cloud Fraction CAM3 with new Microphysics:

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Effect of Freezedry in CAM3 with new Microphysics: DJF Surface Temperature DJF Low Cloud Fraction

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Effect of Freezedry in CAM3 with new Microphysics: JJA Surface Temperature JJA Low Cloud Fraction

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Change in Annual Low Cloud Cover (Standard 2 x CO2)

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Standard 2xCO2 Freezedry 2xCO2

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o Standard 2xCO2 Freezedry 2xCO2

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o Standard 2xCO2 Freezedry 2xCO2 o Standard 2xCO2 Freezedry 2xCO2

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o Standard 2xCO2 Freezedry 2xCO2 o Standard 2xCO2 Freezedry 2xCO2

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Change in JJA Total Grid-box Cloud Liquid Water Path Freezedry 2xCO2 Standard 2xCO2

Conclusions: 

Conclusions Excessive wintertime low cloud cover in CCSM stems from too much moisture in atmosphere (transport or in situ problem?) Forcing reduced low cloud amount in extremely dry air (“freeze drying”) affects only polar regions during winter, except in UW-PBL version Reduced low cloud amount in winter causes surface cooling that alleviates surface temperature biases Freezedry parameterization reduces the TCR in polar regions, despite large increases in low clouds in winter

Bottom Line. . .: 

Bottom Line. . . The freezedry parameterization is a physically justifiable tuning tool that can be used to adjust polar cloud amount when necessary (UW-PBL, UW-PBL II, new microphysics, etc.)

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99% 95% R.H. Arctic Ocean DJF Relative Humidity CAM3 Observed 94% 79%