Vavrus AMWG2007

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Published on January 16, 2008

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Arctic Cloud Biases in CCSM3:  Arctic Cloud Biases in CCSM3 Steve Vavrus Center for Climatic Research University of Wisconsin Slide2:  Observed Annual Cycle of Low Cloud Fraction over Arctic Ocean * * * * * * * * * * * * CAM3 (T42) Slide3:  T42 CAM3 Fixed SSTs Slide4:  DJF Low Cloud Bias DJF 2 m Air Temperature Bias CCSM3 (T85) CCSM3 (T85) Slide5:  Extratropical atmosphere too moist in CAM3 Slide6:  Correlation between monthly biases of PW flux and Arctic low cloud = 0.7 Slide7:  1. CAM3 formula: f = [(RH-RHmin)/(1-RHmin)]2 Possible Formulas for Stratiform Cloud Fraction (f) Slide8:  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) Slide9:  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 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 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 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): :  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 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): :  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 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): :  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) Slide14:  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) Slide16:  Effect of Freezedry in CCSM3: DJF Surface Temperature DJF Low Cloud Fraction Slide17:  Effect of Freezedry in CCSM3: JJA Surface Temperature JJA Low Cloud Fraction Slide19:  CAM3 with UW PBL: DJF Low Cloud Fraction DJF Temperature Bias Slide20:  Effect of Freezedry in CAM3 with UW PBL: DJF Surface Temperature DJF Low Cloud Fraction Slide21:  Effect of Freezedry in CAM3 with UW PBL: JJA Surface Temperature JJA Low Cloud Fraction Slide22:  DJF Low Cloud Fraction CAM3 with new Microphysics: Slide23:  Effect of Freezedry in CAM3 with new Microphysics: DJF Surface Temperature DJF Low Cloud Fraction Slide24:  Effect of Freezedry in CAM3 with new Microphysics: JJA Surface Temperature JJA Low Cloud Fraction Slide25:  Change in Annual Low Cloud Cover (Standard 2 x CO2) Slide26:  Standard 2xCO2 Freezedry 2xCO2 Slide27:  o Standard 2xCO2 Freezedry 2xCO2 Slide28:  o Standard 2xCO2 Freezedry 2xCO2 o Standard 2xCO2 Freezedry 2xCO2 Slide29:  o Standard 2xCO2 Freezedry 2xCO2 o Standard 2xCO2 Freezedry 2xCO2 Slide30:  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.) Slide33:  99% 95% R.H. Arctic Ocean DJF Relative Humidity CAM3 Observed 94% 79%

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