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

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Presentation on theme: "Arctic Cloud Biases in CCSM3 Steve Vavrus Center for Climatic Research University of Wisconsin."— Presentation transcript:

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

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

3 T42 CAM3 Fixed SSTs

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

5 Extratropical atmosphere too moist in CAM3

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

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

8 1. CAM3 formula: f = [(RH-RH min )/(1-RH min )] 2 2. Randall et al. (1996): f = RH p [1-exp(-  q c /(1-RH))] where q c = cloud water content Possible Formulas for Stratiform Cloud Fraction (f)

9 1. CAM3 formula: f = [(RH-RH min )/(1-RH min )] 2 2. Randall et al. (1996): f = RH p [1-exp(-  q c /(1-RH))] where q c = cloud water content 3.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)

10 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

11 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

12 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

13 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)

14 1. CAM3 formula: f = [(RH-RH min )/(1-RH min )] 2 2. Randall et al. (1996): f = RH p [1-exp(-  q c /(1-RH))] where q c = cloud water content 3.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)

15 OBSERVED Annual Cycle of Low Cloud Fraction over Arctic Ocean **** * * * * * * * * CAM3 FREEZEDRY

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

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

18

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

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

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

22 DJF Low Cloud Fraction CAM3 with new Microphysics:

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

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

25 Change in Annual Low Cloud Cover (Standard 2 x CO 2 )

26 OBSERVED Annual Cycle of Low Cloud Fraction over Arctic Ocean **** * * * * * * * * CAM3 FREEZEDRY Standard 2xCO 2 Freezedry 2xCO 2

27 o Standard 2xCO 2 Freezedry 2xCO 2

28 o Standard 2xCO 2 Freezedry 2xCO 2 o Standard 2xCO 2 Freezedry 2xCO 2

29 o Standard 2xCO 2 Freezedry 2xCO 2 o Standard 2xCO 2 Freezedry 2xCO 2

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

31 Conclusions Excessive wintertime low cloud cover in CCSM stems from too much moisture in atmosphere (transport or in situ problem?) 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 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 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 Freezedry parameterization reduces the TCR in polar regions, despite large increases in low clouds in winter

32 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.)

33 99% 95% R.H. Arctic Ocean DJF Relative Humidity CAM3Observed 94% 79%

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