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Arctic Cloud Biases in CCSM3

Arctic Cloud Biases in CCSM3. Steve Vavrus Center for Climatic Research University of Wisconsin. Annual Cycle of Low Cloud Fraction over Arctic Ocean. *. CAM3 (T42). *. *. *. *. *. *. *. *. *. *. *. Observed. T42 CAM3 Fixed SSTs. DJF Low Cloud Bias. CCSM3 (T85).

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Arctic Cloud Biases in CCSM3

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

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

  3. T42 CAM3 Fixed SSTs

  4. DJF Low Cloud Bias CCSM3 (T85) 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. Possible Formulas for Stratiform Cloud Fraction (f) 1. CAM3 formula: f = [(RH-RHmin)/(1-RHmin)]2

  8. Possible Formulas for Stratiform Cloud Fraction (f) 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

  9. Possible Formulas for Stratiform Cloud Fraction (f) 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)

  10. 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

  11. 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): Simulated and Observed Cloud Fraction Downwelling Longwave Radiation Biases Slingo Xu/Randall

  12. 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): Simulated and Observed Cloud Fraction Downwelling Longwave Radiation Biases Slingo Xu/Randall

  13. 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): Simulated and Observed Cloud Fraction Downwelling Longwave Radiation Biases 3. Rossby model “produced the most accurate cloud scheme” in an Arctic RCM intercomparison (Inoue et al., 2006)

  14. Possible Formulas for Stratiform Cloud Fraction (f) • 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)]

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

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

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

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

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

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

  21. CAM3 with new Microphysics: DJF Low Cloud Fraction

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

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

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

  25. Annual Cycle of Low Cloud Fraction over Arctic Ocean * * * CAM3 * * * * * * * * * FREEZEDRY OBSERVED Standard 2xCO2 Freezedry 2xCO2

  26. o Standard 2xCO2 • Freezedry 2xCO2

  27. o Standard 2xCO2 • Freezedry 2xCO2 • o Standard 2xCO2 • Freezedry 2xCO2

  28. o Standard 2xCO2 • Freezedry 2xCO2 • o Standard 2xCO2 • Freezedry 2xCO2

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

  30. 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

  31. 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.)

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

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