January mean equatorial temperature profiles (averaged from 10 S to 10 N) are shown for seven CAM2 simulations using the finite volume dynamical core at 4 degree longitude by 5 degree latitude resolution. The cold point tropopause temperature is also given for each simulation. The simulations are: black (187.8 K) CAM2 control; red (190.8 K) CCM3 parameterizations; magenta (188.5 K) CAM2 prognostic cloud water replaced by CCM3 diagnostic cloud water; blue (189.2) CAM2 evaporation of convective precipitation from Zhang-McFarlane scheme removed, CCM3 had no evaporation; green (188.9 K) CAM2 longwave radiation with updated water vapor continuum replaced by CCM2/3 Ramanathan and Downey parameterization; cyan (190.8) both cloud water and evaporation changed to CCM3 form; yellow (192.7) all three parameterizations returned to CCM3 form.

No single parameterization change is responsible for the cold tropopause in CAM2 and all of the changes are believed to be improved representations of the atmosphere, suggesting that additional changes are required.

 

asr2002_b.jpg


January mean equatorial temperature profiles (averaged from 10 S to 10 N) are shown for three CAM2 simulations using the finite volume dynamical core at 2 degree longitude by 2.5 degree latitude resolution. The simulations are: black (188.9 K) CAM2 control; red (190.4) CAM2 with a new treatment of cloud fraction, water, and ice; magenta (192.2) CAM2 with the new clouds and 42 layers instead of 26 layers. Increasing the horizontal resolution and the vertical resolution both increase the tropopause temperature, as does the new cloud treatment. The actual tropopause temperature is sensitive to the ice crystal optical properties and fall velocities which depend on crystal size. Crystal size has been assumed to depend only on temperature, but this relationship is poorly understood.

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The January zonal mean mixing ratio of cloud ice simulated with the CAM2 prognostic cloud water parameterization and the updated form. The average is taken inside clouds, only when clouds are present. The contour interval is 2.5 mg/kg (or ppmm). The new form produces much greater ice concentrations than the original (Rasch and Kristjansson) form. The small increase in ice near the tropical tropopause is responsible for warming the tropopause.

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