Arctic Cloud Biases in Arctic Cloud Biases in CCSM3 CCSM3 Steve Vavrus Steve Vavrus Center for Climatic Research Center for Climatic Research University of Wisconsin University of Wisconsin
Arctic Cloud Biases in CCSM3
Dec 30, 2015
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). - PowerPoint PPT Presentation
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Arctic Cloud Biases in Arctic Cloud Biases in CCSM3CCSM3
Steve VavrusSteve Vavrus
Center for Climatic ResearchCenter for Climatic Research
University of WisconsinUniversity of Wisconsin
Observed
Annual Cycle of Low Cloud Fraction over Arctic Ocean
* * * * * * * * ** * *
CAM3 (T42)
T42CAM3
Fixed SSTs
DJF Low Cloud Bias
DJF 2 m Air Temperature Bias
CCSM3 (T85)
CCSM3 (T85)
Extratropical atmosphere too moist in CAM3
0
2
4
6
8
10
2 4 6 8 10 12
Precipitable Water Flux into Arctic (70N)
CAM3OBSERVED
Me
rid
ion
al P
W F
lux
acr
oss
70
N (
kg/(
m s
))
Month
0
5
10
15
20
25
30
35
40
1 2 3 4 5 6 7 8 9 10 11 12
Percentage Bias in PW Flux into Arctic (CAM3)
Month
Correlation between monthly biases of PW flux and Arctic low cloud = 0.7
1. CAM3 formula: f = [(RH-RHmin)/(1-RHmin)]2
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(-qc/(1-RH))] where qc = cloud water content
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(-qc/(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 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 winterb) 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 winterb) 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 winterb) 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)
1. CAM3 formula: f = [(RH-RHmin)/(1-RHmin)]2
2. Randall et al. (1996): f = RHp[1-exp(-qc/(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 newmicrophysics (Andrew)]
Possible Formulas for Stratiform Cloud Fraction (f)
OBSERVED
Annual Cycle of Low Cloud Fraction over Arctic Ocean
* * * * * * * * ** * *
CAM3
FREEZEDRY
Effect of Freezedry in CCSM3:
DJF SurfaceTemperature
DJF Low CloudFraction
Effect of Freezedry in CCSM3:
JJA SurfaceTemperature
JJA Low CloudFraction
CAM3 with UW PBL:
DJF Low CloudFraction
DJF TemperatureBias
Effect of Freezedry in CAM3 with UW PBL:
DJF SurfaceTemperature
DJF Low CloudFraction
Effect of Freezedry in CAM3 with UW PBL:
JJA SurfaceTemperature
JJA Low CloudFraction
DJF Low CloudFraction
CAM3 with new Microphysics:
Effect of Freezedry in CAM3 with new Microphysics:
DJF SurfaceTemperature
DJF Low CloudFraction
Effect of Freezedry in CAM3 with new Microphysics:
JJA SurfaceTemperature
JJA Low CloudFraction
Change in Annual Low Cloud Cover (Standard 2 x CO2)
OBSERVED
Annual Cycle of Low Cloud Fraction over Arctic Ocean
* * * * * * * * ** * *
CAM3
FREEZEDRY
Standard2xCO2
Freezedry2xCO2
o Standard 2xCO2
• Freezedry 2xCO2
o Standard 2xCO2
• Freezedry 2xCO2
o Standard 2xCO2
• Freezedry 2xCO2
o Standard 2xCO2
• Freezedry 2xCO2
o Standard 2xCO2
• Freezedry 2xCO2
Change in JJA Total Grid-box Cloud Liquid Water Path
Freezedry 2xCO2 Standard 2xCO2
ConclusionsConclusions Excessive wintertime low cloud cover in CCSM stems Excessive wintertime low cloud cover in CCSM stems
from too much moisture in atmosphere (transport or from too much moisture in atmosphere (transport or in situ problem?)in situ problem?)
Forcing reduced low cloud amount in extremely dry Forcing reduced low cloud amount in extremely dry air (“freeze drying”) affects only polar regions air (“freeze drying”) affects only polar regions during winter, except in UW-PBL versionduring winter, except in UW-PBL version
Reduced low cloud amount in winter causes surface Reduced low cloud amount in winter causes surface cooling that alleviates surface temperature biasescooling that alleviates surface temperature biases
Freezedry parameterization reduces the TCR in Freezedry parameterization reduces the TCR in polar regions, despite large increases in low polar regions, despite large increases in low clouds in winterclouds in winter
Bottom Line. . .Bottom Line. . .
The freezedry parameterization is a physically justifiable tuningtool that can be used to adjust polar cloud amount when necessary(UW-PBL, UW-PBL II, new microphysics, etc.)
99%
95%
R.H.
Arctic Ocean DJFRelative Humidity
CAM3 Observed
94% 79%
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