arctic cloud biases in ccsm3

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)


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)


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



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)



• 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)]


OBSERVED


* * * * * * * * ** * *

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 TemperatureBias

Effect of Freezedry in CAM3 with UW PBL:




CAM3 with new Microphysics:

Effect of Freezedry in CAM3 with new Microphysics:



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

OBSERVED


* * * * * * * * ** * *

CAM3

FREEZEDRY

Standard2xCO2

Freezedry2xCO2

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%

arctic cloud biases in ccsm3

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