1. Two-moment Stratiform Cloud Microphysics & Cloud-aerosol Interactions in CAM
H. Morrison, A. Gettelman (NCAR) , S. Ghan (PNL)
Thanks to: P. Field, S. Massie (NCAR), R. Wood (UW-Seattle)
Morrison/Gettelman
AGU December 2006

2. Motivation Aerosol-cloud interactions Better treatment of ice
Multi-scale modeling (unify treatments)
Method: Extend bulk microphysics (Rasch & Kristjannson, 1998) to do number. New Scheme based on Morrison et al (2005)
Morrison/Gettelman
AGU December 2006

3. N = number concentration q, N Convective Detrainment Aerosol (CCN
q = mixing ratio
N = number concentration
q, N
Convective
Detrainment
Aerosol
(CCN
Number)
Aerosol
(IN
Number)
q, N
Cloud Droplets
(Prognostic)
q, N
Cloud Ice
(Prognostic)
Conversion
Processes
Evap/Cond
Dep/Sub q
Water Vapor
(Prognostic)
Evaporation
Sublimation
Riming
q, N
Rain
(Diagnostic)
q, N
Snow
(Diagnostic)
Sedimentation
Sedimentation
Morrison/Gettelman
AGU December 2006

4. Key features of the new scheme
Two-moment – predicts number concentrations and mixing ratios of cloud water and ice.
Liquid/ice fraction determined by microphysical processes (Bergeron, heterogeneous freezing) instead of simple function of temperature.
Coupled with aerosol by treating droplet activation (Abdul-Razzak and Ghan 1998) and ice nucleation (Cooper 1986).
Diagnostic treatment of rain and snow mixing ratio and number concentration.
Self-consistent treatment of sub-grid cloud water distribution for all relevant microphysics processes – straightforward to couple with diagnostic cloud scheme.
Flexibility to allow independent column approach.
Morrison/Gettelman
AGU December 2006

5. Mixed Phase Processes Observed Simulated (QJRMS, 2004)
why there is a small amount of ice above +5 C and liquid below -40 C (homogeneous freezing threshold in the model). This is because the output temperature is not exactly the same temperature that the microphysics scheme sees - it is affected by other processes. So for example, if the temperature is some value when it is output, by the time the microphysics sees the temperature (and the microphysical variables are output) it may have shifted several degrees due especially to the long time step in CAM.
Morrison/Gettelman
AGU December 2006

6. Particle Sizes
Morrison/Gettelman
AGU December 2006

7. Droplet Number
Morrison/Gettelman
AGU December 2006

8. AVHRR Column Drop Number
Lohmann et al, 1999 from Han et al 1998
Morrison/Gettelman
AGU December 2006

9. Tests of Aerosol effects
Test with observations: Massie et al 2006
Indian ocean case
Test run with sulfur scaled to 30% present
Approx value for pre-industrial
Morrison/Gettelman
AGU December 2006

10. Observed Aerosol effects
CAM sensitivity to aerosols is larger than MODIS for this particular region - but this may not necessarily be true for other regions or globally. Also, since we are using diagnostic aerosol, all variability in CAM aerosol optical depth is due to spatial variability, in MODIS, the aerosol optical depth varies over space and time. So, it is possible that the CAM results may more strongly reflect spatial variability that is not due entirely to changes in aerosol, if there is cross correlation between the CAM aerosol optical depth and other features such as the dynamics.
MODIS data: S. Massie, 2006
Morrison/Gettelman
AGU December 2006

11. Observed Aerosol effects
CAM sensitivity to aerosols is
Larger than MODIS in this region
CAM
MODIS
MODIS data: S. Massie, 2006
Morrison/Gettelman
AGU December 2006

12. ‘Indirect’ Effects Scale sulfate aerosols to 30% present day
See differences in:
Radiative Forcing
Size and number
Liquid water path
Changes to Radiative forcing (Wm-2), using Ghan method:
Size
Number
Total
Direct
Indirect
Num
Size
-3.0
-0.7
-2.3
-1.0
-1.3
Pre-industrial
Present
Morrison/Gettelman
AGU December 2006

13. +10% Change in Liquid water
Present Day Liquid Water
Largest changes in
Storm Tracks where
LWP large.
Little change in
In stratocumulus regions
Preindustrial - Present
most of the change in the mid-latitude storm tracks, which is not surprising given that the highest amounts of liquid water are located there. The fact that we are getting little impact in the deep tropics may reflect the fact that the aerosol doesn't impact droplet number from detrained convection. Interestingly, there seems to be almost no impact on the subtropical stratocumulus regions - this is a very interest result.
Morrison/Gettelman
AGU December 2006

14. Summary/Conclusions New scheme performs well Aerosols affect clouds
Reasonable Re distribution
Reasonable Number distribution
Aerosols affect clouds
Sizes, Number, Liquid water path & Radiation
Model ‘indirect’ effects are too strong
Likey due to fixed size of nucelating aerosols
Need to look more at observations, different regions
Next Steps
Refine aerosol nucleation treatment
Focus on ice phase
Morrison/Gettelman
AGU December 2006