how does CESM 2 calculate the cloud (liq/ice) droplet radius?

[Andy_Wai]

Hello, everyone.

I want to look at cloud microphysics. I am confused about ice/liquid cloud droplet radius. CESM 2.1.3 outputs include :

Name Unit NLEV Time avg Longname
AREI Micron 32 A Average ice effective radius
AREL Micron 32 A Average droplet effective radius

ACTREI Micron 1 A Average Cloud Top ice effective radius
ACTREL Micron 1 A Average Cloud Top droplet effective radius


How does CESM 2.1.3 calculate those parameters? What does the 'average' mean in each parameter?


I checked the CAM5 Scientific Guide at 5. Model Physics — camdoc documentation. (see sections 5.8.3 and 5.8.4). This scientific guide says:
"The effective diameter (De) is defined in a way that is universal for both ice and water clouds, which is essentially the photon path characterizing the PSD (Mitchell 2002):
De = (3/2) * IWC / (

*

)

where

is the ice water content (g/cm:math:^3),

is the bulk ice density (0.917 g/cm

) and

is the total projected area of the PSD (cm:math:^2/cm:math:^3).

The same way is applied to the liquid cloud (only change the 'IWC' to 'LWC' and the density)

"
But how do we know the total projected area of the PSD

at each model layer; can CESM output this variable for ice and liquid cloud?



Thank you for your time.

Andy

 

[andrew]

Average is to collect the variable over time: AREL/FREQL for any averaging period (e.g. a monthly history file) will provide a true average size whenever a cloud is present (or you end up averaging in lots of zeros). Similarily: AREO/FREQI, ACTREI/FCTI, ACTREL/FCTL

Effective radius is the 3rd moment divided by the 2nd moment. See Morrison and Gettelman 2008, J. Climate Equation 5 and the size_dist_params_basic routine in micro_mg_utils.F90 (depending on the CESM version it might be different.

 

[tomasz]

This question might only be marginally relevant here, but do you know whether the sizes of the aerosols from the modal aerosol model (MAM) are taken into account when AREL is calculated?