www.ccsm.ucar.edu

Changing the albedo in CSIM4Changing the albedo in CSIM4

Julie Schramm

CCSM Software Engineering Group

ccsm@ucar.eduhttp://www.cgd.ucar.edu/csm/models/ice-csim4/index.html

www.ccsm.ucar.edu

What is in the albedo parameterization?What is in the albedo parameterization?

• Surfaces: dry snow, melting snow, bare ice, open ocean

• Properties

– two spectral intervals: VIS, NIR

– Direct and diffuse

– Melt ponds are crudely approximated using (Ts) for the bare ice albedo

• Dependencies

– Snow thickness, ice thickness, surface temperature

• Calculated at each ocean grid point, for each thickness category, then area-weighted

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What’s not in the parameterization?What’s not in the parameterization?

• Zenith angle dependence (direct = diffuse)• Variations due to surface topography• Snow age• Explicit melt ponds

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Control Run AlbedoControl Run Albedo

Surface VIS NIR BB

Dry Snow

0.98 0.70 0.85

Wet Snow

0.88 min

0.55 min

0.72 min

Bare Ice

0.78 0.36 0.58

Melting Ice

0.71 min

0.29 min

0.51 min

Open water

NA NA 0.06

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Source code residenceSource code residence

• $CSMROOT/models/ice/csim4/src/source• Source code is in module ice_albedo.F

– 185 lines• Contains subroutine albedos• Modified source code should be put in a separate

directory called src.ice in $SCRIPTS/$CASE/• Albedo parameterization described in Scientific Guide

available at:

http://www.ccsm.ucar.edu/models/ccsm2.0/• User’s Guide also available here

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real (kind=dbl_kind), parameter ::

& albocn = 0.06 ! Ocean albedo... &, albicev = 0.78 ! Visible ice albedo for h > ahmax

&, albicei = 0.36 ! Near-ir ice albedo for h > ahmax

&, albsnowv = 0.98 ! Cold snow albedo, visible

&, albsnowi = 0.70 ! Cold snow albedo, near IR...

Simplest way to modify the albedo:

Base albedos

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Other albedo parametersOther albedo parameters

real (kind=dbl_kind), parameter ::

& snowpatch = 0.02 ! Parameter for fractional snow area

• Fractional area covered with snow : fs = hs/(hs + snowpatch)

• Represents that for relatively thin snow cover, there will be

some areas of bare ice

• Value of 0.02 provides best match with SHEBA data

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Other albedo parametersOther albedo parameters

real (kind=dbl_kind), parameter ::

& ahmax = 0.5 ! Thickness above which ice albedo is constant

(m)

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Temperature dependence parametersTemperature dependence parameters

&, dT_mlt = 1 ! change in temp to give dalb_mlt albedo change (>0)

&, dalb_mlt = -0.075 ! albedo change per dT_mlt change in temp for ice

&, dalb_mltv = -0.100 ! albedo vis change per dT_mlt change in temp for snow

&, dalb_mlti = -0.150 ! albedo nir change per dT_mlt change in temp for snow

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Beyond BasicsBeyond Basics

• Putting modifications into the do loop in ice_albedo.F

• Global grid is decomposed into local subdomains

• Subdomains are dimensioned imt_local x jmt_local

• The physical portion of a subdomain is dimensioned (ilo:ihi, jlo:jhi)

• When adding arrays to this loop, check the dimensions!

do j=jlo,jhi do i=ilo,ihi . . enddo enddo

imt_global

jmt_

glob

al

ilo:ihi

jlo:

jhi

imt_local

jmt_

loca

l

ilo:ihi

jlo:

jhi

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TroubleshootingTroubleshooting

• The model won’t compile. For example, in ice.log file “ice_albedo.F”, 1516-036 (S) Entity fsnow has undefined type.

– Verify that any arrays you’ve added to ice_albedo.F are accessible via one of the use statements.

• I changed some parameters in ice_albedo.F, but the answers aren’t changing…

– Be sure to delete all files in /ptmp/$LOGNAME/$CASE/ice/obj and recompile.

• I’m getting funny lines in the albedo field…

– Make sure that any arrays you add to the do loop are indexed as (ilo:ihi, jlo:jhi) or (imt_local, jmt_local), not (imt_global, jmt_global).