for 2010 repeated captures the annual cycle of Antarctic sea ice The ACCIMA 19992011 simulation has more incident shortwave radiation at the surface that ERAInterim for Antarctic latitudes over most months of the year The shortwave radiation is also excessive compared to ID: 619078
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Slide1
The ACCIMA 1999-2011 simulation loses sea ice in the latter half of the time period. Also, the annual cycle of sea ice are is too large. In contrast, a test COAWST simulation
for 2010 repeated captures
the annual cycle of Antarctic sea ice.Slide2
The ACCIMA 1999-2011 simulation has more incident shortwave radiation at the surface that ERA-Interim for Antarctic latitudes over most months of the year. The shortwave radiation is also excessive compared to Neumayer observations. ERA-Interim has a smaller magnitude error of opposite sign. Correcting this error could reduce sea ice loss in the ACCIMA simulation.Slide3
The downward shortwave radiation received on a plane locally parallel to the Earth’s surface will be a function of the zenith angle – which depends on latitude φ, declination angle δ, and time-of-day h.
Artificially resetting the value for the cosine of the zenith angle will change the solar radiation at the top of the atmosphere – and is expected to proportionately impact the incident solar radiation at the Earth’s surface.
We
tried a
coupled model run with a fractionally-specified change to the cosine of the solar zenith angle. The adjustment is designed to reduced 2/3rds of the difference between ACCIMA and
ERA-Interim at the surface.Slide4
The new Zenith Angle simulation is intended to reduce incident SW radiation at the Earth’s surface to correct for a bias. Top of the atmosphere (TOA) downward shortwave radiation is directly reduced by the specifications for the sensitivity experiment. The incident shortwave radiation at the Earth’s surface (lower panel) is proportionately similar. Specifying the shortwave radiation at the top of the atmosphere impacts the surface shortwave radiation as expected.Slide5
The new Zenith Angle simulation is intended to reduce incident SW radiation at the Earth’s surface to correct for a bias. The seasonal incident longwave radiation (upper left) is only slightly reduced by the change, so we expect the clouds are similar. We see that that SON solar radiation is reduced every year for 2005 –
2009 (upper right).
The seasonal reduction is most in
SON (lower left), the season that has the most bias. There are slight reductions other seasons.Slide6
The new Zenith Angle simulation with reduced incident SW radiation at the Earth’s surface has increased sea ice area (upper left), sea ice volume (upper right) and sea ice thickness (lower left) compared to the earlier ACCIMA simulation. The sea ice area minima for 2007-2010 are
now close to the observed
values.
The annual cycle of sea ice area and the seasonal maxima are much larger than observations.Slide7
ERA-Interim
(multi-year)
Summary
New SimulationSlide8
!------------------------------------------------------------ ! momentum flux !------------------------------------------------------------ ! tau = rhoa(i,j) * ustar
*
ustar
! strx = tau * uatm(i,j) / vmag !
stry = tau * vatm(i,j) / vmag !------------------------------------------------------------ tau = rhoa(i,j) *
ustar
(
ij
) *
rd
(
ij
) ! not the stress at
zlvl
(
i,j
)
strx
(
i,j
) = tau * (
uatm
(
i,j
)-
uvel
(
i,j
)) * 0.6 stry(i,j) = tau * (vatm(i,j)-vvel(i,j)) * 0.6
Modification to ice_atmo.F90This change was included in the ACCIMA run