lowcldfeed-Toulouse-poster-200806

1
Subtropical low cloud feedback in a
superparameterized GCM - a mechanism and CRM
column analogue
Matthew C. Wyant, Christopher S. Bretherton, and
Peter N. Blossey Department of Atmospheric
Sciences, University of Washington, Seattle USA
mwyant,breth,bloss_at_atmos.washington.edu
CRM column analogue for SPCAM cloud response
LTS-sorting illuminates 2K low cloud increase

• The SP-CAM superparameterized GCM
• predicts more low cloud in a warmer climate
• What is the physical mechanism?
• Is this cloud increase sensitive to the coarse
  resolution of the embedded CRMs?

Radiative mechanism for 2K cloud response
More PBL radiative cooling will destabilize the
PBL and force more cumulus convection and perhaps
more associated cloud. Fig. 3c shows that the
2K radiative cooling increase is mainly a
clear-sky response to the increased absolute PBL
humidity (and hence emissivity) associated with a
warmer temperature profile (Fig. 3a). The small
2K increases in relative humidity do not
strongly affect the radiative cooling changes. RH
and cloud changes are viewed as positive
feedbacks on the clear-sky radiative cooling
increases.

• Method
• Make composite forcings/profiles for a cloud
  regime defined with 80-90 percentiles of LTS over
  low-lat ocn column-months, for ctrl and SST2K
  SP-CAM runs.
• Configure SAM6.5 CRM to use identical microphys,
  radiation, resolution, domain orientation as in
  SP-CAM.
• Run CRM to steady-state, look at 2K cloud
  differences

Use percentiles of monthly-mean LTS to sort the
low latitude (30S-30N) oceans (LLO) into
boundary-layer cloud regimes

• LTS better correlated with observed net CRF over
  LLO (r 0.59) than are ?(500 hPa), SST, or EIS.
  It also correlates strongly with inversion height
  and boundary-layer vertical structure.
• Use percentiles since LTS is not
  climate-invariant. A given low cloud fraction is
  associated with higher LTS in a warmer climate in
  which the tropical free-tropospheric temperature
  follows a more stable moist adiabat.

LTS80-90??,q profiles SST
Hor. Advection
(b)
(a)
ctrl 2K

• Introduction
• Superparameterization - a climate model with a
  small cloud-resolving model (CRM) running in
  place of the normal physical parameterizations in
  every grid column.
• Computationally expensive, but may simulate
  turbulent clouds (especially deep convection)
  more realistically.
• SP-CAM (Khairoutdinov and Randall 2005) uses 2D
  CRMs with 32x30 gridpoints,?x 4 km -
  under-resolves boundary-layer Cu, Sc.
• Nevertheless, SP-CAM has a boundary-layer cloud
  climatology that is comparably good to other
  GCMs. Comparing Figs. 1a vs.1b, its biggest bias
  is too much trade cumulus and too little
  stratocumulus.
• Fig. 1c shows lower tropospheric stability LTS
  ?700 - ?1000, which is well correlated to
  observed net CRF over the subtropical oceans
  (Klein and Hartmann 1993) and is a natural
  separator between subtropical cloud regimes. In
  low latitudes, free tropospheric ? is nearly
  uniform and LTS tends to be anticorrelated with
  SST.

Fig. 2a-d shows LTS-sorting applied to the SP-CAM
mean climatology. As in ?(500 hPa)-binning (Bony
et al. 2004 Wyant et al. 2006) the Hadley-Walker
circulation is evident, with low LTS (high SST)
corresponding to extensive deep convective cloud,
deep humidity, rising motion, and less radiative
cooling. The high LTS (low SST) regions have
little cloud or humidity above the moist boundary
layer. Radiative cooling (especially strong in
the boundary layer) drives mean subsidence.
(c)
q nudging
winds
averaging period
cold SST
warm SST
cold SST
warm SST
(d)
25 relative increase in low cloud cover for
high LTS!
(e)
(a)
CRM analogue results
SP-CAM LTS80-90 composites
(b)
(f)
Fig. 3 LTS 80-90 percentile changes of key
variables. Temperature (a) and radiative cooling
(c) profile changes are central to our cloud
increase mechanism.
diverse changes
(a)
(d)
Large-scale dynamics also provide important
feedbacks. The free-tropospheric temperature
profile is remotely forced by deep convection
over the warm parts of the tropics. Changes in
diabatic cooling drive subsidence changes, which
we view here as a local dynamical feedback.
Winds and large-scale horizontal T, RH advection
show negligible 2K changes, and dont appear
important to the low cloud changes.
(g)
(c)
(b)
(e)
1-2 moister PBL

• CRM Deeper moist layer, but similar 2K cloud
  response.
• Mean 2K cloud response depend a bit on wind
  shear, forcing details.

(h)
(d)
(f)
(c)
CRM column analogue for SPCAM - philosophy
more PBL rad cool

• Goal Isolate SP-CAM low cloud response in a
  simpler setting and examine its resolution
  sensitivity.
• Key assumptions (like Zhang Bretherton 2008,
  Caldwell Bretherton 2008)
• Regime-mean 2K cloud response can be recovered
  from regime-mean profile/advective tendency
  changes.
• In low latitudes, strong nonlocal dynamical
  feedbacks counteract changes in column
  temperature profile. These feedbacks are included
  here by allowing mean subsidence to adjust to the
  column diabatic cooling to keep the temperature
  profile close to the SP-CAM composite, using an
  approach that builds on that of Caldwell and
  Bretherton (2008). Here, ? ?0?? where ?0 is
  from SP-CAM and ?? responds to column cooling.
  Additional humidity relaxation is applied above
  the cumulus layer. (NEW CHANGE FROM POSTER AT
  GCSS MEETING.)

low LTS
low LTS
high LTS
high LTS
Fig. 2 Left panels (a-d) Climatology of SP-CAM
LLO column-months sorted into 5-percent bins of
LTS. Right panels (e-h) show 2K changes in
sorted climatology. Purple panels show 80-90th
LTS percentiles, which show the low cloud changes
well and are used for further analysis.
Fig. 1 (a-b) SPCAM vs. observed net cloud
radiative forcing (c) Observed
LTS, (d-f) 2K SPCAM changes.

• LES (?x100 m, ?z40 m, Nx512) Large reduction
  in mean cloud.
• Similar 2K change in low cloud, but with
  different BL structure.

• Wyant et al. (2006) examined SPCAM cloud response
  to an idealized climate warming by comparing
  3.5-year simulations with control SSTs vs.
  SST2K. Low cloud increased over the subtropical
  oceans and high-latitudes, implying a strong
  negative global cloud-radiative feedback
  dCRF/dSST - 0.9 W m-2 K-1 (Fig. 1d-e).
• Fig. 1f shows a correlated 1-2 K LTS increase
  across low latitudes.

The 2K changes in the sorted climatology (Figs.
2e-h) show that in the high-LTS, subsiding
regions, the strong low cloud increases are
accompanied by more radiative cooling, slightly
more PBL relative humidity, but no systematic
changes in lower-tropospheric subsidence.
References Bony, S., J. DuFresne, H. Le Treut,
J.-J. Morcrette, and C. Senior 2004 On dynamic
and thermodynamic components of cloud changes.
Climate Dyn., 22, 71-86. Caldwell, P., and C. S.
Bretherton, 2008 Large eddy simulation of the
diurnal cycle in Southeast Pacific stratocumulus.
J. Climate, submitted. Khairoutdinov, M. F., and
D. A. Randall, 2003 Cloud resolving modeling of
the ARM summer 1997 IOP Model formulation,
results, uncertainties, and sensitivities. J.
Atmos. Sci., 60, 607-625. Khairoutdinov, M., D.
Randall, and C. DeMott, 2005 Simulations of the
atmospheric general circulation using a
cloud-resolving model as a superparameterization
of physical processes. J. Atmos. Sci., 62,
2136-2154. Klein, S., and D. Hartmann, 1993 The
seasonal cycle of low stratiform clouds. J.
Climate, 6, 1587-1606. Wyant, M. C., M.
Khairoutdinov, and C. S. Bretherton, 2006
Climate sensitivity and cloud response of a GCM
with a superparameterization. Geophys. Res.
Lett., 33, L06714, doi10.1029/2005GL025464. Zhang
, M., and C. S. Bretherton, 2008 Mechanisms of
low cloud climate feedback in idealized
single-column simulations with the Community
Atmospheric Model (CAM3). J. Climate, in press.
Thanks ...to Marat Khairoutdinov for the SP-CAM
runs and for providing SAM, and to CMMAP and NSF
for financial support.

• Conclusions
• Subtropical boundary-layer cloud increases
  dramatically in SP-CAM with warmer SST.
• Warming increases radiative destabilization of Cu
  layer, driving more convective cloud.
• CRM analogue ? SP-CAM low cloud overestimated due
  to CRM under-resolution.
• We are about to submit this work to JAMES - email
  one of us or check the JAMES-D website if you
  would like a copy of the manuscript.