Radiative Effect of Clouds on Tropospheric Chemistry: Sensitivity to Cloud Vertical Distributions and Optical Properties

1
Radiative Effect of Clouds on Tropospheric
Chemistry Sensitivity to Cloud Vertical
Distributions and Optical Properties
Paper Number A51C-0062
Hongyu Liu1, James H. Crawford2, Robert B.
Pierce2, David B. Considine2, Jennifer A. Logan3,
Bryan N. Duncan4,5, Peter Norris4,5, Steven E.
Platnick4, Gao Chen2, Robert M. Yantosca3, Mat J.
Evans3,6 1National Institute of Aerospace,
Hampton, VA 2NASA Langley Research Center,
Hampton, VA 3Harvard University, Cambridge, MA
4NASA Goddard Space Flight Center, Greenbelt, MD
5University of Maryland, Baltimore County, MD
6Now at University of Leeds, UK
GEOS-3, 2001
GEOS1-S, 1996
GEOS-4, 2001
4. Intercomparison of GEOS1-S, GEOS-3 and GEOS-4
Cloud ODs
7. Sensitivity to Cloud Absorption of Solar
Radiation
1. Background and Objective

• Clouds play critical roles in influencing
  tropospheric photochemistry through modification
  of solar radiation that determines photolysis
  frequencies. A quantitative understanding of the
  radiative effect of clouds is required for using
  global models to assess anthropogenic
  perturbations to the Earth system.
• However, representation of clouds in global
  models still poses a significant challenge since
  most cloud processes occur on sub-grid scales and
  must be parameterized. Uncertainties in cloud
  distributions and optical properties are
  therefore a limiting factor in model assessments
  of the radiative effect of clouds on global
  tropospheric chemistry.
• We have recently assessed the radiative effect
  of clouds on photolysis frequencies and key
  oxidants in the troposphere with a global 3-D
  chemical transport model (GEOS-CHEM) driven by
  assimilated meteorological observations Liu et
  al., 2005. In this study, we apply the same
  model to examine the sensitivity of this effect
  to the uncertainty associated with the
  distributions and optical properties of clouds.

A cautionary note is presented here that 0.99 is
too low a value for cloud single scattering
albedo (SSA) and is not consistent with current
knowledge of cloud absorption of solar radiation
in the UV wavelength relevant to tropospheric
chemistry. SSA for pure water droplets is between
0.999990 and 0.999999 (i.e., little absorption)
in the UV wavelength range Hu and Stamnes,
1993. SSA for contaminated clouds containing
black carbon is still between 0.999 and 0.9999 at
550nm Chylek et al., 1996. SSA0.99 however was
used in some recent literature of tropospheric
chemistry.
GEOS-3
ISCCP
GEOS1-S (1996)
MODIS
GEOS-4
GEOS1-S
Figure 4. Same as Figure 3, but for ozone
(June). Contour levels are 5, -2, -1, 0, 1, 2,
5, 10. Tropical upper tropospheric ozone is much
less sensitive to the radiative effect of clouds
than previously reported by Tie et al. 2003
using the MOZART-2 model (5 versus 20-30).
SSA0.99
SSA0.999
optically much thinner
GEOS-3 (2001)
GEOS-4 ( 2001)
JO1D
6. Sensitivity to Progressively Adjusted Cloud
Optical Depth
Sensitivity of Global Mean OH to COD
Magnitude(GEOS-3, 2001)
OH
2. Model and Method
Global Average GEOS-3 / GEOS1-S 5.1
GEOS-3 / GEOS-4 1.9
Figure 7. Sensitivity of simulated JO1D and OH
to cloud absorption of solar radiation for June
(GEOS-3). The plots show the percentage changes
in monthly zonal mean JO1D and OH
concentrations when cloud SSA0.99 (left panels)
and SSA0.999 (right panels), relative to
SSA1.0. Results indicate that 1 decrease in SSA
would decrease JO1D and OH concentrations by
10-20 in most of the troposphere.

• GEOS-CHEM was driven with a series of
  meteorological archives (GEOS1-STRAT, GEOS-3, and
  GEOS-4) generated by the Goddard Earth Observing
  System data assimilation system (GEOS DAS) at the
  NASA Global Modeling and Assimilation Office
  (GMAO), which have significantly different cloud
  optical depths (OD) and vertical distributions.
• Ozone-NOx-CO-VOC coupled to aerosol chemistry
  Park et al., 2004.
• Photolysis rate calculation Fast-J radiative
  transfer algorithm Wild et al., 2000 with 3-D
  cloud optical depth and cloud fraction taken from
  GEOS.
• Model extensively evaluated with surface,
  in-situ, and satellite observations
  (http//www-as.harvard.edu/chemistry/trop/geos).
• Sensitivity simulations 1996 (GEOS1-STRAT)
  August 2000 December 2001 (GEOS-3 and GEOS-4)
  4ox5o horizontal resolution. The radiative effect
  of clouds is represented by subtraction of the
  clear-sky simulation from the cloudy-sky
  simulation.

Figure 2. Top left panel Intercomparison of the
June monthly zonal mean column cloud optical
depths in GEOS1-STRAT (1996), GEOS-3 (2001) and
GEOS-4 (2001). MODIS and ISCCP retrievals are for
June 2001. Other panels (color images)
Latitude-height cross sections of cloud optical
depths per kilometer for June. The column cloud
optical depths in GEOS-3 generally agree with the
satellite retrieval products from MODIS and ISCCP
within /-5-30, while those in GEOS1-STRAT and
GEOS-4 are too low by factors of about 5 and 2,
respectively. With respect to vertical
distribution, clouds in GEOS-4 are optically much
thinner in the tropical upper troposphere
compared to those in GEOS1-STRAT and GEOS-3.
8. Summary and Conclusions

• The radiative impact of clouds on global
  tropospheric chemistry is more sensitive to cloud
  vertical distribution than to the magnitude of
  column cloud optical depth.
• Model simulations with each of the three
  (GEOS1-STRAT, GEOS-3, and GEOS-4) cloud
  distributions all show that tropical upper
  tropospheric ozone is much less sensitive to the
  radiative effect of clouds than previously
  reported by Tie et al. 2003 using the MOZART-2
  model (5 versus 20-30).
• JO1D, JNO2, and OH simulations are very
  sensitive to cloud absorption of solar radiation.
  Using 0.99 for cloud single scattering albedo
  (SSA) in global models would decrease simulated
  JO1D, JNO2, and OH concentrations by 10-20
  in most of the troposphere, relative to SSA1.0.
  Realistic values for SSA are between 0.999 and
  1.0.
• With the launchings of CloudSat and CALIPSO
  satellites, a unique dataset of cloud optical /
  physical properties as well as their vertical
  distributions will be available for improving the
  constraints on the model calculated radiative
  effect of clouds on tropospheric chemistry.

5. Sensitivity to Three Cloud Vertical
Distributions and OD
Figure 5. Simulated percentage changes in global
mean OH due to the radiative effect of clouds as
the magnitude of 3-D cloud optical depths is
progressively adjusted. A cloud OD factor of 0.5
corresponds to half of the original GEOS-3 cloud
optical depth with the same 3-D spatial
distributions. Global average effects are modest
for all cloud OD factors, reflecting the opposite
effects of enhanced (weakened) photochemistry
above (below) clouds. Monotonic increases for
January/October reflect the dominant
backscattering from low-level clouds.
Non-monotonic changes for March/June are a result
of the sufficiently large optical depths due to
high clouds which allow less solar radiation to
penetrate down to the lower levels and thus limit
backscattering from low-level clouds. See Figure
6 for latitude-height cross sections of GEOS-3
cloud ODs in different seasons.
GEOS-3, 2001
GEOS1-S, 1996
GEOS-4, 2001
3. Evaluation of GEOS-3 Cloud OD with Satellite
Observations
Probability Distribution Functions
MODIS (MOD08_M3)
MODIS ISCCP GEOS-3
ISCCP (D2)
changes of global mean OH due to cloud
GEOS1-S -1 GEOS-3 1
GEOS-4 14
Figure 3. Simulated percentage changes in
monthly zonal mean OH concentrations due to the
radiative effect of clouds in June. Linear
scaling of cloud optical depth with cloud
fraction in a grid-box is assumed. Our model
calculations indicate that the changes in global
mean OH due to the radiative effect of clouds in
June are about -1 (GEOS1-STRAT), 1 (GEOS-3),
and 14 (GEOS-4), respectively. The effects on
global mean OH are similar for GEOS1-STRAT and
GEOS-3 due to similar vertical distributions of
clouds, even though the column cloud optical
depths in the two archives differ by a factor of
about 5. Clouds in GEOS-4 have a much larger
impact on global mean OH because more solar
radiation is able to penetrate through the
optically thin clouds in the upper troposphere,
increasing backscattering from low-level clouds.
These illustrate that the radiative impact of
clouds on global tropospheric OH is more
sensitive to cloud vertical distribution than to
the magnitude of column cloud optical depth.
GEOS-3
9. References
Global Average GEOS3-OD / MODIS-OD
0.91 GEOS3-OD / ISCCP-OD 1.31
1. Chylek, P., et al., Black carbon and
absorption of solar radiation by clouds, J.
Geophys. Res., 101(D18), 23,365-23,371, 1996. 2.
Hu, Y.X., and K. Stamnes, An accurate
parameterization of the radiative properties of
water clouds suitable for use in climate models,
J. Climate, 6, 728-742, 1993. 3. Liu, H., J.H.
Crawford, R.B. Pierce, P. Norris, S.E. Platnick,
G. Chen, J.A. Logan, R.M. Yantosca, M.J. Evans,
C. Kittaka, Y. Feng, and X. Tie, Radiative effect
of clouds on tropospheric chemistry in a global
three dimensional chemical transport model, J.
Geophys. Res., in revision, Dec. 2005. 4. Park,
R.J., D.J. Jacob, B.D. Field, R.M. Yantosca, and
M. Chin, Natural and transboundary pollution
influences on sulfate-nitrate-ammonium aerosols
in the United States Implications for policy, J.
Geophys. Res., 109, D15204, doi10.1029/2003JD0044
73, 2004. 5. Tie, X., S. Madronich, S. Walters,
R. Zhang, P. Rasch, and W. Collins, Effect of
clouds on photolysis and oxidants in the
troposphere, J. Geophys. Res., 108(D20), 4642,
doi10.1029/2003JD003659, 2003. 6. Wild, O., X.
Zhu, and M.J. Prather, Fast-J Accurate
simulation of in- and below-cloud photolysis in
tropospheric chemical models, J. Atmos. Chem.,
37, 245-282, 2000.
Mean COD (grid-scale) for March 2001
Figure 1. Left panels The global distribution
of GEOS-3 monthly mean (grid-scale, column) cloud
optical depths compared to MODIS and ISCCP
retrievals for March 2001 Liu et al., 2005.
Right panel Same as left panel, but shown as
probability distribution functions. GEOS-3 cloud
optical depths show peaks in the tropics
associated with deep convective clouds and at
midlatitudes associated with extratropical
cyclones in NH and marine stratiform clouds in
SH. These features reasonably agree with MODIS
and ISCCP cloud retrieval products, although
GEOS-3 tends to overestimate cloud ODs in the
tropics and SH midlatitudes Liu et al., 2005.
Also see Figure 2.
Contact Hongyu Liu (hyl_at_nianet.org) URL
http//research.nianet.org/hyl
Figure 6. Latitude-height cross sections of
GEOS-3 monthly mean cloud optical depths for
different seasons.