TOMS Ozone Retrieval Sensitivity to
Assumption of Lambertian Cloud Surface
Part 2. In-cloud Multiple Scattering
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Xiong Liu, Mike Newchurch,1,2 Robert Loughman3 , and Pawan K. Bhartia4
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2.
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Department of Atmospheric Science, University of Alabama in H untsville, Huntsville, Alabama, USA
National Center for Atmospheric Research, Boulder, Colorado, USA
Cooperative Center for Atmospheric Science & Technology, University of Arizona, Tucson, Arizona, USA
NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
Abstract . In part I, we find the assumption of isotropic cloud scattering is fairly good in TOMS
Ozone Absorption Enhancement vs.
Viewing Geometry
ozone retrieval. In this poster, we study the effect of the negl ect of enhanced ozone absorption
by in-cloud multiple scattering on TOMS ozone retrieval. Ozone absorption enhancement in
cloud depends significantly on solar zenith angle and view zenith angle, ozone amount in clouds,
ozone distribution in clouds, and cloud optical thickness. It also depends somewhat on different
cloud types and cloud location. The neglect of ozone absorption enhancement in clouds by
assuming optically thick clouds as Lambertian reflecting surfaces in the ozone retrieval
introduces significant positive errors to the retrieved total ozone, and is the dominating source of
errors, especially at smaller zenith angles.
•Figure 1 shows the enhanced ozone vs. viewing
geometry for a water cloud of optical depth (OD) of 40
between 2-12 km. There is 20.8 DU ozone in cloud.
•The enhanced ozone decreases with the increase of
SZA and VZA F
( igure 1 and Figure 2a) and is
azimuthally independent, ~19 DU at nadir and only
0.15 DU at SZA = 75°and VZA = 70 °. The exchange
of SZA and VZA does not change the amount of
enhanced ozone.
Introduction
•The motivation and objectives of this study are shown in part 1.
•The photon path length in clouds decreases with
increasing SZA and VZA. Furthermore, TOMS
algorithm automatically accounts for the geometrical
path length ( 1/cos(SZA) + 1/cos(VZA) ). These two
factors lead to the dramatic decrease of enhanced
ozone vs. geometrical path length.
•In part 1, we study the effect of assuming cloud scattering as isotropic on TOMS ozone
retrieval. For most conditions, the non-isotropic effect is within ±4 DU for optical thickness ≥ 20,
indicating the assumption of isotropic cloud scattering is fairl y good for optically thick clouds.
•We study the effect of neglect of ozone absorption enhancement due to in-cloud multiple
scattering on TOMS ozone retrieval in this poster.
Figure 1. Enhanced Ozone vs
vs.. Viewing
Geometry. * indicates the SZA.
•The radiative transfer models used in this study are shown in part 1.
Enhanced Ozone vs. Cloud Type and Cloud
Optical Depth
Methodology
•The retrieved ozone difference with and without ozone in clouds in the forward calculation
gives the the enhanced ozone.
•The enhanced ozone vs. viewing geometry is similar for
different types of clouds. The enhanced ozone differs
slightly in magnitude among WC, WCHG, HEX, and
POLY (Figure 2a).
•We study how the enhanced ozone varies with solar zenith angle (SZA) and view zenith angle
(VZA) (SZA ≤ 75 °, VZA ≤ 70 °) , cloud types including water clouds (WC), hexagonal column
ice crystals (HEX), polycrystals (POLY), and water clouds with Henyey-Greenstein phase
function (WCHG), optical thickness of clouds, cloud location, geometrical thickness, and ozone
distribution in clouds.
•The enhanced ozone is smallest for POLY, largest for
WC, due primarily to their difference in the asymmetry
factor g. The smaller g, the less photons interact with
clouds before they are scattered back to atmosphere.
However, their difference is smaller, at most 2 DU
between POLY and WC.
•We also study the vertical distribution of ozone enhancement. Divide a cloud into layers, the
retrieved ozone difference without ozone only in one layer and with ozone in all the layers
approximates the contribution of that layer to the overall ozone enhancement.
•The enhanced ozone decreases with the increase of
cloud optical thickness (Figure 2b) because photons
penetrate less into thicker clouds. At nadir, the enhanced
ozone is 19.2 DU for OD of 10 and 10.9 for OD of 500.
•To represent those tropical high-reflecting clouds, a typical homogeneous cloud is put between
2 - 12 km with an optical thickness of 40.
•Generally, the same amount of ozone as present in the TOMS standard profile is
homogeneously distributed in clouds. The sensitivity of ozone enhancement to ozone amounts
and ozone distribution is studied.
Figure 2. Enhanced Ozone vs
vs.. Cloud type
(a), Cloud Optical Depth (b) and Ozone
Amount in Clouds (c).
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www.nsstc
www.
nsstc.uah.
.uah.edu
edu//
//atmchem
atmchem
Ozone Absorption Enhancement vs.
Ozone Amount and Cloud Location
Vertical Distribution of Ozone Enhancement
•The
enhanced ozone is almost linearly
proportional to the amount of ozone in clouds.
•Figure 5a shows the vertical distribution (20 0.5 -km
layers) of ozone enhancement for WC and OD = 40 at
a few selected angles.
•The enhancement (ratio of enhanced ozone to
input ozone in clouds) actually decreases with
the increase of ozone in clouds (Figure 2c). At
nadir, the enhancement is 0.89 for 5.2 DU in
cloud and 0.84 for 41.6 DU in cloud. The more
absorber in clouds, the less photons penetrate
into clouds.
•The enhanced ozone increases with the
increase of geometrical thickness due primarily
to the increase of the ozone in clouds (Figure 3).
The relative enhancement increases with the
height of cloud location. At nadir, the
enhancement is 0.81 for a cloud at 2-3 km and
0.95 for a cloud at 11-12 km.
•The layer that contributes most is located in the upper
1 km. The weight decreases dramatically deeper into
clouds. This vertical distribution explains the results
shown in Figure 4.
•Figure 5b shows the depth below the cloud top
above which 50% of the ozone enhancement is
contributed vs. geometrical path length for several
clouds.
•The penetration depth decreases with increasing
optical thickness and geometrical path length,
explaining the results Figure 2a and 2b.
Figure 3. Ozone enhancement v s.
s.
cloud location (water clouds, OD = 40).
•Among WC, HEX, and POLY, the penetration depth
is largest for WC and smallest for POLY, consistent
with the results in Figure 2a.
Ozone Absorption Enhancement vs.
Ozone Distribution in Clouds
Figure 5. Vertical weighting functions
of ozone enhancement for WC and
OD = 40 (a), and halfhalf-folding ozone
enhancement depth (b).
•Figure 4a shows six different profiles of ozone
extinction coefficients in clouds. The ozone
distribution above and below clouds is the same
for all. Profile 1 (original L275 profile), profile 2
(homogeneously distribution), 3, and 4 contains
the same amount of ozone, i.e., 20.8 DU. Profile 5
and 6 are similar to profile 2 except that they
contain ozone only in the upper 2 km and the
lower 2 km, respectively.
Figure 4. (a) Six ozone profiles. (b)
Ozone
enhancement
v s.
s.
ozone
distribution in clouds.
•Figure 4b shows that the enhanced ozone
varies greatly with the ozone distribution of ozone
in clouds. The enhanced ozone is smaller for the
original profile compared to profile 2 because less
ozone is distributed in the upper portion. The
highest enhancement is for profile 3. The
enhanced ozone is almost zero for profile 6
because all the ozone is distributed only in the
lower 2 km.
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Summary and Conclusions
•We study the neglect of ozone absorption enhancement in clouds on TOMS ozone retrieval.
•The ozone absorption enhancement is due to the in-cloud multiple scattering, which interacts
with and intensifies ozone absorption. The enhanced ozone depends significantly on solar
zenith angle and view zenith angle, ozone amount in clouds, ozone distribution in clouds, and
cloud optical thickness. It also depends somewhat on different cloud types and cloud location.
•Positive ozone retrieval errors occur without correcting the enhanced ozone. Compared to the
non-isotropic effect, the ozone enhancement in clouds is the dominating sourc es of retrieval
error in the assumption of optically thick clouds as Lambertian surfaces especially for small
zenith angles.
•The treatment of clouds as Lambertian surfaces in the TOMS retrieval algorithm sufficiently
explains the 4-9 DU excess of ozone over cloudy areas over tropical high-reflecting convective
areas. However, more information than is available about ozone distribution in clouds is needed
to accurately characterize individual ozone retrieval errors associated with clouds.
•Further study will be performed on ozone retrieval errors over partial cloudy areas.
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