Download TOMS Ozone Retrieval Sensitivity to Assumption of - nsstc

TOMS Ozone Retrieval Sensitivity to
Assumption of Lambertian Cloud Surface (II)
Xiong
1
Liu,
Mike
1
Newchurch,
Robert
2
Loughman ,
and Pawan K.
3
Bhartia
1. Department of Atmospheric Science, University of Alabama in Huntsville, Huntsville, Alabama, USA
2. Cooperative Center for Atmospheric Science & Technology, University of Arizona, Tucson, AZ, USA
3. NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
Figure 5
ICOAEN Effect vs. Cloud Optical
Properties and Cloud Optical Depth
ICOAEN Effect vs. Ozone Distribution in
Clouds
• Figure 9a shows 7 different ozone profiles in clouds.
• The ICOAEN effect varies slightly with cloud
The ozone distribution above and below clouds is the
same for all. Profiles 1 (original L275 profile), 2 (wellmixed), 3 (homogeneously distribution), 4, and 5
contains the same amount of ozone, i.e., 20.8 DU.
Profiles 6 and 7 are similar to profile 3 except that they
contain ozone only in the upper and lower 2 km.
optical properties (Figure 5).
• Usually the greater the asymmetry factor, the
larger the ICOAEN effect is because the fewer
photons interact with clouds before scattering
back to the atmosphere.
• The ICOAEN effect decreases with the
increase of COD for COD > 10 because photons
penetrate less into thicker clouds. At nadir, the
enhanced ozone is 19.2 DU for OD of 10 and
10.9 DU for OD of 500 (Figure 6).
• Figure 9b shows that the enhanced ozone varies
greatly with ozone distribution in clouds.
Figure 6
• Practically, it is impossible to directly estimate the
ICOAEN effect because of the insufficient knowledge
of cloud parameters and in-cloud ozone distribution.
We indirectly estimate that the ICOAEN effect is
typically 5-13 DU over the Atlantic Ocean and Africa,
and 1-7 DU over the Pacific Ocean.
• When COD is too small, appearing like a clearsky, the retrieved ozone for the case without
ozone in the cloud is overestimated, therefore
decreasing the ICOAEN effect.
Figure 9. (a) Seven ozone profiles. (b) ICOAEN
effect vs. ozone distribution in clouds.
Figure 7
ICOAEN Effect vs. Ozone Amount
and Cloud Location
Vertical Distribution of ICOAEN Effect
• The enhanced ozone is almost linearly
Figure 8
proportional to ozone amount in clouds. The
ratio of enhanced ozone to input ozone in
clouds actually decreases with the increase
of ozone because the more absorber in
clouds, the fewer photons penetrate into
clouds (Figure 7).
• The enhanced ozone increases with the
increase of geometrical thickness due
primarily to the increase of the ozone in
clouds. The relative enhancement increases
with the height of cloud location. At nadir, the
ratio is 0.81 for a cloud at 2-3 km and 0.95 for
a cloud at 11-12 km (Figure8).
1
(b)
• Figure 10a shows the vertical distribution (20
0.5-km layers) of ratios of retrieved to actual incloud ozone for WC of COD 40. The layer that
contributes most is in the upper 1 km, greater
than 1 for smaller viewing geometry. The weight
decreases dramatically deeper into clouds.
• Figure 10b shows the E-folding depth below
the cloud top above which 36.8% of the ozone
enhancement is contributed for several clouds.
Figure 10. Vertical weighting functions of
ICOAEN effect for WC of COD 40 (top), and
E-folding depth for different clouds, SZA =
30° (b).
• The E-folding depth decreases with increasing
COD. Among WC, HEX, and POLY, the Efolding depth is largest for WC and smallest for
POLY, consistent with the results in Figure 5.
2
www.nsstc.uah.edu//atmchem
Overall Retrieval Errors Associated with
Clouds
BCOA Effect
• Figure 13 shows ozone retrieval errors for a 2-12
km WC as a function of COD at SZA=30°, VZA=
30°.
• Figure 11 shows the retrieved ozone below
cloud-bottoms for different clouds at SZA =
30° dramatically deeper into clouds.
• Like the ICOAEN effect, the BCOA effect
varies greatly with viewing geometry, cloud
optical thickness, and ozone amount below
clouds. The BCOA effect becomes significant
only when the COD is smaller and the ozone
amount below cloud-bottom is large.
• At smaller CODs (COD  5), the negative PCM
effect offsets the positive error due primarily to the
ICOAEN effect, leading to the above correct results.
Figure 11. BCOA Effect for different clouds at
SZA=30°.
Overall Retrieval Errors Associated with
Clouds
• Figure 12 shows ozone retrieval errors for a WC of
COD 1 (a) and 40 (b) at 2-12 km.
• At COD 1, the retrieval forcing the effective cloud
fraction to be the forward cloud fraction (fc=1)
contains large errors in total ozone, but the retrieval
using the TOMS algorithm (Rs=80%) leads to very
small errors in total ozone.
• At COD 40, the two retrieval errors converges
because the effective cloud fraction using the TOMS
algorithm is very close to 1. The errors in total ozone
is positive due primarily to the ICOAEN effect.
• With increasing CODs, the negative PCM
decreases in magnitude faster than the positive
error, the overall positive error increases until COD
20-40 when the PCM effect is very small. With the
further increase of COD, the overall error slightly
decreases because the ICOAEN effect slightly
decreases with the increase of COD.
• The use of PCM is fairly good because the
negative PCM effect partly cancel the positive errors
due to other effects. Figures 14a and 14c show that
the retrieval using the TOMS PCM leads to the
above correct results for a WC of COD 1 at 2-3 km,
and 11-12 km, respectively.
Figure 14. Errors varying with VZA for a
WC of COD 1 at 2-3 km (a) and 11-12 km
(c) at VZA= 30°. Rs refers to the assumed
minimum full cloud reflectivity in the
retrieval.
Summary and Conclusions
• We study the assumption of opaque Lambertian surface on TOMS ozone retrieval.
• The assumption of angularly independent cloud reflection is fairly good because the
Ozone Retrieval Error (ORE) is within 1.5% of the total ozone when COD  20.
• Because of the photon penetration and the ICOAEN effect, the assumption of opaque
cloudy surface introduces large OREs even for optically thick clouds. For a water cloud of
COD 40 at 2-12 km with 20.8 DU ozone homogeneously distributed in the cloud, the ORE is
17.8 DU at nadir view. This ICOAEN effect depends greatly on viewing geometry, ozone
amount in the cloud, and ozone distribution in the cloud.
Figure 12. Ozone retrieval errors for a 2-12
km WC of COD 1 (a) and 40 (b) at
SZA=30°. O3,t, O3,a, and O3,b indicate
error in total ozone, ozone above clouds
and ozone below clouds, respectively.
Figure 13. Ozone retrieval errors for a 2-12
km WC as a function of COD at SZA=30°,
3
VZA=30°.
• The TOMS PCM is good because the negative PCM effect arising from the cloud fraction
being underestimated partly cancel positive errors due primarily to the ICOAEN effect. At
COD  5, the negative PCM effect almost offsets the ICOAEN effect, and the TOMS
algorithm retrieves the about correct TOC. With increasing COD up to 20-40, the negative
PCM effect decreases more dramatically than the positive ICOAEN effect, so the overall
positive ORE increases.
Updated on August 30, 2002
4