1 Mike3/papers/tropoz/aguf98 12/2/98 16:30 M. Newchurch 1,2, X. Liu 3, J. H. Kim 4, P. K. Bhartia 5...

1Mike3/papers/tropoz/aguf98 12/2/98 16:30

M. Newchurch1,2, X. Liu3, J. H. Kim4, P. K. Bhartia5

1. U. Alabama in Huntsville, NSSTC 320 Sparkman Dr.,Huntsville, AL 35805

2. NCAR/ACD, 1850 Table Mesa Dr., Boulder, CO United States

3. U. Alabama in Huntsville NSSTC 320 Sparkman Dr., Huntsville, AL 35805

4. Pusan National University, Pusan, Korea, Republic of Korea 5. NASA/GSFC, Greenbelt, MD, United States

Accuracy of TOMS Retrievals over

Cloudy Regions

nsstc.uah.edu/atmchem


Motivation

• We see significant total-ozone-column excess of 10-15 DU over tropical high-altitude, highly reflecting clouds compared to clear observations [Newchurch et al., 2001].

• After accounting for errors involving incorrect cloud height, tropospheric ozone climatology, and considering potential dynamical, photochemical, and NIMBUS-7(N7)/Earth Probe calibration errors, approximately 4~9 DU excesses over cloudy scenes remain unexplained.

• We speculate that the TOMS algorithm approximation of optically thick clouds as opaque Lambertian reflecting surfaces may account for a significant portion of these unexplained excesses.


Objectives

• Cloud surfaces are not Lambertian; the reflection of clouds is angularly dependent. Furthermore, photons penetrate into clouds and the path length is enhanced due to in-cloud multiple scattering, resulting in enhanced ozone absorption. In addition, clouds might be ice (not water) clouds.

• Possible sources of ozone retrieval errors are illustrated in Figure 1. We use radiative transfer codes to address the effects of these aspects on TOMS ozone retrieval for optically thick clouds.

IncidentLight

Ground Surface

Tropopause

Cloudy Sky

Non-LambertianCloud Surface

Photon penetrates intoclouds, and path length isenhanced through multiplescattering, resulting inenhanced ozone absorption.

May be IceClouds?


Radiative Transfer Models and Methodology

• Treat clouds as scattering medium (water clouds, hexagon ice crystals, polycrystals). Calculate the forward backscattered radiance using Polarized Gauss-Seidel Radiative Transfer Code at N7 TOMS six channels.

• Use TOMS version-7 algorithm to retrieve ozone, from which the look-up table is calculated using TOMRAD at 10 pressure levels to reduce radiance interpolation errors.

• Separate the effects due to the neglect of ozone absorption enhancement in clouds from the effect of assuming cloud scattering as isotropic.

• If no ozone in cloud in the forward calculation, ozone absorption and its enhancement do not occur. The difference between the retrieved ozone and the input ozone gives the ozone retrieval error due to assuming cloud scattering as isotropic.

• The retrieved ozone difference with and without ozone in clouds in the forward calculation gives the enhanced ozone.

• Study how these two effects vary with viewing geometry, cloud types, cloud optical thickness, ozone in clouds, ozone distribution in clouds, cloud location, and cloud geometrical thickness.


Assumption of Isotropic Cloud Surface vs. Cloud Optical Depth

• The left panel shows the retrieved total ozone difference as a function of viewing geometry for a water cloud between 2-12 km with a optical depth (OD) of 40. The non-isotropic effect varies with viewing geometry, but is ±4.5 DU for the L275 DU ozone profile illustrated here.

• The non-isotropic effect decreases with increasing cloud OD. When OD 30, the patterns of non-isotropic effect vs. viewing geometry are very similar, but differs significantly when OD 20. When OD increases, clouds act more like Lambertian surfaces, therefore the non-isotropic effect decreases. In addition, at smaller cloud OD, the partial cloud model is used in the ozone retrieval, introducing additional ozone retrieval errors.


Assumption of Isotropic Cloud Surface vs. Cloud Types

• The non-isotropic effect shows different patterns vs. viewing geometry for WC, HEX, POLY, and WCHG.

• For POLY, the total ozone is ±3.5 DU of the original 275 DU. For HEX, the total ozone is within ±4.5 DU of 275 DU except at a few angles where the the error could be -7.5 or +10.9 DU. For WCHG, the total ozone is similar to WC except at anti-solar side and SZA 70, where the error ranges from -10 to -7 DU.


Assumption of Isotropic Cloud Surface vs. Cloud Altitude

•The non-isotropic effect varies a great deal with cloud-top height. The three clouds (2-12 km, 6-12 km, 10-12 km) show almost the same pattern non-isotropic effect, but the three clouds (2-4 km, 2-8 km, 2-12 km) shows very different patterns of non-isotropic effect.

• The non-isotropic effect varies with the ozone profiles. The more ozone above cloud, the larger the non-isotropic effect.


Ozone Retrieval Error Due to The Assumption of Isotropic Cloud SurfaceCloudType

Profile CloudOD

Location(KM)

Error Range(DU)

Avg. Error 1 s.d. (DU)

WC L275 10 2 – 12 -7.2 – 4.9 -4.8 1.9WC L275 20 2 – 12 -4.4 – 4.7 -2.3 1.4WC L275 30 2 – 12 -4.2 – 4.6 -1.2 1.2WC L275 40 2 – 12 -4.1 – 4.4 -0.7 1.1

WC L275 100 2 – 12 -4.0 – 4.0 -0.3 1.1WCHG L275 40 2 – 12 -9.8 – 4.8 -0.6 1.0

HEX L275 40 2 – 12 -7.6 – 10.9 -0.1 1.3POLY L275 40 2 – 12 -2.8 – 3.3 1.4 1.2WC L225 40 2-12 -3.0 – 2.4 -0.9 0.9WC L325 40 2-12 -5.7 – 6.7 -0.7 1.4WC M325 40 2-12 -10.7 – 4.4 -1.7 2.8WC M375 40 2-12 -11.7 – 4.8 -1.8 3.3WC L275 40 6 – 12 -4.1 – 4.4 -0.4 1.0WC L275 40 10 – 12 -4.0 – 4.0 -0.1 0.9WC L275 40 2 – 4 -11.1 – 4.4 -1.0 1.8WC L275 40 2 – 8 -4.0 – 4.9 2.3 0.9


Ozone Absorption Enhancement vs. Viewing Geometry

• The enhanced ozone decreases with increasing zenith angles 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.

• The photon path length in clouds decreases with increasing zenith angles. 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.


Ozone Absorption Enhancement Vs. Cloud Type, Optical Thickness and Ozone Amount

• 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, largest for WC, smallest for POLY.

• The enhanced ozone decreases with increasing cloud optical thickness because photons penetrates less into thicker clouds. At nadir, the enhanced ozone is 19.2 DU for OD of 10 and 10.9 for OD of 500.

• The enhanced ozone is almost linearly proportional to the amount of ozone in clouds. The enhancement (ratio of enhanced ozone to input ozone in clouds) actually decreases with the increase of ozone in clouds.


Ozone Absorption Enhancement Vs. Cloud Altitude

• The enhanced ozone increases with increasing geometrical thickness due primarily to the increase of the ozone in cloud. 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.


Ozone Absorption Enhancement Vs. Ozone Distribution in Clouds

• Six different profiles of ozone extinction coefficients in clouds are shown. 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.

• The enhanced ozone varies greatly with the ozone distribution of ozone in clouds. The enhanced ozone is less 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.


Vertical Distribution of Ozone Absorption Enhancement

• The top panel shows the vertical distribution (20 0.5-km layers) of ozone enhancement for WC and OD = 40 at a few selected angles.

• The layer that contributes most is located in the upper 1 km. The weight decreases dramatically deeper into clouds. This vertical distribution explains why ozone enhancement varies greatly with ozone distribution in clouds.

• The bottom panel shows the depth below the cloud top above which 50% of ozone enhancement is contributed vs. geometrical path length for several clouds.

• The penetration depth decreases with increasing optical thickness and geometrical path length. Among WC, HEX, and POLY, the penetration depth is largest for WC and smallest for POLY.


Summary and Conclusions regardingNon-isotropic effect

• The non-isotropic effect varies with viewing geometry, cloud optical thickness, different types of clouds (different phase function), cloud-top height, and ozone above cloud.

• The non-isotropic effect results from the difference in the ozone absorption enhancement above clouds due to the rayleigh scattering and multiple cloud reflection between the simulated scattered clouds and assumed Lambertian clouds.

• However, for most conditions, the non-isotropic effect is within ±4 DU, indicating the assumption of isotropic cloud scattering is fairly good for clouds with optical thickness 20.


Summary and Conclusions regardingOzone Absorption Enhancement

• The ozone absorption enhancement is due to the in-cloud multiple scattering, which interacts with ozone absorption. The enhanced ozone depends significantly on zenith angles, 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 source 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.


Publications

Kim, J. H., M. J. Newchurch, and Kunhee Han, Distribution of Tropical Tropospheric Ozone Determined Directly from TOMS Measurements, J. Atmos. Sci., in press, 2001.

Newchurch, M. J., X. Liu, J. H. Kim, Lower-tropospheric ozone (LTO) derived from TOMS near mountainous regions, J. Geophys. Res., in press, 2001.

Newchurch, M. J., X. Liu, J. H. Kim, and P. K. Bhartia, On the accuracy of TOMS retrievals over cloudy regions, J. Geophys. Res., in press, 2001.

Newchurch, M. J., D. Sun, and J. H. Kim, Tropical tropospheric ozone derived using Clear-Cloudy Pairs (CCP) of TOMS measurements, J. Atmos. Sci., submitted, 2001.

Newchurch, M. J., D. Sun, and Jae H. Kim, Zonal wave-1 structure in TOMS tropical stratospheric ozone, Geophys. Res. Lett., in press, 2001.

Kim, J. H., and M. Newchurch, Biomass-burning influence on tropospheric ozone over New Guinea and South America, 1455-1461, J. Geophys. Res., 103, 1998.

Kim, J. H., and M. J. Newchurch, Climatology and trends of Tropospheric ozone over the eastern Pacific Ocean: The influence of biomass burning and tropospheric dynamics, Geophys. Res. Lett., 23, 3723-3726, 1996.

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