scholarly journals A new measurement approach for validating satellite-based above cloud aerosol optical depth

2020 ◽  
Author(s):  
Charles K. Gatebe ◽  
Hiren Jethva ◽  
Ritesh Gautam ◽  
Rajesh Poudyal ◽  
Tamas Várnai
Keyword(s):  
Aerosol Optical Depth ◽  
Optical Depth ◽  
Ratio Method ◽  
Cloud Optical Depth ◽  
Airborne Measurements ◽  
Radiative Processes ◽  
Color Ratio ◽  
Satellite Retrievals ◽  
3D Effects ◽  
Measurement Approach

Abstract. The retrieval of aerosol parameters from passive satellite instruments in cloudy scenes is challenging partly because clouds and cloud-related processes may significantly modify aerosol optical depth (AOD) and particle size, a problem that is further compounded by the 3D radiative processes. Recent advances in retrieval algorithms such as the “color ratio” method which utilizes the measurements at a shorter (470 nm) and a longer (860 nm) wavelength have demonstrated the simultaneous derivation of AOD and cloud optical depth (COD) for scenes where absorbing aerosols are found to overlay low-level cloud decks. This study shows simultaneous retrievals of above-cloud aerosol optical depth (ACAOD) and aerosol-corrected cloud optical depth (COD) from airborne measurements of cloud-reflected and sky radiances using the color ratio method. These airborne measurements were taken over marine stratocumulus clouds with NASA's Cloud Absorption Radiometer (CAR) during SAFARI 2000 field campaign offshore of Namibia. The ACAOD is partitioned between the AOD below aircraft (AOD_cloudtop) and above aircraft AOD (AOD_sky). The results show good agreement between AOD_sky and sunphotometer measurements of the above aircraft AOD. The results also show that the use of aircraft-based sunphotometer measurements to validate satellite retrievals of the ACAOD is complicated by the lack of information on AOD below aircraft. Specifically, the CAR-retrieved AOD_cloudtop captures this “missing” aerosol layer caught between the aircraft and cloud top, which is required to quantify above cloud aerosol loading and effectively validate satellite retrievals. In addition, the study finds a strong anticorrelation between the AOD_cloudtop and COD for cases where COD  10, which may be associated with the uncertainties in the color ratio method at lower AODs and CODs. The influence of 3D radiative effects on the retrievals is examined and the results show that at cloud troughs, 3D effects increase retrieved ACAOD by about 3–10 % and retrieved COD by about 25 %. The results show that the color ratio method has little sensitivity to 3D effects at overcast stratocumulus cloud decks. These results demonstrate a novel airborne measurement approach for assessing satellite retrievals of aerosols above clouds, thereby filling a major gap that exists in the global aerosol observations.

scholarly journals A new measurement approach for validating satellite-based above-cloud aerosol optical depth

2021 ◽  
Vol 14 (2) ◽  
pp. 1405-1423
Author(s):  
Charles K. Gatebe ◽  
Hiren Jethva ◽  
Ritesh Gautam ◽  
Rajesh Poudyal ◽  
Tamás Várnai
Keyword(s):  
Aerosol Optical Depth ◽  
Optical Depth ◽  
Ratio Method ◽  
Cloud Optical Depth ◽  
Airborne Measurements ◽  
Color Ratio ◽  
Satellite Retrievals ◽  
3D Effects ◽  
Sun Photometer ◽  
Measurement Approach

Abstract. The retrieval of aerosol parameters from passive satellite instruments in cloudy scenes is challenging, partly because clouds and cloud-related processes may significantly modify aerosol optical depth (AOD) and particle size, a problem that is further compounded by 3D radiative processes. Recent advances in retrieval algorithms such as the “color ratio” method, which utilizes the measurements at a shorter (470 nm) and a longer (860 nm) wavelength, have demonstrated the simultaneous derivation of AOD and cloud optical depth (COD) for scenes in which absorbing aerosols are found to overlay low-level cloud decks. This study shows simultaneous retrievals of above-cloud aerosol optical depth (ACAOD) and aerosol-corrected cloud optical depth (COD) from airborne measurements of cloud-reflected and sky radiances using the color ratio method. These airborne measurements were taken over marine stratocumulus clouds with NASA's Cloud Absorption Radiometer (CAR) during the SAFARI 2000 field campaign offshore of Namibia. The ACAOD is partitioned between the AOD below-aircraft (AOD_cloudtop) and above-aircraft AOD (AOD_sky). The results show good agreement between AOD_sky and sun-photometer measurements of the above-aircraft AOD. The results also show that the use of aircraft-based sun-photometer measurements to validate satellite retrievals of the ACAOD is complicated by the lack of information on AOD below aircraft. Specifically, the CAR-retrieved AOD_cloudtop captures this “missing” aerosol layer caught between the aircraft and cloud top, which is required to quantify above-cloud aerosol loading and effectively validate satellite retrievals. In addition, the study finds a strong anticorrelation between the AOD_cloudtop and COD for cases in which COD < 10 and a weaker anticorrelation for COD > 10, which may be associated with the uncertainties in the color ratio method at lower AODs and CODs. The influence of 3D radiative effects on the retrievals is examined, and the results show that at cloud troughs, 3D effects increase retrieved ACAOD by about 3 %–11 % and retrieved COD by about 25 %. The results show that the color ratio method has little sensitivity to 3D effects at overcast stratocumulus cloud decks. These results demonstrate a novel airborne measurement approach for assessing satellite retrievals of aerosols above clouds, thereby filling a major gap in global aerosol observations.

scholarly journals Validating MODIS above-cloud aerosol optical depth retrieved from “color ratio” algorithm using direct measurements made by NASA's airborne AATS and 4STAR sensors

2016 ◽  
Vol 9 (10) ◽  
pp. 5053-5062 ◽  
Author(s):  
Hiren Jethva ◽  
Omar Torres ◽  
Lorraine Remer ◽  
Jens Redemann ◽  
John Livingston ◽  
...  
Keyword(s):  
Aerosol Optical Depth ◽  
Optical Depth ◽  
Field Experiments ◽  
Ratio Method ◽  
Cloud Optical Depth ◽  
Color Ratio ◽  
Direct Measurements ◽  
Absorbing Aerosols ◽  
Good Agreement

Abstract. We present the validation analysis of above-cloud aerosol optical depth (ACAOD) retrieved from the “color ratio” method applied to MODIS cloudy-sky reflectance measurements using the limited direct measurements made by NASA's airborne Ames Airborne Tracking Sunphotometer (AATS) and Spectrometer for Sky-Scanning, Sun-Tracking Atmospheric Research (4STAR) sensors. A thorough search of the airborne database collection revealed a total of five significant events in which an airborne sun photometer, coincident with the MODIS overpass, observed partially absorbing aerosols emitted from agricultural biomass burning, dust, and wildfires over a low-level cloud deck during SAFARI-2000, ACE-ASIA 2001, and SEAC4RS 2013 campaigns, respectively. The co-located satellite-airborne matchups revealed a good agreement (root-mean-square difference  <  0.1), with most matchups falling within the estimated uncertainties associated the MODIS retrievals (about −10 to +50 %). The co-retrieved cloud optical depth was comparable to that of the MODIS operational cloud product for ACE-ASIA and SEAC4RS, however, higher by 30–50 % for the SAFARI-2000 case study. The reason for this discrepancy could be attributed to the distinct aerosol optical properties encountered during respective campaigns. A brief discussion on the sources of uncertainty in the satellite-based ACAOD retrieval and co-location procedure is presented. Field experiments dedicated to making direct measurements of aerosols above cloud are needed for the extensive validation of satellite-based retrievals.

scholarly journals Validating MODIS Above-cloud Aerosol Optical Depth Retrieved from "Color Ratio" Algorithm using Direct Measurements made by NASA’s Airborne AATS and 4STAR Sensors

2016 ◽  
Author(s):  
Hiren Jethva ◽  
Omar Torres ◽  
Lorraine Remer ◽  
Jens Redemann ◽  
John Livingston ◽  
...  
Keyword(s):  
Root Mean Square Error ◽  
Aerosol Optical Depth ◽  
Optical Depth ◽  
Ratio Method ◽  
Mean Square ◽  
Airborne Measurements ◽  
Color Ratio ◽  
Direct Measurements ◽  
Reflectance Measurements ◽  
Good Agreement

Abstract. We present the first ever validation of above-cloud aerosol optical depth (ACAOD) retrieved from the "color ratio" method applied to MODIS cloudy-sky reflectance measurements using the limited direct measurements made by NASA's airborne AATS and 4STAR sensors. A thorough search of the airborne database collection revealed a total of five events in which airborne sunphotometer, coincident with the MODIS overpass, observed agricultural biomass burning, dust, and wildfire-emitted aerosols above a low-level cloud deck during SAFARI-2000, ACE-ASIA 2001, and SEAC4RS-2013 campaigns, respectively. The co-located satellite-airborne measurements revealed a good agreement (root-mean-square-error 

scholarly journals Satellite Observations of Cloud-Related Variations in Aerosol Properties

Atmosphere ◽  
2018 ◽  
Vol 9 (11) ◽  
pp. 430 ◽  
Author(s):  
Tamás Várnai ◽  
Alexander Marshak
Keyword(s):  
Particle Size ◽  
Aerosol Optical Depth ◽  
Optical Depth ◽  
Cloud Amount ◽  
Orthogonal Polarization ◽  
Radiative Processes ◽  
Moderate Resolution ◽  
Resolution Imaging ◽  
Moderate Resolution Imaging Spectroradiometer ◽  
Aerosol Properties

This paper presents an overview of our efforts to characterize and better understand cloud-related changes in aerosol properties. These efforts primarily involved the statistical analysis of global or regional datasets of Moderate Resolution Imaging Spectroradiometer (MODIS) and Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) aerosol and cloud observations. The results show that in oceanic regions, more than half of all aerosol measurements by passive satellite instruments come from near-cloud areas, where clouds and cloud-related processes may significantly modify aerosol optical depth and particle size. Aerosol optical depth is also shown to increase systematically with regional cloud amount throughout the Earth. In contrast, it is shown that effective particle size can either increase or decrease with increasing cloud cover. In bimodal aerosol populations, the sign of changes depends on whether coarse mode or small mode aerosols are most affected by clouds. The results also indicate that over large parts of Earth, undetected cloud particles are not the dominant reason for the satellite-observed changes with cloud amount, and that 3D radiative processes contribute about 30% of the observed near-cloud changes. The findings underline the need for improving our ability to accurately measure aerosols near clouds.

scholarly journals Ship-based aerosol optical depth measurements in the Atlantic Ocean: Comparison with satellite retrievals and GOCART model

2006 ◽  
Vol 33 (14) ◽  
Author(s):  
A. Smirnov ◽  
B. N. Holben ◽  
S. M. Sakerin ◽  
D. M. Kabanov ◽  
I. Slutsker ◽  
...  
Keyword(s):  
Atlantic Ocean ◽  
Aerosol Optical Depth ◽  
Optical Depth ◽  
Satellite Retrievals ◽  
Gocart Model

scholarly journals Intercomparison of satellite retrieved aerosol optical depth over ocean during the period September 1997 to December 2000

2004 ◽  
Vol 4 (6) ◽  
pp. 8201-8244 ◽  
Author(s):  
G. Myhre ◽  
F. Stordal ◽  
M. Johnsrud ◽  
D. J. Diner ◽  
I. V. Geogdzhayev ◽  
...  
Keyword(s):  
Spatial Correlation ◽  
Aerosol Optical Depth ◽  
Optical Depth ◽  
Entire Period ◽  
Coastal Regions ◽  
Satellite Retrievals ◽  
Field Campaigns ◽  
Good Agreement ◽  
Satellite Instruments

Abstract. Monthly mean aerosol optical depth (AOD) over ocean is compared from a total of 9 aerosol retrievals during a 40 months period. Comparisons of AOD have been made both for the entire period and sub periods. We identify regions where there is large disagreement and good agreement between the aerosol satellite retrievals. Significant differences in AOD have been identified in most of the oceanic regions. Several analyses are performed including spatial correlation between the retrievals as well as comparison with AERONET data. During the 40 months period studied there have been several major aerosol field campaigns as well as events of high aerosol content. It is studied how the aerosol retrievals compare during such circumstances. The differences found in this study are larger than found in a previous study where 5 aerosol retrievals over an 8 months period were compared. However, results in coastal regions are promising especially for aerosol retrievals from satellite instruments dedicated for aerosol research. In depth analyses explaining the differences between AOD obtained in different retrievals are clearly needed. We limit this study to identify differences and similarities and indicate possible sources that affect the quality of the retrievals. This is a necessary first step towards understanding the differences and improving the retrievals.

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