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| Titel |
Looking through the haze: evaluating the CALIPSO level 2 aerosol optical depth using airborne high spectral resolution lidar data |
| VerfasserIn |
R. R. Rogers, M. A. Vaughan, C. A. Hostetler, S. P. Burton, R. A. Ferrare, S. A. Young, J. W. Hair, M. D. Obland, D. B. Harper, A. L. Cook, D. M. Winker |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1867-1381
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Measurement Techniques ; 7, no. 12 ; Nr. 7, no. 12 (2014-12-09), S.4317-4340 |
| Datensatznummer |
250115987
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| Publikation (Nr.) |
 copernicus.org/amt-7-4317-2014.pdf |
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| Zusammenfassung |
The Cloud–Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument
onboard the Cloud–Aerosol Lidar and Pathfinder Satellite Observations
(CALIPSO) spacecraft has provided over 8 yr of nearly continuous vertical
profiling of Earth's atmosphere. In this paper we investigate the V3.01 and
V3.02 CALIOP 532 nm aerosol layer optical depth (AOD) product (i.e the AOD
of individual layers) and the column AOD product (i.e., the sum AOD of the
complete column) using an extensive database of coincident measurements. The
CALIOP AOD measurements and AOD uncertainty estimates are compared with
collocated AOD measurements collected with the NASA High Spectral Resolution
Lidar (HSRL) in the North American and Caribbean regions. In addition, the
CALIOP aerosol lidar ratios are investigated using the HSRL measurements.
In general, compared with the HSRL values, the CALIOP layer AOD are biased
high by less than 50% for AOD < 0.3 with higher errors for higher
AOD. Less than 60% of the HSRL AOD measurements are encompassed within
the CALIOP layer 1 SD uncertainty range (around the CALIOP
layer AOD), so an error estimate is created to encompass 68% of the HSRL
data. Using this new metric, the CALIOP layer AOD error is estimated using
the HSRL layer AOD as ±0.035 ± 0.05 · (HSRL layer AOD) at night
and ±0.05 ± 0.05 · (HSRL layer AOD) during the daytime.
Furthermore, the CALIOP layer AOD error is found to correlate with aerosol
loading as well as aerosol subtype, with the AODs in marine and dust layers
agreeing most closely with the HSRL values. The lidar ratios used by CALIOP
for polluted dust, polluted continental, and biomass burning layers are
larger than the values measured by the HSRL in the CALIOP layers, and
therefore the AODs for these types retrieved by CALIOP were generally too
large.
We estimated the CALIOP column AOD error can be expressed as ±0.05 ± 0.07 · (HSRL column AOD) at night and ±0.08 ± 0.1 · (HSRL
column AOD) during the daytime. Multiple sources of error contribute to both
positive and negative errors in the CALIOP column AOD, including multiple
layers in the column of different aerosol types, lidar ratio errors, cloud
misclassification, and undetected aerosol layers. The undetected layers were
further investigated and we found that the layer detection algorithm works
well at night, although undetected aerosols in the free troposphere
introduce a mean underestimate of 0.02 in the column AOD in the data set
examined. The decreased signal-to-noise ratio (SNR) during the daytime led to poorer performance of
the layer detection. This caused the daytime CALIOP column AOD to be less
accurate than during the nighttime, because CALIOP frequently does not detect
optically thin aerosol layers with AOD < 0.1. Given that the median
vertical extent of aerosol detected within any column was 1.6 km during the
nighttime and 1.5 km during the daytime, we can estimate the minimum
extinction detection threshold to be 0.012 km−1 at night and 0.067 km−1 during the daytime in a layer median sense.
This extensive validation of level 2 CALIOP AOD
products extends previous validation studies to nighttime lighting
conditions and provides independent measurements of the lidar ratio; thus,
allowing the assessment of the effect on the CALIOP AOD of using
inappropriate lidar ratio values in the extinction retrieval. |
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