 |
| Titel |
Adaption of the MODIS aerosol retrieval algorithm using airborne spectral surface reflectance measurements over urban areas: a case study |
| VerfasserIn |
E. Jäkel, B. Mey, R. Levy, X. Gu, T. Yu, Z. Li, D. Althausen, B. Heese, M. Wendisch |
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| ISSN |
1867-1381
|
| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Measurement Techniques ; 8, no. 12 ; Nr. 8, no. 12 (2015-12-15), S.5237-5249 |
| Datensatznummer |
250116724
|
| Publikation (Nr.) |
 copernicus.org/amt-8-5237-2015.pdf |
|
|
|
|
|
| Zusammenfassung |
| MODIS (MOderate-resolution Imaging Spectroradiometer) retrievals of aerosol
optical depth (AOD) are biased over urban areas, primarily because the
reflectance characteristics of urban surfaces are different than that assumed
by the retrieval algorithm. Specifically, the operational "dark-target"
retrieval is tuned towards vegetated (dark) surfaces and assumes a spectral
relationship to estimate the surface reflectance in blue and red wavelengths.
From airborne measurements of surface reflectance over the city of
Zhongshan, China, were collected that could replace the assumptions within
the MODIS retrieval algorithm. The subsequent impact was tested upon two
versions of the operational algorithm, Collections 5 and 6 (C5 and C6). AOD
retrieval results of the operational and modified algorithms were compared
for a specific case study over Zhongshan to show minor differences between
them all. However, the Zhongshan-based spectral surface relationship was
applied to a much larger urban sample, specifically to the MODIS data taken
over Beijing between 2010 and 2014. These results were compared directly to
ground-based AERONET (AErosol RObotic NETwork) measurements of AOD.
A significant reduction of the differences between the AOD retrieved by the
modified algorithms and AERONET was found, whereby the mean difference
decreased from 0.27±0.14 for the operational C5 and 0.19±0.12 for the
operational C6 to 0.10±0.15 and -0.02±0.17 by using the modified C5
and C6 retrievals. Since the modified algorithms assume a higher contribution
by the surface to the total measured reflectance from MODIS, consequently the
overestimation of AOD by the operational methods is reduced. Furthermore, the
sensitivity of the MODIS AOD retrieval with respect to different surface
types was investigated. Radiative transfer simulations were performed to
model reflectances at top of atmosphere for predefined aerosol properties.
The reflectance data were used as input for the retrieval methods. It was
shown that the operational MODIS AOD retrieval over land reproduces the AOD
reference input of 0.85 for dark surface types (retrieved AOD = 0.87 (C5)).
An overestimation of AOD = 0.99 is found for urban surfaces, whereas the
modified C5 algorithm shows a good performance with a retrieved value of
AOD = 0.86. |
| |
|
| Teil von |
|
|
|
|
|
|
|
|