Abstract
Abstract. The daily Level-3 MODIS aerosol optical depth (AOD) product is a global daily spatial aggregation of the Level-2 MODIS AOD (10-km spatial resolution) into a regular grid with a resolution of 1° × 1°. It offers interesting characteristics for surface solar radiation and numerical weather modeling applications. However, most of the validation efforts so far have focused on Level-2 products and only rarely on Level 3. In this contribution, we compare the Level-3 Collection 5.1 MODIS AOD dataset from the Terra satellite available since 2000 against observed daily AOD values at 550 nm from more than 500 AERONET ground stations around the globe. Overall, the mean error of the dataset is 0.03 (17%, relative to the mean ground-observed AOD), with a root mean square error of 0.14 (73%, relative to the same), but these errors are also found highly dependent on geographical region. We propose new functions for the expected error of the Level-3 AOD, as well as for both its mean error and its standard deviation. Additionally, we investigate the role of pixel count vis-à-vis the reliability of the AOD estimates, and also explore to what extent the spatial aggregation from Level 2 to Level 3 influences the total uncertainty in the Level-3 AOD. Finally, we use a radiative transfer model to investigate how the Level-3 AOD uncertainty propagates into the calculated direct normal and global horizontal irradiances.
Highlights
Aerosols play a major role in the earth’s energy budget
We present a global validation of the combined land and ocean Level-3 (L3) MODerate-resolution Imaging Spectroradiometer (MODIS) aerosol optical depth (AOD) product (Collection 5.1) at 550 nm from the Terra satellite
We have presented a global validation of the combined land and ocean Level-3 MODIS AOD product (Collection 5.1) at 550 nm from the Terra satellite, and investigated its suitability for surface solar radiation calculations in numerical weather or radiative transfer modeling
Summary
Aerosols play a major role in the earth’s energy budget. Predicting the climate system invariably requires an evaluation of the aerosol’s radiative forcing, and this, in turn, requires knowledge of the spatial distribution, temporal evolution and optical properties of atmospheric aerosols (Kiehl and Ramanathan, 2006). Kaufman et al (2000) used 7 yr (1993–1999) of AOD data from 50–70 AERONET ground stations to check to what extent measurements acquired at specific times of the day (such as current measurements from satellites Terra and Aqua) could represent the daily average aerosol forcing of climate. For all these stations, they calculated the ratio of the averaged AERONET AOD for the virtual overpass of the satellites (10:00–11:30 LT for Terra’s overpass, and 12:00–13:30 LT for Aqua’s overpass) to the daily averaged AERONET AOD. The dataset encompasses a total of 525 stations, but the number of available stations operating any given day is variable
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