Abstract
This study used the nearly continuous 17-year observation record from the Multi- angle Imaging SpectroRadiometer (MISR) instrument on the National Aeronautics and Space Administration (NASA) Terra Earth Observing System satellite to determine which temporal subsets are long enough to define statistically stable speciated aerosol optical depth (AOD) climatologies (i.e., AOD by particle types) for purposes of climate model evaluation. A random subsampling of seasonally averaged total and speciated AOD retrievals was performed to quantitatively assess the statistical stability in the climatology, represented by the minimum record length required for the standard deviation of the subsampled mean AODs to be less than a certain threshold. Our results indicate that the multi-year mean speciated AOD from MISR is stable on a global scale; however, there is substantial regional variability in the assessed stability. This implies that in some regions, even 17 years may not provide a long enough sample to define regional mean total and speciated AOD climatologies. We further investigated the agreement between the statistical stability of total AOD retrievals from MISR and the Moderate Resolution Imaging Spectroradiometer (MODIS), also on the NASA Terra satellite. The difference in the minimum record lengths between MISR and MODIS climatologies of total AOD is less than three years for most of the globe, with the exception of certain regions. Finally, we compared the seasonal cycles in the MISR total and speciated AODs with those simulated by three global chemistry transport models in the regions of climatologically stable speciated AODs. We found that only one model reproduced the observed seasonal cycles of the total and non-absorbing AODs over East China, but the seasonal cycles in total and dust AODs in all models are similar to those from MISR in Western Africa. This work provides a new method for considering the statistical stability of satellite-derived climatologies and illustrates the value of MISR’s speciated AOD data record for evaluating aerosols in global models.
Highlights
Aerosols exert considerable impacts on air quality and the Earth’s radiation budget, yet neither their mean state in the current climate nor their response to a changing climate are well represented in global climate models [1]
Because the statistical uncertainties increase as the sample sizes decrease, we examined variability of the uncertainty as a function of sampling lengths and determined minimum record lengths required for the observed total and speciated aerosol optical depth (AOD) from Multiangle Imaging SpectroRadiometer (MISR) to meet certain uncertainty levels
We have shown that the length of satellite observation records affects the statistical stability of the climatological AOD, especially the AODs for different types of aerosols
Summary
Aerosols exert considerable impacts on air quality and the Earth’s radiation budget, yet neither their mean state in the current climate nor their response to a changing climate are well represented in global climate models [1]. There have been many studies evaluating the aerosol optical depth (AOD) simulated by climate models using satellite observations. AOD in models may be the most fundamental task for evaluating the direct and indirect radiative effects of aerosols in the models (e.g., Shindell et al [2], Park et al [3], Pan et al [4], Zhu et al [5]). Most of these studies considered the multi-year mean AODs derived from satellite observations as their reference datasets. The use of satellite-derived AOD climatologies for benchmarking climate models has been performed based more on the availability of datasets and temporal overlap with available model runs, rather than any objective standards
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