How Long Is Too Long? Variogram Analysis of AERONET Data to Aid Aerosol Validation and Intercomparison Studies
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Date
2020-09-14
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Citation of Original Publication
Sayer, A. M. (2020). How long is too long? Variogram analysis of AERONET data to aid aerosol validation and intercomparison studies. Earth and Space Science, 7, e2020EA001290. https://doi.org/10.1029/2020EA001290
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Attribution 4.0 International (CC BY 4.0)
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Abstract
Geophysical data sets derived from satellite sensors, ground/airborne instrumentation, andcomputational models are often compared against each other. A common example is the validation ofsatellite aerosol optical depth (AOD) retrievals against measurements from Aerosol Robotic Network(AERONET) Sun photometers. Spatiotemporal mismatch between data set sampling means that uncapturedvariation in the underlying geophysicalfield introduces apparent disagreement into such comparisons,known as representation or collocation matchup uncertainty. This study uses variogram analysis ofAERONET data to estimate temporal mismatch uncertainties and decorrelation time scales for the globalAERONET record. As well as total AOD, thefine‐and coarse‐mode AODs, Ångström Exponent (AE), andfine‐mode fraction (FMF) of AOD are analyzed. Globally, a time difference of 30 min typically induces from0.011–0.035 variation in AOD. For total,fine, and coarse AODs the typical time to decorrelation is around2–10 days. For AE and FMF it is 3–33 days; that is, aerosol systems often persist significantly longer thanindividual events in them. Biomass burning regions tend to show the largest and fastest subdaily AODvariability and also longest times to decorrelation. Some sites show significant season‐to‐season variations inbehavior. These results can be used to inform site‐specific time collocation thresholds for aerosol validationanalyses and account for temporal variation when estimating data set uncertainty. They also haveimplications for comparisons between different satellite products or models, data aggregation, and timeseries analyses. Results are provided on a site‐by‐site basis to facilitate use by other researchers.