Abstract
Abstract. The Visible Infrared Imaging Radiometer Suite (VIIRS) on board the Suomi National Polar-orbiting Partnership (S-NPP) spacecraft provides validated daily global aerosol optical depth (AOD) retrievals; however, a close examination of the VIIRS aerosol product identified residual snow and snowmelt contamination, resulting generally in an overestimation of AOD. The contamination was particularly evident over Northern Hemisphere high-latitude regions during the spring thaw. To improve the product performance, we introduced a new empirical snow and snowmelt screening scheme that combines a normalized difference snow index (NDSI)- and brightness temperature (BT)-based snow test, a snow adjacency test and a spatial homogeneity test (a.k.a. spatial filter). Testing of retrievals for 18 May 2014 indicated that, compared to the previous, visible reflectance anomaly (VRA)-based snow test, the new NDSI- and BT-based snow test screened out an additional 3.44 % of VIIRS AOD retrievals, most of which were over high latitudes experiencing snowmelt. The new snow adjacency test and the homogeneity test degraded another 5.57 % and 0.26 %, respectively, otherwise “good”-quality AOD retrievals. For the VIIRS–AERONET (Aerosol Robotic Network) matchups over Northern Hemisphere high-latitude regions during 3 years of spring (2013–2015), the new scheme also effectively screened out a significant number of the matchups that had anomalously high positive biases attributable to snow and snowmelt contamination. The new snow and snowmelt screening scheme was transferred to the Interface Data Processing Segment (IDPS) VIIRS aerosol algorithm on 22 June 2015. Subsequently no significant snow and snowmelt contamination was found during spring 2016. The scheme is also implemented in the new Enterprise VIIRS aerosol algorithm in the National Oceanic and Atmospheric Administration (NOAA) Enterprise Processing System (EPS) that became operational in 2017.
Highlights
Nowadays with increasing public awareness of air pollution and aerosol climatic effects, satellite observations of global aerosol loading and transport provide valuable information for improving our understanding of the impact of aerosols on weather, climate and public health (Kaufman et al, 2002; Quaas et al, 2008; Al-Saadi et al, 2005; van Donkelaar et al, 2010; Kloog et al, 2011)
Prior to 22 June 2015, the internal snow detection in the operational Interface Data Processing Segment (IDPS) Visible Infrared Imaging Radiometer Suite (VIIRS) aerosol algorithm was based on three tests: (1) the visible reflectance anomaly (VRA), (2) the ratio of top of the atmosphere (TOA) reflectance in the 1240 nm channel (ρ1240, VIIRS band M8) to the 865 nm channel (ρ865, VIIRS band M7) and (3) the surface temperature derived from the split-window technique (Walton et al, 1998)
Adjusting the normalized difference snow index (NDSI) threshold value showed that a higher threshold of 0.10 helps to regain most aerosol optical depth (AOD) retrievals over the heavy-air-pollution pixels that previously went missing in the IDPS algorithm
Summary
Nowadays with increasing public awareness of air pollution and aerosol climatic effects, satellite observations of global aerosol loading and transport provide valuable information for improving our understanding of the impact of aerosols on weather, climate and public health (Kaufman et al, 2002; Quaas et al, 2008; Al-Saadi et al, 2005; van Donkelaar et al, 2010; Kloog et al, 2011). J. Huang et al.: Screening for snow/snowmelt in SNPP VIIRS aerosol optical depth algorithm. With more than 3 years of the validated S-NPP VIIRS aerosol products publicly available and used in various user applications, further in-depth data analyses show that the VIIRS retrievals are consistently overestimating the AOD, when compared with AERONET, over high-latitude regions in the Northern Hemisphere, especially during the spring thaw, when snow is melting. This implies potential snow and snowmelt contamination in the products. For a true snow product, users are directed to the S-NPP VIIRS Snow/Ice products (Key et al, 2013)
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