The MERRA-2 Aerosol Reanalysis, 1980 Onward. Part I: System Description and Data Assimilation Evaluation

Abstract

The Modern-Era Retrospective Analysis for Research and Applications, Version 2 (MERRA-2) updates NASA's previous satellite era (1980 - onward) reanalysis system to include additional observations and improvements to the Goddard Earth Observing System, Version 5 (GEOS-5) Earth system model. As a major step towards a full Integrated Earth Systems Analysis (IESA), in addition to meteorological observations, MERRA-2 now includes assimilation of aerosol optical depth (AOD) from various ground- and space-based remote sensing platforms. Here, in the first of a pair of studies, we document the MERRA-2 aerosol assimilation, including a description of the prognostic model (GEOS-5 coupled to the GOCART aerosol module), aerosol emissions, and the quality control of ingested observations. We provide initial validation and evaluation of the analyzed AOD fields using independent observations from ground, aircraft, and shipborne instruments. We demonstrate the positive impact of the AOD assimilation on simulated aerosols by comparing MERRA-2 aerosol fields to an identical control simulation that does not include AOD assimilation. Having shown the AOD evaluation, we take a first look at aerosol-climate interactions by examining the shortwave, clear-sky aerosol direct radiative effect. In our companion paper, we evaluate and validate available MERRA-2 aerosol properties not directly impacted by the AOD assimilation (e.g. aerosol vertical distribution and absorption). Importantly, while highlighting the skill of the MERRA-2 aerosol assimilation products, both studies point out caveats that must be considered when using this new reanalysis product for future studies of aerosols and their interactions with weather and climate.

Aerosol midlatitude cyclone indirect effects in observations and high-resolution simulations

“…The relationship used in the present study to calculate CDNC from 910 hPa sulfate mass is CDNC = 10 0.41log 10 SO 4 +2.1 , where CDNC is in units of cm −3 and SO4 is in units of µg m −3 . Since MERRA2 aerosol assimilation does not ingest MODIS cloud properties, the CDNC from MODIS should not influence MERRA2 sulfate (Randles et al, 2016). One caveat to using MERRA2 sulfate as a proxy for CDNC when investigating cloud-aerosol adjustments is that MERRA2 does ingest microwave-retrieved rain rates up until 2009 and clearsky microwave WVP into its reanalysis (McCarty et al, 2016).…”

An Unreported Asian Dust (Kosa) Event in Hokkaido, Japan: A Case Study of 7 March 2016

“…Recently, the next version of re-analysis data, called MERRA-2, was released (Bosilovich et al 2015), in which aerosol components were produced using the GOCART module (e.g., Chin et al 2000Chin et al , 2002Ginoux et al 2001;Colarco et al 2010) in the Goddard Earth Observing System, version 5 (GEOS-5) (Rienecker et al 2008). The aerosol data assimilation method uses MODIS, and other stellated-derived and ground-based Aerosol Optical Depth (AOD) data (e.g., Buchard et al 2016b;Randles et al 2016Randles et al , 2017. The aerosol data assimilation method in MERRA-2 (Bosilovich et al 2015, Buchard et al 2016bRandles et al 2016Randles et al , 2017 was advanced from the product of MERRAero (e.g., Provencal et al 2014;Buchard et al 2015Buchard et al , 2016a.…”

“…The aerosol data assimilation method uses MODIS, and other stellated-derived and ground-based Aerosol Optical Depth (AOD) data (e.g., Buchard et al 2016b;Randles et al 2016Randles et al , 2017. The aerosol data assimilation method in MERRA-2 (Bosilovich et al 2015, Buchard et al 2016bRandles et al 2016Randles et al , 2017 was advanced from the product of MERRAero (e.g., Provencal et al 2014;Buchard et al 2015Buchard et al , 2016a. We use the hourly mean MERRA-2 outputs (0.5° × 0.625° in latitude and longitude) and calculated PM 2.5 data with the method proposed by Buchard et al (2016a).…”

Integrated Analysis of Dust Transport and Budget in a Severe Asian Dust Event

“…It places observations from NASA's Earth Observing System (EOS) satellites into climate contexts, thereby updating the MERRA system to include the most recent satellite data (Rienecker et al, 2011;Randles et al, 2016). The original MERRA assimilated only meteorological parameters, including wind and temperature .…”

“…Compared with available observations and models alone, MERRA-2 potentially provides more accurate dust estimations because of the assimilation of various aerosol observations Randles et al, 2016). Therefore, MERRA-2 offers many advantages for estimating and analyzing dust loadings, emission, and deposition in this severe dust event.…”