File Specification for MERRA-2 Climate Statistics Products

The Modern Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2)
contains a wealth of information that can be used for weather and climate studies. By combining
the assimilation of observations with a frozen version of the Goddard Earth Observing System
(GEOS), a global analysis is produced at an hourly temporal resolution spanning from January
1980 through present (Gelaro et al., 2017). It can be difficult to parse through a multidecadal
dataset such as MERRA-2 to evaluate the interannual variability of weather that occurs on a daily
timescale, let alone determine the occurrence of an extreme weather event. Furthermore, it was
recognized that standard metrics were needed to evaluate climate change among climate models
and international research efforts. As a result of these concerns, the Expert Team on Climate
Change Detection and Indices (ETCCDI) developed a set of indices that represent the frequency
and intensity of extreme weather events using a daily time series of 2-m air temperature (T2m)
and precipitation (Alexander et al., 2016). These indices were used as a basis to comprise a list of
fields that represent daily extreme temperature and precipitation events, heatwaves, multi-day
precipitation, as well monthly percentile statistics from the MERRA-2 dataset. Also included in
this data product is a climatological long term mean and standard deviation representing the
interannual variability on a monthly timescale.