From

Kobayashi, S., Y. Ota, Y. Harada, A. Ebita, M. Moriya, H. Onoda, K. Onogi, H. Kamahori, C. Kobayashi, H. Endo, K. Miyaoka, and K. Takahashi , 2015: The JRA-55 Reanalysis: General Specifications and Basic Characteristics. J. Meteorol. Soc. Japan, 93, 5-48, doi: 10.2151/jmsj.2015-001


The analysis of screen-level variables (2-m
temperatures, 2-m relative humidities and 10-m
winds) is performed separately from the atmospheric
analysis component. These variables are analyzed
with a univariate 2-dimensional optimal interpolation
(2D-OI). In JRA-25, departures were estimated
by comparing observations with the first guess at the
analysis time. In JRA-55, observations are compared
with the first guess at the actual observation time and
the departures are applied at the analysis time (the
so-called first guess at the appropriate time (FGAT)
approach).
Temperature and wind observations from islands
are not used because they are not necessarily representative
at the grid scale of JRA-55. Determining
whether an observation is from an island is based
on the 0.25-degree resolution land cover data;
consequently, observations from the coast are also
excluded.
Screen-level analysis fields are not used as initial
conditions for forecasts.