A 4D-Var Analysis System for the California Current: A Prototype for an Operational Regional Ocean Data Assimilation System - Data Assimilation for Atmospheric, Oceanic and Hydrologic Applications - page 347

14.4 CCS Historical Analyses

ROMS CCS is currently being used in conjunction with 4D-Var to construct two
sequences of historical analyses for the circulation along the west coast of North
America. The first of these analyses, referred to as WCRA13, is a 13 year sequence
that spans the period Jan. 1999-Dec. 2011, while the second sequence, referred to
as WCRA31, covers the 31 year period Jan. 1980-Dec. 2011. The two analyses
use identical configurations for ROMS, but differ in the prior surface forcing used.
During the overlapping period 1999-2011 of the two analyses, the observations
assimilated into the model are identical.

14.4.1 WCRA13

In WCRA13, the prior surface forcing is derived from the NRL COAMPS model
introduced in Sect. 14.3 . While the COAMPS fields for the CCS are not available
before Jan. 1999, they do span the full period of WCRA13. The standard height
atmospheric variables are actually derived from four different nests of COAMPS
with horizontal resolution ranging from 3 to 81 km from the inner to the outer nest
( Doyle et al. 2009 ). Only data from the three inner-most nested grids are used in
ROMS and yield surface fields with a resolution of 3-9 km near the coast. This is
the highest resolution atmospheric forcing data set currently available for the CCS
region, and COAMPS verifies well against independent observations, indicating that
it is a high quality product. High horizontal resolution is important for the surface
forcing because many of the important regions of coastal upwelling along the U.S.
west coast are due to topographically enhanced regions of wind stress curl.

14.4.2 WCRA31

In WCRA31, the prior surface forcing is derived from a combination of atmo-
spheric analysis products. The surface winds are taken from the cross-calibrated
multiplatform product (CCMP) of Atlas et al. ( 2011 ) which is a 2D-Var analysis
of all available surface wind observations, using the ECMWF ERA40 reanalysis
as the prior estimate for the period 1987-1999, and the operational ECMWF
analysis after 1999. Consequently, we use 6 hourly sea level pressure, radiation
fluxes, precipitation, and standard height temperature, humidity from the ERA40
analysis so that the ROMS derived heat and fresh water fluxes are consistent with
the prior used for the winds. The resolution of the CCMP wind fields is 25 km,
while that of the ERA-40 reanalysis fields is

2:5 ı . Prior to 1987, the ERA-40
reanalysis fields are used, while after 1987 the ERA-40 reanalysis fields are used
in conjunction with CCMP winds since ERA40 is the prior for the CCMP analyses.