Offline development and initialization of
coupled models; a perspective from the 12km North American Land Data
Assimilation System (NLDASE) project
Brian Cosgrove
NASA/GSFC
Abstract:
The 12km
North American Land Data Assimilation System (NLDASE) project
builds on the uncoupled land surface modeling approach pioneered in the
NLDASE collaborative research effort to investigate two issues which
confront land surface modelers: initialization of coupled NWP model
land
surface states, and offline development of coupled land surface models
(LSMs). LDAS systems have sought to reduce the biases which can
accumulate
in the land surface stores of coupled modeling systems, and thus are
attractive sources of initial land surface conditions for NWP models.
These
uncoupled modeling systems are also much less computationally expensive
to
run than their coupled counterparts, and so are used extensively for
LSM
model development. This talk will focus on the questions which arise
from
these uses--how does such LDAS-based initialization impact subsequent
NWP
model forecasts, and how closely can uncoupled modeling systems mimic
coupled systems when used for LSM development activities? These issues
were
investigated using NCEP's coupled workstation Eta model and NASA's
uncoupled
Land Information System (LIS). The workstation Eta model was configured
to
mirror the operational Eta model, and both the workstation Eta and LIS
systems used the same version of the Noah LSM, and were executed on the
standard operational 12km Eta Arakawa E grid.
Initial results indicate that use of offline LIS land surface
conditions to
initialize the workstation Eta model results in large improvements in
the
RMSE and bias of 2m temperature and relative humidity forecasts over
the
CONUS out to 84 hours. The impacts on surface wind, precipitation, and
upper air forecasts are more mixed. Examination of the
coupled/uncoupled
LSM development issue has also yielded several key conclusions. Chief
among
these is that uncoupled LSM output can significantly diverge from that
of
the parallel coupled system if the offline model surface exchange
coefficient for heat (Ch), surface roughness, and forcing type/interval
are
not matched as closely as possible to that of the coupled system.