Scale- and Variable-Dependent Localization for 3DEnVar Data Assimilation
in the Rapid Refresh Forecast System
Abstract
This study demonstrates the advantages of scale- and variable-dependent
localization (SDL and VDL) on three-dimensional ensemble variational
data assimilation of the hourly-updated high-resolution regional
forecast system, the Rapid Refresh Forecast System (RRFS). SDL and VDL
apply different localization radii for each spatial scale and variable,
respectively, by extended control vectors. Single-observation
assimilation tests and cycling experiments with RRFS indicated that SDL
can enlarge the localization radius without increasing the sampling
error caused by the small ensemble size and decreased associated
imbalance of the analysis field, which was effective at decreasing the
bias of temperature and humidity forecasts. Moreover, simultaneous
assimilation of conventional and radar reflectivity data with VDL, where
a smaller localization radius was applied only for hydrometeors and
vertical wind, improved precipitation forecasts without introducing
noisy analysis increments. Statistical verification showed that these
impacts contributed to forecast error reduction, especially for
low-level temperature and heavy precipitation.