Development of a Two-way Nested LETKF System for Cloud-resolving Model - Data Assimilation for Atmospheric, Oceanic and Hydrologic Applications - page 492

a b

(mm)

(m/s)

Fig. 20.10 Spread distributions of 1 h rainfall ( shaded regions ) and surface wind at the height of
20 m ( vectors ) reproduced by the Inner LETKF at 11 JST of which initial seeds were produced by
using ( a ) the interpolated fields of the Outer LETKF ( L in Fig. 20.4 a) and ( b ) the last analyze fields
of the Inner LETKF ( S in Fig. 20.4 a). Arrows in ( a ) indicate the rainfall regions of which spreads
became smaller when the last analyzed fields of the Inner LETKF were used as the initial seeds

spreads that were seen at several points became smaller, when the analysis fields of
the Inner LETKF ( S ) were used (indicated by thin arrows in Fig. 20.10 a). Because
small-scale convergences might influence the occurrence of local heavy rainfalls,
initial seeds which include small-scale disturbances are desired.
In the CNTL experiment, the initial seeds of the second or later sets of the
Inner LETKF ( A in Fig. 20.4 a) should be close to the initial conditions of the Outer
LETKF ( B in Fig. 20.4 a). Because the initial conditions ( B ) are originated from the
last analyzed fields of the Inner LETKF ( S in Fig. 20.4 a), the smoothed distributions
of the last analyzed fields of the Inner LETKF ( S ) are similar to the interpolated dis-
tributions ( L in Fig. 20.4 a) of the Outer LETKF's initial conditions ( B ). Therefore,
the last analyzed fields of the Inner LETKF ( S ) are used directly as the initial seeds
of the Inner LETKF in the next assimilation set ( A ). In the experiments in which
the no-cost smoother is adopted in producing the initial conditions of the Inner
LETKF, however, the analyzed fields of the no-cost smoother at the initial time of
assimilation are different from the smoothed distributions of the last analyzed fields
of the Inner LETKF ( S ). Therefore, the initial seeds of the Inner LETKF should be
produced by adding the small disturbances, which were extracted from the last ana-
lyzed fields of the Inner LETKF ( S ), to the analyzed fields of the no-cost smoother.

20.5 Impact of Doppler Radar Data and GPS Water Vapor

Data

As mentioned in Sect. 20.2 , convergence of low-level water vapor is essential to
reproduce local heavy rainfalls. In this section, impacts of the GPS data and Doppler
radar data, which provide information of convergence of water vapor, are shown.