Development of a Two-way Nested LETKF System for Cloud-resolving Model - Data Assimilation for Atmospheric, Oceanic and Hydrologic Applications - page 486

20.4 Assimilation Results of Conventional Data and Other

Improvements of Nested LETKF System

20.4.1 Assimilation Results of Conventional Data

Figures 20.5 a, b show the ensemble mean and spread of the rainfalls reproduced
by the Outer LETKF at 15 JST (the end of the second sets of the Inner LETKF
experiments, indicated by a gray triangle in Fig. 20.4 a), just before the occurrence
of the local heavy rainfall. As shown in Sect. 20.2 , the small intense convections
scattered over the western Japan were observed. Due to the large grid interval of
the Outer LETKF (15 km), the reproduced rainfalls were expressed as weak rainfall
regions (Fig. 20.5 a). Although the rainfall intensities were much weaker than the
observed ones, the distribution of rainfall regions was roughly similar to that of the
observed one. In this experiment, any deviation, such as deviations produced from
ensemble forecasts of Global models, was not added to the boundary conditions of
the Outer LETKF. Due to the fixed boundary conditions, the ensemble spreads near
the boundary were small (Fig. 20.5 b). However, the spread around the center of the
domain where the Osaka Plain is located was relatively large, and one Inner LETKF
was deployed around the center of the Outer LETKF.
Next, the ensemble mean distribution of rainfalls analyzed by the Inner LETKF
was compared with the observed one (Figs. 20.5 cand 20.2 b). Although the rainfall
regions were more widely distributed and their rainfall intensities were smaller due
to the averaging procedure, the positions of the reproduced rainfall regions were
similar to the observed regions at 1500 JST (Figs. 20.5 cand 20.2 b). Namely, the
reproduced rainfall regions more than 1 mm/h roughly corresponded to the observed
regions of which rainfall intensities were more than 4 mm/h. Because these well-
reproduced rainfall regions were located in mountainous areas (indicated by circles
in Fig. 20.5 c), these scattered rainfalls at 15 JST are likely related to orographic
effects. As for the rainfall that developed into the local heavy rainfall at Sakai City
(indicated by an arrow in Fig. 20.5 c), it was too small and its intensity was too
weak.
As mentioned, the rainfall intensity of the ensemble mean was smaller than that
of each ensemble member due to the averaging procedure. Because the rainfall
intensity, as well as the position of the rainfalls, is important from the point of
view of disaster prevention, the rainfall distributions of each ensemble members
are shown in the following sections. Figure 20.6 b is the rainfall distributions at
17 JST (indicated by a solid triangle Fig. 20.4 a) reproduced by the Inner LETKF
to show the development of the rainfall at Sakai City. The conventional data was
assimilated in the Outer LETKF and Inner LETKF from 1510 JST to 1700 JST. In
four ensemble members (#001, #004, #007 and #008), the intense rainfall regions
that were developed at Sakai City extended northwestward (indicated by circles in
Fig. 20.6 b). This feature of the analyzed distributions, which was the same as the
observed one at 16 JST, indicates that the local heavy rainfall at Sakai City were