Development of a Two-way Nested LETKF System for Cloud-resolving Model - Data Assimilation for Atmospheric, Oceanic and Hydrologic Applications - page 480

Chapter 20
Development of a Two-way Nested LETKF
System for Cloud-resolving Model

Hiromu Seko, Tadashi Tsuyuki, Kazuo Saito, and Takemasa Miyoshi

Abstract A two-way nested Local Ensemble Transform Kalman Filter (LETKF)
system has been developed to improve the accuracy of numerical forecasts on local
heavy rainfalls. In this system, mesoscale convergence which drives local heavy
rainfalls, is first reproduced by the LETKF with a grid interval of 15 km (Outer
LETKF) which assimilates conventional data. The convection cells associated with
the local heavy rainfall are then reproduced by the higher resolution LETKF with a
grid interval of 1.875 km (Inner LETKF) which assimilates local data. The boundary
conditions of the Inner LETKF are given by the forecast of the Outer LETKF. To
consider the upward cascade effect from storm scale to mesoscale, the forecast
results of the Inner LETKF are reflected into the Outer LETKF every 6 h.
This system was applied to a thunderstorm that caused a local heavy rainfall
event on the Osaka Plain on 5th September 2008. The rainfall distributions similar
to the observed ones were reproduced in a few ensemble members of the Inner
LETKF, although the observed scattered convection cells were expressed as weak
rainfall regions in the Outer LETKF. When the precipitable water vapor or slant-path
water vapor data obtained by GPS and horizontal wind or radial wind data observed
by Doppler radars were assimilated in the Inner LETKF, the number of ensemble
forecasts, which reproduced the local heavy rainfall, increased. The experiments on
the small-scale disturbances in the initial seeds of the Inner LETKF and on the initial
conditions produced by the no-cost smoother showed that these improvements might
enhance the accuracy of local heavy rainfall forecasts.