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

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.