Ingredients improving rainfall forecast in very short-range
- Diabatic initialization and microphysics -

Eunha Lim, Yong-Hee Lee+ and Jong-Chul Ha+
Korea Meteorological Administration, Seoul, Korea
+National Institute of Meteorological Research, Seoul, Korea

To fill the gap from current to 6 hour when mesoscale-model forecast is available, KMA has operated the Korea Local Analysis and Prediction System (KLAPS) since 2008. The gap was filled by nowcasting, based on the persistent method and its variants. KLAPS consists of Weather Research and Forecasting (WRF) and Local Analysis and Prediction System (LAPS). It has two domains: the outer domain (15km resolution) is for providing initial and boundary conditions to the inner domain (5km resolution). A 12hours forecast is performed in the inner domain at every 1 hour. The first guess of the outer domain is nudged to analysis field of global model (25km) during 6 hours using Four Dimensional Data assimilation (FDDA). The initial condition of the inner domain is updated by observations using LAPS.

The 5km model exploits asynoptic data including METAR, PIREP, satellite and radar especially to form a three-dimensional cloud shape, finally to analyze model variables related with cloud in the first guess as follow. First, cloud column model modifies cloud water mixing ratio. Second, cloud type is determined based on first-guess temperature in the cloud region. Third, the prescribed profile (parabolic) of vertical velocity based on cloud type is assigned. The maximum vertical velocity was recalculated by a generic algorithm to enhance the predictability of rainfall forecast. Fourth, other variables, such as wind, geopotential height, are balanced using an omega equation. After introducing the above diabatic initialization, equitable threat score (ETS) for the threshold of 12.5mm/3hr was significantly increased during 12hours especially for first three hour.

The microphysics was changed from WSM6 scheme to WDM6 scheme in 2009. Introducing double moment scheme increases computational cost (about 8%). However the statistical evaluation of precipitation forecast skill over South Korea during summer season confirmed that the forecasted precipitation using the WDM6 scheme was better than that with the WSM6 scheme for both the light and the heavy precipitation categories in terms of ETS.

We will continue developing KLAPS by ingesting new launched satellite (Communication, Ocean and Meteorological Satellite , COMS) and radar data from improved pre-processing system.