Synoptic Verification of Medium-Extended-Range Forecasts of the Northwest Pacific Subtropical High and South Asian High Based on Multi-Center TIGGE Data

Synoptic verification of medium-extended-range forecasts of the Northwest Pacific subtropical high （NWPSH） and South Asian high （SAH） is performed for the summers of 2010-2012 using TIGGE data from four operational centers at the CMA,ECMWF,JMA,and NCEP.The overall activities of the NW-PSH and SAH are examined along with their local characteristics such as the spatial coverage of each high in the East Asian key area （10°-40°N,105°-130°E）,the mean position of the ridge of each high over 110°-122.5°E,the westward extent of the NWPSH ridge,and the eastward extent of the SAH ridge.Focus on the NWPSH and SAH is justified because these two systems have pronounced influences on the summertime persistent heavy rainfall in China.Although the overall activities of both highs are reproduced reasonably well in the TIGGE data,their spatial coverages are reduced in the East Asian key area and both of them are weaker compared with observations.On average,their ridges shift more northward relative to observations.The NWPSH ridge is less westward while the SAH ridge is generally more eastward early in the forecast but too westward later in the forecast.The JMA ensemble prediction system （EPS） produces the best mediumrange （1-10 days） forecasts of the NWPSH based on these metrics,while the ECMWF EPS produces the best medium-range forecasts of the SAH and the most reliable extended-range （11-15 days） forecasts of both highs.Forecasts of the spatial coverage of both highs in the East Asian key area and the mean positions of the ridges are generally valid out to lead times of 7-12 days.By contrast,forecasts of the longitudinal extent of the ridges are typically only valid to lead times of 5-7 days.All the four operational centers＇ models produce excellent forecasts of the mean zonal position of the SAH ridge.The ensemble mean forecast is more reliable than the control forecast over the areas where the NWPSH （20°-30°N,135°-165°E） and SAH （23°-30°N,70°-100°E） are most active.Forecasts of both highs have advantages and disadvantages in the peripheral areas away from their respective center of high activity.

STUDY OF THE MODIFICATION OF MULTI-MODEL ENSEMBLE SCHEMES FOR TROPICAL CYCLONE FORECASTS

This study investigates multi-model ensemble forecasts of track and intensity of tropical cyclones over the western Pacific, based on forecast outputs from the China Meteorological Administration, European Centre for Medium-Range Weather Forecasts, Japan Meteorological Agency and National Centers for Environmental Prediction in the THORPEX Interactive Grand Global Ensemble(TIGGE) datasets. The multi-model ensemble schemes, namely the bias-removed ensemble mean(BREM) and superensemble(SUP), are compared with the ensemble mean(EMN) and single-model forecasts. Moreover, a new model bias estimation scheme is investigated and applied to the BREM and SUP schemes. The results showed that, compared with single-model forecasts and EMN, the multi-model ensembles of the BREM and SUP schemes can have smaller errors in most cases. However, there were also circumstances where BREM was less skillful than EMN, indicating that using a time-averaged error as model bias is not optimal. A new model bias estimation scheme of the biweight mean is introduced. Through minimizing the negative influence of singular errors, this scheme can obtain a more accurate model bias estimation and improve the BREM forecast skill. The application of the biweight mean in the bias calculation of SUP also resulted in improved skill. The results indicate that the modification of multi-model ensemble schemes through this bias estimation method is feasible.