An ensemble Kalman filter(EnKF) combined with the Advanced Research Weather Research and Forecasting model(WRF) is cycled and evaluated for western North Pacific(WNP) typhoons of year 2016. Conventional in situ data, ...An ensemble Kalman filter(EnKF) combined with the Advanced Research Weather Research and Forecasting model(WRF) is cycled and evaluated for western North Pacific(WNP) typhoons of year 2016. Conventional in situ data, radiance observations, and tropical cyclone(TC) minimum sea level pressure(SLP) are assimilated every 6 h using an 80-member ensemble. For all TC categories, the 6-h ensemble priors from the WRF/EnKF system have an appropriate amount of variance for TC tracks but have insufficient variance for TC intensity. The 6-h ensemble priors from the WRF/EnKF system tend to overestimate the intensity for weak storms but underestimate the intensity for strong storms. The 5-d deterministic forecasts launched from the ensemble mean analyses of WRF/EnKF are compared to the NCEP and ECMWF operational control forecasts. Results show that the WRF/EnKF forecasts generally have larger track errors than the NCEP and ECMWF forecasts for all TC categories because the regional simulation cannot represent the large-scale environment better than the global simulation. The WRF/EnKF forecasts produce smaller intensity errors and biases than the NCEP and ECMWF forecasts for typhoons, but the opposite is true for tropical storms and severe tropical storms. The 5-d ensemble forecasts from the WRF/EnKF system for seven typhoon cases show appropriate variance for TC track and intensity with short forecast lead times but have insufficient spread with long forecast lead times. The WRF/EnKF system provides better ensemble forecasts and higher predictability for TC intensity than the NCEP and ECMWF ensemble forecasts.展开更多
In this study,the impact of environmental factors on tropical cyclone(TC)outer-core size was investigated for both migrating and local TCs in the South China Sea during the period 2001–2019.Among all the thermodynami...In this study,the impact of environmental factors on tropical cyclone(TC)outer-core size was investigated for both migrating and local TCs in the South China Sea during the period 2001–2019.Among all the thermodynamic and dynamic factors,the low-level environmental helicity showed the strongest positive correlation with TC outer-core size.Large helicity favors the development and organization of convection in TCs,and the corresponding strong inflow and large angular momentum fluxes into the system is beneficial for the maintenance and enlargement of TC outer-core size.Besides,the asymmetric distribution of helicity may account for the asymmetry of TC outer-core size.Therefore,the environmental helicity,as an integrated dynamic factor,can provide an alternative view on TC outer-core size.展开更多
基金jointly sponsored by the National Key R&D Program of China through Grant No. 2017YFC1501603the National Natural Science Foundation of China through Grant Nos. 41675052 and 41775057。
文摘An ensemble Kalman filter(EnKF) combined with the Advanced Research Weather Research and Forecasting model(WRF) is cycled and evaluated for western North Pacific(WNP) typhoons of year 2016. Conventional in situ data, radiance observations, and tropical cyclone(TC) minimum sea level pressure(SLP) are assimilated every 6 h using an 80-member ensemble. For all TC categories, the 6-h ensemble priors from the WRF/EnKF system have an appropriate amount of variance for TC tracks but have insufficient variance for TC intensity. The 6-h ensemble priors from the WRF/EnKF system tend to overestimate the intensity for weak storms but underestimate the intensity for strong storms. The 5-d deterministic forecasts launched from the ensemble mean analyses of WRF/EnKF are compared to the NCEP and ECMWF operational control forecasts. Results show that the WRF/EnKF forecasts generally have larger track errors than the NCEP and ECMWF forecasts for all TC categories because the regional simulation cannot represent the large-scale environment better than the global simulation. The WRF/EnKF forecasts produce smaller intensity errors and biases than the NCEP and ECMWF forecasts for typhoons, but the opposite is true for tropical storms and severe tropical storms. The 5-d ensemble forecasts from the WRF/EnKF system for seven typhoon cases show appropriate variance for TC track and intensity with short forecast lead times but have insufficient spread with long forecast lead times. The WRF/EnKF system provides better ensemble forecasts and higher predictability for TC intensity than the NCEP and ECMWF ensemble forecasts.
基金This work was supported by the National Key Research and Development Program of China[grant number 2017YFC1501603]the National Natural Science Foundation of China[grant numbers 41675053,42175062,and 61827091].
文摘In this study,the impact of environmental factors on tropical cyclone(TC)outer-core size was investigated for both migrating and local TCs in the South China Sea during the period 2001–2019.Among all the thermodynamic and dynamic factors,the low-level environmental helicity showed the strongest positive correlation with TC outer-core size.Large helicity favors the development and organization of convection in TCs,and the corresponding strong inflow and large angular momentum fluxes into the system is beneficial for the maintenance and enlargement of TC outer-core size.Besides,the asymmetric distribution of helicity may account for the asymmetry of TC outer-core size.Therefore,the environmental helicity,as an integrated dynamic factor,can provide an alternative view on TC outer-core size.