The quasi-biweekly oscillation （QBWO） is the second most dominant intraseasonal mode over the westem North Pacific （WNP） during boreal summer. In this study, the modulation of WNP tropical cyclogenesis （TCG） by ...The quasi-biweekly oscillation （QBWO） is the second most dominant intraseasonal mode over the westem North Pacific （WNP） during boreal summer. In this study, the modulation of WNP tropical cyclogenesis （TCG） by the QBWO and its association with large-scale patterns are investigated. A strong modulation of WNP TCG events by the QBWO is found. More TCG events occur during the QBWO＇s convectively active phase. Based on the genesis potential index （GPI）, we further evaluate the role of environmental factors in affecting WNP TCG. The positive GPI anomalies associated with the QBWO correspond well with TCG counts and locations. A large positive GPI anomaly is spatially correlated with WNP TCG events during a life cycle of the QBWO. The low-level relative vorticity and mid-level relative humidity appear to be two dominant contributors to the QBWO-composited GPI anomalies during the QBWO＇s active phase, followed by the nonlinear and potential intensity terms. These positive contributions to the GPI anomalies are partly offset by the negative contribution from the vertical wind shear. During the QBWO＇s inactive phase, the mid-level relative humidity appears to be the largest contributor, while weak contributions are also made by the nonlinear and low-level relative vorticity terms. Meanwhile, these positive contributions are partly cancelled out by the negative contribution from the potential intensity. The contributions of these environmental factors to the GPI anomalies associated with the QBWO are similar in all five flow patterns--the monsoon shear line, monsoon confluence region, monsoon gyre, easterly wave, and Rossby wave energy dispersion associated with a preexisting TC. Further analyses show that the QBWO strongly modulates the synoptic-scale wave trains （SSWs） over the WNP, with larger amplitude SSWs during the QBWO＇s active phase. This implies a possible enhanced （weakened） relationship between TCG and SSWs during the active （inactive） phase. This study improves our understanding of the modulation of WNP TCG by the QBWO and thus helps with efforts to improve the intraseasonal prediction of WNP TCG.展开更多
The forecast probability of tropical cyclone(TC)genesis in the western North Pacific from 2017 to 2020 was investigated using global ensembles from the Japan Meteorological Agency(JMA),the European Centre for Medium-R...The forecast probability of tropical cyclone(TC)genesis in the western North Pacific from 2017 to 2020 was investigated using global ensembles from the Japan Meteorological Agency(JMA),the European Centre for Medium-Range Weather Forecasts(ECMWF),the U.S.National Centers for Environmental Prediction(NCEP),and the Met Office in the United Kingdom(UKMO).The time of TC genesis was defined as the time the TCs were first recorded in the best-track data(Case 1)and as the time they reached the intensity of a Tropical Storm(Case 2).The results in Case 1 showed that differences between the forecast probability based on each global ensemble were large,even for a 1-day forecast,and that mean probability were from 18%to 74%.The forecasts based on the NCEP had a large frequency bias and overpredicted TC genesis events.The results indicated that the representation of genesis events differed greatly between global ensembles.The effectiveness of multiple ensembles was investigated.The results from the threat score and the false alarm ratio indicated that multiple ensembles had skillful forecasts.When the forecast probability was examined for environmental patterns of synoptic low-level flow,the mean 5-day forecast probability was highest for the pattern in the confluence region.The results also showed that the forecast probability was much larger in Case 2 than in Case 1.In all global ensembles,the mean probability with a lead time of up to 1-week was below 10%for both Case 1 and 2.This result indicates that even with today's operational forecasting systems,it is difficult to regularly predict TC genesis events with a 1-week lead time with high confidence.These results provide a better understanding of TC genesis forecast products in each global ensemble and will be useful information when multiple-ensemble products are created.展开更多
基金jointly supported by the National Natural Science Foundation of China(Grant Nos.41675072,41305050,41275093,41475091 and 41305039)the National Basic Research Program of China(Grant Nos.2013CB430301,2013CB430103 and 2015CB452803)+5 种基金the Jiangsu Provincial Natural Science Fund Project(Grant No.BK20150910)the Natural Science Foundation of the Jiangsu Higher Education Institutions of China(Grant No.14KJA170005)the Priority Academic Program Development of Jiangsu Higher Education Institutions(PAPD)the Project of Global Change and Air–Sea Interaction(Grant No.GASI-03-IPOVAI-04)the base funding of the Atlantic Oceanographic and Meteorological Laboratory(AOML)Earth System Modelling Center Contribution Number 117
文摘The quasi-biweekly oscillation （QBWO） is the second most dominant intraseasonal mode over the westem North Pacific （WNP） during boreal summer. In this study, the modulation of WNP tropical cyclogenesis （TCG） by the QBWO and its association with large-scale patterns are investigated. A strong modulation of WNP TCG events by the QBWO is found. More TCG events occur during the QBWO＇s convectively active phase. Based on the genesis potential index （GPI）, we further evaluate the role of environmental factors in affecting WNP TCG. The positive GPI anomalies associated with the QBWO correspond well with TCG counts and locations. A large positive GPI anomaly is spatially correlated with WNP TCG events during a life cycle of the QBWO. The low-level relative vorticity and mid-level relative humidity appear to be two dominant contributors to the QBWO-composited GPI anomalies during the QBWO＇s active phase, followed by the nonlinear and potential intensity terms. These positive contributions to the GPI anomalies are partly offset by the negative contribution from the vertical wind shear. During the QBWO＇s inactive phase, the mid-level relative humidity appears to be the largest contributor, while weak contributions are also made by the nonlinear and low-level relative vorticity terms. Meanwhile, these positive contributions are partly cancelled out by the negative contribution from the potential intensity. The contributions of these environmental factors to the GPI anomalies associated with the QBWO are similar in all five flow patterns--the monsoon shear line, monsoon confluence region, monsoon gyre, easterly wave, and Rossby wave energy dispersion associated with a preexisting TC. Further analyses show that the QBWO strongly modulates the synoptic-scale wave trains （SSWs） over the WNP, with larger amplitude SSWs during the QBWO＇s active phase. This implies a possible enhanced （weakened） relationship between TCG and SSWs during the active （inactive） phase. This study improves our understanding of the modulation of WNP TCG by the QBWO and thus helps with efforts to improve the intraseasonal prediction of WNP TCG.
文摘The forecast probability of tropical cyclone(TC)genesis in the western North Pacific from 2017 to 2020 was investigated using global ensembles from the Japan Meteorological Agency(JMA),the European Centre for Medium-Range Weather Forecasts(ECMWF),the U.S.National Centers for Environmental Prediction(NCEP),and the Met Office in the United Kingdom(UKMO).The time of TC genesis was defined as the time the TCs were first recorded in the best-track data(Case 1)and as the time they reached the intensity of a Tropical Storm(Case 2).The results in Case 1 showed that differences between the forecast probability based on each global ensemble were large,even for a 1-day forecast,and that mean probability were from 18%to 74%.The forecasts based on the NCEP had a large frequency bias and overpredicted TC genesis events.The results indicated that the representation of genesis events differed greatly between global ensembles.The effectiveness of multiple ensembles was investigated.The results from the threat score and the false alarm ratio indicated that multiple ensembles had skillful forecasts.When the forecast probability was examined for environmental patterns of synoptic low-level flow,the mean 5-day forecast probability was highest for the pattern in the confluence region.The results also showed that the forecast probability was much larger in Case 2 than in Case 1.In all global ensembles,the mean probability with a lead time of up to 1-week was below 10%for both Case 1 and 2.This result indicates that even with today's operational forecasting systems,it is difficult to regularly predict TC genesis events with a 1-week lead time with high confidence.These results provide a better understanding of TC genesis forecast products in each global ensemble and will be useful information when multiple-ensemble products are created.
