The influence of outer-core surface entropy fluxes(SEFs)on tropical cyclone(TC)outer rainband activity is investigated in this study with a fully compressible,nonhydrostatic model.A control simulation and two sensitivity experiments with the outer-core SEF artificially increased and decreased by 20%respectively were conducted to examine the quasi-periodic outer rainband behavior.Larger negative horizontal advection due to the greater radial wind and the positive contribution by asymmetric eddies leads to a longer period of outerrainband activity in the SEF-enhanced experiment.The well-developed outer rainbands in the control and SEF-reduced simulations significantly limit the TC intensity,whereas such an intensity suppression influence is not pronounced in the SEF-enhanced experiment.As diabatic heating in outer rainbands strengthens the outer-core tangential wind,the quasi-periodic activity of outer rainbands contributes to the quasi-periodic variations of the inner-core size of the TCs.
By using monthly NCEP/NCAR meridional gridpoint wind data at the levels of 1000, 850, 700, 600, 500, 400, 300, 200, 150 and 100 hPa from 1948 to 2004, the intensity of global cross-equatorial flows is calculated. The spatial and temporal variation of global cross-equatorial flows at the 850-hPa level are shown and discussed. The results show that the strength of the 850-hPa global cross-equatorial flows represent obvious long-term variation and interdecadal change during the period. Evidence suggests that the cross-equatorial flow of the passages at 45 – 50 °E in June to August, 105 – 115 °E in May to September, 130 – 140 °E in May to September and May to November and 20 – 25 °E in February to April intensified and that the cross-equatorial flow of the passages at 50 – 35 °W in June to August weaken in the past 57 years,with an increase of 0.25m/s/10a for summer Somali Jet and increase of 0.32 m/s/10a for cross- equatorial flow at 130 – 140 °E in May to September The results of Singular Spectrum Analysis (SSA) for the time series indicate that for the cross-equatorial flow at 850 hPa, the interdecadal and long-term trend changes are 35% – 45%, and the interannual variation is no more than 30%, in variance contribution. The results also reveal that the interannual variation of intensity of the summer cross-equatorial flows in the Pacific is significantly correlated with Southern Oscillation. With weak Southern Oscillation, strong cross- equatorial flows in Pacific will happen, though the summer Somali Jet is only a little positively correlated with North Atlantic Oscillation (NAO).
In this study,six intensity forecast guidance techniques from the East China Regional Meteorological Center are verified for the 2008 and 2009 typhoon seasons through an alternative forecast verification technique.This technique is used to verify intensity forecasts if those forecasts call for a typhoon to dissipate or if the real typhoon dissipates.Using a contingency table,skill scores,chance,and probabilities are computed.It is shown that the skill of the six tropical cyclone intensity guidance techniques was highest for the 12-h forecasts,while the lowest skill of all the six models did not occur in 72-h forecasting.For both the 2008 and 2009 seasons,the average probabilities of the forecast intensity having a small error(6 m s-1) tended to decrease steadily.Some of the intensity forecasts had small skill scores,but the associated probabilities of the forecast intensity errors > 15 m s-1 were not the highest.
Based on a high-resolution dataset, this note re-examines the recently developed potential vorticity (PV) metrics for determining extratropical transition (ET) onset and completion times. The PV metrics use average 330-K isentropic potential vorticity (IPV) to determine the ET onset time, defined as the 330-K IPV minimum time. However, the suggested 330-K IPV threshold fails to determine the ET completion time using the 20-km resolution data, and this IPV method cannot resolve reintensifying and weakening tropical cyclone cases due to the absence of differentiation of lower-level IPV tendencies after ET onset between these two groups of cases.
