North Pacific Gyre Oscillation (NPGO) is one of the important modes of decadal variability in the North Pacific sea surface temperature (SST) and sea surface height (SSH). This study investigated the potential influence of NPGO on spatial characteristics of the peak season (July to October) tropical cyclones (TCs) genesis (TCG) number over the western North Pacific (WNP) from 1965 to 2020. We show that NPGO is the first leading Empirical Orthogonal Function (EOF) mode of the North Pacific SST during the TC peak season in the recent 56 years. On the decadal time scale, NPGO has opposite impacts on TCG in the west and east WNP. The relatively weak positive correlation in the west of 140°E and the strong positive correlation in the east of 140°E result in an overall significant negative correlation between NPGO and WNP total TCG number (r = -0.49), which is much more robust than the relationship between Pacific Decadal Oscillation (PDO) and TCG. The critical factors of NPGO affecting TCG are the vertical motion in the west WNP and vertical wind shear (VWS) in the east WNP. The positive NPGO pattern could induce an anomalous off-equatorial vertical circulation, resulting in an upward motion and increased convective precipitation in the west WNP, favoring local TCG. The anomalous convective precipitation enhances the zonal gradient of the atmospheric heat source in the east WNP, increasing VWS. The North Pacific low-level anticyclonic and upper-level cyclonic associated with NPGO further enhance the VWS in the east WNP and lead to the negative low-level relative vorticity, inhibiting local TCG. This study emphasizes the importance of the NPGO’s climate impact in recent decades. The findings here have significant implications for the decadal prediction of WNP TCG change.

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In this paper, the acoustic properties of a hybrid liner placed at the end of an impedance tube are investigated using numerical simulations. The hybrid liner constitutes of three components, a perforated plate, a porous layer and a rectangular back cavity. The presence of the porous layer is to enhance the absorptive performance of a liner. The main objective of the paper is to verify the proposed numerical methodology-a unified linearized Navier-Stokes Equations (LNSE) approach. In the unified LNSE approach, the combination of the Helmholtz Equation, LNSE as well as the equivalent fluid model are solved in different regions of the impedance tube. To achieve this, the continuity of the coupling condition between the LNSE and the Helmholtz equation is examined. Another objective is to analyze the effectiveness of the porous material to the acoustic performance of the liner. Acoustic liner simulations with and without porous material, porous material with different flow resistivity are carried out. A good agreement is found between the numerical results and the measurements previously performed at KTH MWL. 1 Compared to previous work 234 , several improvements have been made in the numerical methodology, such as that the energy equation has been added in order to include the damping due to viscous dissipation as well as the thermal dissipation in the vicinity of the perforated plate.

It has been demonstrated that the tornado-scale vortex (TSV) is one of the fine-scale structures associated with extreme updrafts in the tropical cyclone boundary layer (TCBL), but the relationship between surface wind gusts and TSVs is still unclear. In this study, the one-second model output simulated in the Weather Research and Forecast (WRF) model with the large eddy simulation (WRF-LES) is used to investigate the relationships between TSVs and surface wind gusts. Results show that surface wind gust factors in the regions where TSVs are prevalent are significantly larger than those in other regions. 88% of the maximum gust factors associated with TSVs (vertical velocity larger than 20 m s−1) are larger than 1.25 (gust factors larger than 1.25 account for only 1% of the 1-min gust factors in the TC inner core), and the mean maximum 1-min gust factor associated with a TSV is larger than 1.3, while the mean 1-min gust factor in the TC inner core is only 1.1. The surface gust factors associated with TSVs in tropical cyclone eyewall can reach about 1.8, which can cause severe surface wind hazards. This study suggests that potential risk will increase in the regions where TSVs are prevalent because of the large wind gusts and gust factors. Finer real-time observations are needed to monitor the evolution of TSVs for improving the operational TC-related surface gust warnings.