The statistical relationships between tropical cyclones (TCs) with rapid intensification (RI) and upper-ocean heat content (UOHC) and sea surface temperature (SST) from 1998 to 2016 in the western North Pacific are examined. RI is computed based on four best track datasets in the International Best Track Archive for Climate Stewardship (IBTrACS). The statistical analysis shows that the UOHC and SST are higher in the RI duration than in non-RI duration. However, TCs with high UOHC/SST do not necessarily experience RI. In addition, the UOHC and SST are lower in the storm inner-core region due to storm-induced ocean cooling, and the UOHC reduces more significantly than the SST along the passages of TCs in the lower-latitude regions. Moreover, most of the RI (non-RI) duration is associated with the higher (lower) UOHC, but this is not the case for the SST pattern. Meanwhile, the TC intensification rate during the RI period does not appear to be sensitive to the SST, but shows statistically significant differences in the UOHC. In addition, there is a statistically significant increasing trend in the UOHC underlying TCs from 1998 to 2016. It is also noted that the percentages of the TCs with RI show different polynomial and linear trends based on different calculations of the RI events and RI durations. Finally, it is shown that there is no statistically significant difference in the UOHC, SST, and the percentage of RI among the five categories of ENSO events (i.e., strong El Niño, weak El Niño, neutral, weak La Niña, and strong La Niña).
Typhoon Sinlaku (2008) was a tropical system that affected many countries in East Asia. Besides the loss of life and economic damage, many scientific questions are associated with this system that need to be addressed. A series of numerical simulations were conducted in this study using V3.2 of the advanced research version of the Weather Research and Forecasting (WRF-ARW) model to examine the impacts of different terrain conditions and vortex structures on the eyewall evolution when Sinlaku was crossing Taiwan. The sensitivity experiments using different vortex structures show that a storm of the same intensity with a larger eyewall radius tends to induce stronger wind and rainfall at the outer part of the storm during the terrain-crossing period. This result suggests that the vortex contained with larger angular momentum is more favorable to reform a new eyewall from the contraction of the outer rainband after being affected by terrain. Based on these sensitivity experiments it is suggested that the topography and the tropical cyclone (TC) structure play important roles in regulating the outer tangential wind speed and modulating the unique eyewall evolutions for TCs passing Taiwan. A stronger vortex structure could lead to more precipitation at the outer part of the storm during the terrain influenced period, implying that the forecasters should pay attention to the storm intensity and also the storm structure which is an important dynamic feature that modulates the eyewall evolution and rainfall distribution of a landfalling storm.
This study aims to assess the impact of global warming on intense TCs over the western North Pacific (WNP) through a dynamical downscaling approach. 379 and 179 TCs reaching Category 1 in the High-Resolution Atmospheric Model (HiRAM) are downscaled for use in the Weather Research and Forecasting model at 5-km horizontal resolution in the current climate (1979-2015) and Representative Concentration Pathways 8.5 (RCP8.5) future climate (2074-2100) scenarios, respectively. Inclusion of the downscaling simulations helps better reproduce the probability distribution of the TC lifetime maximum intensity (LMI). In the warmer climate, the top 30 and top 5 % WNP TCs in LMI are projected to be stronger. Such an increase in intensity is statistically significant, and can be primarily explained by enhanced intensification rate (IR). Meanwhile, TCs among the top 5% in LMI can reach higher intensities which cannot be attained in the current climate. After downscaling, the probability of WNP TCs reaching Category 4-5 increases by 6.5 % in the late 21th century, which is 1.7 % higher as compared to the increase projected exclusively by HiRAM. Moreover, for TCs among the top 5 % in LMI, a 233-km and 300-km westward shift of LMI locations is identified in the late 21st century, for simulations with and without applying the downscaling approach, respectively. Both results suggest that very intense TCs would pose a higher threat to the WNP lands under global warming, as they become substantially stronger, and with their LMI locations migrating toward the coast.
The idealized Weather Research and Forecasting (WRF) simulations are conducted to investigate tropical cyclone (TC) size and intensity over the Western North Pacific (WNP) over the past decades, as represented by National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) R-1, European Center for Medium range Weather Forecasting (ECMWF) twentieth century (ERA20C) reanalysis, and National Oceanic and Atmospheric Administration Cooperative Institute for Research in Environmental Sciences 20th Century (CIRES20) Reanalysis V2 data, and under a future climate, as predicted by the Coupled Model Intercomparison Project Phase 5 (CMIP5). Firstly, sensitivity experiments with varying environmental thermodynamic forcing are conducted to examine how thermodynamic conditions affect TC size and intensity. Secondly, distributions of thermodynamic quantities taken from the NCEP/NCAR R-1, ERA20C, CIRES20, and CMIP5 data are used to initialize four more sets of WRF simulations. There is no significant variation in TC size nor intensity over the WNP within the past 90 years based on the idealized downscaling high-resolution WRF model, whereas those simulations initialized based on CMIP5 data show that both the TC size and intensity would increase in the future (2071–2100) of the representative concentration pathway 8.5 (RCP8.5) as compared to those during the current (2010–2040) climate stage of RCP8.5. An explanation for these findings is given by referring to impact of the air–sea thermal disequilibrium and acutely increasing temperature in the TC outflow, while their relation to previous works is also discussed.
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