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Zhiduo Yan
,
Liang Pang
, and
Sheng Dong

Abstract

An increasing number of coastal and offshore structures have been built for coastal protection and marine development in recent years, and these marine structures need to be reasonably designed on the basis of wind speed. In this paper, extreme wind speed estimates are studied in detail by using the best-track datasets of northwestern Pacific Ocean tropical cyclones and ERA5 wind field data. The extreme wind speed fits by five distributions are compared using a blended sample of the wind fields from the ERA5 dataset and parametric wind data. The blend of wind fields improved the data accuracy and extreme value estimation reliability. In addition, the effects of the distribution model, data, threshold, and parameter estimation methods on the calculated results are discussed. The results show that the data had the greatest influences on probability prediction, followed by the distribution model and the parameter estimation method, with the threshold presenting the least influence. In this study, the reliability of the estimates was improved and the uncertainty of the results was analyzed, and the findings provide a wind speed design reference for the northern South China Sea.

Free access
Chung-Chieh Wang
,
Jason Chieh-Sheng Hsu
,
George Tai-Jen Chen
, and
Dong-In Lee

Abstract

This study is the second of a two-part series to investigate two rainfall episodes in the Hovmöller space near Taiwan during the eighth intensive observing period (IOP-8, 12–17 June 2008) of the Southwest Monsoon Experiment/Terrain-influenced Monsoon Rainfall Experiment (SoWMEX/TiMREX). The first episode moved eastward and the second westward, and both caused heavy rainfall in Taiwan. The goal of Part I was to better understand the mechanism and controlling factors for the organization and propagation of the episodes. Here in Part II, the detailed roles played by synoptic conditions and terrain effects are further examined. Three sensitivity tests (at 2.5-km grid spacing) are designed to include only the effects of synoptic evolution (SNP), and those from land–sea distribution–diurnal variations on top of a mean background with/without topography (DIU/DNT). As the benchmark, the control (CTL) experiment captures the 6-day event successfully and is validated in Part I.

In SNP, the two episodes are reproduced with overall similarity to CTL and the observation, and this confirms that the general location/time of rainfall are mainly controlled by synoptic forcing in this case, in contrast to typical warm-season conditions in the central United States. Even so, diurnal effects can still exert discernible impacts and modulate local convective development in many instances, particularly an afternoon enhancement over terrain, and the averaged diurnal cycle in CTL over southeastern China resembles those in DIU/DNT rather than that in SNP (with no land). The steep topography of Taiwan is especially important for its rainfall distribution, including the heavy rainfall on 16 June through processes as postulated by Xu et al.

Full access
Chung-Chieh Wang
,
Jason Chieh-Sheng Hsu
,
George Tai-Jen Chen
, and
Dong-In Lee

Abstract

This paper is the first of a two-part study to investigate two rain-producing episodes in the longitude–time (Hovmöller) space upstream from Taiwan during the eighth intensive observing period (IOP-8, 12–17 June 2008) of the Southwest Monsoon Experiment/Terrain-influenced Monsoon Rainfall Experiment (SoWMEX/TiMREX), with a goal to better understand the mechanism and controlling factors for their organization and propagation. Both in a prefrontal environment, the first episode moved eastward and the second was a rare westward-moving event, and each caused heavy rainfall in Taiwan, on 14 and 16 June, respectively. In Part II, the roles played by synoptic conditions and terrain effects are further examined through sensitivity tests.

With the aid from a successful simulation with a grid spacing of 2.5 km, the structure and organization of convection embedded in the two episodes are shown to be different. With stronger low-level vertical wind shear in its environment, the first episode consisted of well-organized squall-line-type convective systems and propagated eastward mainly through cold-pool dynamics. However, the convection of the second episode was scattered and less organized with weaker vertical shear, and individual cells traveled with background flow toward the north-northeast. Throughout the 6-day case period, the southwesterly low-level jet (LLJ) is found to have much control over the general region of convection, and thus dictates the overall rainfall pattern in the Hovmöller space at the regional scale. The rapid development of the mei-yu front and LLJ over southeastern China during 16–17 June, to the west of the previous location of the jet, is found to result in the westward movement of the second episode.

Full access
Yuan-Zheng Lu
,
Xian-Rong Cen
,
Shuang-Xi Guo
,
Ling Qu
,
Peng-Qi Huang
,
Xiao-Dong Shang
, and
Sheng-Qi Zhou

Abstract

The nominal spatial distribution of diapycnal mixing in the South China Sea (SCS) is obtained with Thorpe-scale analysis from 2004 to 2020. The inferred dissipation rate ε and diapycnal diffusivity K z between 100 and 1500 m indicated that the strongest mixing occurred in the Luzon Strait and Dongsha Plateau regions, with ε ~ 3.0 × 10−8 W kg−1 (ε max = 5.3 × 10−6 W kg−1) and K z ~ 3.5 × 10−4 m2 s−1 (K zmax = 4.2 × 10−2 m2 s−1). The weakest mixing occurred in the thermocline of the central basin, with ε ~ 6.2 × 10−10 W kg−1 and K z ~ 3.7 × 10−6 m2 s−1. The ε and K z in the continental slope indicated that the mixing in the northern part [O(10−8) W kg−1 and O(10−4) m2 s−1, respectively] was comparatively stronger than that in the Xisha and Nansha regions [O(10−9) W kg−1 and O(10−5) m2 s−1, respectively]. The K z in the continental slope region (200–2000 m) decayed at a closed rate from the ocean bottom to the main thermocline when the measured depth D was normalized by the ocean depth H as D/H, whether in the shallow or deep oceans. The diapycnal diffusivity was parameterized as K z = 3.3 × 10−4[1 + (1 − D/H)/0.22]−2 − 6.0 × 10−6 m2 s−1. The vertically integrated energy dissipation was nominally 15.8 mW m−2 for all data and 25.6 mW m−2 for data at stations H < 2000 m. This was about one order of magnitude higher than that in the open oceans (3.0–3.3 mW m−2), which confirmed the active mixing state in the SCS.

Full access
Bosi Sheng
,
Buwen Dong
,
Haolin Wang
,
Mingming Zhang
,
Shuheng Lin
,
Peng Si
,
Fraser C. Lott
, and
Qingxiang Li

Abstract

Precipitation in southern China during April–June 2022 was the highest since 1961. Anthropogenic forcing has reduced the probability of 2022-like Rx30day precipitation by about 45% based on CMIP6 simulations.

Open access