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Qingshan Chen and Ju Ming

Abstract

In this paper, the application of the multilevel Monte Carlo (MLMC) method to numerical simulations of turbulent flows with uncertain parameters is investigated. Several strategies for setting up the MLMC method are presented, and the advantages and disadvantages of each strategy are also discussed. A numerical experiment is carried out using an idealized model for the Antarctic Circumpolar Current (ACC) with uncertain, small-scale bottom topographic features. It is demonstrated that unlike the pointwise solutions, the averaged volume transports are correlated across grid resolutions, and the MLMC method can increase simulation efficiency without losing accuracy in uncertainty assessments.

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Ju-Yu Chen, Silke Trömel, Alexander Ryzhkov, and Clemens Simmer

Abstract

Recent advances demonstrate the benefits of radar-derived specific attenuation at horizontal polarization (A H) for quantitative precipitation estimation (QPE) at S and X band. To date the methodology has, however, not been adapted for the widespread European C-band radars such as those installed in the network of the German Meteorological Service (DWD, Deutscher Wetterdienst). Simulations based on a large dataset of drop size distributions (DSDs) measured over Germany are performed to investigate the DSD dependencies of the attenuation parameter α H for the A H estimates. The normalized raindrop concentration (Nw) and the change of differential reflectivity (Z DR) with reflectivity at horizontal polarization (Z H) are used to categorize radar observations into regimes for which scan-wise optimized α H values are derived. For heavier continental rain with Z H > 40 dBZ, the A H-based rainfall retrieval R(A H) is combined with a rainfall estimator using a substitute of specific differential phase (KDP*). We also assess the performance of retrievals based on specific attenuation at vertical polarization (A V). Finally, the regime-adapted hybrid QPE algorithms are applied to four convective cases and one stratiform case from 2017 to 2019, and compared to DWD’s operational Radar-Online-Aneichung (RADOLAN) RW rainfall product, which is based on Z h only but adjusted to rain gauge measurements. For the convective cases, our hybrid retrievals outperform the traditional R(Z h) and pure R(A H/V) retrievals with fixed α H/V values when evaluated with gauge measurements and outperform RW when evaluated by disdrometer measurements. Potential improvements using ray-wise α H/V and segment-wise applications of the ZPHI method along the radials are discussed.

Open access
Gengxin Chen, Weiqing Han, Yuanlong Li, Michael J. McPhaden, Ju Chen, Weiqiang Wang, and Dongxiao Wang

Abstract

This paper reports on strong, intraseasonal, upper-ocean meridional currents observed in the Indian Ocean between the Bay of Bengal (BOB) and the equator and elucidates the underlying physical processes responsible for them. In situ measurements from a subsurface mooring at 5°N, 90.5°E reveal strong intraseasonal variability of the meridional current with an amplitude of ~0.4 m s−1 and a typical period of 30–50 days in the upper 150 m, which by far exceeds the magnitudes of the mean flow and seasonal cycle. Such prominent intraseasonal variability is, however, not seen in zonal current at the same location. Further analysis suggests that the observed intraseasonal flows are closely associated with westward-propagating eddylike sea surface height anomalies (SSHAs) along 5°N. The eddylike SSHAs are largely manifestations of symmetric Rossby waves, which result primarily from intraseasonal wind stress forcing in the equatorial waveguide and reflection of the equatorial Kelvin waves at the eastern boundary. Since the wave signals are generally symmetric about the equator, similar variability is also seen at 5°S but with weaker intensity because of the inclined coastline at the eastern boundary. The Rossby waves propagate westward, causing pronounced intraseasonal SSHA and meridional current in the upper ocean across the entire southern BOB between 84° and 94°E. They greatly weaken in the western Indian Basin, but zonal currents near the equator remain relatively strong.

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Ke Huang, Weiqing Han, Dongxiao Wang, Weiqiang Wang, Qiang Xie, Ju Chen, and Gengxin Chen

Abstract

This paper investigates the features of the Equatorial Intermediate Current (EIC) in the Indian Ocean and its relationship with basin resonance at the semiannual time scale by using in situ observations, reanalysis output, and a continuously stratified linear ocean model (LOM). The observational results show that the EIC is characterized by prominent semiannual variations with velocity reversals and westward phase propagation and that it is strongly influenced by the pronounced second baroclinic mode structure but with identifiable vertical phase propagation. Similar behavior is found in the reanalysis data and LOM results. The simulation of wind-driven equatorial wave dynamics in the LOM reveals that the observed variability of the EIC can be largely explained by the equatorial basin resonance at the semiannual period, when the second baroclinic Rossby wave reflected from the eastern boundary intensifies the directly forced equatorial Kelvin and Rossby waves in the basin interior. The sum of the first 10 modes can reproduce the main features of the EIC. Among these modes, the resonant second baroclinic mode makes the largest contribution, which dominates the vertical structure, semiannual cycle, and westward phase propagation of the EIC. The other 9 modes, however, are also important, and the superposition of the first 10 modes produces downward energy propagation in the equatorial Indian Ocean.

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Gengxin Chen, Dongxiao Wang, Weiqing Han, Ming Feng, Fan Wang, Yuanlong Li, Ju Chen, and Arnold L. Gordon

Abstract

In the eastern tropical Indian Ocean, intraseasonal variability (ISV) affects the regional oceanography and marine ecosystems. Mooring and satellite observations documented two periods of unusually weak ISV during the past two decades, associated with suppressed baroclinic instability of the South Equatorial Current. Regression analysis and model simulations suggest that the exceptionally weak ISVs were caused primarily by the extreme El Niño events and modulated to a lesser extent by the Indian Ocean dipole. Additional observations confirm that the circulation balance in the Indo-Pacific Ocean was disrupted during the extreme El Niño events, impacting the Indonesian Throughflow Indian Ocean dynamics. This research provides substantial evidence for large-scale modes modulating ISV and the abnormal Indo-Pacific dynamical connection during extreme climate modes.

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Yi-Leng Chen, Yen-Ju Chu, Ching-Sen Chen, Chuan-Chi Tu, Jen-Hsin Teng, and Pay-Liam Lin

Abstract

During 11–12 June 2012, heavy precipitation occurred over the northwestern Taiwan coast (~435 mm) and within the Taipei basin (~477 mm). With the presence of a midlatitude omega-blocking pattern, a persistent cold northerly wind component west of the northeast China low and west of the mei-yu frontal cyclone extends all the way to the subtropics and up to the 700-hPa level. At 2000 LST 11 June, the total precipitable water ahead of the front is elevated (>70 kg m−2) with horizontal southwesterly moisture fluxes >360 g kg−1 m s−1 at the 950-hPa level. The rainfall maximum along the northwestern coast mainly occurs before 0200 LST 12 June, as the convective activities in the frontal zone are enhanced by the localized convergence between the prefrontal southerly barrier jet and environmental airflow. After landfall, the relatively deep (~1.5 km) mei-yu front moves over the mountains (with peaks ~1121 m) along the northern coast and into the Taipei basin. During 0200–0800 LST 12 June, it stalls at the foothills of the Snow Mountains (with peaks ~3886 m) south of the basin under the postfrontal west-northwesterly flow. Rain cells associated with the mei-yu front are enhanced as they move southeastward toward the Snow Mountains. The barrier jet and the rainfall maxima over the northwestern coast and within the Taipei basin are well simulated using the high-resolution WRF Model. With the model terrain removed, the simulated mei-yu front continues to move southward after landfall without reproducing the barrier jet and both observed rainfall maxima.

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Ke Huang, Dongxiao Wang, Weiqing Han, Ming Feng, Gengxin Chen, Weiqiang Wang, Ju Chen, and Jian Li

Abstract

Four-year (2014–17) zonal current data observed by a mooring at (5°N, 90.5°E) in the eastern Indian Ocean show a strong semiannual cycle in the middepth (~1200 m) with distinct vertical structure. This pronounced middepth semiannual variability, however, is inconsistent with the local wind forcing, which shows a predominant annual cycle. The underlying causes for this unique middepth variability along 5°N were elucidated with the addition of a reanalysis product and a continuously stratified linear ocean model. The results suggest that the observed seasonal variability in the middepth zonal flow at 5°N is primarily caused by boundary-reflected Rossby waves forced by the remote semiannual winds along the equator. Contribution from the locally wind-forced Rossby waves is much less. The theoretical Wentzel–Kramers–Brillouin ray paths further verify that the strong semiannual variability of the middepth signals over a moored region in the eastern Indian Ocean is largely a manifestation of the steep angles of propagating energy of the long Rossby waves at semiannual time scale. The annual signals are only significant in the upper and western sections (75°–80°E) as a result of the smooth trajectories of Rossby waves forced by local annual winds. Further analysis reveals that the middepth zonal currents along 5°N are expected to be associated with equatorial symmetric Rossby waves at semiannual period. Consequently, similar zonal flows should also exist in the middepth near 5°S.

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Qiang Wang, Bo Zhang, Lili Zeng, Yunkai He, Zewen Wu, and Ju Chen

Abstract

The properties and heat budget of marine heat waves (MHWs) on the northern South China Sea (SCS) continental shelf are investigated. MHWs with warming amplitudes above 1.5°C occur mainly along the coast, and their temperature anomaly decreases toward the open sea. MHWs with 1°–1.5°C warming and duration < 20 days dominate the northern SCS continental shelf. A heat budget analysis indicates that the main heat source is the sea surface net heat flux. Oceanic processes are dominated by the advection of mean temperature by the anomalous horizontal velocity (advha). The net contribution of advha always cools the upper layer of the ocean, resulting in the decay of MHWs. Active cross-slope water exchanges exist at the east and west sides of the northern SCS continental shelf edge, which makes the dominant contributions to the advha. In the MHW developing phase, the west (east) side makes a positive (negative) contribution to the advha. In the decay phase, both sides make a negative contribution to the advha, resulting in the rapid decay of MHWs. Although the contribution of advha to the heat budget varies along the northern SCS continental shelf edge, its net effect always cools the MHWs over the shelf. These results provide new insight into the characteristics and formation mechanism of MHWs on the northern SCS continental shelf; in particular, they clarify the respective contributions of air–sea flux and oceanic processes to MHWs.

Significance Statement

Marine heat waves (MHWs) are unusual warming events in oceans that heavily affect marine ecosystems and arouse great concern from citizens. MHWs are active in the northern South China Sea (SCS) continental shelf. On the northern SCS continental shelf, the sea surface net heat flux is the main heat source of MHWs, and ocean current anomalies always cool the upper layer of the ocean. Active cross-slope water exchange at the east and west sides of the northern SCS continental shelf edge is the main oceanic way that cools the water on the shelf, eventually resulting in the decay of MHWs.

Open access
Rui Shi, Xinyu Guo, Ju Chen, LiLi Zeng, Bo Wu, and Dongxiao Wang

Abstract

The responses of surface wind stress to the mesoscale sea surface temperature (SST) anomalies associated with the SST front in the northern South China Sea (NSCS) are studied using satellite observations and reanalysis data. Both satellite and reanalysis data explicitly show the linear relationships between the spatial-high-pass filtered wind stress perturbation derivatives and the underlying SST gradient field. However, the noise in the linear relationships is much smaller in the reanalysis data than in the satellite observations. This result is rarely reported in other frontal areas. The wavelet analysis shows that the satellite scatterometer observed numerous high wavenumber perturbations within 100 km in the NSCS, but these perturbations were absent in the reanalysis data. The linear relationship between the perturbation SST gradient and derivative wind stress fields is not significant at this scale, which enhances the noise in the linear relationship. The spatial bandpass-filtered perturbation between 100 and 300 km can give reasonable estimates of the coupling coefficients between the wind stress divergence and downwind SST gradient (α d) and between the wind stress curl and crosswind SST gradient (α c) in the NSCS, with values of 1.33 × 10−2 and 0.95 × 10−2 N m−2 °C−1, respectively.

Open access
Qiang Wang, Weidong Zhou, Lili Zeng, Ju Chen, Yunkai He, and Dongxiao Wang

Abstract

Cross-slope flow plays an important role in the exchange of material, heat, and momentum between the continental shelf and the open sea. In the northern South China Sea (SCS), long-period observations confirm that there is significant cross-slope flow. The variability of this flow is dominated by the intraseasonal component (i.e., the 10–90-day period band) that contributes 74.6% of the total standard deviation. The 10–90-day bandpassed cross-slope flow exhibits almost the same direction vertically in the observed layers, and its first empirical orthogonal function, whose direction is also not changed, contributes 86.7% to its total variance. The strong 10–90-day bandpassed cross-slope flow is phase locked to the boreal winter half year. The intraseasonal variability of cross-slope flow is mainly associated with mesoscale eddies west to the Luzon Strait. The contrasting baroclinic instability growth rates, strong in winter and weak in summer, result in a seasonal cycle of mesoscale eddy kinetic energy, that is, vigorous in winter and weak in summer, which explains the winter phase lock. The interannual variability of baroclinic instability growth rate is mainly determined by the vertical shear of velocity. The strongest vertical shear of velocity from 2014 to 2016 occurred in the winter of 2016/17 and induced the most rapid baroclinic instability growth rate and consequently the largest mesoscale eddy kinetic energy, which resulted in the strongest intraseasonal variability of cross-slope flow. The vertical shear of velocity in the northern SCS is mainly determined by the Luzon Strait transport.

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