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Peng-Qi Huang
,
Yuan-Zheng Lu
, and
Sheng-Qi Zhou

Abstract

A new method is developed to identify the mixed layer depth (MLD) from individual temperature or density profiles. A relative variance profile is obtained that is the ratio between the standard deviation and the maximum variation of the temperature (density) from the sea surface, and the depth of the minimum relative variance is defined as the MLD. The new method is robust in finding the MLD under the influence of random noise (noise level ≤ 5%). A comparison with other available methods, which include the threshold (difference, difference interpolation, gradient, and hybrid methods) and objective (curvature and maximum angle methods) methods, is carried out using the World Ocean Circulation Experiment (WOCE) data. It is found that for a variety of depth sampling resolutions ranging from 0.04 to 25 dbar, the new method and the difference-interpolation method predict MLD values that are closer to the visually inspected ones than those by other methods. Moreover, the quality index (QI) of the MLD that is determined by the new method is the highest when compared with those of the available methods. Also, the application of the new method on the WOCE global dataset yields 94% of MLD values with , substantially higher than those (≤86%) of other methods. Ultimately, it is found that the new method determines very similar MLD values when applied to temperature or density profiles globally because it identifies the base of the mixed layer rather than the uppermost depth of the thermocline. This unique advantage makes the new method applicable in many cases, especially when the density profile is unavailable.

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Ching-Yuang Huang
,
Sheng-Hao Sha
, and
Hung-Chi Kuo

Abstract

The global model FV3GFS is used to simulate Typhoon Lekima (2019), which exhibited track deflection when approaching west-northwestward toward Taiwan. The model successfully simulates the observed northward deflection and the track deflection is produced by topographically induced wavenumber-1 flow with a pair of vorticity gyres around the typhoon center. The gyres tend to rotate counterclockwise about the typhoon center and thus induce an earlier northward and then westward movement. Azimuthal-mean kinetic energy budget of the typhoon indicates that the effect of Taiwan terrain modifies the correlation between the recirculating flow and pressure gradient force east of Taiwan, leading to a slight weakening of the typhoon during the later track deflection. The northward cyclonic deflection in general will be induced for a cyclone to move toward the central to northern terrain such as Lekima. The curvature of the northward cyclonic deflection, however, is large (small) for a northwestbound (nearly westbound) vortex depending on the track-topography-impinging angle. The curvature difference can be explained with the concept of recirculating flow, which is the flow splitting due to topography and rejoins the vortex to produce the wavenumber-1 asymmetry. The cyclonic track curvature of the northwestbound Lekima is larger than that of the westbound Maria (2018) in the FV3GFS simulations. This adds robustness to the conclusion that minor to moderate terrain-related track deflections can be well simulated by the FV3GFS global model near Taiwan.

Open access
Laura X. Huang
,
George A. Isaac
, and
Grant Sheng

Abstract

This study addresses the issue of improving nowcasting accuracy by integrating several numerical weather prediction (NWP) model forecasts with observation data. To derive the best algorithms for generating integrated forecasts, different integration methods were applied starting with integrating the NWP models using equal weighting. Various refinements are then successively applied including dynamic weighting, variational bias correction, adjusted dynamic weighting, and constraints using current observation data. Three NWP models—the Canadian Global Environmental Multiscale (GEM) regional model, the GEM Limited Area Model (LAM), and the American Rapid Update Cycle (RUC) model—are used to generate the integrated forecasts. Verification is performed at two Canadian airport locations [Toronto International Airport (CYYZ), in Ontario, and Vancouver International Airport (CYVR), in British Columbia] over the winter and summer seasons. The results from the verification for four weather variables (temperature, relative humidity, and wind speed and gust) clearly show that the integrated models with new refinements almost always perform better than each of the NWP models individually and collectively. When the integrated model with innovative dynamic weighting and variational bias correction is further updated with the most current observation data, its performance is the best among all models, for all the selected variables regardless of location and season. The results of this study justify the use of integrated NWP forecasts for nowcasting provided they are properly integrated using appropriate and specifically designed rules and algorithms.

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Sheng Huang
,
Weijiang Li
,
Jiahong Wen
,
Mengru Zhu
,
Yao Lu
, and
Na Wu

Abstract

Driven by both climate change and urbanization, extreme rainfall events are becoming more frequent and having an increasing impact on urban commuting. Using hourly rainfall data and “metro” origin–destination (OD) flow data in Shanghai, China, this study uses the Prophet time series model to calculate the predicted commuting flows during rainfall events and then quantifies the spatiotemporal variations of commuting flows due to rainfall at station and OD levels. Our results show the following: 1) In general, inbound commuting flows at metro stations tend to decrease with hourly rainfall intensity, varying across station types. The departure time of commuters is usually delayed by rainfall, resulting in a significant stacking effect of inbound flows at metro stations, with a pattern of falling followed by rising. The sensitivity of inbound flows to rainfall varies at different times, high at 0700 and 1700 LT and low at 0800, 0900, 1800, and 1900 LT because of the different levels of flexibility of departure time. 2) Short commuting OD flows (≤15 min) are more affected by rainfall, with an average increase of 7.3% and a maximum increase of nearly 35%, whereas long OD flows (>15 min) decrease slightly. OD flows between residential and industrial areas are more affected by rainfall than those between residential and commercial (service) areas, exhibiting a greater fluctuation of falling followed by rising. The sensitivity of OD flows to rainfall varies across metro lines. The departure stations of rainfall-sensitive lines are mostly distributed in large residential areas that rely heavily on the metro in the morning peak hours and in large industrial parks and commercial centers in the evening peak hours. Our findings reveal the spatiotemporal patterns of commuting flows resulting from rainfall at a finer scale, which provides a sound basis for spatial and temporal response strategies. This study also suggests that attention should be paid to the surges and stacking effects of commuting flows at certain times and areas during rainfall events.

Restricted access
Yuan-Zheng Lu
,
Shuang-Xi Guo
,
Sheng-Qi Zhou
,
Xue-Long Song
, and
Peng-Qi Huang

Abstract

Thirty-four individual thermohaline sheets are identified at depths of 170–400 m in the Canada Basin of the Arctic Ocean by using the hydrographical data measured with the Ice-Tethered Profilers (ITPs) between August 2005 and October 2009. Each sheet is well determined because the salinity within itself remains very stable and the associated salinity anomaly is markedly smaller than the salinity jump between neighboring sheets. These thermohaline sheets are nested between the Lower Halocline Water (LHW) and Atlantic Water (AW) with lateral coherence of hundreds of kilometers and thickness varying from several to dozens of meters. The physical properties, including temperature, heat flux, and vertical turbulent diffusivity, in the sheet are found to be averagely associated with the AW propagation. Spatially, the thermohaline sheet is in a bowl-shaped distribution, which is deepest in the basin center and gradually becomes shallower toward the periphery. The interaction between the LHW and AW could be demonstrated through the property variances in the sheets. The temperature variances in the upper and lower sheets are correlated with the LHW and AW, respectively, transited at the 15th sheet, whereas the depth variance in the sheet is strongly correlated with the LHW. It is proposed that the sheet spatial distribution is mainly dominated by the Ekman convergence with the Beaufort Gyre, slightly modulated with the AW intrusion.

Significance Statement

The diffusive convection staircases, composed of consecutive steps containing thick mixed layers and relatively thin interfaces, are prominent between the Lower Halocline Water (LHW) and the Atlantic Water (AW) throughout the Canada Basin. This sheet-like structure is in a bowl shape with lateral coherence over hundreds of kilometers. It is proposed that the distribution of the thermohaline sheet is mainly dominated by the Ekman convergence with Beaufort Gyre, as well as the AW intrusion. The present method of thermohaline-sheet identification would have more implications beyond this work. Since the thermohaline sheet remains mostly stable and coherent on a very large spatial–temporal scale, it might play a similar role as the water mass analysis in numerous applications, e.g., climate change.

Restricted access
Chung-Chieh Wang
,
Shin-Yi Huang
,
Shin-Hau Chen
,
Chih-Sheng Chang
, and
Kazuhisa Tsuboki

Abstract

In this study, the performance of a new ensemble quantitative precipitation forecast (QPF) system for Taiwan, with a cloud-resolving grid spacing of 2.5 km, a large domain of 1860 km × 1360 km, and an extended range of 8 days, is evaluated for six typhoons during 2012–13. Obtaining the probability (ensemble) information through a time-lagged approach, this system combines the strengths of high resolution (for QPF) and longer lead time (for hazard preparation) in an innovative way. For the six typhoons, in addition to short ranges (≤3 days), the system produced a decent QPF at a longest range up to days 8, 4, 6, 3, 6, and 7, providing greatly extended lead times, especially for slow-moving storms that pose higher threats. Moreover, since forecast uncertainty (reflected in the spread) is reduced with lead time, this system can provide a wide range of rainfall scenarios across Taiwan with longer lead times, each highly realistic for the associated track, allowing for advanced preparation for worst-case scenarios. Then, as the typhoon approaches and the predicted tracks converge, the government agencies can make adjustments toward the scenario of increasing likelihood. This strategy fits well with the conventional wisdom of “hoping for the best, but preparing for the worst” when facing natural hazards. Overall, the system presented herein compares favorably in usefulness to a typical 24-member ensemble (5-km grid size, 750 km × 900 km, 3-day forecasts) currently in operation using similar computational resources. Requiring about 1500 cores to execute four 8-day runs per day, it is not only powerful but also affordable and feasible.

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Peng-Qi Huang
,
Xian-Rong Cen
,
Yuan-Zheng Lu
,
Shuang-Xi Guo
, and
Sheng-Qi Zhou

Abstract

In this study we examined the applicability of the threshold, curvature, maximum angle, and relative variance methods for identifying the oceanic bottom mixed layer (BML) thickness . Using full-depth temperature profiles along 17 WOCE sections covering the Atlantic, Indian, and Pacific Oceans, we found that the BML thicknesses determined based on the threshold, curvature, and maximum angle methods had wider 95% confidence intervals and much lower quality indexes compared with those based on the visual inspection ( ). The relative variance method appeared to perform better than the other methods because the 95% confidence interval and (0.60) values were closer to those determined based on the visual inspection, although differences were still present. We then proposed an integrated method by optimizing the possible values obtained from the four methods. The BML thicknesses determined using the integrated method were closest to those based on the visual inspection according to the higher (0.64) and more stations (71%) with . Compared with the results in previous studies, the integrated method determined the consistent BML thicknesses in most regions (e.g., the northern Atlantic), and it also effectively identified the BML thicknesses in some regions where the BML was considered to be not readily detectable (e.g., the Madeira Abyssal Plain).

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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.

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Shuang-Xi Guo
,
Xian-Rong Cen
,
Ling Qu
,
Yuan-Zheng Lu
,
Peng-Qi Huang
, and
Sheng-Qi Zhou

Abstract

Flow speed past the measuring probe is definitely needed for the estimation of the turbulent kinetic energy dissipation rates ε and temperature dissipation rates χ based on the Taylor frozen hypothesis. This speed is usually measured with current instruments. Occasional failed work of these instruments may lead to unsuccessful speed measurement. For example, low concentration of suspended particles in water could make the observed speed invalid when using acoustic measuring instruments. In this study, we propose an alternative approach for quantifying the flow speeds by only using the microstructure shear or temperature data, according to the spectral theories of the inertial and dissipation subranges. A dataset of the microstructure profiler, vertical microstructure profiler (VMP), collected in the South China Sea (SCS) during 2017, is used to describe this approach, and the inferred speeds are compared with the actual passing-probe speeds, i.e., the falling speeds of the VMP. Probability density functions (PDFs) of the speed ratios, i.e., the ratios of the speeds respectively inferred from the inertial and dissipation subranges of the shear and temperature spectra to the actual speeds, follow the lognormal distribution, with corresponding mean values of 1.32, 1.03, 1.56, and 1.43, respectively. This result indicates that the present approach for quantifying the flow speeds is valid, and the speeds inferred from the dissipation subrange of shear spectrum agree much better with the actual ones than those from the inertial subrange of shear spectrum and the inertial and dissipation subranges of temperature spectrum. The present approach may be complementary and useful in the evaluation of turbulent mixing when the directly observed speeds are unavailable.

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Yu Zhang
,
Yang Hong
,
Xuguang Wang
,
Jonathan J. Gourley
,
Xianwu Xue
,
Manabendra Saharia
,
Guangheng Ni
,
Gaili Wang
,
Yong Huang
,
Sheng Chen
, and
Guoqiang Tang

Abstract

Prediction, and thus preparedness, in advance of flood events is crucial for proactively reducing their impacts. In the summer of 2012, Beijing, China, experienced extreme rainfall and flooding that caused 79 fatalities and economic losses of $1.6 billion. Using rain gauge networks as a benchmark, this study investigated the detectability and predictability of the 2012 Beijing event via the Global Hydrological Prediction System (GHPS), forced by the NASA Tropical Rainfall Measuring Mission (TRMM) Multisatellite Precipitation Analysis at near–real time and by the deterministic and ensemble precipitation forecast products from the NOAA Global Forecast System (GFS) at several lead times. The results indicate that the disastrous flooding event was detectable by the satellite-based global precipitation observing system and predictable by the GHPS forced by the GFS 4 days in advance. However, the GFS demonstrated inconsistencies from run to run, limiting the confidence in predicting the extreme event. The GFS ensemble precipitation forecast products from NOAA for streamflow forecasts provided additional information useful for estimating the probability of the extreme event. Given the global availability of satellite-based precipitation in near–real time and GFS precipitation forecast products at varying lead times, this study demonstrates the opportunities and challenges that exist for an integrated application of GHPS. This system is particularly useful for the vast ungauged regions of the globe.

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