Artificial upwelling induced by ocean currents —Theory and experiment

A submerged apparatus, which consists of a buoy, several horizontal contraction and expansion tubes (Venturi-type tubes) and a long pipe, is expected to be used to pump the subsurface sea-water (200–300 meter depth) containing abundant nutrients to surface layer (50–100 m) by the dynamic of ocean currents. i.e. an artificial upwelling without energy cost. A preliminary experiment and analysis are undertaken and shows that the capacity of pumping the nutrient-rich sea-water is worth to build a pilot prototype model.     

Change of multifractal thermal characteristics in the western Philippine sea upper layer during internal wave-soliton propagation

The upper layer (above 140 m depth) temperature in the western Philippine Sea near Taiwan was sampled using a coastal monitoring buoy (CMB) with 15 attached thermistors during July 28–August 7, 2005. The data were collected every 10 min at 1, 3, 5, 10, 15, and 20 m using the CMB sensors, and every 15 sec at 15 different depths between 25 m and 140 m. Internal waves and solitons were identified from the time-depth plot of the temperature field. Without the internal waves and solitons, the power spectra, structure functions, and singular measures (representing the intermittency) of temperature field satisfy the power law with multi-scale characteristics at all depths. The internal waves do not change the basic characteristics of the multifractal structure. However, the internal solitons change the power exponent of the power spectra drastically, especially in the low wave number domain; they also break down the power law of the structure function and increase the intermittency parameter. The physical mechanisms causing these different effects need to be explored further.     

An experimental method for determining the frequency-dependent added mass and added mass moment of inertia for a floating body in heave and pitch motions

Most of the existing relevant materials have been obtained from experiments, in which evaluating the added mass at the resonant frequency corresponding to the peak of a frequency-response curve obtained from the “forced” vibration analysis is the most popular technique. In this paper, a simple experimental method was presented where the “free” vibration responses instead of the “forced” ones were used to determine the values of m_ah and I_ap. The main part of the experimental system is composed of a floating body (model) and a spring–shaft shaker. The “free” vibration of this main part was induced by imposing on it an initial displacement (and/or an initial velocity), and from the time histories of displacements information such as the “damped” natural frequencies, damping ratios, sectional added mass coefficients (C_V and C_P) were obtained. Since the displacements of the spring–shaft shaker are “translational” and those of the floating body due to pitch motions are “angular”, a technique for the transformation between the associated parameters of the two components of the main part was presented.     

An Assessment of Structural Measures for Flood-prone Lowlands with High Population Density along the Keelung River in Taiwan

Midstream of the Keelung River Basin in Northern Taiwan has become highly urbanized and densely populated area. Flood inundation along riversides frequently occurred during typhoons or rainstorms. Three protection measures, including constructions of high-level protection levees, a diversion channel, and a detention reservoir, were proposed for flood mitigation. The main purpose of this study is to evaluate the flood mitigation performance of the three proposed structural measures by using combined hydrologic analyses and hydraulic routings. A semi-distributed parallel-type linear reservoirs rainfall-runoff model was used for estimating the surface runoff. Furthermore, a 1-D dynamic channel routing model was coupled with a two-dimensional inundation model to simulate the hydraulic characteristics of river flooding and overland flow. Simulation results of flood stages, runoff peak discharges, and inundation extent under design rainfall scenarios were chosen as the criteria for evaluation. The results showed a diversion channel is superior to the other two measures for flood mitigation of the study area. After the process of environmental impact assessment, a revised diversion channel approach has been approved for construction as the major structural measure.     

Comparison of statistically downscaled precipitation in terms of future climate indices and daily variability for southern Ontario and Quebec,Canada

Given the coarse resolution of global climate models, downscaling techniques are often needed to generate finer scale projections of variables affected by local-scale processes such as precipitation. However, classical statistical downscaling experiments for future climate rely on the time-invariance assumption as one cannot know the true change in the variable of interest, nor validate the models with data not yet observed. Our experimental setup involves using the Canadian regional climate model (CRCM) outputs as pseudo-observations to estimate model performance in the context of future climate projections by replacing historical and future observations with model simulations from the CRCM, nested within the domain of the Canadian global climate model (CGCM). In particular, we evaluated statistically downscaled daily precipitation time series in terms of the Peirce skill score, mean absolute errors, and climate indices. Specifically, we used a variety of linear and nonlinear methods such as artificial neural networks (ANN), decision trees and ensembles, multiple linear regression, and k-nearest neighbors to generate present and future daily precipitation occurrences and amounts. We obtained the predictors from the CGCM 3.1 20C3M (1971–2000) and A2 (2041–2070) simulations, and precipitation outputs from the CRCM 4.2 (forced with the CGCM 3.1 boundary conditions) as predictands. Overall, ANN models and tree ensembles outscored the linear models and simple nonlinear models in terms of precipitation occurrences, without performance deteriorating in future climate. In contrast, for the precipitation amounts and related climate indices, the performance of downscaling models deteriorated in future climate.     

The nonlinear association between ENSO and the Euro-Atlantic winter sea level pressure

A nonlinear projection of the tropical Pacific sea surface temperature anomalies (SSTA) onto the Northern Hemisphere winter sea level pressure (SLP) anomalies by neural networks (NN) was performed to investigate the nonlinear association between El Niño-Southern Oscillation (ENSO) and the Euro-Atlantic winter climate. While the linear impact of ENSO on the Euro-Atlantic winter SLP is weak, the NN projection reveals statistically significant SLP anomalies over the Euro-Atlantic sector during both extreme cold and warm ENSO episodes, suggesting that the Euro-Atlantic climate mainly responds to ENSO nonlinearly. The nonlinear response, mainly a quadratic response to the SSTA, reveals that regardless of the sign of the SSTA, positive SLP anomalies are found over the North Atlantic, stretching from eastern Canada to Europe (with anomaly center located just northwestward of Portugal), and negative anomalies centered over Scandinavia and Norwegian Sea, consistent with the excitation of the positive North Atlantic Oscillation pattern.     

A diagnostic study on the environmental influence of a mesoscale convective system over southern China in Meiyu season

Summary ¶During the Post-TAMEX forecast experiment of Taiwan in 1992, a mesoscale convective system (MCS) developed on June 5–6 over southern China. As this system matured, it produced readily apparent cirrus outflow on satellite imageries while the upper level flow also exhibited a diffluent pattern. The purpose of the current study is to examine the possible changes in its environment associated with the development of this MCS.By using 12-h data from 1200 UTC June 5 to 1200 UTC June 6, objective analyses were performed for a 1°×1° latitude/longitude grid using sounding data and a low-pass filter. To facilitate the diagnosis, a band-pass filter was further applied to separate mesoscale features from macroscale ones, while the apparent heat source and apparent moisture sink defined by Yanai et al (1973) were also calculated.Results suggest that the MCS exerted clearly discernable effects on its environment. The latent heat release led to the development of a warm core and mesoscale high-pressure disturbance at upper levels when the system matured. Ageostrophic winds and diffluent flow patterns together with strong anticyclonic vorticity at 200hPa near the MCS were associated with the mesohigh. After the mature stage, weak cooling occurred above 350hPa, likely due to radiative emission from the cloud top. However, a mid-level cyclonic vortex, often present in MCSs over the North America, was not apparent here due to weak environmental vorticity and small Coriolis parameter f. The level of maximum divergence was initially located at 500hPa, but rose to 200hPa as the MCS matured. In response, the upward motion not only intensified, but the level at which strongest rising occurred also ascended from 700 to 350hPa. Results from the apparent heat source and moisture sink calculation suggest that this slow ascent of maximum heating was partially due to vertical transport of sensible heat by updrafts.During the MCSs mature stage, under the stratiform clouds to the west of the strongest convection, a cold mesohigh formed at the surface due to evaporative cooling in downdrafts, and a gust front appeared along the leading edge of the outflow boundary. A trailing mesolow was also observed, likely due to near-adiabatic warming in drier downdrafts since no precipitation was associated with it.Received April 11, 2002; revised May 27, 2002; accepted July 14, 2002 Published online: April 10, 2003     

Earth's rotation variations: a wavelet analysis

Historical to up‐to‐date data of the minute variations in the solid Earth's rotation are subjected to a comprehensive time‐frequency wavelet analysis. The length‐of‐day for the period 1962–2012 confirms the presence of a prominent, robust 6‐year periodicity and reveals an anomalously strong 18.6‐year tidal oscillation as well as a ~13‐year quasi‐periodic signal. The polar‐motion excitation for the period 1900–2012 validates the existence of the ~26‐year Markowitz wobble and shows an ~8‐year quasi‐periodic signal, but no appreciable 18.6‐year periodicity. Although it is known that exchanges of angular momentum with the geophysical fluids are responsible for the rotational variations of the solid Earth, the exact physical mechanisms involved on interannual‐to‐decadal timescales are still far from understood.     

Tsunami-like solitary waves impinging and overtopping an impermeable seawall: Experiment and RANS modeling

This study investigates tsunami-like solitary waves impinging and overtopping an impermeable trapezoidal seawall on a 1:20 sloping beach. New laboratory experiments are performed for describing three typical cases: a turbulent bore rushes inland and subsequently impacts and overtops the seawall (Type 1); a wave directly collapses on the seawall and then generates overtopping flow (Type 2); and, a wave straightforwardly overtops the seawall crown and collapses behind the seawall (Type 3). A two-dimensional volume of fluid (VOF) type model called the COBRAS (COrnell BReaking And Structure) model, which is based on the Reynolds-Averaged Navier–Stokes (RANS) equations and the k–ε turbulence closure solver, is validated by experimental data and then applied to investigate wave dynamics for which laboratory data are unavailable. Additionally, a set of numerical experiments is conducted to examine the dynamic wave acting force due to waves impacting the seawall. Effects of wave nonlinearity and freeboard are elucidated. Special attention is given to a distinct vortex evolutionary behavior behind the seawall, in which the dynamic properties of entrapped air-bubbles are briefly addressed experimentally and numerically.