Daoyi Chen

Absolute and convective instabilities of plane turbulent wakes in a shallow water layer

In shallow turbulent wake flows (typically an island wake), the flow patterns have been found experimentally to depend mainly on a shallow wake parameter, S = c f D / h in which c f is a quadratic-law friction coefficient, D is the island diameter and h is water depth. In order to understand the dependence of flow patterns on S , the shallow-water stability equation (a modified Orr–Sommerfeld equation) has been derived from the depth-averaged equations of motion with terms which describe bottom friction. Absolute and convective instabilities have been investigated on the basis of wake velocity profiles with a velocity deficit parameter R . Numerical computations have been carried out for a range of R -values and a stability diagram with two dividing lines was obtained, one defining the boundary between absolute and convective instabilities S ca , and another defining the transition between convectively unstable and stable wake flow S cc . The experimental measurements (Chen & Jirka 1995) of return velocities in shallow wakes were used to compute R -values and two critical values, S A =0.79 and S C =0.85, were obtained at the intersections with lines S ca and S cc . Through comparison with transition values observed experimentally for wakes with unsteady bubble (recirculation zone) and vortex shedding, S U and S V respectively, the sequence S C > S A > S U > S V shows vortex shedding to be the end product of absolute instability. This is analogous to the sequence of critical Reynolds numbers for an unbounded wake of large spanwise extent. Experimental frequency characteristics compare well with theoretical results. The observed values of S U and S V for different flow patterns correspond to the velocity profile with R =−0.945, which is located at the end of the wake bubble, and it provides the dominant mode.

Desorption and the two Stages of Drying of Natural Tallgrass Prairie

During the First ISLSCP (International Satellite Land Surface Climatology Project) Field experiment (FIFE) the first stage of drying (with the atmosphere controlling) in these natural grasslands occurred when the soil was moist with a volumetric soil moisture content SM >27% after rainfall. Both at the local scale and at the regional scale, once SM in the top 10 cm of the soil dropped below about 17% and its vertical gradient started to exceed about 1.1%/cm at 5 cm, the cumulative daily evaporation rate could be taken to be proportional to the square root of time, t½, which is not unlike a second stage of drying (with the soil moisture content controlling). Between these two drying stages, there was a transition period which lasted from several days to 2 weeks depending on the soil moisture conditions and on the season. The longer transition periods were observed under conditions of lower net radiation and of higher soil moisture content at depths in excess of, say, 50 cm. The present findings of gradual transitions from the first stage to a desorptive second stage of drying for a surface covered with grass (which can extract water from greater depths) are in contrast to earlier findings of relatively abrupt transitions for bare soil.

Satellite-Sensed Distribution and Spatial Patterns of Vegetation Parameters over a Tallgrass Prairie

Abstract Remotely sensed images of vegetation density expressed as the Normalized Difference Vegetation index (NDVI) recorded during the First ISLSCP (International Satellite Land Surface Climatology Project) Field Experiment were mapped on the same coordinate system covering the 20 km × 20 km experimental site. The temporal behavior of frequency distributions and semivariograms of the NDVI exhibited similarity; these distributions were quite symmetric, except near the extremes. Although values at individual locations showed considerable scatter, when averaged over the experimental area, the NDVI values were well correlated with ground-based measurements of the leaf area index. The persistence tendencies of the spatial patterns of NDVI were evaluated through correlation analysis of the images on successive days. The spatial patterns were preserved well during wet periods; they gradually changed by deep drying of the soil but were restored after heavy rainfall.

Diurnal Variation of Surface Fluxes During Thorough Drying (or Severe Drought) of Natural Prairie

Experimental data recorded over a natural tallgrass prairie during the later stages of drying in the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment-1987 showed (1) that the total daily values of evaporation exhibited a kind of second stage of drying behavior with a t−½ dependency at the daily timescale and (2) that this day-to-day evolution was modulated by the available energy at the surface, that is, the hourly radiation input. This allowed a simple description of the phenomenon by combining a desorptive diffusion-type parameterization for the total daily evaporation or for its dimensionless counterpart (such as Priestley and Taylors α, the evaporative fraction, and a few others), with an assumption of self-preservation in the surface energy budget during the daytime hours. The resulting formulation, which involves two timescales, a daily and an hourly, was able to reproduce daytime hourly flux values over a 2-week period of intensive drying. The method can also be useful in the disaggregation of daily, or even weekly, evaporation into hourly values.

Diagnostics of land surface spatial variability and water vapor flux

To assess the spatial variability of the water vapor (i.e., latent heat) flux LE, it is convenient to scale it with its equilibrium analog LEe. From an analysis of the data from the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment, or FIFE, the spatial distribution of daily values of this dimensionless evaporation, α = LE/LEe, was found to be strongly related to the distributions of soil moisture and of the state of the vegetation. However, the relative strengths of these relationships depended on the soil moisture content and on its distribution. When the mean soil moisture content SM was high, in excess of about 27%, the distribution of evaporation was quite uniform regardless of the vegetation uniformity. In the intermediate range, with 20 < SM < 27%, both soil moisture and vegetation contributed to the spatial distribution of α. This distribution was controlled by the vegetation, when the soil moisture was uniform; under nonuniform soil moisture conditions, however, soil moisture exerted the major control. For SM < 20%, soil moisture normally was nonuniform, and it was found to hold the primary control of the spatial variation of α. The daily distribution of soil moisture content was largely independent from that of the state of the vegetation, due to the different timescales involved.

Wake formation around islands in oscillatory laminar shallow-water flows. Part 1. Experimental investigation

An experimental investigation of oscillatory shallow-water flow around islands has been undertaken to determine the dependence of wake formation on Keulegan-Carpenter number, KC = U o T/D, and stability parameter, S = c f D/h, where U o is amplitude of velocity oscillation, T is oscillation period, D is a representative island diameter, c f is friction coefficient and h is water depth. Two geometries are investigated: a vertical cylinder and a conical island with a small side slope of 8°. Existing experimental results for current flow around the same geometries have shown the influence of the stability parameter. Predominantly laminar flows are investigated and the flows are subcritical. Four modes of wake formation have been identified for both geometries: one with symmetric attached counter-rotating vortices only forming in each half-cycle, one with vortex pairs forming symmetrically in addition in each half-cycle, one with vortex pairs forming with some asymmetry and one with complex vortex shedding. The last results from one of the attached vortices crossing to the opposite side of the body during flow reversal; in the other cases the attached vortices are convected back on the same sides. For convenience these formations are called: symmetric without pairing, symmetric with pairing, sinuous with pairing and vortex shedding. They are shown on KC/S planes for both geometries. Numerical modelling of the flows for the conical island, based on the three-dimensional shallow-water equations with the hydrostatic pressure assumption, is undertaken in Part 2 (Stansby & Lloyd 2001).

Sorption and distribution of asphaltene, resin, aromatic and saturate fractions of heavy crude oil on quartz surface: molecular dynamic simulation.

The molecular scale sorption, diffusion and distribution of asphaltene, resin, aromatic and saturate fractions of heavy crude oil on quartz surface were studied using molecular dynamic simulation. Sorption of saturates on quartz decreased by 31% when temperature increased from 298 to 398K while opposite trend was observed for resins, but insignificant changes were found in asphaltenes and aromatics. Despite of this variety, the main contribution of interactions was van der Waals energy (>90%) irrespective of molecular components and temperatures. The diffusion coefficient of saturates was predicted as 10.8×10(-10)m(2)s(-1) while that of the remaining fractions was about 4×10(-10)m(2)s(-1) at 298K. The most likely oil distribution on quartz surface was that aromatics and saturates transported randomly into and out of the complex consisting of asphaltenes surrounded by resins, which was influenced by temperature. Overall, the knowledge on quartz-oil and oil-oil interactions gained in this study is essential for future risk assessment and remediation activities as previous studies on soil remediation either limited to light oil fractions with <40 carbons or treated the heavy crude oil as a single pseudo entity ignoring the interactions between oil fractions.

Sub-pixel reflectance unmixing in estimating vegetation water content and dry biomass of corn and soybeans cropland using normalized difference water index (NDWI) from satellites

Estimating vegetation cover, water content, and dry biomass from space plays a significant role in a variety of scientific fields including drought monitoring, climate modelling, and agricultural prediction. However, getting accurate and consistent measurements of vegetation is complicated very often by the contamination of the remote sensing signal by the atmosphere and soil reflectance variations at the surface. This study used Landsat TM/ETM+ and MODIS data to investigate how sub‐pixel atmospheric and soil reflectance contamination can be removed from the remotely sensed vegetation growth signals. The sensitivity of spectral bands and vegetation indices to such contamination was evaluated. Combining the strengths of atmospheric models and empirical approaches, a hybrid atmospheric correction scheme was proposed. With simplicity, it can achieve reasonable accuracy in comparison with the 6S model. Insufficient vegetation coverage information and poor evaluation of fractional sub‐pixel bare soil reflectance are major difficulties in sub‐pixel soil reflectance unmixing. Vegetation coverage was estimated by the Normalized Difference Water Index (NDWI). Sub‐pixel soil reflectance was approximated from the nearest bare soil pixel. A linear reflectance mixture model was employed to unmix sub‐pixel soil reflectance from vegetation reflectance. Without sub‐pixel reflectance contamination, results demonstrate the true linkage between the growth of sub‐pixel vegetation and the corresponding change in satellite spectral signals. Results suggest that the sub‐pixel soil reflectance contamination is particularly high when vegetation coverage is low. After unmixing, the visible and shortwave infrared reflectances decrease and the near‐infrared reflectances increase. Vegetation water content and dry biomass were estimated using the unmixed vegetation indices. Superior to the NDVI and the other NDWIs, the SWIR (1650 nm) band‐based NDWI showed the best overall performance. The use of the NIR (1240 nm), which is a unique band of MODIS, was also discussed.

Spatial distribution and pattern persistence of surface soil moisture and temperature over prairie from remote sensing

Abstract Images remotely sensed aboard aircraft during FIFE, namely, PBMR (microwave) soil nwisture and NS001 thermal infrared surface temperature, were mapped on the same coordinate system covering the 20 km×20 km experimental site. For both kinds of image data, the. frequency distributions were close to symmetric, and the area averages compared reasonably well with the ground based measurements. For any image on any given day, the correlation between the remotely sensed values and collocated ground based measurements over the area was usually high in the case of NS001 surface temperature but low in the case of PBMR soil moisture. On the other hand, at any given flux station the correlation between the PBMR and gravimetrie soil moisture over all available days was usually high. The correlation pixel by pixel between images of PBMR on different days was generally high. The preservation of the spatial patterns of soil moisture was also evaluated by considering the correlation. station b y station between ground-based soil moisture measurements on different days; no persistence of spatial pattern. was apparent during wet periods, but a definite pattern gradually established itself toward the end of each drying episode. The spatial patterns of surface: temperature revealed by NS001 were not preserved even within a single day. The cross-correlations among the two kinds of images and the vegetation index NDVI were normally poor. This suggests that different time scales were involved in the different processes of vegetation growth, and of the near-surface soil water and energy budgets.

An object-oriented tool for the control of point-source pollution in river systems

Abstract Computer-aided management of surface-water quality has been a field of continuous progress in the last decades. Object-orientation offers the potential for the development of efficient software tools, capable of dealing with the complexity of water resources and their policy making. In the present work an object-oriented approach has been developed for the analysis of point-source pollution control in river basins. The physical entities of the river basin and the conceptual entities of its water-quality simulation and control have been represented through objects. With the appropriate distribution of responsibilities among these objects and the specification of their collaborations, the assessment and simulation of river water quality can be performed. Alternative strategies of point-source pollution control can be also evaluated, and an optimised control scheme can be suggested. The above analysis has been implemented in the design and development of an easily extended and flexible software tool, using the object-oriented language Smalltalk Express. The tool has been successfully validated through the studies of the South Nation River Catchment in Canada and the Upper Mersey River Catchment in the United Kingdom. In this work the architecture of the software tool is outlined and the employed mathematical analysis along with the results of the case studies are presented.