Jimmy Chi Hung Fung

Improving the wind environment in high-density cities by understanding urban morphology and surface roughness: A study in Hong Kong

Abstract In this study, a high-resolution frontal area density (FAD) map that depicts the surface roughness of urban Hong Kong is produced using a mapping method that takes into account the dense urban morphology and the site wind availability of the territory. Using the MM5/CALMET model simulated wind data of Hong Kong, the FAD map of three urban zones are calculated: podium (0–15m), building (15–60m), and urban canopy (0–60m). The wind tunnel test data is used to correlate the FAD understanding of the three zones. The grid sensitivity test indicates that 200m×200m is the reasonable resolution for the FAD map; the test also establishes that the lower urban podium zone yields the best correlation with the experimental data. The study further establishes that the simpler two-dimensional ground coverage ratio (GCR), which is readily available in the planning circle, can be used to predict the areas average pedestrian level urban ventilation performance of the city. Working with their inhouse GIS team using available data, it allows the planners a way to understand the urban ventilation of the city for decisions related to air paths, urban permeability and site porosity.

Plume dispersion through large groups of obstacles—A field investigation

Abstract A set of field experiments has been conducted at the Cardington experimental site, in the United Kingdom, to investigate the dispersion of a neutrally buoyant plume released upwind of an array of cubes, each of which measured approximately 2 m × 2 m × 2 m. The plume was released below the height of the obstacle array., and a second control plume was released alongside the array. The behaviour of the plume was recorded using both flow visualisation and measurements of gas concentration. A limited number of velocity measurements was also made. It is found that, relative to the control plume, certain mean concentration statistics do not change materially as the plume passes through the obstacle array. These include the form of the cross-sectional profiles, the decay along the centre line and the lateral growth with downstream distance. However, the mean vertical extent of the plume increases by 40–50%. In addition the r.m.s. fluctuations of concentration within the the plume are reduced. A clipped normal distribution is shown to be an appropriate model for the concentration fluctuations within the array plume.

Urban Modification in a Mesoscale Model and the Effects on the Local Circulation in the Pearl River Delta Region

The Pearl River Delta (PRD) region, located in the southern part of Guangdong Province in China, is one of the most rapidly developing regions in the world. The evolution of local and regional sea-breeze circulation (SBC) is believed to be responsible for forming meteorological conditions for high air-pollution episodes in the PRD. To understand better the impacts of urbanization and its associated urban heat island (UHI) on the local- and regional-scale atmospheric circulations over PRD, a number of high-resolution numerical experiments, with different approaches to treat the land surface and urban processes, have been conducted using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5). The results show that an accurate urban land-use dataset and a proper urban land-use parameterization are critical for the mesoscale model to capture the major features of the observed UHI effect and land–sea-breeze circulations in the PRD. Stronger UHI in the PRD increases the differential temperature gradient between urbanized areas and nearby ocean surface and hence enhances the mesoscale SBC. The SBC front consequently penetrates farther inland to overcome the prevailing easterly flow in the western part of inland Hong Kong. Additional sensitivity studies indicate that further industrial development and urbanization will strengthen the daytime SBC as well as increase the air temperature in the lowest 2 km of the atmosphere.

Numerical simulation and process analysis of typhoon-related ozone episodes in Hong Kong

[1] In this study the synoptic patterns conducive to the occurrence of O 3 episodes in Hong Kong are categorized by an inspection of the weather charts over the period of 1999-2003. The synoptic patterns associated with tropical cyclones originating in the North Pacific Ocean and the South China Sea are found to be the most optimal weather conditions for the occurrence of ozone episodes in Hong Kong. A high-resolution version of the regional, three-dimensional, multiscale photochemical air quality model (Pollutants in the Atmosphere and Their Transport in Hong Kong (PATH)), developed by the Hong Kong Environmental Protection Department in 2000, has been employed to investigate the evolution of one type of ozone episode related to tropical cyclones. A nonhydrostatic meteorological model (MM5) was applied with four-dimensional data assimilation to provide necessary meteorological fields to the air quality model. The performances of both the meteorological and chemical models are evaluated by comparing the simulated results with the available observed data. An integrated process rate analysis is used to examine the relative contributions of individual physical and chemical processes in the formation of ozone episodes for obtaining a better understanding of the mechanisms of photochemical smog events in Hong Kong. Results show that about 30% of the total ozone production is due to local chemical production in the lower atmosphere boundary layer, and about 70% is contributed by interregional transport from southern China into Hong Kong. In addition, four main processes, including horizontal advection, vertical transport, photochemical reactions, and deposition are found to have a significant influence on the ground-level concentration of ozone. The sensitivity experiments indicate that the chemical regime for ozone formation in Hong Kong seems to be limited by volatile organic compounds.

A comparison of HYSPLIT backward trajectories generated from two GDAS datasets.

The Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model is widely used to generate backward trajectories in given starting locations. However, differences exist between trajectories generated from the model with different input datasets. In this study, backward trajectories in Hong Kong in the entire year of 2011 are derived by HYSPLIT model. Two sets of Global Data Assimilation System (GDAS) output data associated with different horizontal and vertical resolutions (GDAS1 and GDAS0P5) are used as drivers in an attempt to quantify the differences between the results and discover the underlying reasons responsible for discrepancy. The results reveal that the significant differences between back trajectories generated from the two GDAS datasets can be mainly attributed to different vertical velocity calculation methods due to the absence of vertical velocity in GDAS0P5 dataset. The HYSPLIT trajectories are also sensitive to the horizontal and vertical resolutions of the input meteorological data, but to lesser extents. Results of cluster analysis indicate that when the air mass is from the north, northeast, or west with a long-to-medium range, the HYSPLIT backward trajectories are sensitive to the vertical advection calculation method and data resolution, whereas when the air mass is from the south or southwest with a long range, the trajectories are more likely to remain unchanged with the shifting of vertical velocity or data resolution. By comparing the vertical velocities with the observations and the performance in retrieving PM contributions from different directions, we conclude that GDAS1 dataset is more plausible in backward trajectory analysis in the Pearl River Delta.

Fractal dimensions of lines in chaotic advection

The difference between locally self‐similar fractals, called K fractals, and H fractals, which are globally self‐similar fractals, is recalled . The self‐similar cascade of energy [and enstrophy in the two‐dimensional (2‐D) case], which has been the original incentive for introducing fractals in the study of turbulence, does not seem to generate H‐fractal interfaces in 2‐D turbulent flows, according to the numerical evidence currently available. It only produces spiral singularities that are an example of K‐fractal structures, and presumably a consequence of the structure of the turbulence that is neglected in pictures of the turbulence, based on a self‐similar cascade of energy. It may be that the experimental evidence for fractal interfaces in 3‐D turbulence is, in fact, evidence that these interfaces are K fractal. On the other hand, it could be that only very unsteady flows can produce H‐fractal interfaces through a folding process that the unsteadiness adds to the stretching of the flow [see J. M. Ot...

Estimation of health and economic costs of air pollution over the Pearl River Delta region in China.

The Pearl River Delta region (PRD) is the economic growth engine of China and also one of the most urbanized regions in the world. As a two-sided sword, rapid economic development causes air pollution and poses adverse health effects to the citizens in this area. This work estimated the negative health effects in the PRD caused by the four major ambient pollutants (SO2, NO2, O3 and PM10) from 2010 to 2013 by using a log linear exposure-response function and the WRF-CMAQ modeling system. Economic loss due to mortality and morbidity was evaluated by the value of statistical life (VSL) and cost of illness (COI) methods. The results show that the overall possible short-term all-cause mortality due to NO2, O3 and PM10 reached the highest in 2013 with the values being 13,217-22,800. The highest total economic loss, which ranged from 14,768 to 25,305million USD, occurred in 2013 and was equivalent to 1.4%-2.3% of the local gross domestic product. The monthly profile of cases of negative health effects varied by city and the types of ambient pollutants. The ratio of mortality attributed to air pollutants to total population was higher in urban areas than in rural areas. People living in the countryside should consider the possible adverse health effects of urban areas before they plan a move to the city. The results show that the health burden caused by the ambient pollutants over this region is serious and suggest that tighter control policies should be implemented in the future to reduce the level of air pollution.

Gravitational settling of small spherical particles in unsteady cellular flow fields

Abstract We study the average settling velocity of small spherical particles under gravity through an infinite, periodic, non-divergent unsteady cellular flow field subject to the effects of a Stokes drag force and the inertia of the particles. In particular, the role of the spatial and temporal variations of the flow field on the settling rate is investigated. It is shown that the particles merge into isolated trajectories which are independent of their initial positions and velocities, and the cellular flow causes a net increase in settling velocity in the case V T σ u ⩽ 0.7 (where VT is the terminal velocity in still fluid normalized by the characteristic velocity of the flow) and the maximum decrease in settling rate occurs for V T gs u ≈ 1 when the flow is steady and with one length scale. As the cellular flow field is slightly perturbed with some unsteadiness or with the introduction of more length scales, the difference between the computed settling rate 〈VT〉 in the cellular flow and that in still fluid decreases. In particular when the degree of unsteadiness is greater than some critical value or when the number of length scales is more than four, the computed 〈VT〉 is always less than VT. This is consistent with the results of Fung (1993, J. geophys Res., 98, 20,287–20,297), who studied the settling of particles in a random velocity field. However, for all the parameters studied, particle accumulation is still a recurring feature except for a large unsteady parameter—then the particle motion can be chaotic. The fluid velocity u(xp, t) sampled by the particles along these isolated trajectories is used to explain the difference in settling rate between cellular flow and still fluid.

Assessing Long-Term Trend of Particulate Matter Pollution in the Pearl River Delta Region Using Satellite Remote Sensing

Serious particulate matter (PM) pollution problems in many polluted regions of China have been frequently reported in recent years. Long-term exposure to ambient PM pollution is significantly associated with adverse health effects. Characterizing the long-term trends and variation in PM pollution is a basic requirement for evaluating long-term exposure and for guiding future policies to reduce the effects of air pollution on health. However, long-term, ground-based PM measurements are only available at a few fixed stations. In this study, an algorithm is developed and validated to estimate PM concentrations based on the satellite atmospheric optical depth with 1 km spatial resolution. The long-term trends of PM10 concentrations in the entire Pearl River Delta (PRD) region and different cities are quantified and discussed. From 2001 to 2013, the PM10 pollution of the entire PRD region was dominated by a decreasing trend of -0.15 ± 0.23 μg/m(3)·yr. This decreasing PM10 trend was apparent over 75% of the PRD area, with the most significant decreases observed in the center of the region. However, the remaining 25%, mostly located in the outskirts of the region, showed an increasing PM10 trend. This overall decreasing trend indicates the effectiveness of the control measures applied in the past decade for the primary pollutants.

Examining the Impact of Nitrous Acid Chemistry on Ozone and PM over the Pearl River Delta Region

The impact of nitrous acid (HONO) chemistry on regional ozone and particulate matter in Pearl River Delta region was investigated using the community multiscale air quality (CMAQ) modeling system and the CB05 mechanism. Model simulations were conducted for a ten-day period in October 2004. Compared with available observed data, the model performance for NOx, SO2, PM10, and sulfate is reasonably good; however, predictions of HONO are an order of magnitude lower than observed data. The CB05 mechanism contains several homogenous reactions related to HONO. To improve the model performance for HONO, direct emissions, two heterogeneous reactions, and two surface photolysis reactions were incorporated into the model. The inclusion of the additional formation pathways significantly improved simulated HONO compared with observed data. The addition of HONO sources enhances daily maximum 8-hour ozone by up to 6 ppbV (8%) and daily mean PM2.5 by up to 17 ug/m3 (12%). They also affected ozone control strategy in Pearl River Delta region.