Alastair G. Williams

Bio: Alastair G. Williams is an academic researcher from Australian Nuclear Science and Technology Organisation. The author has contributed to research in topics: Radon & Aerosol. The author has an hindex of 24, co-authored 66 publications receiving 1598 citations. Previous affiliations of Alastair G. Williams include Flinders University & University of Bonn.

FIRE Arctic Clouds Experiment

Abstract: An overview is given of the First ISCCP Regional Experiment (FIRE) Arctic Clouds Experiment that was conducted in the Arctic during April through July, 1998. The principal goal of the field experiment was to gather the data needed to examine the impact of arctic clouds on the radiation exchange between the surface, atmosphere, and space, and to study how the surface influences the evolution of boundary layer clouds. The observations will be used to evaluate and improve climate model parameterizations of cloud and radiation processes, satellite remote sensing of cloud and surface characteristics, and understanding of cloud-radiation feedbacks in the Arctic. The experiment utilized four research aircraft that flew over surface-based observational sites in the Arctic Ocean and Barrow, Alaska. In this paper we describe the programmatic and science objectives of the project, the experimental design (including research platforms and instrumentation), conditions that were encountered during the field experiment, and some highlights of preliminary observations, modelling, and satellite remote sensing studies.

Observations of ice nucleating particles over Southern Ocean waters

Abstract: A likely important feature of the poorly understood aerosol‐cloud interactions over the Southern Ocean (SO) is the dominant role of sea spray aerosol, versus terrestrial aerosol. Ice nucleating particles (INPs), or particles required for heterogeneous ice nucleation, present over the SO have not been studied in several decades. In this study, boundary layer aerosol properties and immersion freezing INP number concentrations (nINPs) were measured during a ship campaign that occurred south of Australia (down to 53 °S) in March‐April 2016. Ocean surface chlorophyll‐a concentrations ranged from 0.11‐1.77 mg m‐3 and nINPs were a factor of 100 lower than historical surveys, ranging from 0.38 to 4.6 m‐3 at ‐20 °C. The INP population included organic heat‐stable material, with contributions from heat‐labile material. Lower INP source potentials of SO seawater samples compared to Arctic seawater were consistent with lower ice nucleating site densities in this study compared to north Atlantic air masses.

Separating remote fetch and local mixing influences on vertical radon measurements in the lower atmosphere

Abstract: Two-point radon gradients provide a direct, unambiguous measure of near-surface atmospheric mixing. A 31-month data set of hourly radon measurements at 2 and 50 m is used to characterize the seasonality and diurnal variability of radon concentrations and gradients at a site near Sydney. Vertical differencing allows separation of remote (fetch-related) effects on measured radon concentrations from those due to diurnal variations in the strength and extent of vertical mixing. Diurnal composites, grouped according to the maximum nocturnal radon gradient ( ΔC max ), reveal strong connections between radon, wind, temperature and mixing depth on subdiurnal timescales. Comparison of the bulk Richardson Number ( Ri B ) and the turbulence kinetic energy (TKE) with the radon-derived bulk diffusivity ( K B ) helps to elucidate the relationship between thermal stability, turbulence intensity and the resultant mixing. On nights with large ΔC max , K B and TKE levels are low and Ri B is well above the ‘critical’ value. Conversely, when ΔC max is small, K B and TKE levels are high and Ri B is near zero. For intermediate ΔC max , however, Ri B remains small whereas TKE and K B both indicate significantly reduced mixing. The relationship between stability and turbulence is therefore non-linear, with even mildly stable conditions being sufficient to suppress mixing. DOI: 10.1111/j.1600-0889.2011.00565.x

The Vertical Distribution of Radon in Clear and Cloudy Daytime Terrestrial Boundary Layers

Abstract: Radon (222Rn) is a powerful natural tracer of mixing and exchange processes in the atmospheric boundary layer. The authors present and discuss the main features of a unique dataset of 50 high-resolution vertical radon profiles up to 3500 m above ground level, obtained in clear and cloudy daytime terrestrial boundary layers over an inland rural site in Australia using an instrumented motorized research glider. It is demonstrated that boundary layer radon profiles frequently exhibit a complex layered structure as a result of mixing and exchange processes of varying strengths and extents working in clear and cloudy conditions within the context of the diurnal cycle and the synoptic meteorology. Normalized aircraft radon measurements are presented, revealing the characteristic structure and variability of three major classes of daytime boundary layer: 1) dry convective boundary layers, 2) mixed layers topped with residual layers, and 3) convective boundary layers topped with coupled nonprecipitating ...

On the use of radon for quantifying the effects of atmospheric stability on urban emissions

Abstract: . Radon is increasingly being used as a tool for quantifying stability influences on urban pollutant concentrations. Bulk radon gradients are ideal for this purpose, since the vertical differencing substantially removes contributions from processes on timescales greater than diurnal and (assuming a constant radon source) gradients are directly related to the intensity of nocturnal mixing. More commonly, however, radon measurements are available only at a single height. In this study we argue that single-height radon observations should not be used quantitatively as an indicator of atmospheric stability without prior conditioning of the time series to remove contributions from larger-scale "non-local" processes. We outline a simple technique to obtain an approximation of the diurnal radon gradient signal from a single-height measurement time series, and use it to derive a four category classification scheme for atmospheric stability on a "whole night" basis. A selection of climatological and pollution observations in the Sydney region are then subdivided according to the radon-based scheme on an annual and seasonal basis. We compare the radon-based scheme against a commonly used Pasquill–Gifford (P–G) type stability classification and reveal that the most stable category in the P–G scheme is less selective of the strongly stable nights than the radon-based scheme; this lead to significant underestimation of pollutant concentrations on the most stable nights by the P–G scheme. Lastly, we applied the radon-based classification scheme to mixing height estimates calculated from the diurnal radon accumulation time series, which provided insight to the range of nocturnal mixing depths expected at the site for each of the stability classes.

An inventory of gaseous and primary aerosol emissions in Asia in the year 2000 : NASA global tropospheric experiment transport and chemical evolution over the pacific (TRACE-P): Measurements and analysis (TRACEP1)

Abstract: [i] An inventory of air pollutant emissions in Asia in the year 2000 is developed to support atmospheric modeling and analysis of observations taken during the TRACE-P experiment funded by the National Aeronautics and Space Administration (NASA) and the ACE-Asia experiment funded by the National Science Foundation (NSF) and the National Oceanic and Atmospheric Administration (NOAA). Emissions are estimated for all major anthropogenic sources, including biomass burning, in 64 regions of Asia. We estimate total Asian emissions as follows: 34.3 Tg SO 2 , 26.8 Tg NO x , 9870 Tg CO 2 , 279 Tg CO, 107 Tg CH 4 , 52.2 Tg NMVOC, 2.54 Tg black carbon (BC), 10.4 Tg organic carbon (OC), and 27.5 Tg NH 3 . In addition, NMVOC are speciated into 19 subcategories according to functional groups and reactivity. Thus we are able to identify the major source regions and types for many of the significant gaseous and particle emissions that influence pollutant concentrations in the vicinity of the TRACE-P and ACE-Asia field measurements. Emissions in China dominate the signature of pollutant concentrations in this region, so special emphasis has been placed on the development of emission estimates for China. China's emissions are determined to be as follows: 20.4 Tg SO 2 , 11.4 Tg NO x , 3820 Tg CO 2 , 116 Tg CO, 38.4 Tg CH 4 , 17.4 Tg NMVOC, 1.05 Tg BC, 3.4 Tg OC, and 13.6 Tg NH 3 . Emissions are gridded at a variety of spatial resolutions from 1° × 1° to 30 s x 30 s, using the exact locations of large point sources and surrogate GIS distributions of urban and rural population, road networks, landcover, ship lanes, etc. The gridded emission estimates have been used as inputs to atmospheric simulation models and have proven to be generally robust in comparison with field observations, though there is reason to think that emissions of CO and possibly BC may be underestimated. Monthly emission estimates for China are developed for each species to aid TRACE-P and ACE-Asia data interpretation. During the observation period of March/ April, emissions are roughly at their average values (one twelfth of annual). Uncertainties in the emission estimates, measured as 95% confidence intervals, range from a low of ±16% for SO 2 to a high of ±450% for OC.

A New Double-Moment Microphysics Parameterization for Application in Cloud and Climate Models. Part I: Description

Abstract: A new double-moment bulk microphysics scheme predicting the number concentrations and mixing ratios of four hydrometeor species (droplets, cloud ice, rain, snow) is described. New physically based parameterizations are developed for simulating homogeneous and heterogeneous ice nucleation, droplet activation, and the spectral index (width) of the droplet size spectra. Two versions of the scheme are described: one for application in high-resolution cloud models and the other for simulating grid-scale cloudiness in larger-scale models. The versions differ in their treatment of the supersaturation field and droplet nucleation. For the high-resolution approach, droplet nucleation is calculated from Kohler theory applied to a distribution of aerosol that activates at a given supersaturation. The resolved supersaturation field and condensation/deposition rates are predicted using a semianalytic approximation to the three-phase (vapor, ice, liquid) supersaturation equation. For the large-scale version of the scheme, it is assumed that the supersaturation field is not resolved and thus droplet activation is parameterized as a function of the vertical velocity and diabatic cooling rate. The vertical velocity includes a subgrid component that is parameterized in terms of the eddy diffusivity and mixing length. Droplet condensation is calculated using a quasi-steady, saturation adjustment approach. Evaporation/deposition onto the other water species is given by nonsteady vapor diffusion allowing excess vapor density relative to ice saturation.

Trimodal Characteristics of Tropical Convection

Abstract: It has long been known that trade wind cumulus and deep cumulonimbus represent primary components of the broad spectrum of cumulus clouds in the Tropics, which has led to the concept of a bimodal distribution of tropical clouds. However, recent analyses of shipboard radar data from Tropical Ocean Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment (COARE) provide evidence of abundant populations of a third cloud type, cumulus congestus. Congestus clouds constitute over half the precipitating convective clouds in COARE and contribute over one-quarter of the total convective rainfall. Global Atmospheric Research Program Atlantic Tropical Experiment studies reveal a similar midlevel peak in the distribution of radar-echo tops. These findings lead to the conclusion that shallow cumulus, congestus,and cumulonimbus are all prominent tropical cumulus cloud types. They are associated with trimodal distributions of divergence, cloud detrainment, and fractional cloudiness in the Tropics. The peaks in the distributions of radar-echo tops for these three cloud types are in close proximity to prominent stable layers that exist over the Pacific warm pool and the tropical eastern Atlantic: near 2 km (the trade stable layer), ;5 km (near 08C), and ;15‐16 km (the tropopause). These stable layers are inferred to inhibit cloud growth and promote cloud detrainment. The 08C stable layer can produce detrainment from cumulonimbi (attendant shelf clouds) and help retard the growth of precipitation-laden and strongly entraining congestus clouds. Moreover, restriction of growth of congestus clouds to just above the 08C level limits further enhancement of cloud buoyancy through glaciation. The three cloud types are found to vary significantly during COARE on the timescale of the 30‐60-day intraseasonal oscillation. The specific roles of clouds of the congestus variety in the general circulation are not yet clear, but some (the shallower ones) contribute to moistening and preconditioning the atmosphere for deep convection; others (the deeper ones) contribute an important fraction of the total tropical rainfall, and both likely produce many midlevel clouds, thereby modulating the radiative heating of the tropical atmosphere.

Statistical Methods For Environmental Pollution Monitoring

Abstract: If you really want to be smarter, reading can be one of the lots ways to evoke and realize. Many people who like reading will have more knowledge and experiences. Reading can be a way to gain information from economics, politics, science, fiction, literature, religion, and many others. As one of the part of book categories, statistical methods for environmental pollution monitoring always becomes the most wanted book. Many people are absolutely searching for this book. It means that many love to read this kind of book.

Surface Heat Budget of the Arctic Ocean

Abstract: A summary is presented of the Surface Heat Budget of the Arctic Ocean (SHEBA) project, with a focus on the field experiment that was conducted from October 1997 to October 1998. The primary objective of the field work was to collect ocean, ice, and atmospheric datasets over a full annual cycle that could be used to understand the processes controlling surface heat exchanges—in particular, the ice–albedo feedback and cloud–radiation feedback. This information is being used to improve formulations of arctic ice–ocean–atmosphere processes in climate models and thereby improve simulations of present and future arctic climate. The experiment was deployed from an ice breaker that was frozen into the ice pack and allowed to drift for the duration of the experiment. This research platform allowed the use of an extensive suite of instruments that directly measured ocean, atmosphere, and ice properties from both the ship and the ice pack in the immediate vicinity of the ship. This summary describes the project goal...