Progress in urban climatology over the two decades since the first publication of the International Journal of Climatology is reviewed. It is emphasized that urban climatology during this period has benefited from conceptual advances made in microclimatology and boundary-layer climatology in general. The role of scale, heterogeneity, dynamic source areas for turbulent fluxes and the complexity introduced by the roughness sublayer over the tall, rigid roughness elements of cities is described. The diversity of urban heat islands, depending on the medium sensed and the sensing technique, is explained. The review focuses on two areas within urban climatology. First, it assesses advances in the study of selected urban climatic processes relating to urban atmospheric turbulence (including surface roughness) and exchange processes for energy and water, at scales of consideration ranging from individual facets of the urban environment, through streets and city blocks to neighbourhoods. Second, it explores the literature on the urban temperature field. The state of knowledge about urban heat islands around 1980 is described and work since then is assessed in terms of similarities to and contrasts with that situation. Finally, the main advances are summarized and recommendations for urban climate work in the future are made. Copyright © 2003 Royal Meteorological Society.

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Two decades of urban climate research: a review of turbulence, exchanges of energy and water, and the urban heat island

Jets, ie collimated streams of matter, occur from the microscale up to the large-scale structure of the universe Our focus will be mostly on surface tension effects, which result from the cohesive properties of liquids Paradoxically, cohesive forces promote the breakup of jets, widely encountered in nature, technology and basic science, for example in nuclear fission, DNA sampling, medical diagnostics, sprays, agricultural irrigation and jet engine technology Liquid jets thus serve as a paradigm for free-surface motion, hydrodynamic instability and singularity formation leading to drop breakup In addition to their practical usefulness, jets are an ideal probe for liquid properties, such as surface tension, viscosity or non-Newtonian rheology They also arise from the last but one topology change of liquid masses bursting into sprays Jet dynamics are sensitive to the turbulent or thermal excitation of the fluid, as well as to the surrounding gas or fluid medium The aim of this review is to provide a unified description of the fundamental and the technological aspects of these subjects

Physics of liquid jets

The conservation and distribution of water substance in atmospheric circulations is considered within a frame of continuity principles, model air flows, and models of microphysical processes. The simplest considerations of precipitation involve its vertical distribution in an updraft column, where condensate appears immediately as precipitation with uniform terminal fallspeed. The study also treats steady two-dimensional air circulations in which time-dependent distributions of water vapor, cloud and precipitation respond to model microphysical processes.

On the distribution and continuity of water substance in atmospheric circulations

Preface Acknowledgements 1. Introduction 2. Atmospheric structure, composition and thermodynamics 3. The continuity and thermodynamic energy equations 4. The momentum equation in Cartesian and spherical coordinates 5. Vertical-coordinate conversions 6. Numerical solutions to partial differential equations 7. Finite-differencing the equations of atmospheric dynamics 8. Boundary-layer and surface processes 9. Radiative energy transfer 10. Gas-phase species, chemical reactions and reaction rates 11. Urban, free-tropospheric and stratospheric chemistry 12. Methods of solving chemical ordinary differential equations 13. Particle components, size distributions and size structures 14. Aerosol emission and nucleation 15. Coagulation 16. Condensation, evaporation, deposition and sublimation 17. Chemical equilibrium and dissolution processes 18. Cloud thermodynamics and dynamics 19. Irreversible aqueous chemistry 20. Sedimentation, dry deposition and air-sea exchange 21. Model design, application and testing Appendix A. Conversions and constants Appendix B. Tables References Index.

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Fundamentals of atmospheric modeling

Since 1951, the biomass of macrozooplankton in waters off southern California has decreased by 80 percent. During the same period, the surface layer warmed—by more than 1.5°C in some places—and the temperature difference across the thermocline increased. Increased stratification resulted in less lifting of the thermocline by wind-driven upwelling. A shallower source of upwelled waters provided less inorganic nutrient for new biological production and hence supported a smaller zooplankton population. Continued warming could lead to further decline of zooplankton.

Climatic Warming and the Decline of Zooplankton in the California Current

The collision, coalescence and breakup of single raindrop pairs were studied at terminal velocities and laboratory pressure (100 kPa) in 761 collision experiments (out of 14 000 attempts). Six size combinations were used with drop pair diameters of [0.18;.0.0395 cm], [0.40; 0.0395 cm], [0.44; 0.0395 cm], [0.18; 0.0715 cm], [0.18; 0.10 cm] and [0.30; 0.10 cm]. For averaging purposes the experiments were repeated over one hundred times for each pair. The new coalescence efficiencies and fragment size distributions in breakup turned out to be consistent with those of McTaggart-Cowan and List (1975b) and permitted the combination of the two data sets into a single data bank spanning essentially the entire range of raindrop sizes. The analysis addressed three main geometric shapes formed by the drops after initial contact, namely, filaments, sheets and disks, and the fragment size distributions after breakup. Significant collisional growth, i.e., coalescence, occurred only when drops <0.06 cm in diame...

Collision, Coalescence and Breakup of Raindrops. Part I: Experimentally Established Coalescence Efficiencies and Fragment Size Distributions in Breakup

The experimental drop collision/breakup results of Low (1977) and Low and List (1982) and McTaggart-Cowan and List (1975b), taken at laboratory pressure and terminal drop speeds, were parameterized for future use in cloud and precipitation modeling. The primary analyses of the 10 representative raindrop pairs were based on the three main geometric shapes generally assumed by the drop pairs after their initial contact and before breakup (or coalescence): filaments, sheets and disks. Relationships for the average total fragment number for each category are given. The fragment number distributions resulting from the collisions in each classification were fitted as sums of normal and log-normal distributions with the parameters of each distribution being related to the drop sizes and physical quantities derived from them (like the collision kinetic energy, CKE). Each collision was then weighted according to the individual contribution and summed to give the probability of occurrence of each breakup t...

Collision, Coalescence and Breakup of Raindrops. Part II: Parameterization of Fragment Size Distributions

Drag coefficients and Best numbers of models of six planar snow crystals, two conical graupel and two conical small-hail particles were determined experimentally in glycerin-water mixtures and salt solutions. The Reynolds number (Re) range covered for the crystals was 0.1 to 200 and for the conical models 10 to 2000. It was found that the drag coefficients of dendritic shapes differed by factors of up to 4 from that of a disc of equal thickness and at an equal Reynolds number. The drag ratio is roughly constant with Re and linearly related to the ratio of the respective surface areas. The drag coefficients of the conical models assumed values between 0.5 and 2.0. During steady fall they decreased with increasing Re; however, as soon as oscillations started this trend reversed. Since tumbling does occur for larger graupel and small hail at Re > 300–1000 their main characteristic motions and frequencies are also discussed. Values for oscillation frequencies are given and the motions are described i...

Free-Fall Behavior of Planar Snow Crystals, Conical Graupel and Small Hail

The collision and subsequent breakup of water drops moving essentially vertically and at terminal velocity has been studied for five drop pairs: the diameters Ds of the large drops were 4.8, 3.6 and 3.0 mm; the diameters D3 of the small drops were 1.8 mm and 1.0 mm. 712 collisions were obtained in 25,000 individually recorded attempts. Three distinct types of collision-breakup were found with the following occurrence: necks 27%, sheets 55% and disks 18%. Bag breakups were insignificant with <0.5%. All types are defined and corresponding examples shown. Fragment size and number distributions for the different types and the overall situation give further reasons for the breakdown into the different types. The disk collision has been found to be the major cause for the depletion in number of large drops, hence the cutoff of large drops in rain. The results also form the first data bank for numerically modelling the evolution of raindrop size spectra and the Langmuir chain process.

Collision and Breakup of Water Drops at Terminal Velocity.

Photographic and visual observations of a large number (> 104) of collisions between two water drops, one of cloud droplet size (70 μ diameter) and one of raindrop size (1.0–3.5 mm diameter) reveal the conditions under which a collision may result in coalescence, bouncing, or partial coalescence. Coalescence occurs for 85–95% of collisions at velocities equivalent to the difference in terminal velocities of the drop-droplet pair. Bouncing always results from collisions in which the droplet center trajectory does not intersect the collector drop surface. Partial coalescence occurs infrequently and is restricted to narrow ranges of velocities and incident angles.

Barriers to coalescence are the existence of an air film between the approaching surfaces and the ability of the drop surface to deform on collision. Sufficient electrical attraction, long contact time, or high impact velocity can overcome these barriers and insure coalescence.

Surface tension variations of ±15% from the value for distilled water did not affect the coalescence process at velocities equivalent to natural conditions. Electric charges of like or unlike sign on the drop pair are found to enhance coalescence once a threshold charge of 10−2 esu on the collector drop is attained (with a droplet charge of −10−5 esu); higher drop charges insure 100% coalescence.

Values of coalescence efficiency E2 calculated from experiments, lead to an expression E2 = R2/(R + r)2, which is valid at least for drop radii 400 μ<R<2000 μ, and droplet radii 20 μ<r<100 μ. Sample calculations show that, because not every collision results in coalescence, previous growth rates are overestimates. In summary, it is no longer considered adequate to assume a coalescence efficiency of unity for all drop-droplet collisions.

Roland List

Papers

Collision, Coalescence and Breakup of Raindrops. Part I: Experimentally Established Coalescence Efficiencies and Fragment Size Distributions in Breakup

Collision, Coalescence and Breakup of Raindrops. Part II: Parameterization of Fragment Size Distributions

Free-Fall Behavior of Planar Snow Crystals, Conical Graupel and Small Hail

Collision and Breakup of Water Drops at Terminal Velocity.

The Coalescence process in raindrop growth