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

An efficient time-dependent equation for predicting ground surface temperature devised by Bhumralkar (1975) and Blackadar (1976) is tested against a 12-layer soil model and compared with five other approximate methods in current use. It is found to be generally superior if diurnal forcing is present and very much superior to the use of the insulated surface assumption. An analogous method of predicting ground surface moisture content is presented which allows the surface to become moist quickly during rainfall or to become drier than the bulk soil while evaporation occurs. These improved methods are not of much relevance unless the main influences of a vegetation layer are included. An efficient one-layer foliage parameterization is therefore developed that extends continuously from the case of no shielding of the ground by vegetation to complete shielding. It includes influences of both ground and foliage albedos and emissivities, net leaf area index, stomatal resistance, retained water on the foliage, and several other considerations. When it is tested against data for wheat measured by Penman and Long (1960), it appears quite adequate despite the many simplifying assumptions. The parameterization predicts that errors of up to a factor of 2 in evapotranspiration can be incurred by ignoring the presence of a vegetation layer.

Efficient prediction of ground surface temperature and moisture, with inclusion of a layer of vegetation

Canopy temperatures, obtained by infrared thermometry, along with wet- and dry-bulb air temperatures and an estimate of net radiation were used in equations derived from energy balance considerations to calculate a crop water stress index (CWSI). Theoretical limits were developed for the canopy air temperature difference as related to the air vapor pressure deficit. The CWSI was shown to be equal to 1 - E/Ep, the ratio of actual to potential evapotranspiration obtained from the Penman-Monteith equation. Four experimental plots, planted to wheat, received postemergence irrigations at different times to create different degrees of water stress. Pertinent variables were measured between 1340 and 1400 each day (except some weekends). The CWSI, plotted as a function of time, closely paralleled a plot of the extractable soil water in the 0- to 1.1-m zone. The usefulness and limitations of the index are discussed.

Canopy temperature as a crop water stress indicator

one: Plant stand architecture.- 1.1 Role of Phytometric Investigations in The Studies of Plant Stand Architecture and Radiation Regime.- 1.2 Principal Phytometric Characteristics of Stands.- 1.2.1 Phytometric characteristics of leaf and other plant organs.- 1.2.2 Phytometric characteristics of an individual plant.- 1.2.3 Phytometric characteristics of a pure stand.- 1.2.4 Plant stand as a horizontal layer.- 1.3 Phytometrical Methods.- 1.3.1 Determination of leaf area.- 1.3.2 The measurement of leaf orientation.- 1.3.3 Inclined point quadrats method.- 1.3.4 Stratifying clip method.- 1.3.5 The methods of statistical measurements.- 1.3.6 Numerical methods for determination of foliage area vertical distribution.- 1.3.7 A rational method for determining phytometric characteristics of stand architecture and productivity.- 1.4 Statistical Characteristics of A Stand.- 1.4.1 Parameters of statistical characteristics.- 1.4.2 Correlation between statistical characteristics.- 1.5 Spatial Distribution of Phytoelements in Stands.- 1.5.1 General.- 1.5.2 Space-time variability of transition functions.- 1.5.3 Vertical distribution of phytomass and phytoarea.- 1.5.4 Horizontal distribution of phytomass and phytoarea.- 1.6 Foliage Area Orientation in Stands.- 1.6.1 General.- 1.6.2 Distribution functions of leaf inclination and azimuth orientation.- 1.6.3 G-function of leaf orientation.- 1.7 Plant Stand Architecture, Photosynthesis and Productivity.- two: Radiation regime in plant stand.- II.1 Radiation Field in a Plant Stand and The Problem of Its Mathematical Modelling.- II.1.1 General.- II. 1.2 Basic characteristics of the radiation field.- II. 1.3 Optical characteristics of phytoelements.- II. 1.4 Optical characteristics of plate medium.- II. 1.5 The radiation transfer equation for an optically anisotropic plate medium.- IL 1.6 Plant stand as a plate medium.- IL 1.7 The radiation transfer equation for a plant stand.- II. 1.8 Leaf and plant stand absorption functions.- II. 1.9 Statistical character of the radiation field in plant stands.- 11.2 Incident Radiation.- II.2.1 General.- IL2.2 Incoming direct solar radiation.- 11.2.3 Incoming diffuse sky radiation.- 11.2.4 Incoming total solar radiation.- 11.2.5 Incoming long-wave radiation of the atmosphere.- 11.2.6 Photosynthetically active radiation.- 11.3 Optical Properties of Phytoelements.- 11.3.1 General.- 11.3.2 Optical models of the leaf.- 11.3.3 Scattering phase function of the leaf.- 11.3.4 Spectral optical properties of phytoelements.- 11.3.5 Integral coefficients of leaf reflection, transmission and absorption for short-wave radiation and PAR.- 11.3.6 Optical properties of phytoelements in the long-wave spectral region.- 11.4 Penetration of Direct Solar Radiation into a Plant Stand.- 11.4.1 General.- 11.4.2 Statistical character of penetration of direct radiation in a plant stand. Penumbras.- 11.4.3 Theoretical expressions for direct solar radiation penetration.- 11.4.4 Penetration theory for direct solar radiation in horizontally inhomogeneous plant stands.- 11.4.5 Calculated penetration of direct solar radiation and its dependence on various factors.- 11.4.6 Methods of experimental investigation.- 11.4.7 Experimental data on penetration.- 11.5 Penetration of Diffuse Sky Radiation Into Plant Stand.- 11.5.1 General.- 11.5.2 Penetration formulae.- 11.5.3 Calculation of intensities and zonal radiation.- 11.5.4 Calculation of downward fluxes.- 11.5.5 Method of hemispherical photographs.- 11.5.6 Statistical character of the penetration of diffuse sky radiation.- 11.6 Scattering of Radiation Inside Plant Stands.- 11.6.1 General.- 11.6.2 Scattering and absorption coefficients for an elementary volume in a plant stand. Scattering phase function.- 11.6.3 Solution of radiation transfer equation for horizontal leaves.- 11.6.4 The Schwarzschild approximation for modified radiative transfer equation.- 11.6.5 Approximation for single scattering.- 11.6.6 Leaf scattering coefficient ?L and the complementary radiation field.- 11.6.7 Calculation of the complementary PAR field.- 11.6.8 Calculation of the complementary NIR field.- 11.7 Total Radiation Field in Plant Stands.- 11.7.1 General.- 11.7.2 Intensities of total radiation field.- 11.7.3 Total vertical fluxes.- 11.7.4 Angular distribution of total radiation flux.- 11.7.5 Leaf absorption in total radiation field.- 11.7.6 Radiation in a plant stand with horizontal leaves.- 11.7.7 Errors of the approximate methods of calculation.- 11.7.8 New theories.- 11.7.9 Monte Carlo simulation models.- 11.8 Semiempirical Formulae for Total Radiation Fluxes.- 11.8.1 General.- 11.8.2 Exponential and binomial semiempirical formulae.- 11.8.3 New semiempirical formulae.- 11.9 Albedo of Plant Stand.- 11.9.1 General.- 11.9.2 Formulae for the albedo and brightness coefficient.- 11.9.3 Albedo and its dependence on various factors.- 11.9.4 Comparison of calculated and experimental data.- 11.10 Calculation of Long-Wave Radiation in A Stand.- 11.11 Net Radtation in Plant Stands.- Conclusion.- Supplement. Description of Field Experiments.- References.- Author Index.

The radiation regime and architecture of plant stands

Estimating crop water deficit using the relation between surface-air temperature and spectral vegetation index

A Linke-Feussner radiometer was used to measure the radiative temperature of natural surfaces, with small errors caused by the divergence of radiative flux and by its angular variation. On typical cloudless summer days, the maximum temperature of tall crops and of open water was close to maximum air temperature; a bare soil surface exceeded air temperature by 20°C; short grass was the coldest surface at night. A linear variation of net (total) radiation with net short-wave radiation was established for several tall crops, and the increase in long-wave loss per unit increase of net radiation (heating coefficient) was 0·08. Daily totals of net radiation as a percentage of solar radiation income were: 37 per cent (bare soil), 41 per cent (short grass), 46 per cent (tall crop), and 53 per cent (water). 
 
 
 
When the aerodynamic character of a crop is known, the effective resistance of the stomata to water-vapour diffusion can be related theoretically to the difference between surface and air temperature. For rough vegetation, estimated stomatal resistance is 0·4 to 0·8 sec cm−1 and the estimated heating coefficient is almost independent of wind speed at about 0·1.

The radiation balance of bare soil and vegetation - radiative temperature in the heat balance of natural surfaces

Measured radiative flux divergence near the ground at night - discussion

A series of 100-mb temperature maps was drawn up for the winters 1949-1950 to 1958-1959 using 10-day mean data. These maps were used to summarize the large-scale events of each winter and of the ten winters in general. The results suggest the existence of two major patterns, both highly persistent. 
 
(i) An asymmetric pattern with a warm ridge over east Siberia-Alaska and an elliptical cold-core vortex over Eurasia. 
 
(ii) An eccentric bi-polar distribution, the trough line extending from central Canada to central Siberia. 
 
The Aleutian ridge and Eurasian trough are quasi-stationary features, common to both regimes, while the Canadian and North Atlantic sectors experience the greatest change from one regime to the other - alternating warm ridge and cold trough. Both the regional climatology of the arctic winter stratosphere and the variations in the sequence of events from year to year for the Arctic as a whole, are described in terms of these two features and two alternating patterns. 
 
The striking asymmetry of both patterns in the stable stratosphere appears to require an external forcing agent, and this analysis tends to highlight the role of tropospheric activity.

L. C. W. Bonacina

Papers

The radiation balance of bare soil and vegetation - radiative temperature in the heat balance of natural surfaces

Measured radiative flux divergence near the ground at night - discussion

Structure of arctic winter stratosphere over a 10-yr period