A study of the sensitivity of bare soil evaporation schemes to soil surface wetness,using the coupled soil moisture and surface temperature prediction model,BARESOIL

Summary The performance of evaporation schemes with and approach and their combination within resistance representation of evaporation from bare soil surface is discussed. For this purpose nine schemes, based on different functions of or , on the ratio of the volumetric soil moisture content and its saturated value are used.The quality of the chosen schemes has been evaluated using the results of time integration by the coupled soil moisture and surface temperature prediction model, BARESOIL, using in situ data. A sensitivity analysis was made using two sets of data derived from the volumetric soil moisture content of the top soil layer. One with values below the wilting point (0.17 m³m^–3) and the second with values above 0.20m³m^–3. Data sets were obtained at the experimental site Rimski anevi, Yugoslavia, from the bare surface of a chernozem soil.With 4 Figures     

In Search of Warmer Climates? The Impact of Climate Change on Flows of British Tourists

This paper investigates the impact of climate change on the chosendestinations of Britishtourists. Destinations are characterised in terms of attractors includingclimate variables, traveland accommodation costs. These variables are used to explain the currentobserved pattern ofoverseas travel in terms of a model based upon the idea of utilitymaximisation. The approachpermits the trade-offs between climate and holiday expenditure to be analysedand effectivelyidentifies the optimal climate for generating tourism. The findings are usedto predict the impactof various climate change scenarios on popular tourist destinations.     

A Green Planet Versus a Desert World: Estimating the Maximum Effect of Vegetation on the Land Surface Climate

We quantify the maximum possible influence of vegetation on the global climate by conducting two extreme climate model simulations: in a first simulation (desert world), values representative of a desert are used for the land surface parameters for all non glaciated land regions. At the other extreme, a second simulation is performed (green planet) in which values are used which are most beneficial for the biosphere's productivity. Land surface evapotranspiration more than triples in the presence of the green planet, land precipitation doubles (as a second order effect) and near surface temperatures are lower by as much as 8 K in the seasonal mean resulting from the increase in latent heat flux. The differences can be understood in terms of more absorbed radiation at the surface and increased recycling of water. Most of the increase in net surface radiation originates from less thermal radiative loss and not from increases in solar radiation which would be expected from the albedo change. To illustrate the differences in climatic character and what it would imply for the vegetation type, we use the Köppen climate classification. Both cases lead to similar classifications in the extra tropics and South America indicating that the character of the climate is not substantially altered in these regions. Fundamental changes occur over Africa, South Asia and Australia, where large regions are classified as arid (grassland/desert) climate in the desert world simulation while classified as a forest climate in the green planet simulation as a result of the strong influence of maximum vegetation on the climate. This implies that these regions are especially sensitive to biosphere-atmosphere interaction.     

Farm-Level Adaptation to Climatic Variability and Change: Crop Diversification in the Canadian Prairies

Among other foci, recent research on adaptation to climatic variability and change has sought to evaluate the merit of adaptation generally, as well as the suitability of particular adaptations. Additionally, there is a need to better understand the likely uptake of adaptations. For example, diversification is one adaptation that has been identified as a potential farm-level response to climatic variability and change, but its adoption by farmers for this reason is not well understood. This paper serves two purposes. The first is to document the adoption of crop diversification in Canadian prairie agriculture for the period 1994–2002, reflect upon its strengths and limitations for managing a variety of risks, including climatic ones, and gauge its likely adoption by producers in response to anticipated climate change. The second purpose is to draw on this case to refine our current understanding of climate change adaptation more generally. Based upon data from over 15 000 operations, it was determined that individual farms have become more specialized in their cropping patterns since 1994, and this trend is unlikely to change in the immediate future, notwithstanding anticipated climate change and the known risk-reducing benefits of crop diversification. More broadly, the analysis suggests that suitable and even possible climate change adaptations need to be more rigorously assessed in order to understand their wider strengths and limitations.     

Basis for integration of conventional observations of cloud into global nephanalyses

Water vapour is the largest and, radiatively, the most important trace gas in the Earth's atmosphere. Cloud amount and cloud optical depth feedbacks are, as yet, poorly understood and improvements in model parameterization schemes await an adequate observational data base. Satellite retrievals do not, and will not for some time, provide more than snapshot (a few months to a few years) records. Conventional surface-based observations of cloudiness could complement the global coverage offered by satellite retrievals if a sound relationship between the two observational measurements could be found. Observations underline the importance of the vertical dimension of clouds which affects the observational geometries from satellites and the surface. A new basis for the relationship between the (vertical) earthview of cloud amount and the (whole dome) skycover of cloud amount has been sought. Over four and a half thousand all-sky camera photographs, representing a considerable range of seasonal and climatological conditions, have been analyzed to give rise to a database from which predictive relationships for earthview, E, and skycover, S, have been established.Cubic functions are the most soundly based both physically and empirically. We find: S=0.647+2.192E–0.461E ²+0.037E ³ and E=-0.001+0.082S+0.269S ²–0.019S ³ for the prediction of skycover from earthview and earthview from skycover, respectively. If earthview is required from skycover observations then ES could be used with little additional error. Hence, conventional surface observations of skycover could be compared directly with satellite-derived earthview values. More importantly, these results do not support the widespread assumption that conventional (surface) observations of cloud amount always exceed the earthview could retrieval. Furthermore, climate model predictions of total cloud amount may also be interpreted via these relationships. GCM-predicted layer cloud amounts can be synthesized into modelled E values using the random overlap formula and hence it is possible to construct modelled S values which are directly comparable with conventionally observed cloud climatologies. The baseline observation of skycover by clouds therefore provides a valuable validation tool for both satellite programmes and climate models.     

Bioaccumulation of Cobalt in Parmelia sulcata

Six locations across mainland Portugal were selected for exposing Parmelia sulcata, for a one-year period (8 months for one site), with simultaneous measurement of total (dry + wet) deposition (one-month periods). The exposed lichens and the total (dry + wet) deposition were analysed for cobalt contents by INAA (instrumental neutron activation analysis) and ICP-MS (inductively coupled plasma mass spectroscopy), respectively. The designated wet deposition was evaluated through the collected water volume; the designated dry deposition was assessed after the (dried) residual mass of the wet deposition. An excellent agreement between Co contents in exposed lichens and the cumulative (1) Co contents in the dry deposition, (2) dry deposition, and (3) wet deposition has been found for the locations with alternate drought and precipitation months, high dry deposition, and high Co contents in the latter. Continuous rainfall was found to hinder the Co accumulation in the lichen due to its release from the lichen and/or lower Co contents in the dry deposition. At three locations, P. sulcata Co contents, after subtraction of the background (before exposure), equalled or exceeded the Co contents in the cumulative dry deposition at the end of the exposure time. The optimal exposure period for this species likely depends on the exposure conditions.     

On Trajectory Curvature as a Selection Criterion for Valid Lagrangian Stochastic Dispersion Models

Recently Wilson and Flesch (Boundary-Layer Meteorology, 84, 411-426, 1997) suggested that the average increment d z to the orientation = arctan(w/u) of the Lagrangian velocity-fluctuation vector can be used to distinguish the better Lagrangian stochastic models within the well-mixed class. Here it is demonstrated that the specification of d z constitutes neither a sufficient or universally applicable criterion to distinguish the better Lagrangian stochastic models within the well-mixed class. The hypothesis made by Wilson and Flesch that Lagrangian stochastic models with /P_E irrotational are zero-spin models, having d z=0, is proven     

A statistical-dynamical downscaling procedure for global climate simulations

Summary A statistical-dynamical downscaling procedure for global climate simulations is described. The procedure is based on the assumption that any regional climate is associated with a specific frequency distribution of classified large-scale weather situations. The frequency distributions are derived from multi-year episodes of low resolution global climate simulations. Highly resolved regional distributions of wind and temperature are calculated with a regional model for each class of large-scale weather situation. They are statistically evaluated by weighting them with the according climate-specific frequency. The procedure is exemplarily applied to the Alpine region for a global climate simulation of the present January climate.List of Symbols west-east mesh size in geographic coordinates south-north mesh size in geographic coordinates N number of large-scale weather classes n number of regional-scale event classes p pressure P probability Ø large-scale event regional-scale event q _v specific humidity potential temperature u west-east wind component v south-north wind componentAbbreviations AGL above ground level - LT local time - UTC universal time coordinated With 13 Figures     

Adapting stochastic weather generation algorithms for climate change studies

While large-scale climate models (GCMs) are in principle the most appropriate tools for predicting climate changes, at present little confidence can be placed in the details of their projections. Use of tools such as crop simulation models for investigation of potential impacts of climatic change requires daily data pertaining to small spatial scales, not the monthly-averaged and large-scale information typically available from the GCMs. A method is presented to adapt stochastic weather generation models, describing daily weather variations in the present-day climate at particular locations, to generate synthetic daily time series consistent with assumed future climates. These assumed climates are specified in terms of the commonly available monthly means and variances of temperature and precipitation, including time-dependent (so-called transient) climate changes. Unlike the usual practice of applying assumed changes in mean values to historically observed data, simulation of meteorological time series also exhibiting changes in variability is possible. Considerable freedom in climate change scenario construction is allowed. The results are suitable for investigating agricultural and other impacts of a variety of hypothetical climate changes specified in terms of monthly-averaged statistics.     

Equilibrium and transient climatic warming induced by increased atmospheric CO2

The change in the Earth's equilibrium global mean surface temperature induced by a doubling of the CO₂ concentration has been estimated as 0.2 to 10 K by surface energy balance models, 0.5 to 4.2 K by radiative-convective models, and 1.3 to 4.2 K by general circulation models. These wide ranges are interpreted and quantified here in terms of the direct radiative, forcing of the increased CO₂, the response of the climate system in the absence of feedback processes, and the feedbacks of the climate system. It is the range in the values of these feedbacks that leads to the ranges in the projections of the global mean surface warming. The time required for a CO₂-induced climate change to reach equilibrium has been characterized by an e-folding time _e with values estimated by a variety of climate/ocean models as 10 to 100 years. Analytical and numerical studies show that this wide range is due to the strong dependence of _e on the equilibrium sensitivity of the climate model and on the effective vertical thermal diffusivity of the ocean model. A coupled atmosphere-ocean general circulation model simulation for doubled CO₂ suggestes that, as a result of the transport of the CO₂-induced surface heating into the interior of the ocean, _e 50 to 100 years. Theoretical studies for a realistic CO₂ increase between 1850 and 1980 indicate that this sequestering of heat into the ocean's interior is responsible for the concomittant warming being only about half that which would have occurred in the absence of the ocean. These studies also indicate that the climate sytem will continue to warm towards its as yet unrealized equilibrium temperature change, even if there is no further increase in the CO₂ concentration.     

Regular and seasonal behaviour of atmospheric radio noise field strength (ARNFS) over low latitude station calcutta

Summary The electromagnetic radiation of cloud discharge known as atmospheric radio noise field strength (ARNFS) shows a gradual fall from a frequency of 9 kHz to 81 kHz as studied over a period of two years at Calcutta, very close to Bay of Bengal. The main characteristic features of ARNFS at Calcutta are that-(i) ARNFS shows that midday median value is smaller than midnight median value in all months, (ii) level of daily minimum is higher in February and monsoon compared to other seasons, (iii) sunrise effect and sunset effect are well correlated with local sunrise and sunset times, (iv) the magnitude of sunrise fade and sunrise fade rate are maximum in April and lowest during winter period, (v) the magnitude of sunset fade is higher in premonsoon and postmonsoon while it is lowest in monsoon, (vi) number of occurrence of both sunrise effect and sunset effect is remark-ably smaller in monsoon. The positions of the sun and of atmospheric sources are jointly the causes of seasonal and diurnal variations. The missing of sunrise effect and sunset effect are due to local cloud activity and variation of electron density during geomagnetic storms.With 7 Figures     

Properties of spatial temperature derivatives in the atmospheric surface layer

A pair of parallel cold wires separated in either the vertical or lateral direction was used to obtain the three components _x, _y, _z of the temperature derivative in the streamwise, lateral and vertical directions, respectively. The average absolute skewness values of _x and _z are nonzero and approximately equal, while the skewness of _y is approximately zero. These results appear to be consistent with the presence of a large, three-dimensional organised structure in the surface layer. There is an apparent low-frequency contamination in the spectral density of _y and _z due mainly to small errors in estimating the sensitivity of the cold wires. The temperature derivatives were high-pass filtered, the filter being set to remove possible contributions from the large structure and to minimise low-frequency sensitivity contamination. The filtered rms ratios \~_x/\~_y and \~_x/\~_z were in the range 0.7 to 0.9, a result in qualitative agreement with that obtained in the laboratory boundary layer by Sreenivasan et al. (1977). The skewness of filtered _x or _z is negligible, consistent with local isotropy of small-scale temperature fluctuations and in support of the high wavenumber spectral isotropy discussed in Antonia and Chambers (1978).     

Vegetation responses and feedbacks to climate: a review of models and processes

Vegetation changes both in stationary and changing climates. Such changes can significantly affect hydrological and climate dynamics. Probabilistic, inferential, empirical, statistical, threshold, ecophysiological, and mechanistic vegetation models provide tools and ideas to construct coupled climate and vegetation schemes to study climate/vegetation feedbacks. Their logic is discussed, typical applications are presented, and their usefulness is assessed. Developing coupled climate and vegetation schemes also implies tackling scaling issues explicitly. Just as the Courant-Friedrichs-Lewy (CFL) criterion guarantees that information is not transferred faster through space than time in climate models, information should be transmitted fast enough in vegetation models for the landscape to register vegetation responses. To guarantee that this is the case, a migration criterion, or m criterion, is proposed. The CFL criterion and the m criterion set formal constraints on the design of coupled atmosphere and vegetation schemes. In particular, the ratio of climate and vegetation space scales should be approximately five orders of magnitude less than the ratio of climate and vegetation time scales.     

Quantifying the Impact of Global Climate Change on Potential Natural Vegetation

Impacts of climate change on vegetation are often summarized in biome maps, representing the potential natural vegetation class for each cell of a grid under current and changed climate. The amount of change between two biome maps is usually measured by the fraction of cells that change class, or by the kappa statistic. Neither measure takes account of varying structural and floristic dissimilarity among biomes. An attribute-based measure of dissimilarity (V) between vegetation classes is therefore introduced. V is based on (a) the relative importance of different plant life forms (e.g. tree, grass) in each class, and (b) a series of attributes (e.g. evergreen-deciduous, tropical-nontropical) of each life form with a weight for each attribute. V is implemented here for the most used biome model, BIOME 1 (Prentice, I. C. et al., 1992). Multidimensional scaling of pairwise V values verifies that the suggested importance values and attribute weights lead to a reasonable pattern of dissimilarities among biomes. Dissimilarity between two maps (V) is obtained by area-weighted averaging of V over the model grid. Using V, present global biome distribution from climatology is compared with anomaly-based scenarios for a doubling of atmospheric CO2 concentration (2 × CO2), and for extreme glacial and interglacial conditions. All scenarios are obtained from equilibrium simulations with an atmospheric general circulation model coupled to a mixed-layer ocean model. The 2 × CO2 simulations are the widely used OSU and GFDL runs from the 1980's, representing models with low and high climate sensitivity, respectively. The palaeoclimate simulations were made with CCM1, with sensitivity similar to GFDL. V values for the comparisons of 2 × CO2 with present climate are similar to values for the comparisons of the last interglacial and mid-Holocene with present climate. However, the two simulated 2 × CO2 cases are much more like each other than they are to the simulated interglacial cases. The largest V values were between the last glacial maximum and all other cases, including the present. These examples illustrate the potential of V in comparing the impacts of different climate change scenarios, and the possibility of calibrating climate change impacts against a palaeoclimatic benchmark.     

Recurrence statistics of concentration fluctuations in plumes within a near-neutral atmospheric surface layer

This study examines the statistical properties of the concentration derivative, , for a dispersing plume in a near-neutrally stratified atmospheric surface layer. Towards this goal, the probability density function (pdf) of , and the conditional pdf of given a fixed concentration level, , have been measured. These pdfs are found to be modeled well by a generalizedq-Gaussian (gqG) distribution with intermittency exponent,q, equal to 0.3 and 3/4, respectively. These results highlight the strong intermittency effect (patchiness) of the small-scale concentration eddy structures in the plume. The distribution of time intervals between successive high peaks in the squared derivative process, x², is found to be well approximated by a power-law distribution, implying that occurrences of these high peaks are much more clustered than would be predicted by a Poisson or shot-noise process. The results are used to improve models for the joint pdf of and , and for the expected number of upcrossings per unit time interval of a fixed concentration level that have been proposed by Kristensenet al. (1989). The predictions of the improved models are in accord with observations, and suggest that the intercorrelation between and must be explicitly incorporated if good estimates of the upcrossing intensity are to be obtained.     

Climate engineering a review of aerosol approaches to changing the global energy balance

As global greenhouse warming continues to intensify, it is likely that demands to employ technologies of climate engineering will become increasingly insistent. This paper addresses the possibility of canceling the radiative effects of the increasing greenhouse gases through solar reflectors. Two promising approaches, according to COSEPUP (1992), are the employment of aerosols in the stratosphere, directly as reflectors, or in the troposphere, for the seeding of clouds to increase cloud amounts and brightness. Besides technological and economic feasibility, such schemes could be relatively reversible, and describing their impact may be within the reach of future scientific study.The climate system is not yet sufficiently understood for such actions to be warranted. However, there is considerable potential for an increased understanding of what such actions might do through the study of the role of similar aerosols already added to the climate system. In particular, the most intense volcanoes (e.g. Pinatubo) supply the stratosphere with enough aerosol over a period of a year or two to cancel out greenhouse warming from a resulting doubling of carbon dioxide. Furthermore, the addition of sulfate aerosols to the troposphere from the burning of fossil fuel may already be canceling out globally up to half of the greenhouse-gas warming. These comparisons suggest that at least 10 times as much sulfate aerosol would be needed in the troposphere as would be needed in the stratosphere for a comparable climatic effect. A better understanding of the role of the already-present aerosols is a prerequisite for further progress in the use of aerosols for climate engineering. The links between the horizontal and vertical distribution of radiative sources and sinks and various atmospheric feedback processes, especially those related to the hydrological cycle and the consequent global and regional responses, are also needed.     

Outcome and value uncertainties in global-change policy

Choices among environmental policies can be informed by analysis of the potential physical, biological, and social outcomes of alternative choices, and analysis of social preferences among these outcomes. Frequently, however, the consequences of alternative policies cannot be accurately predicted because of substantial outcome uncertainties concerning physical, chemical, biological, and social processes linking policy choices to consequences. Similarly, assessments of social preferences among alternative outcomes are limited by value uncertainties arising from limitations of moral principles, the absence of economic markets for many environmental attributes, and other factors. Outcome and value uncertainties relevant to global-change policy are described and their magnitudes are examined for two cases: stratospheric-ozone depletion and global climate change. Analysis of information available in the mid 1980s, when international ozone regulations were adopted, suggests that contemporary uncertainties surrounding CFC emissions and the atmospheric response were so large that plausible ozone depletion, absent regulation, ranged from negligible to catastrophic, a range that exceeded the plausible effect of the regulations considered. Analysis of climate change suggests that, important as outcome uncertainties are, uncertainties about values may be even more important for policy choice.     

Global Warming Potentials: 1. Climatic Implications of Emissions Reductions

The use of Global Warming Potentials (GWPs) to calculate equivalent carbon dioxide emissions reductions in the climate change context is examined. We find that GWPs are accurate only for short time horizons. Over long time horizons their use implicitly leads to tradeoffs between near-term and long-term climate change. For one of the most policy-relevant cases, comparing reductions in methane and carbon dioxide, the long-term effect on climate of reducing methane emissions is relatively small, at variance with the common perception based on published GWP values.     

Spatial Variability of Turbulent Fluxes in the Roughness Sublayer of an Even-Aged Pine Forest

The spatial variability of turbulent flow statistics in the roughness sublayer (RSL) of a uniform even-aged 14 m (= h) tall loblolly pine forest was investigated experimentally. Using seven existing walkup towers at this stand, high frequency velocity, temperature, water vapour and carbon dioxide concentrations were measured at 15.5 m above the ground surface from October 6 to 10 in 1997. These seven towers were separated by at least 100m from each other. The objective of this study was to examine whether single tower turbulence statistics measurements represent the flow properties of RSL turbulence above a uniform even-aged managed loblolly pine forest as a best-case scenario for natural forested ecosystems. From the intensive space-time series measurements, it was demonstrated that standard deviations of longitudinal and vertical velocities (_u, _w) and temperature (_T) are more planar homogeneous than their vertical flux of momentum (u_* ²) and sensible heat (H) counterparts. Also, the measured H is more horizontally homogeneous when compared to fluxes of other scalar entities such as CO₂ and water vapour. While the spatial variability in fluxes was significant (>15 %), this unique data set confirmed that single tower measurements represent the canonical structure of single-point RSL turbulence statistics, especially flux-variance relationships. Implications to extending the moving-equilibrium hypothesis for RSL flows are discussed. The spatial variability in all RSL flow variables was not constant in time and varied strongly with spatially averaged friction velocity u_*, especially when u_* was small. It is shown that flow properties derived from two-point temporal statistics such as correlation functions are more sensitive to local variability in leaf area density when compared to single point flow statistics. Specifically, that the local relationship between the reciprocal of the vertical velocity integral time scale (I_w) and the arrival frequency of organized structures (/h) predicted from a mixing-layer theory exhibited dependence on the local leaf area index. The broader implications of these findings to the measurement and modelling of RSL flows are also discussed.     

Experiments On Buoyant Plume Dispersion In A Laboratory Convection Tank

Plume dispersion in the convective boundary layer (CBL) is investigated experimentally in a laboratory convection tank. The focusis on highly-buoyant plumes that loft near or become trapped in the CBL capping inversion and resistdownward mixing. Such plumes are defined by dimensionless buoyancy fluxes F_* 0.1, where F_* = F_b/(U w_* ² z_i), F_b is the stack buoyancy flux,U is the mean wind speed, w_* is the convective velocity scale, and z_i is the CBL depth. The aim is to obtain statistically-reliable mean (C) and root-mean-square (rms, _c) concentration fields as a function of F_* and the dimensionless distance X = w_*x/(U z_i), where x is the distance downstream of the source.The experiments reveal the following mainresults: (1) For 3 X 4and F_* 0.1, the crosswind-integrated concentration (CWIC) fields exhibit distinctly uniform profiles below z_i with a CWIC maximum aloft, in contrast to the nonuniform profiles obtained earlier by Willis and Deardorff. (2) The lateral dispersion (_y) variation with X is consistent with Taylor's theory for _* 0.1 and a buoyancy-enhanced dispersion, _y/z_i F_* ^1/3X^2/3, forF_* = 0.2 and 0.4. (3) The entrapment, the plume fraction above z_i, has a mean (E) that follows a systematic variationwith X and F_*, and a variability (_e/E) that is broad ( 0.3 to 2) near the source but subsides to 0.25 far downstream. (4) Vertical profiles of the concentration fluctuation intensity (c/C) are uniform for z < z_i and X > 1.5, but exhibit significant increases: (a) at the surface and close to the source (X 1.5), and(b) in the entrainment zone. (5) The cumulative distribution functions (CDFs) of the scaled concentration fluctuations (c/_c) separate into mixed-layer and entrainment-layer CDFs for X 2, with the mixed-layer group collapsing to a single distribution independent of z.These are the first experiments to obtain all components of the lateral and vertical dispersion parameters (rms meander, relative dispersion, total dispersion) for continuous buoyant releases in a convection tank. They also are the first tank experiments to demonstrate agreement with field observations of: (1) the scaled ground-level concentration along the plume centreline, and (2) the dimensionless lateral dispersion _y/z_i of buoyant plumes.