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A. Henderson-Sellers

Abstract

Canopy-plus-soil “big-leaf” models of the land surface now exist and are being incorporated in global climate models. These big-leaf models could be the basis for the incorporation of an interactive land biosphere into global models. However, their use depends upon satisfactory specification of the distribution of plants (and soils), and while such distributions can be obtained for the present-day, they must be manufactured for all other climatic scenarios. Thus, one prerequisite for the incorporation of an interactive land biosphere into global climate models is the successful prediction of the natural vegetation (or “climax” vegetation) for climatic scenarios such as the doubling of atmospheric carbon dioxide. (It is also necessary to predict agricultural land-use classes, but that is outside the scope of this paper.) A highly generalized (nine classes) grouping of Holdridge life zones has been used here as a means of investigating three-step “coupling” of a land-surface scheme into a global climate model. The investigation includes the derivation of nine “natural” ecosystems for the present day climate using temperature and precipitation derived from three experiments undertaken with the NCAR community climate model. These predictions differ from one another and both differ significantly from the prescribed classification groupings of ecosystem complexes used with one big-leaf, land-surface scheme—the Biosphere Atmosphere Transfer Scheme (BATS). On the other hand, these highly generalized groupings show relatively little sensitivity to the temperature changes induced by doubling atmospheric CO2 and even the inclusion of the precipitation disturbances in the doubled-CO2 scenario causes changes in the ecosystem “predictions” that are only of similar degree, or smaller, than the differences between sets of life-zone classes generated from present-day or doubled-CO2 climates. This result implies that the combined global field of “annual bio-temperature and precipitation” is not yet consistently predicted by GCMS, not that vegetation is likely to be insensitive to climate change.

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A. Henderson-Sellers

Abstract

A simple model of urban mixing height is developed and yields an equation for the height of the inversion h′ as a function of distance downwind into the city in the case of non-planar topography. Since many British cities are not situated on a flat terrain the consideration of the effect of topographical features may be of particular importance. The model, which is demonstrated to be valid for most times of the day, has been used in urban studies permitting rapid calculation of mixed-layer depths, ground-level concentrations and distribution of aerosols.

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A. Henderson-Sellers

Abstract

Layer cloud amounts for the months of January and July 1979 have been extracted from a compressed version of the U.S. Air Force's 3D-nephanalysis. These layer cloud amounts are discussed in the context of the total cloud amount climatology described by Hughes and Henderson-Sellers. Low cloud (≲2000 m) is the predominant cloud type in most regions. High cloud (≳6000 m) amounts are generally less than ∼10% and amounts are significantly smaller than the cirrus cloud amounts of London. It seems likely that some high level cloud has been incorrectly archived as middle level cloud as a result of: (i) the assumption of cloud emissivities of one in the retrieval algorithm and (ii) the latitudinally-invarient definition of the middle/high cloud boundary. There is a reasonable level of agreement between zonally averaged low level and total cloud amounts derived here and those of London. West coast stratus is correctly identified as low cloud but the polar cloud amounts exhibit a seasonal cycle opposite to that seen in traditional climatologies.

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A. Henderson-Sellers

Abstract

Land-surface schemes developed for incorporation into global climate models include parameterizations that are not yet fully validated and depend upon the specification of a large (20–50) number of ecological and soil parameters, the values of which are not yet well known. There are two methods of investigating the sensitivity of a land-surface scheme to prescribed values: simple one-at-a-time changes or factorial experiments. Factorial experiments offer information about interactions between parameters and are thus a more powerful tool. Here the results of a suite of factorial experiments are reported. These are designed (i) to illustrate the usefulness of this methodology and (ii) to identify factors important to the performance of complex land-surface schemes. The Biosphere-Atmosphere Transfer Scheme (BATS) is used and its sensitivity is considered (a) to prescribed ecological and soil parameters and (b) to atmospheric forcing used in the off-line tests undertaken. Results indicate that the most important atmospheric forcings are mean monthly temperature and the interaction between mean monthly temperature and total monthly precipitation, although fractional cloudiness and other parameters are also important. The most important ecological parameters are vegetation roughness length, soil porosity, and a factor describing the sensitivity of the stomatal resistance of vegetation to the amount of photosynthetically active solar radiation and, to a lesser extent, soil and vegetation albedos. Two-factor interactions including vegetation roughness length are more important than many of the 23 specified single factors. The results of factorial sensitivity experiments such as these could form the basis for intercomparison of land-surface parameterization schemes and for field experiments and satellite-based observation programs aimed at improving evaluation of important parameters.

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P. Irannejad and A. Henderson-Sellers

Abstract

The land surface water balance components simulated by 20 atmospheric global circulation models (AGCMs) participating in phase II of the Atmospheric Model Intercomparison Project (AMIP II) are analyzed globally and over seven Global Energy and Water Cycle Experiment Coordinated Enhanced Observing Period basins. In contrast to the conclusions from analysis of AMIP I, the results presented here suggest that the group average of available AGCMs does not outperform all individual AGCMs in simulating the surface water balance components. Analysis shows that the available reanalysis products are not appropriate for evaluation of AGCMs’ simulated land surface water components. The worst simulation of the surface water budget is in the Murray–Darling, the most arid basin, where all the reanalyses and seven of the AGCMs produce a negative surface water budget, with evaporation alone exceeding precipitation and soil moisture decreasing over the whole AMIP II period in this basin. The spatiotemporal correlation coefficients between observed and AGCM-simulated runoff are smaller than those for precipitation. In almost all basins (except for the two most arid basins), the spatiotemporal variations of the AGCMs’ simulated evaporation are more coherent and agree better with observations, compared to those of simulated precipitation. This suggests that differences among the AGCMs’ surface water budget predictions are not solely due to model-generated precipitation differences. Specifically, it is shown that different land surface parameterization schemes partition precipitation between evaporation and runoff differently and that this, in addition to the predicted differences in atmospheric forcings, is responsible for different predictions of basin-scale water budgets. The authors conclude that the selection of a land surface scheme for an atmospheric model has significant impacts on the predicted continental and basin-scale surface hydrology.

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G. Thomas and A. Henderson-Sellers

Abstract

The temporal and spatial scales that characterize surface hydrologic processes provide conceptual and practical difficulties to the development of parameterization schemes for incorporation into climate models. In particular, there is a requirement to develop process descriptions applicable to large areas but that can model (capture) day-to-day and even hour-to-hour temporal change We compare two recently proposed methods of simulating subgrid-scale heterogeneity in precipitation distribution. These schemes diverge significantly when the fractional areal extent of the precipitation falls below about 0.2. We have also examined two recently proposed parameterizations of surface hydrologic processes in the context of basin-scale data from the Hunter Valley in southeastern Australia. We find that although both models capture the predominant characteristics of the annual and monthly surface runoff adequately, the day-to-My variability in the observed flow requires a more explicit identification and treatment of the predominant runoff-generating processes.

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A. Henderson-Sellers and K. McGuffie

Abstract

The Burger distribution, which is characterized by two parameters, mean cloud amount and scale distance, is evaluated in terms of its usefulness for representing cloud amount frequency distributions, and as a means of rescaling these frequency distributions to areal extents of the sky other than those from which the input observations were derived. It is found that the Burger distribution performs almost as well (rms errors of ∼3% absolute frequency) as the beta distribution (rms errors of ∼2% absolute frequency) when the conventional method of calculating mean cloud amount is employed. The Burger distribution performs as well as the beta distribution when the calculation of mean cloud is correlated to take into account observing practice. The advantages of the Burger distribution include the prediction of nonzero values for clear and overcast conditions and the potential for areal extent of sky rescaling of cloud amount frequency distributions. It is found that scaling down (e.g., from conventional surface observations of the full-sky dome to a near-zenith view commensurate with a nadir satellite retrieval) is highly successful with rms errors similar to those obtained in fitting to the observations. However, scaling up from zenith to full-dome views is less successful with rms errors up to 11% (absolute frequency).

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K. McGuffie and A. Henderson-Sellers

Meteorological whole-sky photography can be traced back to just after the turn of the century. Capturing an objective and well-determined view of the cloud cover over the whole-sky dome has been one of the principal goals of subsequent developments. Three types of photographic systems have been devised: refracting, reflecting, and moving film systems. A moving film apparatus seems to have been the first to capture a whole-sky view, but the technology has not advanced far since then. Reflecting systems are the cheapest for do-it-yourself enthusiasts, but refracting systems are readily purchased. The problem of selecting the most useful method for projection of the sky onto the film has arisen many times in the last 70 yr. Although an equidistant projection system makes relative distance determination easier, cloud amount can be most readily determined from a photograph produced by an equal-area projection system. If such a system is not used, the grid superimposed on the image must correct for areal distortion. Recent literature describing the use of “fish-eye” lenses in forest and urban micrometeorology might benefit from cross-referencing with the meteorologists' problems reviewed here. For meteorological and climatalogical application, such as intercomparison with satellite-derived cloud amounts, it must be noted that the precise nature of lens projection for an automated system probably has a much smaller effect than the observer-perceived sky shape on conventional reports.

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N. A. Hughes and A. Henderson-Sellers

Sampling and averaging strategies are as significant an influence upon the resulting cloud climatologies as the resolution of the original cloud archives. An investigation of total cloud amount data, as represented by the U.S. Air Force 3-dimensional nephanalysis, illustrates the effects of temporal and spatial processing. Analysis of mean cloud amount as a function of the standard deviation provides a quantitative method for determining cloud size and assessing regional time series of variability. Careful definition of spatially and temporally homogeneous cloud climatology regions facilitates stratified sampling and could obviate the need for averaged data.

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A. Henderson-Sellers and N. A. Hughes

A one-year global total cloud amount climatology has been compiled from the U.S. Air Force's 3D-nephanalysis cloud archive. The derived cloud distributions are shown to be reliable and in good agreement with major features of the general circulation.

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