TY - JOUR
T1 - Executive summary of the Tucson Aggregation Workshop
AU - Michaud, Jene D.
AU - Shuttleworth, W. James
N1 - Funding Information:
Our assessment of workshop findings and recommendations is based on the papers and discussions which took place at the Tucson Aggregation Workshop, held in Tucson, Arizona, 23-24 March 1994. The contribution of workshop participants (approaching a hundred, far to numerous too mention by name) and of workshop speakers (Han Dolman, Karen Humes, Pave1 Kabat, Susan Moran, Joel Noilhan, Roger Pielke, Mike Raupach, Steve Running, Piers Sellers, and Eric Wood) to the content of this summary was therefore fundamental to its writing. Financial and administrative support for the Tucson Aggregation Workshop was provided by the Biological Aspects of Hydrologic Cycle (BAHC) Core Project of the International Geosphere-Biosphere Program (IGBP) and the Intema-tional Satellite Land Surface Climatology Program (ISLSCP) of the International World Climate Program. The workshop and, in major part, the subsequent publication of this Special Issue was further substantially supported by the US National Aeronautical and Space Administration (2485MD/BGE-001 l), under its Terrestrial Ecology and Modeling Program, which is under the direction of Diane Wickland.
Funding Information:
The effect of land surface heterogeneity on the atmosphere and on the surface energy balance has attracted widespread interest because understanding this effect is fundamental to a comprehensive knowledge of regional and global hydrometeorological processes. Moreover, many investigators are concerned that inadequate treatment of heterogeneity may weaken confidence in large-scale models which do not resolve heterogeneity at scales smaller than the model grid. Interest in heterogeneity has been spurred further by several technical advances, not the least of which is the availability of satellite data. Remote sensing technology offers high-resolution data to quantify regional and global heterogeneity and make areal-average measurements representing the effective areal-average value of surface parameters. Computational advances and increased interest in climate, and therefore in the modeling of land-atmosphere interactions, have also promoted interest in surface heterogeneity. ‘Aggregation’ generally refers to spatial averaging of some heterogeneous surface variable such as albedo, soil hydraulic properties, soil moisture, fraction of vegetation cover, surface temperature, surface reflectance, sensible heat flux, latent heat flux, surface resistance, aerodynamic resistance, or aspects of topography; or it refers to spatial averaging of some near-surface meteorological field such as temperature, humidity or precipitation. There is the question of how to ‘average’ (arithmetic or logarithmic being two of the possibilities), and the size of the region over which averaging should be performed. This size depends on the degree of heterogeneity and whether there is a causal relationship between the variable being averaged and the quantity to be calculated in the model. There is a possibility that aggregation will fail when a heterogeneous variable has a non-linear relationship with some other variable of interest. Moreover, aggregation strategies may be dependent on model formulation. Aggregation is a more limited enterprise than ‘scaling’, because scaling seeks to find a basis for relating a phenomenon at one scale to an analogous phenomenon at other scales. However, several papers in this issue do address scaling. Interest in the ‘aggregation problem’ is motivated largely either by the desire to make efficient use of highly resolved spatial data, or by the desire to proceed confidently without utilization of detailed data. In other words, it seeks to address the question, ‘How can we model spatially variable processes using a grid size which is coarse enough to be economical, yet fine enough that results are not affected by sub-grid-scale variability?’ However, the topic of aggregation is equally pertinent to the question of adequate spatial resolution of measurements; hence there is a need to investigate the effect of spatial resolution on the accuracy of remotely sensed measurements. In March 1994, a workshop was convened in Tucson, Arizona, to assess the state of the art in aggregation research. The workshop was sponsored jointly by the Biological Aspects of Hydrologic Cycle (BAHC) Core Project of the International Geosphere-Biosphere Program and the International Satellite Land Surface Climatology Project (ISLSCP) of the World Climate Research Program. In some part, the organization of the workshop, and in major part the subsequent publication of this was also supported by the US National Aeronautical and Space Administration (NASA), under its Mission to Planet Earth’s Terrestrial Ecology and Modeling Program.
PY - 1997/3/15
Y1 - 1997/3/15
N2 - 'Aggregation' refers to spatial averaging of some heterogeneous surface variable to obtain an effective value representative of an area. The effect of surface heterogeneity on interactions between land and atmosphere is relevant to near-surface hydrology, ecology, and climate, and is the common theme of the papers in this issue. Even though the full effect of heterogeneity must be neglected owing to limited spatial resolution of large-scale models, it is important to understand when and how the presence of heterogeneity requires recognition in any aggregate representation. In March 1994, a workshop, which has come to be known as the 'Tucson Aggregation Workshop', was convened to assess the state of the art in aggregation research, and the papers in this issue are the product of that workshop. The principal findings of the workshop can be summarized as follows: 1. substantial progress has been made in producing aggregated representations of flat terrain. Simple aggregation rules applied to surface properties have given rise, in some studies, to simulated surface energy fluxes that are within 10%, of fluxes produced from models with full representation of heterogeneity. 2. Aggregation rules are relatively straightforward in the case of patch scale heterogeneity (variability on the order of hundreds to thousands of meters) of vegetative characteristics which control surface exchanges, although aggregation of soil hydraulic properties and possibly of soil moisture remains problematic. In addition, some of the effects of meso-scale heterogeneity (variability on the order of 10-100 km) in surface cover will need to be addressed through more complicated types of parameterization. 3. There is convincing evidence that the regional energy balance (over, say, 105 km2) is insensitive to gentle topography, provided that surface vegetatiton and water availability are uniform, but in mountainous terrain the influence of topography on near-surface meteorology must be considered. 4. It appears that the value of simple combinations of remotely sensed radiances representing areal-average measurements are influenced only slightly by unresolved variability, although the averaging of some derived variables based on these radiances offers a greater challenge, especially with sparse canopies.
AB - 'Aggregation' refers to spatial averaging of some heterogeneous surface variable to obtain an effective value representative of an area. The effect of surface heterogeneity on interactions between land and atmosphere is relevant to near-surface hydrology, ecology, and climate, and is the common theme of the papers in this issue. Even though the full effect of heterogeneity must be neglected owing to limited spatial resolution of large-scale models, it is important to understand when and how the presence of heterogeneity requires recognition in any aggregate representation. In March 1994, a workshop, which has come to be known as the 'Tucson Aggregation Workshop', was convened to assess the state of the art in aggregation research, and the papers in this issue are the product of that workshop. The principal findings of the workshop can be summarized as follows: 1. substantial progress has been made in producing aggregated representations of flat terrain. Simple aggregation rules applied to surface properties have given rise, in some studies, to simulated surface energy fluxes that are within 10%, of fluxes produced from models with full representation of heterogeneity. 2. Aggregation rules are relatively straightforward in the case of patch scale heterogeneity (variability on the order of hundreds to thousands of meters) of vegetative characteristics which control surface exchanges, although aggregation of soil hydraulic properties and possibly of soil moisture remains problematic. In addition, some of the effects of meso-scale heterogeneity (variability on the order of 10-100 km) in surface cover will need to be addressed through more complicated types of parameterization. 3. There is convincing evidence that the regional energy balance (over, say, 105 km2) is insensitive to gentle topography, provided that surface vegetatiton and water availability are uniform, but in mountainous terrain the influence of topography on near-surface meteorology must be considered. 4. It appears that the value of simple combinations of remotely sensed radiances representing areal-average measurements are influenced only slightly by unresolved variability, although the averaging of some derived variables based on these radiances offers a greater challenge, especially with sparse canopies.
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U2 - 10.1016/s0022-1694(96)03126-5
DO - 10.1016/s0022-1694(96)03126-5
M3 - Article
AN - SCOPUS:0031104151
SN - 0022-1694
VL - 190
SP - 176
EP - 181
JO - Journal of Hydrology
JF - Journal of Hydrology
IS - 3-4
ER -