Effective parameters for surface heat fluxes in heterogeneous terrain

The relations between most land-surface characteristics and surface heat fluxes are typicallynon-linear. Because the ground surface is heterogeneous at all scales, it is important to accountfor these non-linear relations. EVective parameters are often applied for that purpose. Steady-state simulations were used in this paper to thoroughly analyse the eVective parameters impactunder a broad range of atmospheric conditions. The eVect of diVerent types of aggregatingfunctions on the accuracy of various eVective parameters is also examined. The authors foundthat linear averaging of leaf area index and soil water content gives higher latent and lowersensible heat fluxes than the corresponding flux averaging over all surface types existing in onesquare grid. Linear averaging of roughness length under unstable conditions provides higherlatent and lower sensible heat fluxes than flux averaging, whereas under stable conditions giveshigher sensible and lower latent heat fluxes. Non-linear functions result to be more useful thanlinear functions to compute the eVective value of those parameters which aVect the surface heatfluxes independently of the atmospheric stability (e.g., leaf area index and soil water content,and unlike roughness length)

Comparison of three land-surface schemes with the Fourier amplitude sensitivity test (FAST)

This paper explores which are the land-surface parameters playing a key role in three surface schemes, namely the land-atmosphere interactive dynamics (LAID), the interaction soil-biosphere-atmosphere (ISBA) and the biosphere-atmosphere transfer scheme (BATS). The Fourier amplitude sensitivity test (FAST) was used for that purpose. This test estimates the relative contribution of model input parameters to the variance of surface heat fluxes. This analysis demonstrates that, for the three considered schemes, four parameters can explain most of the variance of surface heat fluxes under a broad range of environmental conditions. Soil wetness plays a predominant roˆ le for the heat fluxes. Roughness length is the most important parameter for the momentum flux. Leaf area index, in vegetated land, and texture, mainly in bare land, also have a significant impact on the fluxes. Roughness length is usually more important for sensible heat flux than for latent heat flux, and is mostly important under stable atmospheric conditions. Soil wetness and vegetation parameters are the dominant parameters under buoyant conditions

Coupled land surface/hydrologic/atmospheric models

The topics covered include the following: prototype land cover characteristics data base for the conterminous United States; surface evapotranspiration effects on cumulus convection and implications for mesoscale models; the use of complex treatment of surface hydrology and thermodynamics within a mesoscale model and some related issues; initialization of soil-water content for regional-scale atmospheric prediction models; impact of surface properties on dryline and MCS evolution; a numerical simulation of heavy precipitation over the complex topography of California; representing mesoscale fluxes induced by landscape discontinuities in global climate models; emphasizing the role of subgrid-scale heterogeneity in surface-air interaction; and problems with modeling and measuring biosphere-atmosphere exchanges of energy, water, and carbon on large scales

Protecting climate with forests

Policies for climate mitigation on land rarely acknowledge biophysical factors, such as reflectivity, evaporation, and surface roughness. Yet such factors can alter temperatures much more than carbon sequestration does, and often in a conflicting way. We outline a framework for examining biophysical factors in mitigation policies and provide some best-practice recommendations based on that framework. Tropical projects-avoided deforestation, forest restoration, and afforestation-provide the greatest climate value, because carbon storage and biophysics align to cool the Earth. In contrast, the climate benefits of carbon storage are often counteracted in boreal and other snow-covered regions, where darker trees trap more heat than snow does. Managers can increase the climate benefit of some forest projects by using more reflective and deciduous species and through urban forestry projects that reduce energy use. Ignoring biophysical interactions could result in millions of dollars being invested in some mitigation projects that provide little climate benefit or, worse, are counter-productive

The climatic impacts of land surface change and carbon management, and the implications for climate-change mitigation policy

http://www.sciencedirect.com/science/journal/14693062Strategies to mitigate anthropogenic climate change recognize that carbon sequestration in the terrestrial biosphere can reduce the build-up of carbon dioxide in the Earth’s atmosphere. However, climate mitigation policies do not generally incorporate the effects of these changes in the land surface on the surface albedo, the fluxes of sensible and latent heat to the atmosphere, and the distribution of energy within the climate system. Changes in these components of the surface energy budget can affect the local, regional, and global climate. Given the goal of mitigating climate change, it is important to consider all of the effects of changes in terrestrial vegetation and to work toward a better understanding of the full climate system. Acknowledging the importance of land surface change as a component of climate change makes it more challenging to create a system of credits and debits wherein emission or sequestration of carbon in the biosphere is equated with emission of carbon from fossil fuels. Recognition of the complexity of human-caused changes in climate does not, however, weaken the importance of actions that would seek to minimize our disturbance of the Earth’s environmental system and that would reduce societal and ecological vulnerability to environmental change and variability

Conceptual aspects of a statistical‐dynamical approach to represent landscape subgrid‐scale heterogeneities in atmospheric models

Land‐surface characteristics play a key role in the partition of energy received at the Earth's surface and, as a result, have a major impact on the atmosphere. Yet the representation of land‐surface processes in atmospheric models is not realistic. Actual state‐of‐the‐art parameterizations of land surfaces do not account for the landscape heterogeneity found on the resolvable scale of these models and are based on a large amount of empirical constants that in practice can be difficult to estimate. An alternative parameterization based on a statistical‐dynamical approach is suggested here. With this approach, the most important characteristics of the soil‐plant‐atmosphere system that affect the partition of energy (e.g., plant stomatal conductance, soil humidity, surface roughness) would be represented by a probability density function (pdf) rather than by a single “representative” value. Typically, such pdf's are characterized by two to four parameters. A primary simplified version of this parameterization was used to estimate the land‐surface energy fluxes that are produced at the grid scale by various distributions of stomatal conductance under a broad range of environmental conditions. To demonstrate the potential of the approach, results were compared with the same fluxes calculated with a big leaf model using the mean stomatal conductance that corresponds to the distributions. Large absolute and relative differences are obtained between the two schemes for many combinations of stomatal conductance pdf's and environmental conditions. These differences are due mainly to the nonlinearity of the processes involved in the redistribution of the energy absorbed at the ground surface. These numerical experiments demonstrate the importance of accounting for landscape heterogeneity in the simulation of land‐surface energy fluxes, and demonstrate the potential benefits of adopting a statistical‐dynamical approach for the representation of land‐surface processes in atmospheric models

Measurement of Atmospheric Boundary Layer Turbulent Fluxes with the Duke University Helicopter Observation Platform (HOP) in Support of CHATS

Abstract : The main objective of the Canopy Horizontal Array Turbulence Studies (CHATS) was to provide the relevant data needed to improve the parameterization of sub-filter-scale (SFS) processes in the roughness sub-layer that is affected by canopy-atmosphere interactions. For that purpose, NCAR deployed in March-June 2007 an array of sonic anemometers at different heights and lateral separations in a homogeneous walnut orchard in the Central Valley of California so as to capture the influence of the wake-scale motions in the lee of vegetation. Together with this array, other instruments (including a high tower and a new eye-safe lidar) were operating during the field campaign

Lettenmaier receives 2000 Hydrology Section Award

Dennis P. Lettenmaier was awarded the Hydrology Section Award at the Fall Meeting Hydrology Section Reception, which was held on December 16, 2000, in San Francisco, California. The medal recognizes outstanding contributions to the science of hydrology