Roni Avissar

Trading Water for Carbon with Biological Carbon Sequestration

Carbon sequestration strategies highlight tree plantations without considering their full environmental consequences. We combined field research, synthesis of more than 600 observations, and climate and economic modeling to document substantial losses in stream flow, and increased soil salinization and acidification, with afforestation. Plantations decreased stream flow by 227 millimeters per year globally (52%), with 13% of streams drying completely for at least 1 year. Regional modeling of U.S. plantation scenarios suggests that climate feedbacks are unlikely to offset such water losses and could exacerbate them. Plantations can help control groundwater recharge and upwelling but reduce stream flow and salinize and acidify some soils.

A Parameterization of Heterogeneous Land Surfaces for Atmospheric Numerical Models and Its Impact on Regional Meteorology

Abstract Natural land surfaces are usually heterogeneous over the resolvable scales considered in atmospheric numerical models. Therefore, model surface parameterizations that assume surface homogeneity may fail to represent the surface forcing accurately. In this paper, a parameterization of the subgrid-scale forcing of heterogeneous land surfaces for atmospheric numerical models is suggested. In each surface grid element of the numerical model similar homogeneous land patches located at different places within the element are regrouped into subgrid classes. Then, for each one of the subgrid classes, a sophisticated micrometeorological model of the soil-plant-atmosphere system is applied to assess the surface temperature, humidity, and fluxes to the atmosphere. The global fluxes of energy between the grid and the atmosphere are obtained by averaging according to the distribution of the subgrid classes. In addition to the surface forcing, detailed micrometeorological conditions of the patches are assessed...

Mechanisms of long-distance dispersal of seeds by wind

Long-distance dispersal (LDD) is central to species expansion following climate change, re-colonization of disturbed areas and control of pests. The current paradigm is that the frequency and spatial extent of LDD events are extremely difficult to predict. Here we show that mechanistic models coupling seed release and aerodynamics with turbulent transport processes provide accurate probabilistic descriptions of LDD of seeds by wind. The proposed model reliably predicts the vertical distribution of dispersed seeds of five tree species observed along a 45-m high tower in an eastern US deciduous forest. Simulations show that uplifting above the forest canopy is necessary and sufficient for LDD, hence, they provide the means to define LDD quantitatively rather than arbitrarily. Seed uplifting probability thus sets an upper bound on the probability of long-distance colonization. Uplifted yellow poplar seeds are on average lighter than seeds at the forest floor, but also include the heaviest seeds. Because uplifting probabilities are appreciable (as much as 1–5%), and tree seed crops are commonly massive, some LDD events will establish individuals that can critically affect plant dynamics on large scales.

Influence of landscape structure on local and regional climate

This paper discusses the physical linkage between the surface and the atmosphere, and demonstrates how even slight changes in surface conditions can have a pronounced effect on weather and climate. Observational and modeling evidence are presented to demonstrate the influence of landscape type on the overlying atmospheric conditions. The albedo, and the fractional partitioning of atmospheric turbulent heat flux into sensible and latent fluxes is shown to be particularly important in directly affecting local and regional weather and climate. It is concluded that adequate assessment of global climate and climate change cannot be achieved unless mesoscale landscape characteristics and their changes over time can be accurately determined.

Evaluation of vegetation effects on the generation and modification of mesoscale circulations

Abstract The purpose of the present study is to evaluate (i) the effect of vegetated surfaces on modifying sea breeze and daytime thermally induced upslope flows, and (ii) the generation of thermally induced flow by vegetated areas contrasted by bare soil area. In order to address these objectives, the following tasks were carried out: 1) previous documented studies with implication for (i) and (ii) are reviewed; 2) the main features of the thermal balance of vegetated surfaces are outlined qualitatively; 3) a quantitative evaluation of the various components in the thermal balance based on documented observational studies is provided; and 4) scale analyses and numerical model simulations are used to provide quantitative evaluations of the circulations involved with (i) and (ii) for several illustrative cases. The study suggests that the impact of vegetated surfaces in those cases is highly dependent on the environmental conditions as well as vegetation characteristics. For ideal environmental conditions ...

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.

Representation of heterogeneity effects in Earth system modeling: Experience from land surface modeling

The land surface is characterized by pronounced spatial heterogeneity that spans a wide range of scales. This heterogeneity affects the surface energy and water budgets, as well as the land-atmosphere exchanges of momentum, heat, water and other constituents, through a number of highly nonlinear processes. The resolution of present-day Earth (or climate) system models is still too coarse to explicitly capture the effects of surface heterogeneity, which therefore needs to be parameterized within the framework of complex and nonlinear land surface process schemes. The effects of surface heterogeneity are here grouped in two categories, which we define as “aggregation” and “dynamical” effects. Models of aggregation effects attempt to calculate the contribution of different subgrid scale surface types to the grid box average energy and water budgets and surface-atmosphere exchanges. Such models have been based on discrete approaches, whereby heterogeneity is described in terms of a finite number of subgrid “tiles” or “patches,” and on continuous approaches, in which heterogeneity is described in terms of probability density functions. Subgrid scale aggregation has been shown to especially affect the surface latent and sensible heat fluxes, the simulation of snow, and the dynamics of soil moisture and runoff. Dynamical heterogeneity effects are associated with microscale and mesoscale circulations induced by heterogeneous surfaces. These circulations can influence boundary layer structure, cloud formation, precipitation, and vertical transfer of momentum, energy, and water up to the midtroposphere. In the last decade or so, the importance of land surface heterogeneity representation has been increasingly recognized in a large number of new studies. This paper reviews and critically discusses different approaches that have been proposed to represent aggregation and dynamical effects of surface heterogeneity and their incorporation in land surface process schemes. Some of the methodologies discussed in this paper are of general nature and therefore can be of interest for problems of subgrid scale process description in other geophysical disciplines.

Three-dimensional numerical study of shallow convective clouds and precipitation induced by land surface forcing

A state-of-the-art mesoscale atmospheric model was used to investigate the three-dimensional structure and evolution of shallow convective clouds and precipitation in heterogeneous and homogeneous domains. In general, the spatial distribution of clouds and precipitation is strongly affected by the landscape structure. When the domain is homogeneous, they appear to be randomly distributed. However, when the landscape structure triggers the formation of mesoscale circulations, they concentrate in the originally dry part of the domain, creating a negative feedback which tends to eliminate the landscape discontinuities, and spatially homogenize the land water content. The land surface wetness heterogeneity of the domain and the toted amount of water vapor present in the atmosphere (locally evapotranspired and/or advected) affect the precipitation regime. In general, the upward motion of mesoscale circulations generated by landscape heterogeneities is stronger than thermal cells induced by turbulence. Furthermore, their ability to transport moist, warm air to higher elevations increases the amount of water that can be condensed and precipitated. The evolution of shallow convective clouds and precipitation consists of a “build-up phase” during which turbulence is predominant and responsible for the moistening of the atmosphere. In heterogeneous domains, it is also responsible for the creation of horizontal pressure gradients leading to the generation of mesoscale circulations. This phase occurs during the morning hours. From about 1200 until 1600 LST, clouds develop and most of the precipitation is produced. This is the “active phase.” After 1600 LST, the horizontal thermal and pressure gradients, which fed the energy necessary to create and sustain the mesoscale circulations, gradually disappear. This is the “dissipation phase.” The differences and similarities obtained between three-dimensional and two-dimensional simulations were also studied. These simulations indicate that, unless the landscape presents a clear two-dimensional structure, the use of such a two-dimensional model is not appropriate to simulate this type of clouds and precipitation. Conversely, two-dimensional simulations can be confidently used, provided that the simulated domain presents a two-dimensional heterogeneity.

Global Hydroclimatological Teleconnections Resulting from Tropical Deforestation

Abstract Past studies have indicated that deforestation of the Amazon basin would result in an important rainfall decrease in that region but that this process had no significant impact on the global temperature or precipitation and had only local implications. Here it is shown that deforestation of tropical regions significantly affects precipitation at mid- and high latitudes through hydrometeorological teleconnections. In particular, it is found that the deforestation of Amazonia and Central Africa severely reduces rainfall in the lower U.S. Midwest during the spring and summer seasons and in the upper U.S. Midwest during the winter and spring, respectively, when water is crucial for agricultural productivity in these regions. Deforestation of Southeast Asia affects China and the Balkan Peninsula most significantly. On the other hand, the elimination of any of these tropical forests considerably enhances summer rainfall in the southern tip of the Arabian Peninsula. The combined effect of deforestation ...

Atmospheric Disturbances Caused by Human Modification of the Landscape

This study documents significant atmospheric effects over the U.S. central plains caused by human modification of the landscape. Using observations and an atmospheric model, it is shown here that diurnal, thermally induced circulations occur during summer over a 250 × 250 km region in Oklahoma and Kansas. Furthermore, it is shown that the driving force behind these circulations is the landscape heterogeneity resulting from differential land use patterns, that such atmospheric phenomena are characteristic of surfaces with this type of heterogeneity and not limited to infrequent days when unusual wind or other meteorological conditions prevail, and that the net effect of these motions is significant, not only locally, but also at the regional and global scales.