On the narrative form of simulations.

Understanding complex physical systems through the use of simulations often takes on a narrative character. That is, scientists using simulations seek an understanding of processes occurring in time by generating them from a dynamic model, thereby producing something like a historical narrative. This paper focuses on simulations of the Diels-Alder reaction, which is widely used in organic chemistry. It calls on several well-known works on historical narrative to draw out the ways in which use of these simulations mirrors aspects of narrative understanding: Gallie for "followability" and "contingency"; Mink for "synoptic judgment"; Ricoeur for "temporal dialectic"; and Hawthorn for a related dialectic of the "actual and the possible". Through these reflections on narrative, the paper aims for a better grasp of the role that temporal development sometimes plays in understanding physical processes and of how considerations of possibility enhance that understanding

Calibrating AGN Feedback in Clusters

Whether caused by AGN jets, shocks, or mergers, the most definitive evidence for heating in cluster cores comes from X-ray spectroscopy. Unfortunately such spectra are essentially limited to studying the emission spectrum from the cluster as a whole. However since the same underlying emission measure distribution produces both the observed CCD and RGS spectra, X-ray imaging can still provide spatial information on the heating process. Using Chandra archival data for a sample of 9 clusters, we demonstrate how imaging data can be used to constrain departures from a canonical, isobaric cooling flow model as a function of position in a given cluster. The results of this analysis are also shown for the deep archival exposure of the Perseus cluster. Such heating maps can provide constraints on both the location and magnitude of the heating in the cores of clusters. When combined with detections and spectral index maps from low-frequency radio observations, these maps can be used to distinguish between different models for heating in these objects.Comment: 4 pages, 2 figures; to appear in the proceedings of "The Monsters' Fiery Breath", Madison, Wisconsin 1-5 June 2009, Eds. Sebastian Heinz & Eric Wilcot

Convergence Analysis of the Fast Subspace Descent Methods for Convex Optimization Problems

The full approximation storage (FAS) scheme is a widely used multigrid method for nonlinear problems. In this paper, a new framework to design and analyze FAS-like schemes for convex optimization problems is developed. The new method, the Fast Subspace Descent (FASD) scheme, which generalizes classical FAS, can be recast as an inexact version of nonlinear multigrid methods based on space decomposition and subspace correction. The local problem in each subspace can be simplified to be linear and one gradient descent iteration (with an appropriate step size) is enough to ensure a global linear (geometric) convergence of FASD.Comment: 33 page

Bioeconomic meta-modelling of Indonesian agroforests as carbon sinks

In many areas of developing countries, economic and institutional factors often combine to give farmers incentives to clear forests and repeatedly plant food crops without sufficiently replenishing the soils. These activities lead to large-scale land degradation and contribute to global warming through the release of greenhouse gases into the atmosphere. We investigate whether agroforestry systems might alleviate these trends when carbon-credit payments are available under the Clean Development Mechanism of the Kyoto Protocol. A meta-modelling framework is adopted, comprising an econometric-production model of a smallholding in Sumatra. The model is used within a dynamic-programming algorithm to determine optimal combinations of tree/crop area, tree-rotation length, and firewood harvest. Results show the influence of soil-carbon stocks and discount rates on optimal strategies and reveal interesting implications for joint management of agriculture and carbon.bio-economic meta-modelling, Indonesia, agroforestry, carbon credits, dynamic programming, Environmental Economics and Policy, Resource /Energy Economics and Policy,

Mascons as structural relief on a lunar Moho

Mascons as structural relief on lunar Moh

Optimal Land-Use Decisions in the Presence of Carbon Payments and Fertilizer Subsidies: An Indonesian Case Study

The Clean Development Mechanism of the Kyoto Protocol provides the opportunity for smallholders to receive financial rewards for adopting tree-based systems that are sustainable. In this paper a meta-model is developed to simulate interactions between trees, crops and soils under a range of management regimes for a smallholding in Sumatra. The model is used within a dynamic-programming algorithm to determine optimal tree/crop areas, tree-rotation lengths, firewood-harvest and fertilizer application rates for a landholder faced with deteriorating land quality and opportunities to receive carbon credits and fertiliser subsidies. It is found th at profit maximising management strategies depend on initial soil quality. For example, incentives to participate in carbon projects only exist when the soil is degraded because the opportunity cost of the forgone crop production is low. Also, when soil-carbon stocks are low only trees should be grown and residues added to the soil to increase carbon stocks until a threshold level is reached, when it becomes optimal to switch to a steady-state system of crops with fertiliser. In this case, tree rotation lengths depend on carbon and fertiliser prices; where increases in these prices decrease the opportunity cost of growing trees and allow for longer rotations. If, however, the initial soil-carbon stock is high, the profit-maximising strategy is to grow only crops and use fertiliser, which initially depletes the soil of carbon until a steady state is reached and maintained.Land Economics/Use,