How dynamical models can learn from the data��an example with a simplified ENSO model

Most dynamical models of the natural system contain a number of empirical parameters which reflect our limited understanding of the simulated system or describe unresolved subgrid-scale processes. While the parameterizations basically introduce some uncertainty to the model results, they also hold the prospect of tuning the model. In general, a deterministic tuning is related to an inversion of the model which is often impossible or requires considerable computing effort for most climate models. Another way to adjust the model parameters to a specific observed process is stochastic fitting where a set of parameters and model output are taken as random variables. Here, we present a dynamical?Cstatistical approach with a simplified model of the El Ni?o?CSouthern Oscillation (ENSO) cycle whose parameters are adjusted to simulated and observed data by means of Bayesian statistics. As ENSO model, we employ the Schop?CSuarez delay oscillator model. Monte Carlo experiments highlight the large sensitivity of the model results to varied model parameters and initial values. The statistical adjustment is done by Bayesian model averaging of the Monte Carlo experiments. Applying the method to simulated data, the posterior ensemble mean is much closer to the reference data than the prior ensemble mean. The learning effect of the model is evident in the leading empirical orthogonal functions and statistically significant in the mean state. When the method is applied to the observed ENSO time series, the ENSO model in its classical setup is not able to account for the temporally varying periodicity of the observed ENSO phenomenon. An improved setup with continuous adjustment periods and extended parameter range is developed in order to allow the model to learn from the data gradually. The improved setup leads to promising results during the twentieth century and even a weak forecast skill over 6?months. Thus, the described method offers a promising tool for data assimilation in dynamical weather and climate models. However, the simplified ENSO model is barely appropriate for operational ENSO forecasts owing to its limited physical complexity.     

The potential effects of climate change on malaria transmission in Africa using bias-corrected regionalised climate projections and a simple malaria seasonality model

Climatic conditions such as relatively cold temperatures and dryness are able to limit malaria transmission. Climate change is therefore expected to alter malaria spread. A previous assessment of the potential impacts of climate change on the seasonality of malaria in Africa is revisited. Bias-corrected regional climate projections with a horizontal resolution of 0.5° are used from the Regional Model (REMO), which include land use and land cover changes. The malaria model employed is the climate-driven seasonality model (MSM) from the Mapping Malaria Risk in Africa project for which a comparison with data from the Malaria Atlas Project (MAP) and the Liverpool Malaria Model (LMM), and a novel validation procedure lends more credence to results. For climate scenarios A1B and B1 and for 2001–2050, REMO projects an overall drying and warming trend in the African malaria belt, that is largely imposed by the man-made degradation of vegetation. As a result, the malaria projections of the MSM show a decreased length of the malaria season in West Africa. The northern Sahel is no more longer suitable for malaria in the projections and shorter malaria seasons are expected for various areas farther south. In East Africa, higher temperatures and nearly unchanged precipitation patterns lead to longer transmission seasons and an increase in highland malaria. Assuming constant population numbers, an overall increase in person-months of exposure of up to 6 % is found. The results of this simple seasonality model are similar to previous projections from the more complex LMM. However, a different response to the warming of highlands is found for the two models. It is concluded that the MSM is an efficient tool to assess the climate-driven malaria seasonality and that an uncertainty analysis of future malaria spread would benefit from a multi-model approach.     

Numerical simulations for the non-linear Molodensky problem

We present a boundary element method to compute numerical approximations to the non-linear Molodensky problem, which reconstructs the surface of the Earth from the gravitational potential and the gravity vector. Our solution procedure solves a sequence of exterior oblique Robin problems and is based on a Nash-Hörmander iteration. We apply smoothing with the heat equation to overcome a loss of derivatives in the surface update. Numerical results show the error between the approximation and the exact solution in a model problem.     

Fluid venting activity on the Costa Rica margin: new results from authigenic carbonates

Carbonate precipitates on mounds and along tectonic scarps off the Costa Rica margin are manifestations of subduction-induced dewatering. The long-term dewatering history is recorded in mineralogical, petrological and isotope signals of carbonates recovered from these sites. The carbonates are strongly depleted in ¹³C (–11 to –53 PDB) and enriched in ¹⁸O (+4 to +8 PDB). Thermogenic methane and biogenic methane were identified as sources of the carbon. Chemoherm carbonates and seepage-associated carbonates formed in a focused flow regime have lighter ¹³C values, while others formed in a more diffusive flow regime have slightly enriched C isotope values. Three fluid components were inferred based on the calculation of equilibrium ¹⁸O: clay dehydration water, gas hydrate water and seawater. Calculated equilibrium ¹⁸O values of carbonates from different down-core depths as well as from different precipitation stages show that the ¹⁸O of the precipitating fluid is progressively depleted with time. Dolostones showing a methane-C source and a well constrained O-isotope signature are thought to have formed at depth in the sediment and subsequently became exhumed. Glauconitic sandstones cemented by methane-derived carbonate provide evidence that fluid and solid material have been expelled by the mud volcano.     

Sedimentation in the floodplains of the Mekong Delta,Vietnam Part II: deposition and erosion

Deposition and erosion play a key role in the determination of the sediment budget of a river basin, as well as for floodplain sedimentation. Floodplain sedimentation, in turn, is a relevant factor for the design of flood protection measures, productivity of agro‐ecosystems, and for ecological rehabilitation plans. In the Mekong Delta, erosion and deposition are important factors for geomorphological processes like the compensation of deltaic subsidence as well as for agricultural productivity. Floodplain deposition is also counteracting the increasing climate change induced hazard by sea level rise in the delta. Despite this importance, a sediment database of the Mekong Delta is lacking, and the knowledge about erosion and deposition processes is limited. In the Vietnamese part of the Delta, the annually flooded natural floodplains have been replaced by a dense system of channels, dikes, paddy fields, and aquaculture ponds, resulting in floodplain compartments protected by ring dikes. The agricultural productivity depends on the sediment and associated nutrient input to the floodplains by the annual floods. However, no quantitative information regarding their sediment trapping efficiency has been reported yet. The present study investigates deposition and erosion based on intensive field measurements in three consecutive years (2008, 2009, and 2010). Optical backscatter sensors are used in combination with sediment traps for interpreting deposition and erosion processes in different locations. In our study area, the mean calculated deposition rate is 6.86 kg/m² (≈ 6 mm/year). The key parameters for calculating erosion and deposition are estimated, i.e. the critical bed shear stress for deposition and erosion and the surface constant erosion rate. The bulk of the floodplain sediment deposition is found to occur during the initial stage of floodplain inundation. This finding has direct implications on the operation of sluice gates in order to optimize sediment input and distribution in the floodplains. Copyright © 2013 John Wiley & Sons, Ltd.     

Meteorites from the United Arab Emirates: Description,weathering, and terrestrial ages

We report on the first meteorite search campaign in the United Arab Emirates (UAE). The geology and proximity of our search region suggest that it is the north‐western extension of the Oman meteorite fields. We found 26 ordinary chondrites, bringing the total number of official meteorites from the UAE to 28. The campaign was organized and conducted in close cooperation with the UAE government and the main masses of the meteorites remained in the country where they will become part of an exhibition. The bulk composition of five meteorite and three soil samples indicates an uptake of U, Mo, Sr, Ba, Li, and Pb from the soil into the meteorites during terrestrial weathering. Terrestrial ages determined from ¹⁴C decay of 21 meteorites range from recent falls to 24.4 ka, with two meteorites having >37 ka and approximately 39 ka, respectively. Weak correlations between weathering degree, meteorite bulk chemical composition, and terrestrial age suggest highly localized weathering conditions, possibly related to abundant occurrences of sabkhas in the search region.