Joanna M. Nield

Modelling vegetated dune landscapes

This letter presents a self-organising cellular automaton model capable of simulating the evolution of vegetated dunes with multiple types of plant response in the environment. It can successfully replicate hairpin, or long-walled, parabolic dunes with trailing ridges as well as nebkha dunes with distinctive deposition tails. Quantification of simulated landscapes with eco-geomorphic state variables and subsequent cluster analysis and PCA yields a phase diagram of different types of coastal dunes developing from blow-outs as a function of vegetation vitality. This diagram indicates the potential sensitivity of dormant dune fields to reactivation under declining vegetation vitality, e.g. due to climatic changes. Nebkha simulations with different grid resolutions demonstrate that the interaction between the (abiotic) geomorphic processes and the biological vegetation component (life) introduces a characteristic length scale on the resultant landforms that breaks the typical self-similar scaling of (un-vegetated) bare-sand dunes.

Estimating aerodynamic roughness over complex surface terrain

[1] Surface roughness plays a key role in determining aerodynamic roughness length (zo) and shear velocity, both of which are fundamental for determining wind erosion threshold and potential. While zo can be quantified from wind measurements, large proportions of wind erosion prone surfaces remain too remote for this to be a viable approach. Alternative approaches therefore seek to relate zo to morphological roughness metrics. However, dust-emitting landscapes typically consist of complex small-scale surface roughness patterns and few metrics exist for these surfaces which can be used to predictzofor modeling wind erosion potential. In this study terrestrial laser scanning was used to characterize the roughness of typical dust-emitting surfaces (playa and sandar) where element protrusion heights ranged from 1 to 199mm, over which vertical wind velocity profiles were collected to enable estimation of zo. Our data suggest that, although a reasonable relationship (R 2 >0.79) is apparent between 3-D roughness density and zo, the spacing of morphological elements is far less powerful in explaining variations in zo than metrics based on surface roughness height (R 2 >0.92). This finding is in juxtaposition to wind erosion models that assume the spacing of larger-scale isolated roughness elements is most important in determining zo. Rather, our data show that any metric based on element protrusion height has a higher likelihood of successfully predicting zo. This finding has important implications for the development of wind erosion and dust emission models that seek to predict the efficiency of aeolian processes in remote terrestrial and planetary environments.

Coupling leeside grainfall to avalanche characteristics in aeolian dune dynamics

Avalanche (grainflow) processes are fundamental drivers of dune morphodynamics and are typically initiated by grainfall accumulations. In sedimentary systems, however, the dynamism between grainfall and grainflow remains unspecified because simple measurements are hampered by the inherent instability of lee slopes. Here, for the first time, terrestrial laser scanning is used to quantify key aspects of the grainfall process on the lee (slip face) of a barchan sand dune. We determine grainfall zone extent and flux and show their variability under differing wind speeds. The increase in the downwind distance from the brink of peak grainfall under stronger winds provides a mechanism that explains the competence of large avalanches to descend the entire lee slope. These findings highlight important interactions between wind speed, grainfall, and subsequent grainflow that influence dune migration rates and are important for correct interpretation of dune stratigraphy.

The dynamism of salt crust patterns on playas

Playas are common in arid environments and can be major sources of mineral dust that can influence global climate. These landforms typically form crusts that limit evaporation and dust emission, modify surface erosivity and erodibility, and can lead to over prediction or underprediction of (1) dust-emission potential and (2) water and heat fluxes in energybalance modeling. Through terrestrial laser scanning measurements of part of the Makgadikgadi Pans of Botswana (a Southern Hemisphere playa that emits significant amounts of dust), we show that over weeks, months, and a year, the shapes of these surfaces change considerably (ridge thrusting of >30 mm/week) and can switch among continuous, ridged, and degraded patterns. Ridged pattern development changes the measured aerodynamic roughness of the surface (as much as 3 mm/week). The dynamic nature of these crusted surfaces must be accounted for in dust entrainment and moisture balance formulae to improve regional and global climate models.

Surface moisture−induced feedback in aeolian environments

Aeolian dune development is influenced by feedback between surface properties and sediment transport, yet little is known about the larger scale temporal and spatial natures of this relationship. Surface moisture is particularly influential, and is generally recognized in aeolian environments for its ability to increase the critical shear velocity required to entrain sediment in beach settings or, alternatively, to sustain vegetation and stabilize surfaces at a dune-field scale. However, conceptual models and field work have alluded to its importance in protodune initiation, while field observations infer that seasonal moisture input may contribute to residual dune ridge formation at the dune-field scale. This has the potential to reveal geomorphic adaptation to variations in climate, and identify a recognizable signature in the rock record. This article presents a simulation model that produces geomorphological features similar to field observations and is capable of examining the implications of surface and transport feedback at both scales. Results (1) reveal the control of surface moisture at different temporal scales, (2) display complexity in the development of multiple spatial scales within a cellular automaton framework, (3) highlight the importance of transient sand strips and sediment supply frequency in aeolian transport dynamics and protodune development, and (4) explore the relationship and significance of feedback duration, development time, and bedform spatial scale in the development of incipient dunes. This study illustrates the importance of considering geomorphic feedback when assessing the influence of surface moisture in aeolian process–dominated systems.

Are visibility-derived AOT estimates suitable for parameterizing satellite data atmospheric correction algorithms?

Aerosol optical thickness (AOT) is an important parameter in radiative transfer models (RTMs) used for atmospheric correction of remotely-sensed data. It is often estimated from horizontal visibility measurements by use of the Koschmieder formula or other related methods built into RTMs. This article assesses the accuracy of this estimation, in the context of atmospheric correction, by comparing AERONET AOT data with AOT estimates from UK Met Office visibility data at a site in Hampshire, UK. Root mean square errors are calculated for a number of visibility categories (0–10, 10–20, 20–30, and 30–40 km) and are found to be high for all visibilities (ranging from half to more than double the mean AOT for each category). For all visibilities km, these errors are significantly higher than those from other AOT estimation methods. Simulations performed with the 6S RTM show that the effect of these errors on satellite-level radiances are large (up to 36 ), and the change in vegetation indices (normalized difference vegetation index and atmospherically resistant vegetation index) is smaller, but still significant. It is recommended that estimations of AOT based upon visibility measurements are only used if no alternatives are available, and that great caution is used when estimation is performed for visibilities km.

Spatial variability of the atmosphere over southern England, and its effect on scene-based atmospheric corrections

Earth observation data acquired in the optical region require atmospheric correction before they can be used quantitatively. Most operational methods of atmospheric correction assume that the atmospheric properties are uniform across the image, but this assumption is unlikely to be valid for large images. This study aims to characterize the spatial variation in atmospheric properties over a typical mid-latitude area (southern England), and to assess the errors that would result from applying a scene-based atmospheric correction to data collected under this variable atmosphere. Two key atmospheric properties – aerosol optical thickness (AOT) and precipitable water content (PWC) – are assessed over two clear days in June 2006, and results show an AOT range of approximately 0.1–0.5 and a PWC range of 1.5–3.0 cm. Radiative transfer modelling shows that errors in reflectance of up to 1.7 percentage points, and up to a 5% change in normalized difference vegetation index, can be caused by AOT variability, but that PWC variability has minimal effects. Sensitivity analyses also show that the high uncertainty of many data sources used to provide AOT values for atmospheric correction may also lead to significant errors in the resulting products. The spatial variability of the atmosphere cannot be ignored, and we are in need of operational, generic methods to perform a spatially variable atmospheric correction.

Early-stage aeolian protodunes: bedform development and sand transport dynamics

Early-stage aeolian bedforms, or protodunes, are elemental in the continuum of dune development and act as essential precursors to mature dunes. Despite this, we know very little about the processes and feedback mechanisms that shape these nascent bedforms. Whilst theory and conceptual models have offered some explanation for protodune existence and development, until now, we have lacked the technical capability to measure such small bedforms in aeolian settings. Here, we employ terrestrial laser scanning to measure morphological change at the high frequency and spatial resolution required to gain new insights into protodune behaviour. On a 0.06 m high protodune, we observe vertical growth of the crest by 0.005 m in two hours. Our direct measurements of sand transport on the protodune account for such growth, with a reduction in time-averaged sediment flux of 18% observed over the crestal region. Detailed measurements of form also establish key points of morphological change on the protodune. The position on the stoss slope where erosion switches to deposition is found at a point 0.07 m upwind of the crest. This finding supports recent models that explain vertical dune growth through an upwind shift of this switching point. Observations also show characteristic changes in the asymmetric cross section of the protodune. Flow-form feedbacks result in a steepening of the lee slope and a decline in lower stoss slope steepness (by 3°), constituting a reshaping of protodune form towards more mature dune morphology. The approaches and findings applied here, a) demonstrate an ability to quantify processes at requisite spatial and temporal scales for monitoring early-stage dune evolution, b) highlight the crucial role of form-flow feedbacks in enabling early-stage bedform growth, alluding to a fluctuation in feedbacks that require better representation in dune models, and c) provide a new stimulus for advancing understanding of aeolian bedforms.

A prospectus for future geomorphological investigation of the Namib Sand Sea

The Namib Sand Sea in southern Africa offers an ideal location in which to consider general questions about the evolution of sand seas, about the fluxes of sand through contemporary dune fields and about the patterns of dune form that are created. This paper aims to provide a concise account of the approaches and techniques that are currently being used and will be used in the future to address these questions. The paper considers the techniques employed to investigate wind climate, the morphometry of the dunes, the internal structure of dune sediments, the age of the dunes and the potential to model dune development.

Breakwater morphological modelling: predicting equilibrium morphologies using entropy based techniques

In order to design suitable breakwaters for beach protection, morphological modelling is required. This modelling can be utilised to determine the effectiveness of different structures and the position that sand is likely to move to when a stable, equilibrium morphology develops. The majority of morphological models currently used are time-step based. They involve the interaction of wave, current, sediment transport and sediment balance modules. Sediment is transported over a small amount of time, after which the modules are updated and sediment transport calculations begin again. In this paper a new method is put forward, where the need to time step is avoided. Different morphologies are compared directly, based on an objective function, where morphologies closer to the desired equilibrium have a smaller net sediment transport. Morphologies are modified using global optimisation techniques. This new method is able to predict sediment deposition in the lee of detached shore parallel breakwaters.