AGU - December 2018
AGU 2018
This was definitely an AGU that will be remembered. It was a novelty that AGU was at Washington and the whole “band” (Chuang, Jennifer, Joao, Laura, and Xuelin) was present with poster and oral presentations! Washington welcomed us with sunny cold days (“un frío que te …”) and a flu that hit a lot of participants, including our very own Jennifer and Chuang. With about 25,000 participants in total, it was impossible to follow all the potentially interesting/relevant presentations - but we tried our best. “Our” presentations went all well and here is a little summary of what we presented. On day 1, Jennifer described her analysis of the Shoreshop workshop. There is obviously a lot of interest around this project and hopefully the community will be able to organize a follow up. On day 2 and 3, Chuang, Xuelin and Joao presented posters on their more recent findings on ripples, edge waves and the NZ wave climate. One thing to note, the poster hall was HUGE (see the picture on the side) and each day there were thousands of posters on display. I decided to simply walk through the posters and stop to look at details in very few cases. There was just too much going on. On day 4, Laura presented her most recent work (finished the night before?) on tropical cyclones and I presented a comparison of machine learning techniques to predict shoreline change.
Overall, I have seen a lot of presentations. If only one could be picked as the most stimulating one, I would certainly choose the talk from Paul Carling titled Storm-wave development of shore-normal grooves (gutters) on a steep sandstone beach face (check his recent paper: https://www.sciencedirect.com/science/article/pii/S0272771418300556). I agree with Paul that groove formation must involve some fast and strong feedback between flow and morphology. But what controls the spacing of the pattern? I have no idea! Observations at other sites could help but it is definitely one of those puzzles that challenge our understanding of sediment transport and beyond.
Finally, AGU remains a great social event to meet old friends (who are indeed getting older) and discuss new projects. This is also what AGU is about. I thought AGU was big enough when it reached 10,000 participants and this time the number was more than doubled. I begin to feel that we are beyond saturation. It was totally impossible to follow everything going on. The list of presentations I missed is longer than that of those I attended. Still, it is incredible how smoothly the conference ran considering the number of people involved.
See you all next year in San Francisco (where pizza and jazz are way better than in Washington). Happy 2019.
Giovanni
Contact: Giovanni Coco at g.coco@auckland.ac.nz
Altimeter based wave height correction - September 2018
Directional correction of modeled sea and swell wave heights using satellite altimeter data
Last September, I published an article about a new technique that uses satellite altimeter data to correct wave height from hindcasts according to the wave direction and type, e.g., sea or swell.
Recent hindcasts can split the wave spectra information into many partitions, one for windsea waves and the remaining for swell waves, that can be present in a sea state. This approach prevents the loss of information that occurs in most of the available hindcasts which contain only a bulk representation (i.e., an aggregated wave height and one mean direction) of the wave spectra. As no correction was available for partitioned hindcasts, we developed this methodology and compared the correction results with the results obtained from a standard (bulk) correction. The highlights of the proposed technique are:
- The correction is buoy independent, based only on satellite altimeter data.
- The technique corrects simultaneous sea and swell partitions present in a sea state.
- The correction for sea and swell waves changes according to the wave direction.
- The proposed correction outperforms a similar technique applied for bulk sea states.
- The technique helped identifying systematic errors of the wave hindcast.
- The method could be applied worldwide to correct any partitioned hindcast.
We are currently performing another assessment of the correction by downscaling the corrected and non-corrected offshore waves and comparing them with the nearshore wave buoys available in New Zealand. The article entitled "Directional correction of modeled sea and swell wave heights using satellite altimeter data" (Albuquerque, J., Antolínez, J. A. A., Rueda A., Méndez, J. F. and Coco, G., 2018, Directional correction of modeled sea and swell wave heights using satellite altimeter data, September 2018, Ocean Modelling 131, DOI: 10.1016/j.ocemod.2018.09.001) can be found in the link below.
Stay tuned as we will soon make the hindcasts available online for the whole of New Zealand!
Contact: João Albuquerque at j.dealbuquerque@auckland.ac.nz
PIV-V3V Experiments with fixed ripple bedforms
Wave-generated ripples are bedforms characterized wavelength and height of the order of centimeters. Large areas of ripples often contain pattern irregularities and defects (e.g., terminations, bifurcations and doubling) that impact fluid flow structure and lead to the formation of vortices and turbulence attenuation in the near bottom region. Flow structure is controlled by both neighboring and nonlocal bedforms as a result of sheltering effects. Vortices and other coherent structures result in high shear stress and lift forcing which play a key role in sediment suspension, sediment transport, ripple migration and other related nonlinear dynamics (e.g., lateral linking, merge, splitting, defect creation and repulsion). Neglecting these dynamics can result in significant underestimation of sediment suspension, sediment transport, ripple geometry, and ripple development. However, the flow structure over different transient ripple geometry is still not well understood.
We use 3D volumetric Particle Image Velocimetry (3DPIV) in a laboratory setup using 3D-printed bedforms (with various defect geometries, and both flat and rough surfaces) to study turbulence and coherent flow structures. The results shed light on the sheltering effect by the neighboring ripples and the role of bed roughness, with significant implications for the morphodynamics of ripples.
Contact: Chuang Jin at cjin987@aucklanduni.ac.nz
Shoreshop - June 2018
A wide range of numerical models are implemented in the prediction of the shoreline evolution and few comparative studies exist to assess their relative strengths and weaknesses. We devised a non-competition (Shorecast) and subsequent workshop (Shoreshop 2018) to bridge this gap.
In February 2018 we invited world experts in numerical modelling of shoreline evolution to participate to a non-competitive competition: Shorecast. The competition was centred on a question: Can you predict shoreline evolution at Tairua beach for the period 2014-2016?
We provided scientists with 14 years (1999-2013) of daily shoreline evolution from video images and wave/tide characteristics from numerical modelling of Tairua beach. We asked them to predict shoreline evolution for the period 2014-2016. For that period, we only provided the wave characteristics and actual shoreline evolution was not given. Subsequently, participants were not biased by knowing the actual answer and were, therefore, unable to tune/fit the model. We also asked scientists to predict the evolution of Tairua beach until 2100 but discussed these predictions in purely qualitative way.
Contributors from 18 institutions convened on June 6-8 at the University of Auckland to present and discuss the “blind” predictions for Tairua beach. Models were subjected to intense scrutiny and discussions ranged from the details of modelling techniques to the role of climate models in long term shoreline prediction. Interestingly, almost all models displayed evident agreement with the observations but they also indicated shortcomings that need to be addressed. These shortcomings become more evident when attempting to predict over a longer temporal horizon (2100).
The shoreshop initiative does not end with the workshop. The interest raised has gone well beyond the initial expectations and future shoreshops are planned.
The workshop was sponsored by the Hazard Hub (University of Auckland) and has been organized within the framework of the GNS-Hazard Platform project: Climate change impacts on coastal hazards. The shorecast competition was made possible thanks to the support and data sharing from Waikato Regional Council, MetOcean, NIWA, and the University of Waikato.
Contact: Jennifer Montaño at jmon177@aucklanduni.ac.nz