105 research outputs found
Synergy of wind wave model simulations and satellite observations during extreme events
In this study, the quality of wave data provided by the
new Sentinel-3A satellite is evaluated and the sensitivity of the wave model
to wind forcing is tested. We focus on coastal areas, where altimeter data
are of lower quality and wave modelling is more complex than for the open
ocean. In the first part of the study, the sensitivity of the wave model to
wind forcing is evaluated using data with different temporal and spatial
resolution, such as ERA-Interim and ERA5 reanalyses, the European Centre for
Medium-Range Weather Forecasts (ECMWF) operational analysis and short-range
forecasts, German Weather Service (DWD) forecasts and regional atmospheric
model simulations (coastDat). Numerical simulations show that
the wave model forced using the ERA5 reanalyses and that forced using the
ECMWF operational analysis/forecast demonstrate the best capability over the
whole study period, as well as during extreme events. To further estimate the
variance of the significant wave height of ensemble members for different
wind forcings, especially during extreme events, an empirical orthogonal
function (EOF) analysis is performed. In the second part of the study, the
satellite data of Sentinel-3A, Jason-2 and CryoSat-2 are assessed in
comparison with in situ measurements and spectral wave model (WAM)
simulations. Intercomparisons between remote sensing and in situ observations
demonstrate that the overall quality of the former is good over the North Sea
and Baltic Sea throughout the study period, although the significant wave
heights estimated based on satellite data tend to be greater than the in situ
measurements by 7 to 26 cm. The quality of all satellite data near
the coastal area decreases; however, within 10 km off the coast,
Sentinel-3A performs better than the other two satellites. Analyses in which
data from satellite tracks are separated in terms of onshore and offshore
flights have been carried out. No substantial differences are found when
comparing the statistics for onshore and offshore flights. Moreover, no
substantial differences are found between satellite tracks under various
metocean conditions. Furthermore, the satellite data quality does not depend
on the wind direction relative to the flight direction. Thus, the quality of
the data obtained by the new Sentinel-3A satellite over coastal areas
is improved compared to that of older satellites.</p
An Improved and Homogeneous Altimeter Sea Level Record from the ESA Climate Change Initiative
Sea Level is a very sensitive index of climate change since it integrates the impacts of ocean warming and ice mass loss from glaciers and the ice sheets. Sea Level has been listed as an Essential Climate Variable (ECV) by the Global Climate Observing System (GCOS). During the past 25 years, the sea level ECV has been measured from space by different altimetry missions that have provided global and regional observations of sea level variations. As part of the Climate Change Initiative (CCI) program of the European Space Agency (ESA) (established in 2010), the Sea Level project (SL_cci) aimed at providing an accurate and homogeneous long-term satellite-based sea level record. At the end of the first phase of the project (2010-2013), an initial version (v1.1) of the sea level ECV has been made available to users (Ablain et al., 2015).
During the second phase (2014-2017), improved altimeter standards have been selected to produce new sea level products (called SL_cci v2.0) based on 9 altimeter missions for the period 1993-2015 (https://doi.org/10.5270/esa-sea_level_cci-1993_2015-v_2.0-201612). Corresponding orbit solutions, geophysical corrections and altimeter standards used in this v2.0 dataset are described in details in Quartly et al. (2017). The present paper focuses on the description of the SL_cci v2.0 ECV and associated uncertainty and discusses how it has been validated. Various approaches have been used for the quality assessment such as internal validation, comparisons with sea level records from other groups and with in-situ measurements, sea level budget closure analyses and comparisons with model outputs. Compared to the previous version of the sea level ECV, we show that use of improved geophysical corrections, careful bias reduction between missions and inclusion of new altimeter missions lead to improved sea level products with reduced uncertainties at different spatial and temporal scales. However, there is still room for improvement since the uncertainties remain larger than the GCOS requirements. Perspectives for subsequent evolutions are also discussed
An atmosphere–wave regional coupled model: improving predictions of wave heights and surface winds in the southern North Sea
The coupling of models is a commonly used approach when addressing the
complex interactions between different components of earth systems. We
demonstrate that this approach can result in a reduction of errors in wave
forecasting, especially in dynamically complicated coastal ocean areas, such
as the southern part of the North Sea – the German Bight. Here, we study the effects of coupling of an atmospheric model (COSMO) and a wind wave
model (WAM), which is enabled by implementing wave-induced drag in the
atmospheric model. The numerical simulations use a regional North Sea coupled
wave–atmosphere model as well as a nested-grid high-resolution German Bight wave model. Using one atmospheric and two wind wave models simultaneously
allows for study of the individual and combined effects of two-way coupling
and grid resolution. This approach proved to be particularly important under
severe storm conditions as the German Bight is a very shallow and dynamically
complex coastal area exposed to storm floods. The two-way coupling leads to a
reduction of both surface wind speeds and simulated wave heights. In this
study, the sensitivity of atmospheric parameters, such as wind speed and
atmospheric pressure, to the wave-induced drag, in particular under storm
conditions, and the impact of two-way coupling on the wave model performance,
is quantified. Comparisons between data from in situ and satellite altimeter
observations indicate that two-way coupling improves the simulation of wind
and wave parameters of the model and justify its implementation for both
operational and climate simulations
Potential impacts of climate and environmental change on the stored water of Lake Victoria Basin and economic implications
The changing climatic patterns and increasing human population within the Lake Victoria Basin (LVB), together with overexploitation of water for economic activities call for assessment of water management for the entire basin. This study focused on the analysis of a combination of available in situ climate data, Gravity Recovery and Climate Experiment (GRACE), Tropical Rainfall Measuring Mission (TRMM) observations, and high resolution Regional Climate simulations during recent decade(s) to assess the water storage changes within LVB that may be linked to recent climatic variability/changes and anomalies. We employed trend analysis, principal component analysis (PCA), and temporal/spatial correlations to explore the associations and covariability among LVB stored water, rainfall variability, and large-scale forcings associated with El-Niño/Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD). Potential economic impacts of human and climate-induced changes in LVB stored water are also explored.Overall, observed in situ rainfall from lake-shore stations showed a modest increasing trend during the recent decades. The dominant patterns of rainfall data from the TRMM satellite estimates suggest that the spatial and temporal distribution of precipitation have not changed much during the period of 1998–2012 over the basin consistent with in situ observations. However, GRACE-derived water storage changes over LVB indicate an average decline of 38.2 mm/yr for 2003–2006, likely due to the extension of the Owen Fall/Nalubale dam, and an increase of 4.5 mm/yr over 2007–2013, likely due to two massive rainfalls in 2006–2007 and 2010–2011. The temporal correlations between rainfall and ENSO/IOD indices during the study period, based on TRMM and model simulations, suggest significant influence of large-scale forcing on LVB rainfall, and thus stored water. The contributions of ENSO and IOD on the amplitude of TRMM-rainfall and GRACE-derived water storage changes, for the period of 2003–2013, are estimated to be ~2.5 cm and ~1.5 cm, respectively
Understanding extreme sea levels for broad-scale coastal impact and adaptation analysis
One of the main consequences of mean sea level rise (SLR) on human settlements is an increase in flood risk due to an increase in the intensity and frequency of extreme sea levels (ESL). While substantial research efforts are directed towards quantifying projections and uncertainties of future global and regional SLR, corresponding uncertainties in contemporary ESL have not been assessed and projections are limited. Here we quantify, for the first time at global scale, the uncertainties in present-day ESL estimates, which have by default been ignored in broad-scale sea-level rise impact assessments to date. ESL uncertainties exceed those from global SLR projections and, assuming that we meet the Paris agreement goals, the projected SLR itself by the end of the century in many regions. Both uncertainties in SLR projections and ESL estimates need to be understood and combined to fully assess potential impacts and adaptation needs
- …