13 research outputs found
Global coastal attenuation of wind-waves observed with radar altimetry
Coastal studies of wave climate and evaluations of wave energy resources are mainly regional
and based on the use of computationally very expensive models or a network of in-situ data.
Considering the significant wave height, satellite radar altimetry provides an established
global and relatively long-term source, whose coastal data are nevertheless typically flagged
as unreliable within 30 km of the coast. This study exploits the reprocessing of the radar
altimetry signals with a dedicated fitting algorithm to retrieve several years of significant
wave height records in the coastal zone. We show significant variations in annual cycle
amplitudes and mean state in the last 30 km from the coastline compared to offshore, in
areas that were up to now not observable with standard radar altimetry. Consequently, a
decrease in the average wave energy flux is observed. Globally, we found that the mean
significant wave height at 3 km off the coast is on average 22% smaller than offshore, the
amplitude of the annual cycle is reduced on average by 14% and the mean energy flux loses
38% of its offshore value
A global lake and reservoir volume analysis using a surface water dataset and satellite altimetry
Lakes and reservoirs are crucial elements of the
hydrological and biochemical cycle and are a valuable resource for
hydropower, domestic and industrial water use, and irrigation. Although their
monitoring is crucial in times of increased pressure on water resources by
both climate change and human interventions, publically available datasets of
lake and reservoir levels and volumes are scarce. Within this study, a time
series of variation in lake and reservoir volume between 1984 and 2015 were
analysed for 137 lakes over all continents by combining the JRC Global
Surface Water (GSW) dataset and the satellite altimetry database DAHITI. The
GSW dataset is a highly accurate surface water dataset at 30 m resolution
compromising the whole L1T Landsat 5, 7 and 8 archive, which allowed for
detailed lake area calculations globally over a very long time period using
Google Earth Engine. Therefore, the estimates in water volume fluctuations
using the GSW dataset are expected to improve compared to current techniques
as they are not constrained by complex and computationally intensive
classification procedures. Lake areas and water levels were combined in a
regression to derive the hypsometry relationship (dh ∕ dA) for all
lakes. Nearly all lakes showed a linear regression, and 42 % of the lakes
showed a strong linear relationship with a R2 > 0.8, an
average R2 of 0.91 and a standard deviation of 0.05. For these lakes and
for lakes with a nearly constant lake area (coefficient of variation
< 0.008), volume variations were calculated. Lakes with a poor
linear relationship were not considered. Reasons for low R2 values were
found to be (1) a nearly constant lake area, (2) winter ice coverage and
(3) a predominant lack of data within the GSW dataset for those lakes. Lake
volume estimates were validated for 18 lakes in the US, Spain, Australia and
Africa using in situ volume time series, and gave an excellent Pearson
correlation coefficient of on average 0.97 with a standard deviation of
0.041, and a normalized RMSE of 7.42 %. These results show a high
potential for measuring lake volume dynamics using a pre-classified GSW
dataset, which easily allows the method to be scaled up to an extensive
global volumetric dataset. This dataset will not only provide a historical
lake and reservoir volume variation record, but will also help to improve our
understanding of the behaviour of lakes and reservoirs and their
representation in (large-scale) hydrological models.</p
Ecosystem services in changing social-ecological systems
In Lautze, Jonathan; McCartney, Matthew; Gibson, J. (Eds.). The Omo-Turkana Basin: cooperation for sustainable water management. Abingdon, Oxon, UK: Routledge - Earthsca
DAHITI – an innovative approach for estimating water level time series over inland waters using multi-mission satellite altimetry
Satellite altimetry has been designed for sea level monitoring over open
ocean areas. However, for some years, this technology has also been used to
retrieve water levels from reservoirs, wetlands and in general any inland
water body, although the radar altimetry technique has been especially
applied to rivers and lakes. In this paper, a new approach for the estimation
of inland water level time series is described. It is used for the
computation of time series of rivers and lakes available through the web
service "Database for Hydrological Time Series over Inland Waters"
(DAHITI). The new method is based on an extended outlier rejection and a
Kalman filter approach incorporating cross-calibrated multi-mission altimeter
data from Envisat, ERS-2, Jason-1, Jason-2, TOPEX/Poseidon, and SARAL/AltiKa,
including their uncertainties. The paper presents water level time series for
a variety of lakes and rivers in North and South America featuring different
characteristics such as shape, lake extent, river width, and data coverage. A
comprehensive validation is performed by comparisons with in situ gauge data
and results from external inland altimeter databases. The new approach yields
rms differences with respect to in situ data between 4 and 36 cm for lakes
and 8 and 114 cm for rivers. For most study cases, more
accurate height information than from other available altimeter databases
can be achieved
Using a Tandem Flight Configuration between Sentinel-6 and Jason-3 to Compare SAR and Conventional Altimeters in Sea Surface Signatures of Internal Solitary Waves
Satellite altimetry has been providing a continuous record of ocean measurements with numerous applications across the entire range of ocean sciences. A reference orbit has been used since 1992 with TOPEX/Poseidon, which was repeated in the Jason missions, and in the newly launched Sentinel-6 Michael Freilich (in November 2020) to continually monitor the trends of sea level rise and other properties of the sea surface. These multidecadal missions have evolved alongside major technological advances, whose measurements are unified into a single data record owing to continuous intercalibration and validation efforts. However, the new Sentinel-6 provides synthetic aperture radar (SAR) processing, which improves the along-track resolution of conventional altimeters from a few kilometres (e.g., for Jason-3) to about 300 m. This means a major leap in sampling towards higher frequencies of the ocean spectrum, which inevitably means reconciling the assumption of a uniform Brown surface between the footprints of the larger kilometre-scale conventional altimetry and those of the finer-scale SAR altimetry. To explore this issue, this study uses the vantage point of the Sentinel-6/Jason-3 tandem phase to compare simultaneous sea surface signatures of large-scale Internal Solitary Waves (ISWs) between SAR and conventional altimetry. These waves can modulate the sea surface into arrayed sections of increased and decreased roughness with horizontal scales up to 10 km, which inflict sharp transitions between increased and decreased backscatter in the radar altimeters. It is found that Sentinel-6 can provide more detailed structures of ISWs in standard level-2 products, when compared with those from the conventional Jason-3 (similarly to previous results reported from the SAR altimeter from Sentinel-3). However, a new and striking feature is found when comparing the radar backscatter between Sentinel-6 and Jason-3, which are in opposite phases in the ISWs. These intriguing results are discussed in light of the intrinsically different acquisition geometries of SAR and conventional altimeters as well as possible implications thereof
Coastal sea level anomalies and associated trends from Jason satellite altimetry over 2002-2018 [Data paper]
Climate-related sea level changes in the world coastal zones result from the superposition of the global mean rise due to ocean warming and land ice melt, regional changes caused by non-uniform ocean thermal expansion and salinity changes, and by the solid Earth response to current water mass redistribution and associated gravity change, plus small-scale coastal processes (e.g., shelf currents, wind & waves changes, fresh water input from rivers, etc.). So far, satellite altimetry has provided global gridded sea level time series up to 10-15km to the coast only, preventing estimation of sea level changes very close to the coast. Here we present a 16-year-long (June 2002 to May 2018), high-resolution (20-Hz), along-track sea level dataset at monthly interval, together with associated sea level trends, at 429 coastal sites in six regions (Northeast Atlantic, Mediterranean Sea, Western Africa, North Indian Ocean, Southeast Asia and Australia). This new coastal sea level product is based on complete reprocessing of raw radar altimetry waveforms from the Jason-1, Jason-2 and Jason-3 missions