Wind speeds over the world’s oceans have increased over the past two decades, as have wave heights. Studies of climate change typically consider measurements or predictions of temperature over extended periods of time. Climate, however, is much more than temperature. Over the oceans, changes in wind speed and the surface gravity waves generated by such winds play an important role. We used a 23-year database of calibrated and validated satellite altimeter measurements to investigate global changes in oceanic wind speed and wave height over this period. We find a general global trend of increasing values of wind speed and, to a lesser degree, wave height, over this period. The rate of increase is greater for extreme events as compared to the mean condition.

https://researchbank.swinburne.edu.au/file/bca43746-6f16-4ad2-a4b7-27f8d8d60fed/1/PDF (Published version).pdf

Global trends in wind speed and wave height over the past 25 years

The new level of precision and global coverage provided by satellite altimetry is rapidly advancing studies of ocean circulation. It allows for new insights into marine geodesy, ice sheet movements, plate tectonics, and for the first time provides high-resolution bathymetry for previously unmapped regions of our watery planet and crucial information on the large-scale ocean features on intra-season to interannual time scales. Satellite Altimetry and Earth Sciences has integrated the expertise of the leading international researchers to demonstrate the techniques, missions, and accuracy of satellite altimetry, including altimeter measurements, orbit determination, and ocean circulation models. Satellite altimetry is helping to advance studies of ocean circulation, tides, sea level, surface waves and allowing new insights into marine geodesy. Satellite Altimetry and Earth Sciences provides high resolution bathymetry for previously unmapped regions of our watery planet. Satellite Altimetry and Earth Sciences is for a very broad spectrum of academics, graduate students, and researchers in geophysics, oceanography, and the space and earth sciences. International agencies that fund satellite-based research will also appreciate the handy reference on the applications of satellite altimetry.

Satellite altimetry and Earth sciences :a handbook of techniques and applications

The Greenland Ice Sheet has been a major contributor to global sea-level rise in recent decades1,2, and it is expected to continue to be so3. Although increases in glacier flow4-6 and surface melting7-9 have been driven by oceanic10-12 and atmospheric13,14 warming, the magnitude and trajectory of the ice sheet's mass imbalance remain uncertain. Here we compare and combine 26 individual satellite measurements of changes in the ice sheet's volume, flow and gravitational potential to produce a reconciled estimate of its mass balance. The ice sheet was close to a state of balance in the 1990s, but annual losses have risen since then, peaking at 345 ± 66 billion tonnes per year in 2011. In all, Greenland lost 3,902 ± 342 billion tonnes of ice between 1992 and 2018, causing the mean sea level to rise by 10.8 ± 0.9 millimetres. Using three regional climate models, we show that the reduced surface mass balance has driven 1,964 ± 565 billion tonnes (50.3 per cent) of the ice loss owing to increased meltwater runoff. The remaining 1,938 ± 541 billion tonnes (49.7 per cent) of ice loss was due to increased glacier dynamical imbalance, which rose from 46 ± 37 billion tonnes per year in the 1990s to 87 ± 25 billion tonnes per year since then. The total rate of ice loss slowed to 222 ± 30 billion tonnes per year between 2013 and 2017, on average, as atmospheric circulation favoured cooler conditions15 and ocean temperatures fell at the terminus of Jakobshavn Isbrae16. Cumulative ice losses from Greenland as a whole have been close to the rates predicted by the Intergovernmental Panel on Climate Change for their high-end climate warming scenario17, which forecast an additional 70 to 130 millimetres of global sea-level rise by 2100 compared with their central estimate.

/pdf/mass-balance-of-the-greenland-ice-sheet-from-1992-to-2018-2ir4asxtop.pdf

Mass balance of the Greenland Ice Sheet from 1992 to 2018

https://backend.orbit.dtu.dk/ws/files/247661618/1_s2.0_S0273117721000594_main.pdf

Altimetry for the future: building on 25 years of progress

. We combine satellite observations and numerical models to
show that Earth lost 28 trillion tonnes of ice between 1994 and 2017. Arctic
sea ice (7.6 trillion tonnes), Antarctic ice shelves (6.5 trillion tonnes),
mountain glaciers (6.1 trillion tonnes), the Greenland ice sheet (3.8
trillion tonnes), the Antarctic ice sheet (2.5 trillion tonnes), and
Southern Ocean sea ice (0.9 trillion tonnes) have all decreased in mass.
Just over half (58 %) of the ice loss was from the Northern Hemisphere,
and the remainder (42 %) was from the Southern Hemisphere. The rate of
ice loss has risen by 57 % since the 1990s – from 0.8 to 1.2 trillion
tonnes per year – owing to increased losses from mountain glaciers,
Antarctica, Greenland and from Antarctic ice shelves. During the same
period, the loss of grounded ice from the Antarctic and Greenland ice sheets
and mountain glaciers raised the global sea level by 34.6  ±  3.1 mm.
The majority of all ice losses were driven by atmospheric melting (68 %
from Arctic sea ice, mountain glaciers ice shelf calving and ice sheet
surface mass balance), with the remaining losses (32 % from ice sheet
discharge and ice shelf thinning) being driven by oceanic melting.
Altogether, these elements of the cryosphere have taken up 3.2 % of the
global energy imbalance.

/pdf/review-article-earth-s-ice-imbalance-1hhp2yavip.pdf

Review Article: Earth's ice imbalance

Evaluation of the precision of different Delay-Doppler Processor (DDP) algorithms using CryoSat-2 data over open ocean

Swath mode processing of CryoSat-2 Synthetic Aperture Radar Interferometric (SARIn) mode has been used to monitor elevation of areas with complex topography such as over ice sheet and ice cap margins. Swath processing relies on an accurate measure of the angle of arrival of the measured echo and, therefore, requires custom strategies in order to resolve the ambiguous phase measurement. In mountainous regions of complex terrain, it may be necessary to apply different phase ambiguities across a waveform record when returns come from different scatters distributed perpendicularly to the CryoSat-2 ground tracks. In this letter, we present modifications to the conventional swath processing method whereby a multisurface retracker is first applied to the record in order to identify potential different scattering surfaces. Phase ambiguity is then independently resolved for each of these subsurfaces. The improvements with this new method over the Karakoram glaciers are a 10% increase in the number of measurements with improvements of almost 50% for individual glaciers and a reduction in the median absolute deviation of the elevations from 20.18 to 14.69 m.

/pdf/multisurface-retracker-for-swath-processing-of-5617hn1uw7.pdf

Multisurface Retracker for Swath Processing of Interferometric Radar Altimetry

Measuring the Lake Level Evolution in the Qinghai-Tibet Plateau with Radar Altimeters

CryoSat-2 is the first satellite mission carrying a high pulse repetition frequency radar altimeter with interferometric capability on board. Across track interferometry allows the angle to the point of closest approach to be determined by combining echoes received by two antennas and knowledge of their orientation. Accurate information of the platform mispointing angles, in particular of the roll, is crucial to determine the angle of arrival in the across-track direction with sufficient accuracy. As a consequence, different methods were designed in the CryoSat-2 calibration plan in order to estimate interferometer performance along with the mission and to assess the roll’s contribution to the accuracy of the angle of arrival. In this paper, we present the comprehensive approach used in the CryoSat-2 Mission to calibrate the roll mispointing angle, combining analysis from external calibration of both man-made targets, i.e., transponder and natural targets. The roll calibration approach for CryoSat-2 is proven to guarantee that the interferometric measurements are exceeding the expected performance.

Roll Calibration for CryoSat-2: a comprehensive approach

The inland water level plays an essential role in water balance management. This paper presents methodology to retrieve water level from radar altimeter data including Cryosat-2 and Jason-2 over lakes in Western Africa. The idea is to combine both Level-1 and Level-2 data to retrieve water level more precisely and automatically over small lakes. From Level-1 data, the waveform is analyzed for data filtering and the possibility of water level retrieval. Then an iteration method is proposed to retrieve the height from Level-2 altimeter data with a strict water mask. The preliminary results of water level time series, derived from Level-2 data, in Lake Volta and Lake Kainji are presented in this paper, and the results are compared with the data from DAHITI (Database for Hydrological Time Series of Inland Waters). The standard deviation is controlled below 0.3 meters.

Albert Garcia-Mondejar

Papers

Evaluation of the precision of different Delay-Doppler Processor (DDP) algorithms using CryoSat-2 data over open ocean

Multisurface Retracker for Swath Processing of Interferometric Radar Altimetry

Measuring the Lake Level Evolution in the Qinghai-Tibet Plateau with Radar Altimeters

Roll Calibration for CryoSat-2: a comprehensive approach

Inland water level retrieval over western africa with radar altimeters