3 research outputs found
A Facet-Based Numerical Model for Simulating SAR Altimeter Echoes from Heterogeneous Sea Ice Surfaces
Cryosat-2 has provided measurements of pan-Arctic
sea ice thickness since 2010 with unprecedented spatial coverage
and frequency. However, it remains uncertain how the Ku-band
radar interacts with the vast range of scatterers that can be present
within the satellite footprint, including sea ice with varying
physical properties and multi-scale roughness, snow cover, and
leads. Here, we present a numerical model designed to simulate
delay-Doppler SAR (Synthetic Aperture Radar) altimeter echoes
from snow-covered sea ice, such as those detected by Cryosat-2.
Backscattered echoes are simulated directly from triangular facetbased models of actual sea ice topography generated from
Operation IceBridge Airborne Topographic Mapper (ATM) data,
as well as virtual statistical models simulated artificially. We use
these waveform simulations to investigate the sensitivity of SAR
altimeter echoes to variations in satellite parameters (height, pitch,
roll) and sea ice properties (physical properties, roughness,
presence of water). We show that the conventional Gaussian
assumption for sea ice surface roughness may be introducing
significant error into the Cryosat-2 waveform retracking process.
Compared to a more representative lognormal surface, an echo
simulated from a Gaussian surface with rms roughness height of
0.2 m underestimates the ice freeboard by 5 cm – potentially
underestimating sea ice thickness by around 50 cm. We present a
set of ‘ideal’ waveform shape parameters simulated for sea ice and
leads to inform existing waveform classification techniques. This
model will ultimately be used to improve retrievals of key sea ice
properties, including freeboard, surface roughness and snow
depth, from SAR altimeter observations
Snow property controls on modelled Ku-band altimeter estimates of first-year sea ice thickness: Case studies from the Canadian and Norwegian Arctic
Uncertainty in snow properties impacts the accuracy
of Arctic sea ice thickness estimates from radar altimetry. On firstyear sea ice (FYI), spatiotemporal variations in snow properties
can cause the Ku-band main radar scattering horizon to appear
above the snow/sea ice interface. This can increase the estimated
sea ice freeboard by several centimeters, leading to FYI thickness
overestimations. This study examines the expected changes in Kuband main scattering horizon and its impact on FYI thickness
estimates, with variations in snow temperature, salinity and
density derived from 10 naturally occurring Arctic FYI Cases
encompassing saline/non-saline, warm/cold, simple/complexly
layered snow (4 cm to 45 cm) overlying FYI (48 cm to 170 cm).
Using a semi-empirical modeling approach, snow properties from
these Cases are used to derive layer-wise brine volume and
dielectric constant estimates, to simulate the Ku-band main
scattering horizon and delays in radar propagation speed.
Differences between modeled and observed FYI thickness are
calculated to assess sources of error. Under both cold and warm
conditions, saline snow covers are shown to shift the main
scattering horizon above from the snow/sea ice interface, causing
thickness retrieval errors. Overestimates in FYI thicknesses of up
to 65% are found for warm, saline snow overlaying thin sea ice.
Our simulations exhibited a distinct shift in the main scattering
horizon when the snow layer densities became greater than 440
kg/m3
, especially under warmer snow conditions. Our simulations
suggest a mean Ku-band propagation delay for snow of 39%,
which is higher than 25%, suggested in previous studies