Introduction
Estimating ice sheet mass balance requires knowledge of
the gains and losses to the system. The IPCC estimates
a 0.40±0.15 m sea-level rise by the year 2100 (scenario RCP2.6
). Assumptions in assessing snow accumulation (the
primary positive input to ice sheets) account for a portion of the
uncertainty in potential sea-level rise. Studies using ice cores
provide records of past accumulation, while climate models
reconstruct
accumulation in regions lacking in situ measurements. Regional Climate
Models (RCMs) provide widespread spatial coverage of recent
annual snow accumulation (1958–2015), but these estimates interpolate
between ground-truthed point locations .
The simplification of accumulation rates from point-based measurements
may be overcome using ice-penetrating radar. Radar stratigraphy studies
measure snow accumulation
by combining in situ measurements (snow density, ice core chemistry)
with continuous isochronal layers within the dry-snow zone of ice sheets.
The depth of a given radar layer depends on the density
of the snowpack through which the radar signal travels. Using
density to calculate the depth and age of these layers allows for
the estimation of an accumulation rate. Given this density and signal
propagation relationship, we assume that the most accurate
radar-derived snow accumulation rates result from radar surveys
collected simultaneously with in situ density measurements.
The European Space Agency's 2006 Airborne SAR/Interferometric Radar
Altimeter System (ASIRAS) campaign detected annual accumulation layers
along the Expéditions Glaciologiques Internationales au
Groenland (EGIG) traverse across central Greenland
. Previous accumulation estimates using
ASIRAS layers along EGIG incorporate a maximum of two density profiles
to calculate ASIRAS travel time (and thus determine ASIRAS layer depth)
. Concurrent to the 2006 radar campaign,
conducted detailed measurements of near-surface
density to 13 m depth using a neutron-probe (NP). Eight NP sites along
a 250 km segment of the EGIG route provide ground-truthed
density measurements for calculating ASIRAS travel time through the ice
sheet's surface (Fig. ). We present annual
accumulation rates from 1959 to 2004 derived from NP density-adjusted
ASIRAS layers (ASIRAS-NP accumulation record). We compare our
ASIRAS-NP accumulation record to previous measurements of annual
accumulation from shallow cores
and three RCMs.
Detailed density, collected in conjunction with an airborne radar survey,
offers the opportunity to understand radar-derived accumulation rates'
sensitivity to density. We test the ASIRAS accumulation rate's sensitivity
to density using simple Herron and Langway modeled density profiles
in place of NP density measurements (ASIRAS-HL). Modeled
density combined with radar layers may offer an alternative for deriving
accumulation rates from radar layers lacking ground-truthed density.
Field locations along a portion of the Expédition Glaciologique
Internationale au Groenland (EGIG) traverse. The 250 km segment of
ASIRAS radar data discussed in this paper spans 2700 to 3200 m
elevation. Black crosses show neutron-probe density T-sites
from . White squares □ show shallow
cores. The filled triangle shows the location of Summit
Station for reference. 100 m contour intervals are displayed. Scale bar accurate at 71∘ N latitude.
Background
EGIG line
Major joint European expeditions across central Greenland were conducted in
1958–1959 (EGIG1) and 1967–1968 (EGIG2) .
Shallow cores drilled along the route led to the first isotopic climate curves
produced by . report
a tritium based mean annual accumulation from 1959 to 1969 of 0.329 m
water equivalent accumulation per year (m w.e.a-1) at site
T31 (Station Centrale) and 0.243 (m w.e.a-1) at site
T43 (Crête). found mean annual accumulation
from 1946 to 1955 of 0.355 (m w.e.a-1) at T31.
A 1990–1992 reconstruction of the EGIG line by
Technische Universität Braunschweig led to glacio-meteorological,
isotopic/chemical, and snow accumulation studies
. These studies found accumulation rates
between 0.47 and 0.44 mw.e.a-1 at the
western portion of EGIG, decreasing to 0.25 to 0.20 (mw.e.a-1)
along the eastern edge of the EGIG line. report no
long-term temporal changes in accumulation rates from 1955 to 1995.
Site T21 marks the approximate transition to the dry snow zone, with air
temperatures upslope from T21 rarely exceeding 0 ∘C
(Fig. ) . observed
a transition in surface conditions approximately 10 km uphill from
site T31. Elevations upslope from T31 experience less persistent
winds, leaving a smooth surface with undisturbed summer surface hoar
. Down-slope from T31 the upper snow layer appears
to be wind-packed, with sastrugi marking the surface .
ASIRAS radar
The European Space Agency originally designed ASIRAS to serve as a prototype for
the CryoSat Mission . ASIRAS uses a Ku-band radar
altimeter to measure ice sheet surface elevation and detect sub-surfaces
layers. described the internal reflection horizons
observed by ASIRAS as corresponding to density interfaces. ASIRAS radar has
a carrier frequency of 13.5GHz and instrument bandwidth
B=1GHz. The radar transect discussed in this paper was collected
using low altitude mode (LAM). ASIRAS-LAM has Ns=4096 echo samples, an
uncompressed pulse length of Tuc=80 µs, an instrument
sampling frequency of Fs=37.5MHz, resulting in a range
bin resolution of ΔR=0.109 m, using the range-resolution
equation from :
ΔR=TucFsc2BNs,
where c is the speed of light, ∼299 792 458ms-1.
measured annual accumulation from 1995 to 2002 at site
T21 using ASIRAS radar. The study used a single NP density profile, T21, to
calculate radar travel time through snowpack and depth to the sub-surface
layers. establish ASIRAS internal reflection horizons as
annual accumulation layers, correlating high density winter peaks with local
peaks in radar power return. ASIRAS accumulation layers, examined down to
10 m depth, produced a mean annual accumulation of
0.47 mw.e.a-1 (similar to ).
found decreasing accumulation with increasing elevation
and short-scale variability in line with and
. observe upward curved reflectors in areas
of steep slope and accumulation-driven layering, in agreement with
.
collected simultaneous ground-based density measurements
and ASIRAS overflights in the percolation zone along EGIG (downslope from this
study). The dominant second peak observed in the ASIRAS waveform results
from a heterogeneous zone of metamorphosed snow and ice lenses below the
winter snowpack . From this finding,
demonstrate ASIRAS's ability to derive winter accumulation rates. Similar to
, found a strong correlation
between surface gradient and accumulation rate, with higher accumulation
rate in the plateau areas and lower accumulation rate on slopes.
presented mean accumulation rates for the period 1998–2003,
calculated from six ASIRAS layers along 200 km
of the EGIG route. used published T21 NP density values
from and a Summit density profile (located
150 km north of the EGIG line) to linearly interpolate density
values, calculate radar travel time through snow/firn layers, calculate layer depth and
accumulation rates along the EGIG line. report an average
accumulation of 0.36 mw.e.a-1 with high spatial and temporal
variability, and a decreasing accumulation gradient from west to east
along EGIG consistent with previous studies of , ,
, and the interannual variability observed by
. presented a 15–20 % increase in
accumulation above 3000 m over the past 20–25 years compared
to .
use ASIRAS layers down to approximately 15 m depth
along EGIG to assess firn compaction. They use an automatic method to
identify ASIRAS layers and confirm the layers represent isochrones spaced
1 year apart . Accumulation rates are not reported, as the
study focused on firn compaction.
EGIG-Ground in situ measurements
Shallow core accumulation
drilled 11 shallow firn cores (referred to as T-sites,
Fig. ), 8 to 10 m depth, spaced 50 km apart along the
EGIG line. Using hydrogen peroxide (H2O2) analysis and snow
density to identify the seasonal signal, report annual
accumulation from 1969 to 1989 for T-sites along the EGIG line. Their
accumulation rates fit with the general understanding of water vapor
transport and snow accumulation across the Greenland Ice Sheet:
higher accumulations near the coast (0.44 m w.e.a-1)
gradually decreasing with increasing elevation (0.25 m w.e.a-1 at
the east end of EGIG). found large variations of
local annual accumulation rates from 1979 to 1989, with typical standard
deviations of 10 to 25 %. Changes in annual accumulation correlated
from site to site. Temporally, annual accumulation increased slightly in
central Greenland and decreased slightly at middle and low elevations.
drilled 18 shallow firn cores along the EGIG line
during field campaigns in 1990 and 1992. Annual accumulation rates from
1984 to 1989 were determined by counting seasonally varying tracers
δ18O and major ions. Measurements were collected at
approximately eight samples per year. The 1990 campaign used seasonally
varying hydrogen peroxide (H2O2) analysis (in collaboration with
) to determine summer maxima of flourimetric profiles.
observed distinct winter/summer pairings in the upper
snow. found layer thickness and accumulations similar to
(0.47–0.43 mw.e.a-1), along the western portion
of EGIG , decreasing to the east (0.25 to
0.20 mw.e.a-1, though fewer measurements exist to support the
eastern accumulation rates). argue for accumulation
variations along EGIG due to large scale topographic valleys and ridges.
The short timespan of the study, 1984 to 1989, limited the identifiable
temporal trend .
Neutron-probe densities and accumulation
collected near-surface in situ neutron-probe (NP) density
measurements along a 365 km section of the EGIG line in the spring
and autumn of 2004 and spring and summer of 2006 . The 2006
traverse coincided with airborne observations of sub-surface layers using ASIRAS.
The probe used by consists of an annular radioactive
americium-241/beryllium source of fast neutrons around a cylindrical detector
of slow (thermal) neutrons . The fast neutrons lose energy
by scattering as they move through the snow. The count rate of slow neutrons
arriving back at the detector per unit time relates to the density of the
snow . derived theoretical calibration
equations for count rate and snow density. See ,
for descriptions of NP data collection.
collected 17 “T-site” NP density
profiles and accumulation rates spanning ice-sheet elevations of 1940 to
3201 m, (eight NP T-sites of this study are mapped on
Fig. ). The NP measured snow density from the surface
down to approximately 13 m depth at the T-sites along the EGIG
traverse. snow-pit data show that very thin ice layers
can occur at all sites but are not resolved by the NP measurements.
observe “that the density peaks lie in winter snow but
are formed during the following summer, when warmer temperatures
promote densification in the near-surface layer (e.g., )” .
describe two primary phases of the seasonal density cycle
in the dry-snow region: temperature driven densification and accumulation rate
driven compaction. Densification increases late spring to early summer as
temperatures warm, with rates decreasing after a period of maximum firn
compaction ends in late summer. The snow accumulation rate then
dominates compaction through winter. This transition in near-surface snow
from late-summer to autumn marks the annual seasonal change detected
by ASIRAS . The snow thickness between
density peaks, adjusted to mean water-equivalent using density measurements,
defines the NP estimate of annual accumulation .
Methods
ASIRAS-traced layers
We focus on a 250 km segment of the EGIG line spanning eight NP
T-sites (Fig. ). We trace 48 layers down to 20 m depth
from a 29 April 2006 ASIRAS flight radargram (Fig. ). Our radar
profile starts at T21a (0 km distance)
2700 m elevation and ends 250 km to the east beyond the
ice divide (3200 m elevation). Using SAR processed level_1b ASIRAS
data, we apply the following signal processing
techniques: waveform alignment, stacking, and gain. Each column of the
radargram represents the centered mean of the surrounding 100 columns (hence
100 columns “stacked” into one record, representing approximately three
horizontal meters). The ASIRAS signal weakens with increasing depth through
the snowpack; thus we apply a ramped gain to the signal to enhance the visual
contrast of the radargram. The ramped gain resembles an exponential gain,
resulting in a 3× enhancement of layer intensity at 15 m
depth and an 8× enhancement at 20 m depth.
ASIRAS radargram of a portion of the 47 traced internal
reflection horizons, or layers, down to 20 m depth. The
uppermost layer represents the 2005 accumulation year. Distance along EGIG
corresponds with gray line in Fig. , with 0 km starting
at 2700 m on the western slope and 250 km ending below
3200 m on the eastern slope of the Greenland Ice Sheet.
The left axis shows depth for NP and layers.
The right axis shows two-way travel time of an ASIRAS radar pulse. See
Appendix Fig. for full extent of layers across EGIG.
We trace layers by tracking the maximum reflected power. The trace progresses
by searching the adjacent column for a maximum power reflected within the
vertical range of a moving window. Automated layer tracing occurs one layer
at a time with visual inspection and user approval of the final traced layer.
The shallowest (1st) layer represents the 2005 accumulation surface and the
deepest (48th) traced layer represents the 1959 accumulation year (October
1958 to September 1959) (Fig. ). Layers fade in intensity around 16 m
depth along the western section of EGIG (0–75 km) and at 14 m depth
along the rest of the line (75–250 km). The exact
depth depends on the ASIRAS electromagnetic wave speed v
(ms-1) through the snowpack. Electromagnetic wave speed
v relates to the real part of the dielectric permittivity
ϵr (dimensionless) which in the near-surface can be
related to density ρ (kgm-3) by ():
v=cϵrϵr=(1+8.45×10-4ρ)2,
where c is the speed of light in a vacuum (∼ 299 792 458 ms-1).
Signal travel time will change based on the density of the snowpack.
Dense coastal snowpack slows the signal speed compared to a less dense
interior snowpack. Near the coast 45 nanoseconds (ns) of travel time equals
5.10 m depth, while the same travel time in the interior would equal
5.15 m depth. ASIRAS corrected with a coastal density profile gives
a depth of 20.03 m at 190 ns. The same 190 ns
travel time reaches 20.24 m depth using the interior NP profile.
The near-surface difference in travel time being unresolvable given
ASIRAS's 0.109 m range-bin resolution (Sect. ).
ASIRAS-NP and ASIRAS-HL accumulation rates have the same radar-time
layer positions. The records differ only in the snow density used to adjust
the radar signal propagation, which determines the depth, and therefore
thickness of an annual accumulation layer. ASIRAS-NP, discussed
in Sect. , interpolates densities between eight
NP density profiles (Sect. ) to calculate layer depths.
ASIRAS-HL, discussed in Sect. , uses
ASIRAS layers with HL density profiles to calculate layer depths.
Density profiles
Neutron-probe density
Detailed density profiles allow for more accurate calculations of ASIRAS
radar travel time through the snowpack. Previous studies along EGIG rely on a maximum of two density profiles
to constrain radar travel time for determination of layer depth. We
expand the range of ground-truthed NP density measurements
by combining 16 previously published density
profiles from eight T-sites (T21a, T23, T27, T31, T35, T39, T41) in the dry-snow
zone above 2700 m (Fig. ). The deepest NP measurements are
11 m, while our deepest ASIRAS layer reaches 30 m depth. We
calculate densities below 11 m using the centimeter resolution
density GISP2 B-core at Summit. The easternmost
NP density measurement (T41) at 10 m depth
(0.538 gm-3) is similar to the GISP2 B-core density value at
10 m core depth (0.529 gm-3). The western edge of EGIG, at
2700 m elevation, has a 10 m density of
0.559 gm-3. We assume similar rates of densification below
10 m depth at T21a, T41, and GISP2. We append the GISP2
profile at 10 m depth to start where T21a NP density profile ends.
Herron and Langway model density
Logistical challenges for both ice coring and NP logging limit spatially
detailed density measurements. 's simple empirical model of polar
snow densification provides an alternative for estimating ice sheet density
in the absence of in situ measurements. The model allows us to generate a density
profile at any point along EGIG with three input parameters: mean annual
accumulation A, mean annual temperature T, and initial surface snow
density ρ0. The model has two stages of densification for
depths above and below the “critical density”
ρ=0.55 Mgm-3. The “critical density” marks the transition from
first-stage rapid densification due to grain settling and packing to second-stage
slowed densification with depth . The model equations used
for density ρ at depth h for the two stages of densification:
Pre-critical: Post-critical: ρh=ρiζ01+ζ0ρh=ρiζ11+ζ1ζ0=expρik0hζ1=expρik1(h-h0.55)A0.5+lnρ0ρi-ρ0+ln0.55ρi-0.55k0=11⋅exp-10 160R⋅Tk1=575⋅exp-21 400R⋅T,
where h0.55=1ρik0[ln0.55ρi-0.55-ρ0ρi-ρ0], k0 and k1
are Arrhenius-type rate constants, and gas constant R=8.314JK-1mol-1, T= temperature
in Kelvin and the density of ice ρi=0.917 Mgm-3.
We calculate densities along EGIG using mean annual accumulation from ,
temperatures calculated using latitude and elevation dependent lapse rates
from , and 's T-site
surface densities as inputs to 's model. We use densities
generated from 's model to adjust radar travel time and derive
the ASIRAS-HL accumulation rate as a comparison to the NP-derived ASIRAS-NP
accumulation rates.
Accumulation rates from ASIRAS
ASIRAS-NP: accumulation rate using neutron-probe densities
The 16 NP density profiles at eight T-sites (T21a, T23, T27, T31, T35,
T39, T41) bound by the combined T21a and GISP2 b-core densities on the west
and the T41 and GISP2 b-core on the east, provide the anchor points for
interpolating depth-density values at every point along the EGIG line. We
calculate annual accumulation rates from 1959 to 2004 at the T-sites
(Table ). We refrain from calculating a 2005 annual accumulation
due to its proximity to the April 2006 ice sheet surface at time of radar and NP
collection. The NP densities represent the most detailed density
measurements along EGIG, correcting ASIRAS travel time
through the snowpack to create the ASIRAS-NP accumulation rates.
ASIRAS-HL: accumulation rate using Herron and Langway densities
Taking detailed NP density measurements remains beyond the scope of most
radar surveys. We explore using simple model densities (HL) to
produce accumulation rates from ASIRAS radar layers (ASIRAS-HL). We examine
the accumulation rate difference when using modeled density data (ASIRAS-HL)
and using NP density (ASIRAS-NP). Using spatially continuous input
parameters of accumulation, temperature, and surface density, we generate HL
density profiles for each radar trace (one approximately every 3 m
horizontally, Sect. ) along the 250 km segment of EGIG.
We examine accumulation rate sensitivity to density by reducing the number of
HL density profiles used to correct layer depth along the 250 km EGIG
segment. Distance intervals and their corresponding number of density
profiles per the 250 km EGIG segment are the following: 250 km (HL250,
1 profile), 125 km (HL125, 2 profiles), 50 km (HL50,
5 profiles), 30 km (HL30, 8 profiles), and 15 km (HL15, 17
profiles). We linearly interpolate between the HL profiles to obtain a density profile
at every point along EGIG.
EGIG-Ground accumulation measurements
We combine shallow firn core records from and NP based
accumulation rates calculated by to establish EGIG-Ground
records spanning 1978–2004 at T21, T27, T31, T41. Site T43 has 12 annual
accumulations from 1976 to 1988 but no NP measurements .
cores generally span 1978 to 1988 while the
accumulation rates span the mid-1980s to 2004. We use the
mean of the two records at site T41 where the studies overlap. Sites T21,
T27, and T31 do not have overlapping records, with no accumulation rates for
years 1989 and 1990 at T21 and T27 and 1989 for T31. These EGIG-Ground
records serve as a basis for comparison of accumulation rates derived from
ASIRAS layers (Fig. ).
Mean annual accumulation rates at T21, T27, T31, and T41 from
ASIRAS-NP, three Regional Climate Models (MM5, MAR, RACMO2.3), and combined
EGIG-Ground measurements from and .
ASIRAS-NP and EGIG-Ground accumulation rates from 1985 to 2004
are not statistically different at the four sites. The RCMs underestimate
accumulation compared to ASIRAS-NP, but succeed in tracking the
general accumulation trend across the 250 km EGIG segment.
ASIRAS-NP comparison to Regional Climate Models
We validate ASIRAS-NP accumulation rates using EGIG-Ground point
accumulation records , then compare our results to
accumulation rates from three Regional Climate Models (RCMs):
Modèle Atmosphérique Régional (MAR) , the
polar version of the Regional Atmospheric Climate Model
(RACMO2.3) , and the calibrated Fifth Generation
Mesoscale Model modified for polar climates (Polar MM5) .
As discussed in Sects. and ,
ASIRAS waveform results from a heterogeneous zone of metamorphosed snow
and thin ice lenses below the winter snowpack .
Therefore we define the ASIRAS-NP accumulation year as representing
October to September. Comparisons between ASIRAS-NP and the RCMs
occur by sampling ASIRAS-NP at the RCM grid points along the
250 km EGIG segment (Fig. ). MAR annual snow accumulation
spans 1958 to 2013 . Data obtained for this study
defines the accumulation year as January to December. MAR has a
25 km spatial resolution, resulting in approximately 11 MAR
accumulation estimates along the 250 km EGIG line.
Mean water-equivalent accumulations rates from 1985 to 2004 along
EGIG line. The solid blue line shows this study's ASIRAS-NP accumulation derived from
ASIRAS layers and NP densities. Black squares depict mean accumulation
from T-sites with NP/Core EGIG-Ground measurements from
and spanning 1985–2004 (T21, T27, T31, T41). Snow accumulation
from three Regional Climate Models, Polar MM5, MAR, and RACMO2.3 is plotted
for comparison. The models generally underestimate accumulation compared to
ASIRAS-NP and EGIG-Ground in situ estimates. Radar-derived accumulation rates
are highest near the coast where density values have the largest range.
Standard uncertainties displayed are for accumulation values from 1985 to
2004.
RACMO2.3 estimates monthly cumulated total surface mass balance (SMB) (with
SMB defined as precipitation minus sublimation, snow erosion, and runoff) for the
period 1958–2013 . We sum RACMO2.3 monthly accumulation
values from October to September to align annual accumulation with the ASIRAS-NP
defined accumulation year of (Sect. ). The RACMO2.3 spatial
resolution of 11 km results in 24 modeled accumulations along
EGIG.
re-sample the Polar MM5's 24 km horizontal
grid output to a 1.25 km equal-area grid using bilinear
interpolation . Polar MM5's hydrologic year spans
from 14 September to 15 September. The 24 km original spacing results in
approximately 11 Polar MM5 data points along the 250 km EGIG line.
The 1.25 km grid down sampled to our 2 km grid
results in 125 points along EGIG. Using generic mapping tools (GMTs)
nearneighbor command with bilinear interpolation
we interpolate from the 1.25 km grid to a 2 km grid spacing.
Results
ASIRAS-NP accumulation rate
The layers detected by ASIRAS with depths calculated using NP profiles,
provide a spatially continuous record of accumulation across 250 km
of the EGIG route. We trace layers to 30 m depth and report
accumulation rates for 46 layers spanning 1959–2004
(Fig. ). The mean accumulation rate for the entire 250 km
EGIG segment is 0.337 mw.e.a-1 from 1959 to 2004. We focus our
reported results on the period 1985 to 2004, during which EGIG-Ground
measurements exist for comparison. Figure displays spatial and
temporal variations in layers across the EGIG segment.
Temporally, accumulation rates increase over time, with the onset of increase
occurring in the mid-1970s. From 1959 to 1964, mean accumulation was
0.277 mw.e.a-1. From 1965 to 1974, mean accumulation was
0.270 mw.e.a-1. From 1975 to 1984, mean accumulation was
0.327 mw.e.a-1. Mean ASIRAS-NP accumulation from 1985 to 1994
was 0.328 mw.e.a-1. Accumulation for the period 1995 to 2004
(0.382 mw.e.a-1) increases by 16 % compared to the
previous 10-year period. Table summarizes the
10-year mean accumulation results. See Table
for detailed accumulations from 1959 to 2004.
Summary table of mean accumulation rates (meters water equivalent) 1959–2004.
Period
ASIRAS-NP
MM5
MAR
RACMO
1959–2004
0.337±0.03
0.288±0.04
0.329±0.03
0.279±0.06
1995–2004
0.382±0.03
0.307±0.05
0.347±0.02
0.306±0.04
1984–1995
0.328±0.02
0.279±0.04
0.326±0.02
0.279±0.06
1975–1984
0.327±0.01
0.299±0.04
0.345±0.03
0.283±0.06
1965–1974
0.270±0.01
0.272±0.04
0.316±0.03
0.257±0.06
1959–1964
0.277±0.01
0.276±0.04
0.300±0.02
0.263±0.05
Spatially, accumulation decreases with increasing elevation and distance from
the coast. Mean annual accumulation for 1985 to 2004 at T21a (0 km)
is 0.455 mw.e.a-1, gradually decreasing to
0.378 mw.e.a-1 at T31, 0.297 mw.e.a-1 at T41,
and 0.254 mw.e.a-1 at the 250 km mark
(Table 2). Using the transition in surface conditions occurring
near T31 to divide EGIG, we examine accumulation rate changes over time above
and below T31. Below T31, mean accumulation increased by 20 % over the
10-year period 1995 to 2004 (0.465 mw.e.a-1) compared
to the 1985 to 1994 period (0.387 mw.e.a-1). Above T31 accumulation
increased by 13 % over the 10-year period 1995 to 2004
(0.335 ma-1) compared to the 1985 to 1994 period
(0.296 ma-1).
ASIRAS-NP and RCMs vs. EGIG-Ground
The published EGIG-Ground in situ accumulation records discussed in
Sect. serve as the “known” accumulation rate to
which we compare our ASIRAS-NP record and the RCMs. We focus on four sites (T21, T27, T31,
T41) that have accumulation records from both and
. Figure presents these comparisons at the
four T-sites. We compare the full records year by year using a nonparametric
Wilcoxon sign-rank test designed for two populations with paired
observations. The differences of the paired observations will
have a distribution whose median is zero at the 5 % significance
level if the two populations are not statistically different.
The zero median of the paired and differenced records indicate the ASIRAS-NP
and EGIG-Ground accumulations come from identical populations. The paired
yearly accumulations for RCMs and EGIG-Ground at the four T-sites are significantly
different based on a Wilcoxon signed-rank test. In addition to the year-to-year comparisons, no statistical difference exists
between EGIG-Ground and ASIRAS-NP mean accumulation from 1985 to 2004
at sites T21, T27, T31, T41 (Table ). We conducted an analysis of variance test to compare
means of the 20-year (1985–2004) annual snow accumulation for
ASIRAS-NP and EGIG-Ground, and the RCMs and ASIRAS-NP accumulation for
1978 to 2004. Significance differences were determined for alpha <0.05. Pearson's
correlation coefficients for ASIRAS-NP and EGIG-Ground
and RCM accumulations for the entire record (1959–2004) range
from 0.56 to 0.19 (Table ).
Summary table of mean accumulation rates (m w.e. a-1), 1985–2004, for
Fig. .
Study
T21
T27
T31
T41
EGIG-Ground
0.488±0.03
0.405±0.02
0.385±0.02
0.294±0.01
ASIRAS
0.455±0.02
0.409±0.02
0.378±0.01
0.297±0.01
MM5
0.370±0.01
0.333±0.01
0.311±0.06
0.257±0.06
MAR
0.373±0.02
0.357±0.02
0.344±0.01
0.322±0.01
RACMO2.3
0.390±0.02
0.348±0.02
0.318±0.01
0.233±0.01
Mean percentage difference between ASIRAS-NP and RCMs for periods 1985–1994, 1995–2004, 1985–2004.
Period
RACMO
MAR
MM5
EGIG
1985–2004
18 %
3 %
17 %
1995–2004
17 %
4 %
13 %
1985–1994
13 %
-4 %
21 %
Below T31
1985–2004
16 %
16 %
20 %
1995–2004
17 %
17 %
16 %
1985–1994
9 %
8 %
25 %
Above T31
1985–2004
18 %
-4 %
15 %
1995–2004
17 %
-3 %
12 %
1985–1994
16 %
-10 %
19 %
ASIRAS-NP vs. Regional Climate Models
Regional Climate Models RACMO2.3, MAR, and Polar MM5, generally
underestimate mean annual snow accumulation along EGIG compared
to ASIRAS-NP (Fig. ). ASIRAS-NP and the RCM accumulation
rates from 1978 to 2004 positively correlate at T-sites T21, T23, T27,
and T41 for MAR, MM5, and RACMO2.3 (Table ). An
ANOVA comparison of means for 1959 to 2004 shows
RCM and ASIRAS-NP mean accumulations are statistically different at T-sites T21, T27, T31, and
T41. Using a Wilcoxon signed-rank test, year-to-year comparisons show
significant statistical differences between all three RCMs and ASIRAS-NP at the
T-sites. RCM mean accumulations along EGIG are
significantly lower for the time period coincident with EGIG-Ground
measurements, 1985 to 2004 (Fig. ). We focus on the period 1985 to
2004 to calculate standard uncertainty (σn, where
σ = standard deviation and n=20).
Mean RACMO accumulation (0.297 mw.e.a-1) from 1985 to 2004
across the entire 250 km EGIG segment is 18 % lower than
ASIRAS-NP (0.356 mw.e.a-1) (Table ). Mean ASIRAS-NP and RACMO
accumulations rates from 1985 to 2004 differ spatially along EGIG (Fig. ). Using the
transition in surface conditions occurring near T31 (3000 m elevation)
to divide EGIG, RACMO underestimates accumulation by 16 % downslope
from T31 compared to ASIRAS-NP. Above T31, mean RACMO accumulation
is 18 % lower than ASIRAS-NP. Accumulation rates change over time with
differing rates above and below T31. RACMO underestimates accumulation
down-slope of T31 by 17 % compared to ASIRAS-NP from 1995 to 2004.
Below T31 from 1985 to 1994 RACMO accumulation estimates are 9 %
lower than ASIRAS-NP. Above T31, RACMO underestimates accumulation
by 16 % for the 1985–1994 period and 17 % for 1995–2004, relative to ASIRAS-NP.
Mean MAR accumulation (0.336 mw.e.a-1) from 1985 to 2004
across the entire 250 km EGIG segment is 3 % lower than
ASIRAS-NP (0.355 mw.e.a-1). Spatially, MAR
both underestimates and overestimates accumulation along EGIG. MAR
underestimates accumulation by 16 % downslope from T31 compared to
ASIRAS-NP. Above T31, mean MAR accumulation rates are 4 % higher than
ASIRAS-NP. Accumulation rates vary over time with differing rates above and
below T31. MAR accumulations from 1995 to 2005 are 17 % lower than
ASIRAS-NP accumulations down-slope of T31. Below T31 from 1985 to 1994
MAR accumulations are 8 % lower than ASIRAS-NP. East of T31,
MAR accumulation is 10 % higher for 1985–1994 and 3 % higher
for 1995–2004, relative to ASIRAS-NP (Table ).
Mean Polar MM5 accumulation (0.293 mw.e.a-1) from 1985 to 2004
across the entire 250 km EGIG segment is 17 % lower than
ASIRAS-NP (0.356 mw.e.a-1). Mean ASIRAS-NP and Polar MM5
accumulations rates from 1985 to 2004 differ spatially along EGIG. Using the
transition in surface conditions occurring near T31 (3000 m elevation)
to divide EGIG, Polar MM5 underestimates accumulation by 20 % downslope
from T31 compared to ASIRAS-NP. Above T31, mean Polar MM5 accumulation
is 15 % lower than ASIRAS-NP. Accumulation rates change over time with
differing rates above and below T31. Polar MM5 underestimates accumulation
down-slope of T31 by 16 % compared to ASIRAS-NP from 1995 to 2004.
Below T31 from 1985 to 1994 Polar MM5 accumulation estimates are 25 %
lower than ASIRAS-NP. Above T31, Polar MM5 underestimates accumulation
by 12 % for the 1985–1994 period and 19 % for 1995–2004, relative to ASIRAS-NP.
ASIRAS-NP vs. ASIRAS-HL: accumulation rate sensitivity to density
Mean percentage accumulation differences between ASIRAS-NP and ASIRAS-HL
decrease with increasing age/depth of the layers (Fig. ).
Figure plots the mean difference between ASIRAS-NP and
ASIRAS-HL for the upper five layers (2000–2004), period (1985–1989), and the 20-year
period 1985–2004 layers. On average the deeper the layer, the lower the difference
between ASIRAS-NP and ASIRAS-HL accumulation rates. The upper five layers
differ by 4 % on average. The 1985–1989 period differs by 3.2 %.
Overall, for the period 1985–2004, mean ASIRAS-HL accumulation is 4.5 %
lower than ASIRAS-NP accumulation.
Mean percentage accumulation difference between ASIRAS-NP and
ASIRAS-HL for the upper five (2000–2004), lower five (1985–1989), and
1985–2004 layers. In general, differences in accumulation decrease with
increasing depth/age of the layers. ASIRAS-HL accumulation differs from
ASIRAS-NP accumulation by 4.5 % for the 1985–2004 period. The low mean
differences across the 250 km EGIG segment indicate that modeled
densities provide accurate accumulation estimates in radar survey regions
lacking in situ density measurements.
We test sensitivity to density by limiting the number of HL density profiles
along EGIG and interpolating density values between the profiles. Using one
density profile (HL250 km) for the entire 250 km EGIG segment
results in a 10 % difference in ASIRAS-NP and ASIRAS-HL. Incorporating
two density profiles (HL125 km) halves the accumulation difference
from 10 to 5 %. The ASIRAS-NP and ASIRAS-HL accumulation difference
reduces to 3 % for HL50 km (5 profiles), HL30 km (8
profiles), and HL15 km (17 profiles).
Discussion
ASIRAS-NP accumulation rate
The ASIRAS layers combined with NP density data improve understanding of
accumulation between T-sites, showing detailed peaks and valleys in
accumulation as seen and attributed to topography by ,
, , . The undulating
layers observed in Fig. reinforce ice core observations of high
spatial variability . Spatial variability decreases with
increasing depth, as layers undergo compaction. The fluctuations of layer
depth and vertically aligned dips and peaks may indicate surface accumulation
anomalies . A gradual horizontal migration of undulations
over time could produce spatially periodic accumulation rates, as described
by . Undulations preserved from year to year are visible
east of T27 at 60 km and from 125 to 175 km along the EGIG
line (Fig. and Fig. ). The oscillations are visible along the 250km
EGIG segment in the long term mean temporal accumulation rate in
Fig. . Visual inspection of layer thickness for a given
year (Figs. and ) allows us to argue for high
confidence in the extreme values measured by ASIRAS-NP.
In comparison to historical records, and
observed accumulation increases of 19 and 10 % over the last 30 and
52 years, respectively, in high-elevation interior Greenland. We
report a 16 % increase in accumulation for the period 1995–2004 compared
to 1985–1994 and a 41 % increase in accumulation for the period 1965–1974 (Table ).
We observe an east–west gradient along EGIG of increasing
accumulation, with lower accumulation increases in the east and higher increases to the
west. The east–west gradient strengthens from 1995 to 2004, when ASIRAS-NP is
20 % higher than ASIRAS-HL below T31 and 13 % higher above T31.
ASIRAS-NP vs. EGIG-Ground
The patterns observed by , ,
at the T-sites align with the overall trend observed by
along EGIG of decreasing accumulation from the coast to the
interior. The year-to-year comparisons from Fig. using Paired
Wilcoxon signed-rank span every year with observations for both ASIRAS-NP and
EGIG-Ground. Year-to-year comparisons show that ASIRAS-NP tracks the
EGIG-Ground measurements consistently. EGIG-Ground accumulation minima and
maxima are not always consistent across the EGIG route for a given year
(e.g., the T31 record's maxima occurs in 1995, while T27 and T41 records have near
minimum values for 1995). We attribute site to site accumulation fluctuation
in the EGIG-Ground record to the limited spatially extent of a given shallow core or
snowpit. Accumulation extremes seen in the ASIRAS-NP record are consistent
across the T-sites (low in 1998, high in 1996).
ASIRAS-NP vs. Regional Climate Models
Polar MM5 underestimates accumulation relative to ASIRAS-NP and EGIG-Ground
(Fig. ). Mean Polar MM5 accumulation from 1985 to 2004 along EGIG
is 0.06 m (17 %) lower than ASIRAS-NP measurements. The mean
0.06 m difference falls within Polar MM5's standard deviation of
accumulation along EGIG (0.025–0.075) . Polar MM5's consistent
underestimate of accumulation relative to ASIRAS-NP and EGIG-Ground
may be explained by the measurements used to tune Polar MM5.
added spatial and temporal resolution to Greenland ice
sheet accumulation by calibrating the Polar MM5 using firn cores and
meteorological stations data. re-sample the Polar MM5's
24 km horizontal grid output to a 1.25 km equal-area grid
using bilinear interpolation . The
24 km original spacing results in approximately eight Polar MM5 data
points along the 250 km EGIG line. These eight Polar MM5 points were
tuned from a network of cores and automatic weather stations and thus were
not forced to correspond exactly with cores along EGIG.
omit the majority of the T-site accumulation rates along EGIG, including only
sites T31, T41, and T43. ASIRAS-NP provides detailed accumulation
measurements every 3 m, nearly continuous tracking along EGIG
relative to Polar MM5. The increased spatial resolution may contribute to
the difference in accumulation rates.
Yearly comparisons of the entire record of ASIRAS-NP and Polar MM5 snow
accumulations (Fig. ) show positive correlations for T21a, T23,
T27, T41, T43 (Table ). Correlations between ASIRAS-NP and Polar
MM5 demonstrate the model's utility for predicting the relative year-to-year
accumulation trend. ASIRAS-NP and Polar MM5 both track the general
coast to interior accumulation gradient as elevation increases (Fig. ).
describe a noticeable change in surface conditions near site T31.
Above T31, summer surface hoar appears undisturbed, possibly from less
persistent winds. Down-slope from T31 katabatic winds pack the upper snow
layer and form sastrugi, which may influence spatial variability and preservation of
accumulation layers. Signal preserved in the upper ASIRAS-NP accumulation layers
would be absent in the Polar MM5 record, possibly explaining Polar MM5's 25 % accumulation underestimate
compared to ASIRAS-NP below T31. Elevations upslope from T31 experience less
persistent winds, leaving a smooth surface with undisturbed summer surface
hoar, with a mean Polar MM5 accumulation 18 % lower than ASIRAS-NP. The
spatial gradient has a noticeable temporal component when comparing ASIRAS-NP
and Polar MM5. From 1985 to 1994, Polar MM5 accumulation is 28 % lower below
T31 and 21 % lower above T31 compared to ASIRAS-NP. The east–west gradient along EGIG strengthens
from 1995 to 2004, when Polar MM5 is 23 % lower than ASIRAS-NP below T31 and
16 % lower above T31. Polar MM5's recent accumulation rates near EGIG
rely on firn cores drilled prior to 1995 and limited automatic weather
stations at high elevation. Thus recent observed increases in accumulation at
high elevation due to increased moisture availability from warming
may not appear in the Polar MM5 record.
ASIRAS-NP vs. ASIRAS-HL: accumulation rate sensitivity to density
Subtracting the ASIRAS-HL and ASIRAS-NP accumulation rates tests the
radar-derived accumulation rate's sensitivity to density. The ASIRAS layers'
position in radar time remains constant between the ASIRAS-NP and ASIRAS-HL.
Density, which determines radar velocity and therefore water-equivalent
depth, is the lone variable between ASIRAS-NP and ASIRAS-HL accumulation
records. The largest differences in accumulation occur where NP
and HL densities differ most. NP density profiles provide detailed vertical
resolution of seasonal density fluctuation. Seasonal density fluctuations are
most prominent in the near surface layers and in areas with large variability
in temperature and accumulation (e.g., coastal, lower elevations).
Though the simple three parameter model cannot capture the
detailed seasonal density variations, the model's generalized density in the
near-surface generates ASIRAS-HL accumulation rates within 4.5 % of
ASIRAS-NP. NP and HL densities resemble each other most at deeper depths as
compaction smooths seasonal fluctuations in density. Thus the deeper layers
have the smallest mean percentage accumulation difference (3 %)
(Fig. ). The low (4.5 %) mean accumulation differences
along EGIG indicate that modeled density values provide reasonable
accumulation estimates in areas with low variability in density and where
detailed density profiles are unavailable. Below 11 m depth, differences are related to
Summit GISP2b-Shifted and Summit GISP2b shallow density core bounding the
west (0 km) and east (250 km) margins, respectively, of the
EGIG line. No dominant spatial pattern of accumulation differences emerges
from west to east. The mean of the lower five accumulation years (1985–1989)
account for the smallest accumulation differences from 60 to 250 km.
The largest differences along EGIG occur on the east end
(225–250 km) where ASIRAS-NP is constrained by a Summit density
core. Abrupt jumps in mean percent accumulation difference occur where
the number of layers is included in the average change.
Recall the spatially continuous nature of the density inputs for the
ASIRAS-HL accumulation record (HL density profiles spaced
3 m apart). These density inputs were driven by highly resolved HL
model inputs of accumulation, temperature and surface density. ASIRAS-HL
accumulation accuracy relative to ASIRAS-NP may be due to these model inputs.
We test this possibility with the HL250 km, HL125 km, etc.,
accumulation records, which rely on a limited number of density profiles and
interpolation. The moderate reduction from 10 to 5 % accumulation
difference for HL250 km and HL125 km is likely due to the
linear gradients (increasing accumulation downslope, decreasing elevation,
increasing temperature) of the HL model input parameters along the
250 km EGIG segment. The two density profiles of HL125 km
cover both the lower and upper range of the gradients. The interpolation
between these two contain the majority of density variation seen in the
ASIRAS-NP, thus accounting for the 4 % mean accumulation difference
between ASIRAS-HL125 km and ASIRAS-NP. An average 3.2 % percent
accumulation difference can be obtained using 5 HL profiles at 50 km
spacing. This finding stands to improve accuracy for radar-derived
accumulation rates and serve as a guideline for correcting the wealth of
IceBridge radar data.
Conclusions
Point-based measurements such as ice cores and weather stations
provide the basis for current accumulation estimates. Models and
interpolation between these points provide spatially continuous estimates of
accumulation. Radar-detected annual accumulation layers offer a physical
observation connecting point-based measurements. Detailed NP density
measurements provide accurate radar travel time velocities and exact
densities for water-equivalent conversion, improving accuracy of annual
accumulation rates from ASIRAS. We report spatially continuous annual
accumulation rates from 1959 to 2004 along a 250 km segment of EGIG.
Our ASIRAS-NP rates are not statistically different from EGIG-Ground point
measurements spanning 1985–2004. Polar MM5 and RACMO2.3 consistently
underestimate accumulation by 17 % along EGIG compared to ASIRAS-NP.
MAR underestimates by as much as 16 % and overestimates by 10 %.
Overall, Regional Climate Models Polar MM5, MAR, and RACMO2.3
succeed in capturing the general trend of accumulation seen by
ASIRAS-NP, but they underestimate the total amount of snow.
The ASIRAS-NP observed increases in mean accumulation may relate
to increased warming and availability of moisture at higher elevations.
The similarity between ASIRAS and EGIG-Ground demonstrates that the ASIRAS
layers, adjusted with NP density, produce accurate estimates of accumulation
along a continuous 250 km segment of the EGIG line. We recognize the
challenge of obtaining detailed density measurements and demonstrate the use
of simple HL models to derive adequate accumulation estimates in the dry snow region.
Using profiles at 50 km intervals produces ASIRAS-HL
accumulation rates within 3 % of ASIRAS-NP estimates. High resolution airborne
radar systems operated in dry snow regions of ice sheets, such as
those onboard Operation IceBridge, calibrated with a minimal number of
modeled density profiles, may produce accumulation rates within the
uncertainty of accumulation best-estimates using detailed density profiles.
Data availability
The original raw ASIRAS data must be accessed via the European Space Agency's EO Data Access site:
https://earth.esa.int/web/guest/data-access/how-to-access-eo-data/how-to-access-earth-observation-data-distributed-by-esa (),
https://directory.eoportal.org/web/eoportal/airborne-sensors/asiras ().
The neutron probe data can be accessed by contacting Elizabeth (Liz) Morris
http://www.spri.cam.ac.uk/people/morris/
(emm36@cam.ac.uk).
See the Assets tab
for DOIs for the ASIRAS-NP and ASIRAS-HL accumulation rates and ASCII
versions of Table 1, Table 2, and Table A1.