The correct derivation of paleotemperatures from ice cores requires exact
knowledge of all processes involved before and after the deposition of snow
and the subsequent formation of ice. At the Antarctic deep ice core drilling
site Dome C, a unique data set of daily precipitation amount, type, and stable
water isotope ratios is available that enables us to study in detail
atmospheric processes that influence the stable water isotope ratio of
precipitation. Meteorological data from both automatic weather station and a
mesoscale atmospheric model were used to investigate how different
atmospheric flow patterns determine the precipitation parameters. A
classification of synoptic situations that cause precipitation at Dome C was
established and, together with back-trajectory calculations, was utilized to
estimate moisture source areas. With the resulting source area conditions
(wind speed, sea surface temperature, and relative humidity) as input,
the precipitation stable isotopic composition was modeled using the
so-called Mixed Cloud Isotope Model (MCIM). The model generally
underestimates the depletion of
Ice cores from the vast ice sheets of Greenland and Antarctica have proven to be of high value in paleoclimate research. Of particular importance is the use of stable water isotope ratios as proxy for deriving past temperatures. However, it has been shown that the calibration of the “paleothermometer” is not as straightforward as originally assumed. Various factors apart from air temperature influence the stable isotope ratio, both before and after the deposition of the snow that develops into ice by metamorphosis. Postdepositional processes were thought to occur mainly within the snow pack, firn, or ice. Recent studies have shown, however, that the interaction between the uppermost layers of the snowpack and the overlying atmosphere between precipitation events also plays an important role. This was found in both Greenland (Steen-Larsen et al., 2013; Bonne et al., 2014) and Antarctica (Casado et al., 2016a, b; Ritter et al., 2016; Touzeau et al., 2016) as well as in laboratory experiments (Ebner et al., 2017).
In this study we focus on the processes before deposition, namely atmospheric processes related to moisture transport and precipitation formation. The precipitation data used here enable us to exclude postdepositional processes to study the purely atmospheric influence on precipitation. Since the stable water isotope ratio changes during evaporation and condensation processes (Dansgaard, 1964), it is important to know as much as possible about the history of the precipitation observed at an ice core drilling site, specifically moisture source, moisture transport paths, and meteorological conditions at both the moisture source and the deposition site. Precipitation measurements in Antarctica are rare due to the large technical difficulties of measuring precipitation at extremely low temperatures or high wind speeds. However, at the deep-drilling location Dome C on the East Antarctic plateau, a series of precipitation data has been collected that includes not only precipitation amounts but also precipitation type and stable isotope ratios. This unique data set can be combined with a full meteorological data set including radiosonde data, automatic weather station (AWS) data, and atmospheric model data. This, for the first time, allows us to study in detail the synoptic conditions that lead to precipitation at Dome C and how they are related to the precipitation stable isotope ratios. We compare our results to those of a similar study carried out by Dittmann et al. (2016) for Dome Fuji, Dronning Maud Land (DML), another deep ice core drilling site, where a 1-year series of combined stable isotope and precipitation data is available. In both studies exactly the same methods were used for calculation of transport pathways and isotopic fractionation as well as for synoptic analysis, which is highly valuable as often past studies have site-specific approaches, making comparisons very challenging.
Since the ground-breaking work of Dansgaard (1964), stable water isotopes
have become one of the most important parameters measured in ice cores. An
empirical linear relationship was found between the annual mean air
temperature (derived from the 10
A variety of models is used to simulate isotopic fractionation, from simple Rayleigh-type distillation models to fully three-dimensional atmospheric circulation models. So far, most models are still based on the early theories developed by Jouzel and Merlivat (1984). Ciais and Jouzel (1994) extended this theory to mixed clouds in their Mixed Cloud Isotope Model (MCIM), which is described further in the methods section.
Kavanaugh and Cuffey (2003) developed a model of intermediate complexity
(ICM), more complex than simple Rayleigh-type models but not as
sophisticated as general circulation models (GCM), to study how variations
in single climate parameters or in fundamental characteristics of isotopic
distillation affect the stable isotope ratio of polar precipitation.
Schoenemann and Steig (2016) applied their model to
In the discussion of sea level rise, the possibility of a mitigation of sea level rise by increased Antarctic precipitation, the most important component of the surface mass balance, is often mentioned (e.g., Church et al., 2013). However, the relationship between stable isotope ratios and precipitation or accumulation is yet fully understood. Most commonly, the assumption of a positive correlation between stable isotope ratio (as proxy for air temperature) and accumulation rate has been used based on the relationship between temperature and saturation water vapor pressure (Clausius–Clapeyron). However, contrasting results are found in the recent literature. While Frieler et al. (2015), using both model and ice core data, state that Antarctic accumulation increases with rising air temperature, Fudge et al. (2016) found that the relationship between accumulation and temperature has not been constant over the past 30 000 years in West Antarctica. They stated that atmospheric dynamics play a more important role than thermodynamics, which had also been found by Altnau et al. (2015) and Schlosser et al. (2014) in coastal DML.
In the past, precipitation in the interior of the Antarctic continent was
only poorly understood because only a few meteorological observatories have
existed in continental Antarctica. A analysis of satellite imagery has
brought only limited progress due to the difficulty of distinguishing
between clouds and the snow surface (Massom et al., 2004). Since the
improvement of global and mesoscale atmospheric models, however, our
knowledge has advanced considerably. Noone et al. (1999) studied
precipitation conditions in DML using ECMWF reanalysis
data. They found that 89
Synoptic events with blocking anticyclones were also described by Scarchilli et al. (2011), Massom et al. (2004), and Hirasawa et al. (2000). At the deep-drilling site Dome Fuji, while warm air advection combined with orographic lifting sometimes was not sufficient for precipitation formation, it did cause the removal of the prevalent temperature inversion layer by cloud formation that increased the downward long-wave radiation and by turbulent mixing (Enomoto et al., 1998; Hirasawa et al., 2000). Also, increased amounts of diamond dust can be observed after a synoptic snowfall event when moisture levels are still higher than on average (Hirasawa et al., 2013; Dittmann et al., 2016; Schlosser et al., 2016).
Dittmann et al. (2016) analyzed the only other daily precipitation
For ice core interpretation, these findings are important since they contradict the older assumption that precipitation in the interior Antarctica is predominantly diamond dust and thus exhibits only a weak seasonality. This implies that all seasons are represented evenly in the ice core. If, however, the synoptic snowfall occurs preferably in certain seasons and/or this preference is not constant in different climates, potentially a cold or warm bias would be found in the temperatures derived from stable water isotopes of an ice core. An understanding of the atmospheric circulation and its influence on precipitation conditions at deep-drilling sites is therefore essential for a correct interpretation of the ice core proxy data.
For Antarctica, only very few studies exist, that combine daily
precipitation
Dome C (75.106
Precipitation has been measured and sampled at Dome C since 2006 (with some
interruptions in the early time period) and this sampling is ongoing. A
wooden platform of approximately 1 m height, covered by a polystyrene/teflon
plate, is used to measure daily precipitation amounts. The elevated platform
is surrounded by a rail of 5
Furthermore, the crystal type of the precipitation is analyzed, so that
diamond dust, drift snow, and regular snowfall can be distinguished. Diamond
dust forms due to radiative cooling of almost saturated air and consists of
very fine needles. Mixing of a warmer, moister air mass with cold air can
also lead to supersaturation of the cold air and consequent ice crystal
formation. Synoptic snowfall is marked by various types of snow crystals
that depend mainly on air temperature during crystal formation, whereas
drift snow can be recognized by broken crystals. Also, a mixing of crystal
types can be observed. Note that the precipitation amounts are so small that
errors in quantification can amount to 100
We note that
A detailed description of the measurements and a first analysis of the stable isotope data are provided by Stenni et al. (2016).
Radiosonde data from the meteorological station at Dome C are used to determine the temperature at both the top of the surface inversion layer and the condensation level. The upper-air data are provided by the Meteo-Climatological Observatory of the Italian Antarctic Research Program (PNRA). Since the beginning of the measurements in 2005, a radiosonde has been launched every day at 12:00 UTC, unless excessive wind speeds prevent it. For each standard pressure level, geopotential height, air temperature, humidity, and wind are measured, and the data are delivered as TEMP files to the WMO (World Meteorological Organisation) Global Telecommunication System (GTS).
The current AWS, named Dome C II, was
installed by the Antarctic Meteorological Research Center (AMRC) in 1995.
The AMRC and AWS programs are sister projects of the University of
Wisconsin-Madison, which are funded by the United States Antarctic Program
(USAP). USAP provides real-time and archived weather observations and
satellite imagery and supports a network of AWSs across Antarctica. At the
AWS, standard meteorological variables, namely air temperature, surface
pressure, wind speed and direction, and humidity, are measured. AWS data can
be found at
The AMPS (Powers et al., 2003, 2012) is a real-time numerical weather prediction system run to provide guidance for the weather forecasters of the USAP. It has been operated by the National Center for Atmospheric Research (NCAR) in support of the USAP since 2001, at first employing the polar version of the Fifth-Generation Pennsylvania State University/NCAR Mesoscale Model (Polar MM5). Since 2006 AMPS has used the Weather Research and Forecasting (WRF) model. The performance of WRF in AMPS and in Antarctica has been verified in a number of previous studies (see, e.g., Bromwich et al., 2005, 2013; Deb et al., 2016), while model output from AMPS has supported various Antarctic investigations (e.g., Powers, 2007; Nigro et al., 2011, 2012). The AMPS archive is the repository of gridded output from AMPS from over the years (Powers et al., 2012), and WRF gridded output from the archive has supported numerous studies (Seefeldt and Cassano, 2008, 2012; Schlosser et al., 2010a, 2016). Here, AMPS archive data from the period 2008–2010 are used here in analyses of the meteorological conditions affecting Dome C and its precipitation.
For the period analyzed in this study, the AMPS WRF configuration consisted
of a nested domain setup with grids of 45 and 15
In this study, AMPS archive data are utilized to investigate the synoptic
situation that lead to precipitation and to estimate moisture sources for
the precipitation events. Fully three-dimensional 5-day back trajectories
were calculated with the RIP4 software (Stoelinga, 2009) and together with
500
Trajectories were calculated for three different arrival levels: 300,
500, and 600
The so-called MCIM is a simple Rayleigh-type model that, however, allows the co-existence of water droplets and ice crystals and, as such, is the consequent further development of the basic distillation model established by Jouzel and Merlivat (Jouzel and Merlivat, 1984; Merlivat and Jouzel, 1979). It is still widely used in ice core studies and also is the basis for implementation of stable isotopes in GCMs or climate models. The model calculates fractionation in an isolated air parcel between the initial evaporation and the final precipitation. In contrast to a pure Rayleigh model, an adjustable part of the condensate stays in the cloud. In a likewise adjustable range of temperatures, both liquid droplets and ice crystals occur in the cloud, which causes additional kinetic fractionation processes due to the Bergeron–Findeisen effect: because of the different saturation vapor pressure with respect to ice and water, the actual vapor pressure lies between the saturation vapor pressure above water and that above the ice. This means a subsaturated environment for liquid water but a supersaturated environment for ice. This results in a net transport of water vapor from the droplets to the ice, with fractionation during evaporation from the droplets and deposition (i.e., negative sublimation) on the ice crystals. No fractionation is associated with freezing of liquid droplets since the freezing is rapid (Ciais and Jouzel, 1994). The initial isotopic composition of the vapor after the first evaporation is calculated assuming a balance between evaporation and condensation. Details about MCIM can be found in Ciais and Jouzel (1994) and Dittmann et al. (2016).
Histogram of daily precipitation amounts for
Frequency of the different precipitation types of observed precipitation.
Figure 1 shows a histogram of daily precipitation amounts at Dome C for the
period 2008–2010 derived from (a) measurements and (b) AMPS archive data. It
shows a positively skewed distribution: in both model and observations, a
large number of extremely small amounts are observed compared to only a few
events with more than 0.2
Note that Fig. 2 only displays the number of days with the observed
precipitation type and does not take into account snowfall amounts. Snowfall
days at higher temperatures are less frequent than those at temperatures
below
Synoptic patterns classification:
500
Figure 3 displays the wind direction at the AWS Dome C II for (a) all days
and (b) only observations with wind speeds above 10
Based on mainly 500 Blocking anticyclone Figure 4a shows the 500 Weak anticyclone with northwesterly flow Figure 4b displays similar fields as in Fig. 4a, the 500 Anticyclone with northeasterly flow In Fig. 4c a special case of the earlier examples is shown: specifically,
the flow here is northeasterly rather than northwesterly. In this synoptic
pattern, often a cutoff low or upper-level low is situated north or
slightly northwest of the coast of Wilkes Land. The flow is directed around
the cutoff low towards Dome C. While the distance to the coast is similar
for a northwesterly and a northeasterly flow, some dynamic lifting of the
air mass above the ocean might be involved in addition to the orographic
lifting. This should be studied in a future investigation. Splitting of flow In contrast to conditions determined from studies for Dome Fuji and Kohnen
Station in DML, Dome C relatively often experiences a
situation where the planetary waves are amplified, but the flow is split
into a zonal part, in which Dome C is situated, and a meandering part with
the strong trough and ridge in the amplified flow staying north of the
Dome C region. While this leads to reduced advection of warm and moist air to
Dome C, it can still cause precipitation formation. As the air mass
originates farther south than in the cases described above, the meridional
exchange of heat and moisture is smaller. Flow from West Antarctica Another situation that has not been found at other deep-drilling sites is
that relatively warm and moist air is advected to Dome C from the
Amundsen–Bellingshausen seas across Marie Byrd Land. In the 500 Post-event increased moisture Several cases, for which AMPS shows very low or no precipitation, exhibit
increased amounts of measured precipitation at Dome C. The precipitation was
classified as diamond dust, but the events showed amounts that were
atypically high for this type of precipitation. It was found that these
cases, which did not show the northerly flow connected to advection of
relatively warm and moist air, usually occurred after a synoptic snowfall
event had happened. This implies that the available moisture was still
increased, and AMPS shows a fairly large, isolated area of weak
precipitation almost centered at Dome C.
Sea level pressure from AMPS (domain 1) for 3 May 2007, 00:00 UTC.
AMPS wind speed
The AMPS wind direction for synoptic precipitation events only, identified
in the AMPS data, is displayed in Fig. 6. Contrary to the average conditions
displayed in Fig. 3, which have a pronounced preference for the southeast
sector, for snowfall events the most frequent wind direction is NNW to NW,
with almost no cases displaying flow from the SW sector. Also, the highest
wind speeds (12–14
Observed wind speed at AWS vs. observed and modeled 24 h precipitation at Dome C.
This wind influence also becomes clear from Fig. 7, in which the relationship between precipitation amounts and wind speed is illustrated. Precipitation amounts are related to wind speed for (a) observations and (b) AMPS archive data. Again, it has to be considered that days with high wind are mostly related to synoptic snowfall events that have high precipitation amounts in AMPS but cannot be seen in the observation since the snow has been blown off the measuring platform and thus not been recorded. Thus, Fig. 7b seems to be more realistic than Fig. 7a, with larger precipitation amounts at correspondingly higher wind speeds. Surface mass balance data from firn cores and a stake array suggest that AMPS precipitation has a positive bias, whereas the total amounts measured at the platform are too low, which seems plausible considering the above mentioned mass losses due to removal of snow from the platform by the wind. Since all three methods have considerable error possibilities, we refrain from a more specific numeric quantification of these findings.
Observed and modeled
Figure 8a shows observed
In Fig. 9 the observed and modeled
The moisture sources for arrival levels 600 and 500
Estimated moisture source areas for arrival levels
It should be kept in mind, though, that MCIM is a relatively simple model
with various strong simplifications, such as assumptions of a single
moisture source, a single temperature inversion, and a humidity inversion
parallel to the temperature inversion. Additionally, it is assumed that the
500
Dome C and Dome Fuji are both deep ice core drilling sites with the oldest
ice ever drilled on earth (800 000 and 720 000 years, respectively; EPICA
community members, 2004; Motoyama, 2007). At 3810
Whereas daily precipitation measurements are available at Dome Fuji for only 1 year, the Dome C series is a multiyear time series having been continued to the present. For our study, the years 2008–2010 were analyzed. In addition to the type of data used in the Dome Fuji study, at Dome C upper-air data and crystal type data were available for our study. The synoptic situations responsible for precipitation are fairly similar for both stations (basically related to amplified Rossby waves); however, cases specific to either Dome C or Dome Fuji do occur: Whereas for Dome Fuji this refers to a situation with moisture advection from the south (via Kohnen Station, another deep-drilling site; Schlosser et al., 2010a, b), for Dome C the moisture is advected via West Antarctica and the flow related to the ASL. The latter is of special importance for glacial periods when the topography of the ice sheet was different from today.
The case of splitting of the flow (Fig. 4d) did not occur in the Dome Fuji study, but, given the shortness of the investigation period, we cannot rule out the possibility that this situation also occurs in the Dome Fuji area.
Our study generally confirms the results of the Dome Fuji study. Both studies pointed out that the synoptic situation of amplified waves with strongly developed troughs and ridges lead to increased meridional exchange of heat and moisture. For interpretation of stable isotope profiles from ice cores, this means that a more northern moisture source does not necessarily mean a stronger depletion in heavy isotopes since the temperature difference between moisture source and deposition site is reduced. Also, based on daily values or precipitation events, no correlation between deuterium excess and moisture source conditions could be found for either location. Most earlier studies deal with longer time periods (from at least monthly–seasonal to glacial–interglacial changes).
The first and only multiyear data series of daily precipitation amounts, precipitation type, and stable isotope ratios at an Antarctic deep ice core drilling site was combined with output from a mesoscale atmospheric model and a simple isotope model to study the influence of the precipitation regime on the corresponding stable water isotope ratios.
Here we present the first complete classification of synoptic patterns for precipitation events at Dome C for 2008–2010. Snowfall events with precipitation amounts an order of magnitude larger than diamond dust precipitation were often associated with amplification of Rossby waves in the circumpolar trough with increased meridional transport of heat and moisture.
In contrast to other deep-drilling sites in East Antarctica, such as Dome Fuji (Dittmann et al., 2016) and Kohnen (Schlosser et al., 2010a, b), at Dome C in some cases a moisture transport from West Antarctica across the continent occurred. This is particularly interesting due to its relation to the ASL. Strength and location of the ASL have a strong influence on meridional exchange of heat and moisture in West Antarctica (Raphael et al., 2016).
The
Note that diamond dust is not parameterized in the WRF model used in AMPS. Nevertheless the model output used here yields only 6 days with no precipitation at all in the study period.
Modeled stable isotope ratios showed a “warm” bias compared to the observations, which was also found in previous similar studies (e.g., Steen-Larsen et al., 2017).
However, using the condensation temperature at Dome C derived from radiosonde data as model input (rather than the temperature at the top of the inversion layer or the temperature at the arrival levels of the calculated trajectories) did not improve the correlation between observed and modeled isotope ratios; in fact, the correlation coefficient decreased considerably and was no longer significant, most likely because the condensation temperature determined from the radiosonde data displayed only a weak annual cycle. More detailed studies of vertical humidity and temperature profiles during precipitation are necessary to understand this result. However, at present, no explanation for this can be offered. The assumption generally used in ice core studies (e.g., Stenni et al., 2016) that the temperature at the top of the inversion layer represents the condensation temperature could not be proven.
No correlation was found between observed deuterium excess and relative humidity at the estimated moisture source, which is contradictory to measurements by Uemura et al. (2008) and Steen-Larsen et al. (2014). Whether this has general physical reasons or is due to the fact that we studied individual events or to errors in moisture source estimation cannot be determined with the given data set.
It was also found that a more northern moisture source does not – as commonly assumed – necessarily mean stronger depletion of heavy isotopes, since the advection of warm air associated with snowfall events reduces the temperature difference between oceanic moisture source and deposition site and thus reduces the strength of the distillation. This confirms the recent results of Dittmann et al. (2016) found at the deep-drilling site Dome Fuji for a 1-year time period.
With the extension of the data series in the future it will be possible to
calculate statistically significant delta–
The precipitation amount and stable isotope data are available as a Supplement to Stenni et al. (2016) at
BS is responsible for the precipitation measurements and stable isotope analysis, MV and AC for the crystal analysis, and PG and CS for the radiosonde data provision and analysis. AD carried out the stable isotope modelling, with contributions by VMD, and the comparisons of observations with modeled meteorological and isotope data. ES did the analysis of synoptic patterns, where AMPS data analysis was supported by JGP and KWM. The manuscript was prepared by ES, AD, JGP, and KWM with constructive comments of the other co-authors.
The authors declare that they have no conflict of interest.
This study was funded by the Austrian Science Funds (FWF) under grants
P24223 and P28695. AMPS is supported by the US National Science
Foundation, Division of Polar Programs. The precipitation measurements at
Dome C have been carried out in the framework of the Concordia station and
ESF PolarCLIMATE HOLOCLIP projects. We appreciate the support of the
University of Wisconsin-Madison Automatic Weather Station Program with the
Dome C II data set (NSF grant numbers ANT-0944018 and ANT-12456663).
Radiosonde data and information were obtained from IPEV/PNRA Project
“Routine Meteorological Observation at Station Concordia” (