Extraordinary blowing snow transport events in East Antarctica

Claudio Scarchilli 0 1 2 3 Massimo Frezzotti 0 1 2 3 Paolo Grigioni 0 1 2 3 Lorenzo De Silvestri 0 1 2 3 Lucia Agnoletto 0 1 2 3 Stefano Dolci 0 1 2 3 0 C. Scarchilli M. Frezzotti (&) P. Grigioni L. De Silvestri L. Agnoletto ENEA, Rome, Italy 1 S. Dolci Consorzio P.N.R.A. S.C.r.l., Rome, Italy 2 S. Dolci CNR, Rome, Italy 3 C. Scarchilli L. Agnoletto Dipartimento di Scienze della Terra, Universita` di Siena , Siena, Italy In the convergence slope/coastal areas of Antarctica, a large fraction of snow is continuously eroded and exported by wind to the atmosphere and into the ocean. Snow transport observations from instruments and satellite images were acquired at the wind convergence zone of Terra Nova Bay (East Antarctica) throughout 2006 and 2007. Snow transport features are well-distinguished in satellite images and can extend vertically up to 200 m as first-order quantitatively estimated by driftometer sensor FlowCaptTM. Maximum snow transportation occurs in the fall and winter seasons. Snow transportation (drift/blowing) was recorded for *80% of the time, and 20% of time recorded, the flux is [10-2 kg m-2 s-1 with particle density increasing with height. Cumulative snow transportation is *4 orders of magnitude higher than snow precipitation at the site. An increase in wind speed and transportation (*30%) was observed in 2007, which is in agreement with a reduction in observed snow accumulation. Extensive presence of ablation surface (blue ice and wind crust) upwind and downwind of the measurement site suggest that the combine processes of blowing snow sublimation and snow transport remove up to 50% of the precipitation in the coastal and slope convergence area. These phenomena represent a major negative effect on the snow accumulation, and they are not sufficiently taken into account in studies of surface mass balance. The observed wind-driven ablation explains the inconsistency between atmospheric model precipitation and measured snow accumulation value. 1 Introduction Since the first expedition of Scott, Priestley, and Mawson, the slope and coastal areas of the East Antarctic Ice Sheet are known as the area of our planet with the highest winds and blowing snow. Nowhere else on Earth does a single meteorological element (wind) has such an overwhelming influence on the climate of an entire continent (Wendler et al. 1993). Strong katabatic winds blow throughout the year, and a large but unknown fraction of the snow that falls on the ice sheet is continuously exported to the atmosphere and the Southern Ocean. These processes constitute a significant negative term in the surface mass balance (SMB). SMB is given by (Dery and Yau 2002): Fig. 1 Satellite image in false colour (MODIS Terra, 9 November 2004) of the Terra Nova Bay area showing automatic weather station (AWS) locations, blue ice area (dark blue) and wind crust area (light blue). Contour lines every 250 m and snow accumulation survey site are indicated. Nansen Ice Sheet (NIS), Mario Zucchelli Station (MZS), Midpoint (MdPt) where SMB indicates the surface mass balance, P denotes precipitation (snowfall), E is evaporation (surface sublimation), QM represents snowmelt, QS is the blowing snow sublimation, QT is the snow transport and rQT is the divergence of snow transport. The extreme environmental conditions and remote location of Antarctica have long inhibited the systematic study of its climate and snow accumulation processes. Measurement of blowing snow in Antarctica is very difficult and limited, and data are only available for a few sites (e.g., Mann et al. 2000; Nishimura and Nemoto 2005). Atmospheric models estimate that the horizontal divergence of snow by wind transport is of minor significance for integrated ice sheet SMB because the model simulations assume that katabatic winds tend to remove mass from the interior regions of the continent and displace it to coastal/convergence areas (Dery and Yau 2002; van den Broeke et al. 2006). Moreover, the blowing snow process and direct export into the ocean are not explicitly included in numerical weather forecasting and general circulation models (Gallee et al. 2001; Genthon and Krinner 2001; Krinner et al. 2007; van den Broeke et al. 2006). On the contrary, analysis of satellite images and field observations show that the wind crust and blue ice areas (Fig. 1), characterised by SMB from nil (-10 kg m-2 year-1 \ wind crust B 10 kg m-2 year-1) to strong negative (-500 kg m-2 year-1 \ blue ice area B 10 kg m-2 year-1), are very well-distributed on the katabatic wind convergence area and the coastal/slope area, representing more than 50% of the surface of the slope and coastal area within 300 km of Terra Nova Bay (Frezzotti et al. 2002b, 2004). Snow accumulation surveys show that ablation by wind-driven processes represent from 20% to more than 80% of the solid precipitation, and that depositional processes are very rare and negligible in the SMB (Frezzotti et al. 2004, 2007). Gallee et al. (2005) compared snow accumulation measurements and model outputs for the same time periods, the model is found to simulate well the spatial and temporal variability of the SMB when take in account wind erosion. In this study, field measurements are integrated with satellite image analysis to make the first-order quantitative estimation of blown snow exported to the atmosphere and ocean in a slope/coastal katabatic wind convergence area, the Terra Nova Bay (East Antarctica). 2 Site characteristics and instruments The Larsen Glacier (Fig. 1; 74 570S, 161 460E; about 1,350 m above sea level in altitude) is located between the Reeves and David Glaciers, about 50 km inland from Terra Nova Bay (Northern Victoria Land, East Antarctica). Katabatic winds that frequently exceed 25 m s-1 blow from the south and converge along the David, Reeves, and Priestley valleys in Terra Nova Bay, deflecting from the S-N to W-E and NW-SE directions (Bromwich et al. 1990). (Bintanja 2001). However, field snowdrift experiments in Antarctica (Mann 1998; Bintanja and Reijmer 2001) find little or no deviation from a logarithmic fit in snowdrift conditions. The g(z) continuous profiles are evaluated by fitting the g discrete values to the power law for steady state conditions (Shiotanai and Arai 1967): where g(zr) is the density at a reference height zr, u* the friction velocity and wfaveraged is the mean falling velocity of a snow particle. It is considered as independent of particle size (Mann et al. 2000) and it is calculated as the second parameter in the previous fitting. The wfaveraged and u* are considered independent of height (Bintanja 1998). The transport rate QT (kg m-1 s-1), is calculated as: The site is at the limit of the Antarctic plateau before an orographic jump of more than 1,200 m over less than 20 km from the plateau to the coast. Another instrument was installed at midpoint (MdPt), a plateau site about 600 km from the coast (Fig. 1: 75 320S, 145 510E; 2,510 m above sea (...truncated)