266
A. Pellegrini et al.
consequently, on ocean-atmosphere energy exchange and température. Ledley [6]
pointed out that the mechanisms causing this influence are: (1) snow acts as an
insulator, keeping the sea ice relatively warm and thin during the winter and thus
easier to melt away; (2) snow has a lower volumétrie spécifie heat than the ice,
causing it to cool and warm more rapidly than ice; and (3) snow generally has a
higher albedo than sea ice, reflecting more solar radiation and thus producing
thicker sea ice. As a resuit, the addition of a constant précipitation rate resulting
in a snow cover on sea ice produces a cooling effect in the polar régions.
However, such changes are not easily monitored by direct observations. Gauge
déterminations of snowfall are strongly contaminated by wind transport of previously deposited snow. In addition, snowfall amounts at high polar élévations are
often too small for confident measurement. Glaciological accumulation values are
a potentially important source of précipitation data, but the requirements for
broad areal coverage, the lack of annual time resolution in many régions and the
uncertain impact of wind reworking are problematic [7].
In the following sections a method to identify épisodes of snowfall over North
Victoria Land, and particularly over Terra Nova Bay, is described. In Section 2
some characteristics of the Antarctic continent are presented. This is followed in
Section 3 by a depiction of the space-time distribution of relative humidity in the
studied area. Section 4 présents the method used to analyse épisodes of précipitation. In Sections 5 and 6 some cases of mesoscale cyclogenesis and
blowing/drifting snow are analysed. In Section 7 the agreement between the
direct observations of précipitation over Terra Nova Bay area and the épisodes of
probably snowfall obtained by analysing the main meteorological parameters
recorded at the different stations is explored. Section 8 summarises some conclusive considérations.
2 Study Area
Antarctica is essentially an ice-covered mountain surrounded by océan with an
average élévation of 2500 m and with very low moisture content. Zonal mean précipitable water at 80°S is only 1.5 kg m-2 in the annual average according to
Peixoto and Oort [8]. The Antarctic Plateau is one of the two largest deserts of the
world, the other one being the Sahara. Almost the entire South Polar Desert area
comprises East Antarctic, while West Antarctica gets somewhat more précipitation. However, research on précipitation is hampered by the low rate of occurrence and the interférence from wind-blown surface drift. Besides this, wherever
a sutficient amount of snow is available, strong winds can drastically modify the
surface, changing it from a smooth plane to a field of sastrugi, or compiling and
moving snow dunes, increasing the surface roughness considerably [9].
Figure 1 shows the Victoria Land area, identifying points of interest and showing automatic weather station (AWS) sites. In this area, the Terra Nova Bay (TNB)
Station (Italy) and McMurdo Station (USA) are located and both Antarctic
Programmes deployed AWSs in the région: squares mark US ones, circles the
Italian ones.
1 he meteorological measurements at these stations (including the manned sta-
A. Pellegrini et al.
consequently, on ocean-atmosphere energy exchange and température. Ledley [6]
pointed out that the mechanisms causing this influence are: (1) snow acts as an
insulator, keeping the sea ice relatively warm and thin during the winter and thus
easier to melt away; (2) snow has a lower volumétrie spécifie heat than the ice,
causing it to cool and warm more rapidly than ice; and (3) snow generally has a
higher albedo than sea ice, reflecting more solar radiation and thus producing
thicker sea ice. As a resuit, the addition of a constant précipitation rate resulting
in a snow cover on sea ice produces a cooling effect in the polar régions.
However, such changes are not easily monitored by direct observations. Gauge
déterminations of snowfall are strongly contaminated by wind transport of previously deposited snow. In addition, snowfall amounts at high polar élévations are
often too small for confident measurement. Glaciological accumulation values are
a potentially important source of précipitation data, but the requirements for
broad areal coverage, the lack of annual time resolution in many régions and the
uncertain impact of wind reworking are problematic [7].
In the following sections a method to identify épisodes of snowfall over North
Victoria Land, and particularly over Terra Nova Bay, is described. In Section 2
some characteristics of the Antarctic continent are presented. This is followed in
Section 3 by a depiction of the space-time distribution of relative humidity in the
studied area. Section 4 présents the method used to analyse épisodes of précipitation. In Sections 5 and 6 some cases of mesoscale cyclogenesis and
blowing/drifting snow are analysed. In Section 7 the agreement between the
direct observations of précipitation over Terra Nova Bay area and the épisodes of
probably snowfall obtained by analysing the main meteorological parameters
recorded at the different stations is explored. Section 8 summarises some conclusive considérations.
2 Study Area
Antarctica is essentially an ice-covered mountain surrounded by océan with an
average élévation of 2500 m and with very low moisture content. Zonal mean précipitable water at 80°S is only 1.5 kg m-2 in the annual average according to
Peixoto and Oort [8]. The Antarctic Plateau is one of the two largest deserts of the
world, the other one being the Sahara. Almost the entire South Polar Desert area
comprises East Antarctic, while West Antarctica gets somewhat more précipitation. However, research on précipitation is hampered by the low rate of occurrence and the interférence from wind-blown surface drift. Besides this, wherever
a sutficient amount of snow is available, strong winds can drastically modify the
surface, changing it from a smooth plane to a field of sastrugi, or compiling and
moving snow dunes, increasing the surface roughness considerably [9].
Figure 1 shows the Victoria Land area, identifying points of interest and showing automatic weather station (AWS) sites. In this area, the Terra Nova Bay (TNB)
Station (Italy) and McMurdo Station (USA) are located and both Antarctic
Programmes deployed AWSs in the région: squares mark US ones, circles the
Italian ones.
1 he meteorological measurements at these stations (including the manned sta-