Meteorological Conditions During Snowfall at Terra Nova Bay (Antarctica)
269
extends from west to east into the sea with the Drygalski Ice Tongue. The Reeves
Névé, which is fed by the East Antarctic Ice Sheet, and the Priestley Névé, which is
nourished by the Talos Dôme, flow into the Reeves and Priestley Glaciers, respectively. These two glaciers end in the Nansen Ice Sheet, an ice shelf which is pinned
to Inexpressible Island and to Northern Foothills. Other main glaciers, such as the
Aviator, the Campbell and the Mawson, and numerous tributary glaciers flow into
the main outlet glaciers.
Ice accumulation in central East Antarctica has very low values, resulting in
less than 2.5 cm/year1 in depth. Due to orographie lifting of moist air masses
flowing southward toward the edge of the ice sheet, and to decreasing distance
from the moisture source, accumulation rates increase toward the coast. Another
factor determining accumulation is the air température; moisture-carrying
capacity of air diminishes strongly as its température decreases. So we can expect
that when air températures increase snowfall will also increase, although a quantitative relationship has not been determined [15],
3 Humidity of Air Over North Victoria Land
At a given point, the snow surface can accumulate mass by précipitation, condensation, déposition and wind transport; on the other hand, snow can be removed
by évaporation, sublimation and wind transport. Considering ail these factors, up
to the présent the problem of finding some way to record the amount of snowfall
has not been solved. Schwerdtfeger [9] reports that it has sometimes been
observed that a gage was empty after an intensive snowfall and strong winds,
whereas plenty of wind-driven snow had entered the collecter on a day without
any real précipitation. However, significant progress has recently been made in
techniques to directly and indirectly détermine Antarctic précipitation amounts.
In this chapter an approach is proposed to identify épisodes of précipitation
over North Victoria Land, in particular the Terra Nova Bay area.
Précipitation characteristics at the coast and in the high interior are very different. Coastal snowfall is episodic in association with synoptic-scale features
such as cyclones and fronts [10]. In the interior, the portion of précipitation associated with clouds decreases, and the semicontinuous fall of ice crystals from
cloud-free skies becomes progressively more important as élévation increases. In
fact, in Antarctica the cloud-forming processes are weakly developed and the rôle
of the air mass clouds in précipitation is small. Consequently, one can infer that
the entire snow accumulation in Antarctica, estimated to be 2480 km' water year ',
is due to this advection of water vapour from the oceanic régions. Thanks to this
advection of water vapour, 61.7% of the world's reserves of freshwater are concentrated in Antarctica [16]. So information on humidity is indispensable for a
study of the laws of moisture transport in the atmosphère, the processes of cloud
and précipitation formation and heat balance [17].
Considering the monthly and annual averages of relative humidity at ail available stations, some general features can be evidenced:
- The monthly trend of relative humidity, showing minimum values during the
winter season and a maximum during summer, is similar to the monthly tren
269
extends from west to east into the sea with the Drygalski Ice Tongue. The Reeves
Névé, which is fed by the East Antarctic Ice Sheet, and the Priestley Névé, which is
nourished by the Talos Dôme, flow into the Reeves and Priestley Glaciers, respectively. These two glaciers end in the Nansen Ice Sheet, an ice shelf which is pinned
to Inexpressible Island and to Northern Foothills. Other main glaciers, such as the
Aviator, the Campbell and the Mawson, and numerous tributary glaciers flow into
the main outlet glaciers.
Ice accumulation in central East Antarctica has very low values, resulting in
less than 2.5 cm/year1 in depth. Due to orographie lifting of moist air masses
flowing southward toward the edge of the ice sheet, and to decreasing distance
from the moisture source, accumulation rates increase toward the coast. Another
factor determining accumulation is the air température; moisture-carrying
capacity of air diminishes strongly as its température decreases. So we can expect
that when air températures increase snowfall will also increase, although a quantitative relationship has not been determined [15],
3 Humidity of Air Over North Victoria Land
At a given point, the snow surface can accumulate mass by précipitation, condensation, déposition and wind transport; on the other hand, snow can be removed
by évaporation, sublimation and wind transport. Considering ail these factors, up
to the présent the problem of finding some way to record the amount of snowfall
has not been solved. Schwerdtfeger [9] reports that it has sometimes been
observed that a gage was empty after an intensive snowfall and strong winds,
whereas plenty of wind-driven snow had entered the collecter on a day without
any real précipitation. However, significant progress has recently been made in
techniques to directly and indirectly détermine Antarctic précipitation amounts.
In this chapter an approach is proposed to identify épisodes of précipitation
over North Victoria Land, in particular the Terra Nova Bay area.
Précipitation characteristics at the coast and in the high interior are very different. Coastal snowfall is episodic in association with synoptic-scale features
such as cyclones and fronts [10]. In the interior, the portion of précipitation associated with clouds decreases, and the semicontinuous fall of ice crystals from
cloud-free skies becomes progressively more important as élévation increases. In
fact, in Antarctica the cloud-forming processes are weakly developed and the rôle
of the air mass clouds in précipitation is small. Consequently, one can infer that
the entire snow accumulation in Antarctica, estimated to be 2480 km' water year ',
is due to this advection of water vapour from the oceanic régions. Thanks to this
advection of water vapour, 61.7% of the world's reserves of freshwater are concentrated in Antarctica [16]. So information on humidity is indispensable for a
study of the laws of moisture transport in the atmosphère, the processes of cloud
and précipitation formation and heat balance [17].
Considering the monthly and annual averages of relative humidity at ail available stations, some general features can be evidenced:
- The monthly trend of relative humidity, showing minimum values during the
winter season and a maximum during summer, is similar to the monthly tren