Meteorological Conditions During Snowfall at Terra Nova Bay (Antarctica)
275
0° DIR â 45° or 225° < DIR < 360°: coupling this condition with an RH è 70%,
from 2 to 24 3-hourly consecutive observations were found to occur.
Recalling that both conditions required relative humidity at station 7352 higher than 70%, we assumed that when wind direction indicates air blowing from the
sea, précipitation is likely to occur, while in the case of air coming from the continent high humidity values can be due to drifting/blowing snow within the air
flow. Two typical situations of marine and continental flow (Fig. 5a, b) were
analysed: winds at surface level in Fig. 5a could lead to the hypothesis of cyclogenesis over the south-western Ross Sea, while the situation reported in Fig. 5b can
be related to blowing or drifting snow, as indicated by the high wind speeds and
by the low relative humidity recorded at stations other than 7352.
5 Cases of Mesoscale Cydogenesis
When polar air masses move over relatively warm water, the fluxes of sensible and
latent heat from the océan surface can warm a deep layer of the atmosphère [20],
If the heating rate is not too great, baroclinie instability is the primary mechanism
for formation and development of mesoscale cyclones [20,21 ]. On the other hand,
diabatic heating helps to reduce the static stability of the near-surface atmosphère,
and mesoscale cyclones can grow faster and with a shorter wavelength [22].
The East Antarctic Plateau is located to the west of Ross Sea, and thus the prevailing westerly upper level flow [23] descends about 2000 m from the high
Plateau toward the océan. In a homogeneous incompressible fluid with constant
density the potential vorticity is conserved. Carrasco and Bromwich [20] applied
this concept to the Victoria Land/Ross Sea area, observing that a column of air
over the Plateau expands vertically when it goes down onto the Ross Sea. This
increase in depth implies a création of négative (cyclonic) relative vorticity.
Therefore the air column acquires cyclonic circulation over the western Ross Sea,
offshore from Victoria Land.
The synoptic situation related to this phenomenon could be analysed by means
of the Ertel potential vorticity maps [24]. Observing the maps corresponding to
épisodes of intrusion of moist air mass from the sea toward the continent présents a cell of négative isentropic potential vorticity over the région of North
Victoria Land; instead when katabatic winds occur, an area with large values of
potential vorticity develops offshore and over Victoria Land, and the contours
tend to bunch together at the edge of the région. These air masses resuit in
extremely dry conditions and conditions move at very high speed.
In addition to the broad-scale analysis, some case studies of mesoscale cyclogenesis over the Ross Sea presented in current literature hâve been considered,
together with concurrent analysis of our data. Carrasco and Bromwich [20] examined four mesoscale cyclones that formed near Terra Nova Bay between 16 and 20
February 1988, describing their évolution thus:
- At 12:00 UTC, on 16 February a mesoscale vortex was located next to
Inexpressible Island. Then the authors observed that the weak south-westerly
winds recorded at AWS 7351 between 15:00 and 21:00 UTC on 16 February may
signal the passage of the vortex along the Victoria Land coast.
275
0° DIR â 45° or 225° < DIR < 360°: coupling this condition with an RH è 70%,
from 2 to 24 3-hourly consecutive observations were found to occur.
Recalling that both conditions required relative humidity at station 7352 higher than 70%, we assumed that when wind direction indicates air blowing from the
sea, précipitation is likely to occur, while in the case of air coming from the continent high humidity values can be due to drifting/blowing snow within the air
flow. Two typical situations of marine and continental flow (Fig. 5a, b) were
analysed: winds at surface level in Fig. 5a could lead to the hypothesis of cyclogenesis over the south-western Ross Sea, while the situation reported in Fig. 5b can
be related to blowing or drifting snow, as indicated by the high wind speeds and
by the low relative humidity recorded at stations other than 7352.
5 Cases of Mesoscale Cydogenesis
When polar air masses move over relatively warm water, the fluxes of sensible and
latent heat from the océan surface can warm a deep layer of the atmosphère [20],
If the heating rate is not too great, baroclinie instability is the primary mechanism
for formation and development of mesoscale cyclones [20,21 ]. On the other hand,
diabatic heating helps to reduce the static stability of the near-surface atmosphère,
and mesoscale cyclones can grow faster and with a shorter wavelength [22].
The East Antarctic Plateau is located to the west of Ross Sea, and thus the prevailing westerly upper level flow [23] descends about 2000 m from the high
Plateau toward the océan. In a homogeneous incompressible fluid with constant
density the potential vorticity is conserved. Carrasco and Bromwich [20] applied
this concept to the Victoria Land/Ross Sea area, observing that a column of air
over the Plateau expands vertically when it goes down onto the Ross Sea. This
increase in depth implies a création of négative (cyclonic) relative vorticity.
Therefore the air column acquires cyclonic circulation over the western Ross Sea,
offshore from Victoria Land.
The synoptic situation related to this phenomenon could be analysed by means
of the Ertel potential vorticity maps [24]. Observing the maps corresponding to
épisodes of intrusion of moist air mass from the sea toward the continent présents a cell of négative isentropic potential vorticity over the région of North
Victoria Land; instead when katabatic winds occur, an area with large values of
potential vorticity develops offshore and over Victoria Land, and the contours
tend to bunch together at the edge of the région. These air masses resuit in
extremely dry conditions and conditions move at very high speed.
In addition to the broad-scale analysis, some case studies of mesoscale cyclogenesis over the Ross Sea presented in current literature hâve been considered,
together with concurrent analysis of our data. Carrasco and Bromwich [20] examined four mesoscale cyclones that formed near Terra Nova Bay between 16 and 20
February 1988, describing their évolution thus:
- At 12:00 UTC, on 16 February a mesoscale vortex was located next to
Inexpressible Island. Then the authors observed that the weak south-westerly
winds recorded at AWS 7351 between 15:00 and 21:00 UTC on 16 February may
signal the passage of the vortex along the Victoria Land coast.