WORLD METEOROLOGICAL ORGANIZATION
Distr.: RESTRICTED
____________________
EC/WGAM-VIII/INF-4
(11.XI.2002)
EXECUTIVE COUNCIL
_________
WORKING GROUP ON ANTARCTIC METEOROLOGY
ITEM: 6.1
EIGHTH SESSION
GENEVA, 25 – 27 NOVEMBER 2002
Original: ENGLISH
ACCUMULATION OF SNOW AND ICE AND THE BOUNDARY LAYER
CLIMATE OF ANTARCTICA
Prepared by Hugh Hutchinson and Doug Shepherd
Summary and Purpose of the Document
The Appendix to this document is a paper prepared by the chairman on the effects
of Snow and Ice in Antarctica on Climate. It is provided for Information.
Appendix: “Accumulation of Snow and Ice and the Boundary Layer Climate of Antarctica.”
EC/WGAM-VIII, INF-4, Appendix
ACCUMULATION OF SNOW AND ICE AND THE BOUNDARY LAYER
CLIMATE OF ANTARCTICA
Climatic importance
Understanding basic attributes of the Earth's atmosphere is fundamental to understanding our
changing climate. Antarctica's icecap, sea ice and the general circulation of the Southern Ocean are
known to have major impacts on global climate.
Precipitation over Antarctica is recognised as an important climatic variable. Precipitation in polar
regions has been forecast to increase with potential increases in global temperature, hence monitoring
the cryosphere is an important component of detecting global change. The rate of accumulation of snow
and ice is necessary information for the assessment of the stability and motion of the Antarctic ice
sheets. A detailed knowledge of precipitation over the Antarctic continent is necessary for studies of the
mass balance of the ice sheets, to ensure that numerical models are performing correctly at high
southern latitudes and for the interpretation of ice cores. However, although we can get estimates of
snow accumulation by a number of techniques, we have little knowledge of the atmospheric processes
that give rise to snowfall or the synoptic situations responsible for most of the accumulation.
Atmospheric modeling techniques
In contrast to gauge measurements, additional methods using atmospheric techniques have
been examined in the last two decades, such as the derived moisture budget from rawin-sonde data. In
recent years, the enhancement of meteorological data assimilation methods, including satellite data, has
led to the use of more reliable atmospheric numerical analyses and models for the study of Antarctic
precipitation and its variability. The Greenland Ice Sheet offers much better prospects for the success of
atmospheric methods than Antarctica, because it lies just east of the data rich North America continent,
and it is surrounded by a ring of upper air stations. (Bromwich, 1995).
Precipitable water
The precipitable water (sometimes called the total precipitable water) of a column of air is the
depth of water (in mm) or alternatively expressed as the total mass of water (kg/m2) that would be
obtained if all the water vapour in the column, were condensed on to a horizontal plane of unit area
cross-section. Precipitable water is a useful measure of the water vapour content of an air column. The
term is not, however, to be regarded as implying that the amount of water may, in fact, be precipitated by
an actual physical process.
The depth of precipitable water of an atmospheric column, of p1 (hPa) at the bottom and p2 (hPa)
at the top and of mean mixing ratio r (g/kg), is given in tenths of millimetres, by the approximate formula:
Precipitable water = r(p1-p2)/g
Where:
g is the gravitional acceleration (9.8 m/s2).
The so-called “precipitable water” for an entire atmospheric column is found by applying the
formula to selected successive layers. The water-vapour content of the air normally decreases rapidly
with height. Near sea level, over middle and tropical latitudes there is enough water vapour in the
overlying atmosphere, even in cloudless conditions, to account for almost complete absorption of
terrestrial radiation in the wavelengths other than those of the atmospheric window (approximately 8 to
12 microns).
In the wet season of northern Australia the precipitable water can reach a value of 60 mm in a
tropical depression that is part of the NW monsoon. In middle latitudes in summer precipitable water is
highly variable and generally in the range from 5 to 40 mm, depending on the synoptic meteorological
situation over a particular region.
The amount of precipitable water in the column above the ice plateau of East Antarctica is
extremely small, less than the dry air over the Arctic sea ice. A typical value for the maritime air off the
Antarctic coast is less than 5 mm, while over the continental interior of Antarctica the precipitable water
is normally less than 1 mm. Near the coast of Antarctica the amount of precipitable water is greater than
normal, if the prevailing trajectory taken is transporting relatively moist air from more northern latitudes
towards the coast, especially from the open ocean rather than the sea ice zone. Above the elevated
EC/WGAM-VIII, INF-4, Appendix, p. 2
continental interior of East Antarctica the precipitable water is frequently less than 1 mm, except for the
unusual incursion of moist air from more northern latitudes sometimes associated with a degenerating
depression from the mid-latitude westerlies. The very dry air inland is almost constantly descending as it
moves in surface boundary layer towards the coast. as a continental outflow, focused by glacial valleys.
Precipitable water is measured by balloon-borne radiosondes. Precipitable water can be fairly
accurately estimated from moisture measurements e.g. dew point in the surface layer of the atmosphere,
since the amount of water vapour in the atmosphere decreases rapidly with height. GPS satellite
measurements can be used to indicate the amount of precipitable water in the slant column between a
polar orbiting GPS satellite and a ground reference point. Remote sensing from a geostationary
meteorological satellite can give an indication of the mid troposhere circulation of water vapour on a
global scale.
Drifting and blowing snow
According to Bintanja (1998), drifting and blowing snow are regularly occurring phenomena over
snow-covered surfaces. When the surface wind becomes stronger than a certain threshold wind speed,
the drag force becomes sufficently large to overcome inter-particle bonds and gravity, and the surface
particles are lifted from the surface. For moderately strong winds, the particles remain confined close to
the surface and periodically bounce on the surface, this is commonly referred to as saltation. For
stronger winds, turbulent motions are able to further lift the particles against gravity. As a result, the
particles are able to leave the saltation layer and become fully suspended. Travelling with the speed of
the wind in the direction of the wind, drifting snow particles can be transported over considerable
distances. The horizontal mass flux associated with drifting snow can be of appreciable magnitude,
causing a considerable redistribution of the precipitated snow, as has been shown by observation and
theoretical calculations of snow drift transport rates.
Sublimation
During drifting snow, continuous sublimation of the snow particles takes place as the ambient air
is generally below saturation with respect to the water-vapour pressure at the temperature of the
suspended snow particle. Turbulent fluctuations in vertical velocity, which provide the drag required to
keep the particle in the air, continuously ventilate the particle, thereby significantly enhancing the particle
to air moisture transfer. Since water vapour transport occurs on all sides of each floating snow particle
(the surface-area/mass ratio of a suspended snow particle is very large), sublimation of drifting snow
seems to be a potentially more efficient ablation mechanism than sublimation of the rigid snow/ice
surface, which was demonstrated by Pomeroy (1989).
Over Antarctica, wind speeds are regularly high enough to induce drifting snow, especially in
coastal regions where cold, dry katabatic winds almost continuously blow down the relatively steep ice
slope. Sublimation of drifting snow in Antarctica is a regularly occurring phenomenon, which makes it
necessary to quantify its contribution to the surface mass balance. Sublimation of suspended snow is
negligible in the interior of Antarctica where wind speeds and temperatures are low, whereas near the
windy and relatively warm coast its contribution is significant (up to 17 cm w.e. per year). Snowdriftsublimation is highest during the summer, when temperatures are highest, in spite of the fact that wind
speeds are not as high as in winter. Snowdrift sublimation is one of the major terms in the surface mass
balance in Antarctica, particularly in coastal regions.
Accumulation
Glaciological data have been synthesized into a single, long-term annual accumulation depiction
for the Antarctic continent. Compilations produced have evolved as the number of observations has
increased. These depictions generally indicate a strong dependance of accumulation on prevailing
moisture advection and topography. There are relatively large values for the east Antarctic coastal
escarpment grading to desert-like conditions over the interior plateau of east Antarctica. Giovinetto and
Bentley (1985) made a manual synthesis of over 1200 data points. Now known as GB85, it differed from
previous works in that the lowest accumulation values of less than 50 mm/yr extend over a much larger
area than was previously depicted (Bromwich 1988). GB85 is still frequently used for model and
analyses validation. More recent studies have provided results that are in significant agreement.
Accumulation and precipitation are related using (Bromwich, 1998):
B=P–E–D–M
EC/WGAM-VIII, INF-4, Appendix, p. 3
Where:
B is accumulation
P is precipitation
E is the net sublimation minus deposition of hoarfrost.
D is the divergence of snow drift.
M is the divergence of meltwater runoff.
All terms are areal average and time average. Vaughan, D. G. et al. (1999), have made a
reassessment of net surface mass balance in Antarctica.
The Antarctic ice sheet and sea level
The great ice sheet covering Antarctica locks up vast quantities of water from the oceans, and
thus has the potential to affect global sea level considerably. In the short term, it is the precipitation rate
over the ice sheet that determines the impact of the ice sheet on sea level. Comparisons with radiosonde
data and Antarctic accumulation observations lead to the overall conclusion that the Australian GASP
analyses for the period 1989-92 do a surprisingly good job depicting the atmospheric moisture fluxes,
and their convergences, despite the coarse vertical resolution. (Budd et al., 1995). The atmospheric
moisture budget from ECMWF for the period 1985-95 seems to also be a realistic simulation of the
Antarctic atmosphere (Cullather et al., 1998).
Spatial and temporal variability of Antarctic precipitation as determined from atmospheric
models
Difficulties in obtaining accurate estimates of Antarctic precipitation are well known. Direct in
situ gauge measurements are complicated by wind biases and the presence of an unlimited snow field.
The introduction of blowing snow creates the problem of distinguishing snow that has been precipitated
from that which has been picked up by the wind and transported. Additionally, over the interior of the
Antarctic plateau, daily snowfall amounts are less than the minimum gauge resolution.
In contrast to gauge measurements, additional methods using atmospheric techniques have
been examined in the last two decades, such as the derived moisture budget from rawinsonde data. In
recent years, the enhancement of meteorological data assimilation methods, including satellite data, has
led to the use of more reliable atmospheric numerical analyses and models for the study of Antarctic
precipitation and its variability.
In a paper by Cullather et al (1998), the authors expand on results presented in Bromwich et al.
(1995) by examining the spatial representation of net precipitation (precipitation minus
evaporation/sublimation) derived from the atmospheric moisture budget of the European Centre for
Medium Range Weather Forecasts (ECMWF) analyses. A useful exercise is to inter-compare fields
derived from atmospheric numerical methods with synthesized observational data sets from glaciological
and direct measurement methods. An inter-comparison offers a means of validation as well as a
qualitative measure of how well the various fields are known. Appraisal of the spatial depiction and
regional variability offered by available atmospheric methods is relevant to other efforts in examining
Antarctic ice sheet mass balance, including the use of satellite altimetry data. This evaluation is also of
interest to modelers in assessing the quality and sources of available validation data. Some key issues
addressed by this study are:



What are the qualitative and quantitative differences in the spatial distributions of the data
sets?
What is the annual and inter-annual variability of Antarctic precipitation on regional
scales?
Are the causes for the observed variations in the atmospheric moisture budget apparent?
The continental surface temperature inversion
Surface temperature is markedly height dependent. There is a close correspondence between
surface temperature and inversion strength, both for individual stations on a day-to-day or month-tomonth basis; and for all stations for the winter season. The world's lowest surface temperature yet
recorded (on 21 July 1983) is minus 89.6oC at Vostok station, 1,300 km from the coast and at an
elevation of 3,488 m. August is the coldest month with a mean temperature of minus 70oC and an
EC/WGAM-VIII, INF-4, Appendix, p. 4
average total cloud cover of 3 oktas. In all months the average wind speed is about 5 m/sec with the
most frequently observed wind direction being WSW (down slope).
The surface temperature inversion over the Antarctic continent results from the radiative heat
loss from the ice surface particularly during the polar night of winter. Phillpot and Zillman (1970) found
that the average strength of the surface temperature inversion over the Antarctic continent in winter is
about 25oC on the high plateau area of East Antarctica and this grades to be about 5oC near the coast.
Winter mean depths of 500-700 m are found at high plateau stations while the depth at McMurdo it is in
the range 400-500 m. Coastal stations that are not on ice shelves such as Mawson and Davis have a
depth of 300-400m in winter. There is also a definite seasonal variation of inversion strength and depth.
In some cases the surface temperature occasionally falls to near minus 80oC at Vostok in winter and the
strength of the surface temperature inversion exceeds 30oC. Then the depth of the boundary layer (as
defined by the height above the ice surface of the highest temperature in the troposphere) may be 1.5
km.
Schwertfeger 1984 noted that acoustic soundings at Amundsen Scott indicate a well-defined
ground based shear zone of some 40-300 m deep, well below the height of the highest tropospheric
temperature. Radiosonde measurements leave no doubt that above this pronounced surface inversion
layer in the interior, normally there is a rather thick layer between say 500 and 1500m above the ice
surface in which the temperature changes little with height.
Katabatic winds
The surface wind regime is one of the most characteristic features of Antarctic climatology
(Parish and Bromwich, 1987). Nowhere on any other continent has one single element such an
overwhelming influence of the climate of the continent as a whole . The model of the Antarctic wind
regime described below requires validation, and certainly the relevance of this scale of motion to the
general circulation of the atmosphere is still open to debate. For example, researchers have tried to
determine the interrelationship, if any, between the katabatic flow and the circumpolar vortex found
during autumn, winter and spring in the upper levels of the troposphere and in the lower stratosphere.
The strong radiative cooling of the ice sheet significantly influences the Antarctic surface winds.
The gravity driven flow moves very slowly at first away from high elevation areas of the ice cap,
accelerating as it moves towards the coast. The configuration of the ice topography provides an
extensive elevated cold air source. Lower lying glacial basins cause strong confluence of airflows. Near
the coast the very cold (dense), high velocity airflow is confined to a layer about 600 metres thick, with
the highest speed winds at a height of about 200 metres above the ice surface.
These katabatic winds blow with remarkable constancy in direction, forced to the left of the line
of maximum ice slope as a result of the Earth's rotation (the Coriolis force). The confluence of cold air
drainage currents from the interior is responsible for the extremely strong katabatic wind speeds in the
Adelie Land region. Parish and Wendler (1991) showed how the annual mean surface wind speed slowly
accelerates over more than a thousand kilometres from Dome C on the high ice plateau until within 100
km of the coast. The monthly mean speed is about 25 m/s at Port Martin in March and at Cape Denison
the monthly mean speed is greater than 20m/s at Cape Denison between March and October. In some
glacial valleys confluence is particularly strong, and katabatics can reach around 40 m/s on some days
for hundreds of kilometres as the airflow flow makes it way towards the coast. There is often, however, a
decrease in speed, due to ice surface roughness, within 100km of the coastal escarpment.
Once katabatic winds have reached the Antarctic coast their down slope driving force has been
lost. Rapid deceleration and dissipation occurs within a short distance offshore because glacial valley
convergence in the airflow is replaced by divergence of the shallow airstream that was once katabatic.
Surface wind velocities are frequently less than 5m/s at elevated sites such as Dome C, Vostok
and Dome Fuji as there is often no synoptic scale weather system of significance in the vicinity and the
katabatic flow is negligible. However surface temperatures are extremely cold at high elevations above
sea level, especially in winter and early spring. Dumont d'Urville or Mawson are almost constantly windy.
Casey has periods of extremely strong surface winds interspersed with periods of relatively light winds,
due in part to the topographic influence of Law Dome on surface winds driven by intense low-pressure
systems passing by offshore. Davis has in general the most benign wind climate of the three ANARE
stations on the Antarctic continent.
EC/WGAM-VIII, INF-4, Appendix, p. 5
Katabatic winds and their connection to the general circulation.
The flow of air down the ice slopes brings about a compensatory subsidence of dry, relatively
warm air from the atmosphere above the katabatic wind level, thus developing, maintaining or
strengthening the surface temperature inversion. The strength and thickness of the surface temperature
inversion on the high plateau of East Antarctica can build up in mid to late winter to extraordinary
magnitudes, according to radiosonde measurements e.g. Vostok prior to 1992. It is possible that the
surface temperature inversion over the interior of the Antarctic Continent may be slow to respond to any
global warming over the next century. Accordingly meteorological studies may be necessary to monitor
the possible warming of upper levels of the troposphere, above the surface temperature inversion. The
only inland staffed station currently making routine balloon-borne vertical profile soundings of the
troposphere and lower stratosphere in Antarctica is Amundsen-Scott at South Pole.
References
Bintanja, Richard (1998)
The contribution of snowdrift sublimation to the surface mass balance of Antarctica.
Annals of Glaciology 27, 251-259. The International Glaciology Society.
Bromwich, D. H., (1979)
Precipitation and accumulation estimates for East Antarctica, derived from rawinsonde information. PhD. Thesis.
University of Wisconsin – Madison, 142 pp. Not cited
Bromwich, D.H. (1988)
Snowfall in high southern latitudes.
Rev. Geophys., 26, 149-168.
Bromwich, D.H. (1990)
Estimates of Antarctic precipitation.
Nature, 343, 627-629.
Bromwich, D.H., and F. M. Robasky, (1993).
Recent precipitation trends over the polar ice sheets.
Meteor. Atmos. Phys., 51, 259-274. Not cited
Bromwich, D.H, F.M. Robasky, R.L. Cullather and M.L. Van Woert. (1995)
The atmospheric hydrological cycle over the Southern Ocean and Antarctica from operational numerical analyses.
Monthly Weather Review, 123(12), 3518-3538.
Budd W. F., P.A. Reid and L. Minty. (1995).
Antarctic moisture flux and net accumulation from global atmospheric analyses.
Annals of Glaciology, 21, 149-156. The International Glaciology Society.
Cullather R.L, D.H. Bromwich and M. L. Van Woert. (1997). Not cited.
Interannual variations in Antarctic precipitation related to El-Nino/Southern Oscillation.
Journal of Geophysical Research , Volume 101, No. D14, 19,109-19,118.
Cullather R. L., D.H. Bromwich and M. L. Van Woert (1998).
Spatial and temporal variability of Antarctic precipitation from atmospheric methods.
Journal of Climate, Vol. 11, No. 3, 334-367.
Giovinetto, M.B. and C.R. Bentley (1985).
Surface balance in ice drainage systems of Antarctica.
Antarctic Journal, U.S., 20, 6-13.
Parish, T. R. and D. H. Bromwich (1987).
The surface wind field over the Antarctic ice sheets,
Nature 328, 51-54.
Parish, T.R. and G. Wendler (1991).
The katabatic wind regime at Adelie Land, Antarctica.
International Journal of Climatology, Vol. 11, 97-107.
Phillpot H. and J. Zillman (1970).
EC/WGAM-VIII, INF-4, Appendix, p. 6
The surface temperature inversion over the Antarctic continent.
Journal Geophysics Research, 75, 4161-9.
Pomeroy, J.W. (1989).
A process-based model of snow drifting.
Annals of Glaciology, 13, 237-240.
Smith I.N., W.F. Budd, and P.A.Reid (1998). Not cited.
Model estimates of Antarctic accumulation rates and their relationship to temperature changes.
Annals of Glaciology, 27, 246-250. The International Glaciology Society.
Schwertfeger (1984)
Weather and climate of the Antarctic, developments in atmospheric sciences. Elsevier Science Publishing
Company Inc. New York. 261pp.
Vaughan, D. G., J. L. Bamber, M. Giovinetto, J. Russell, A. P. R. Cooper (1999),
Reassessment of net surface mass balance in Antarctica
J. Climate, 12, 933-46.
EC/WGAM-VIII, INF-4, Appendix, p. 7
OBSERVATIONS OF SNOWFALL, DRIFTING AND BLOWING SNOW AT
MAWSON, DAVIS AND CASEY AND THE CLIMATE RECORD
Hugh Hutchinson and Doug Shepherd
Bureau of Meteorology
Tasmania and Antarctica Regional Office
Hobart, Tasmania 7001, Australia
h.hutchinson@bom.gov.au
d.shepherd@bom.gov.au
Visual observations of snowfall recorded in WMO present weather code
Difficulties in obtaining accurate estimates of Antarctic precipitation are well known.
Direct in situ gauge measurements are complicated by wind biases and the presence of an
unlimited snow field. The introduction of blowing snow creates the problem of distinguishing
precipitation from that which has been picked up by the wind and transported. Additionally,
over the interior of the Antarctic plateau, daily snowfall amounts are less than the minimum
gauge resolution.
When making a three-hourly visual synoptic observation, the duty Meteorological
Observer selects the Detailed Description of the Weather which best describes the type of
weather occurring at the time of observation or within the hour preceding it. The WMO
SYNOP Code describes in detail all the weather elements that can occur at any place around
the world but the codes are still adequate for polar regions in almost all circumstances.
With respect to the reporting of Present Weather there are 100 categories to choose
from. The higher code figure describes weather elements that are relatively more significant
and hence takes precedence if more than one weather phenomenon takes precedence.
Cloud type associated with snowfall
The reporting of snow showers associated with Low Cloud 9 (Cumulonimbus) or
Low Cloud 2 (Large Cumulus) is rare in Antarctica compared with frozen precipitation that is
not showers and is associated with Middle Cloud 2 (Nimbostratus) and sometimes Low
Cloud 5 (usually extensive Stratocumulus that is not formed by the spreading out of
Cumulus).
Snow showers


86 – Moderate or heavy snow shower.
85 – Slight snow shower.
Snowfall
Frozen precipitation (excluding snow showers and hail) is reported as present
weather codes 70 to 75.






75 – heavy continuous snow.
74 – heavy intermittent snow.
73 – moderate continuous snow.
72 – moderate intermittent snow.
71 – slight continuous snow.
70 – slight intermittent snow.
Snowfall is classified as:
Slight
Indicated by small sparse snow flakes, and generally the visibility is reduced
but not less than 1000 metres.
EC/WGAM-VIII, INF-4, Appendix, p. 8
Moderate
Heavy
Indicated by large numerous flakes and generally the visibility is between 400
metres to 1000 metres.
Numerous flakes of all sizes and general visibility is reduced to less than 400
metres.
Blowing and drifting snow
Present weather codes from 36 and 37 describe drifting (below eye level) snow and
codes 38 and 39 blowing (above eye level) snow.




39 – Thick above eye level.
38 – Slight or moderate above eye level.
37 – Thick below eye level.
36 – Slight or moderate below eye level.
Recording the incidence of snowfall in any 24 hours period
If snowfall is observed or is known to have occurred during each 24 hours period
then this is indicated in the WMO Phenomena number 53. Accordingly we find that the our
Antarctic records show that:



Snowfalls are less frequent at Mawson than at Casey or Davis. The
mean number of 24 hours periods that snow has been observed at
Mawson is 60 during the 1954-1999 period of records. The frequency of
snowfalls being observed is slightly higher in December and January
than the other months.
Snowfalls at Davis are more frequent than at Mawson but less frequent
than at Casey. The annual average number of days of snowfall at Davis
is about 137 days during the period 1969 to 1999. The months with
most frequent snowfalls are March, April and May. November,
December and January have the least number of days of snowfall.
The mean number of days per year that snow has been observed to fall
or is known to have occurred at Casey is around 180. The months with
least days when snow has fallen are November, December and
January.
The history of Australian weather observation sites
Mawson
Detailed surface and upper air meteorological observations have been made
regularly at Mawson since the station was established in 1954. The climatic record thus
constitutes the longest continuous record of meteorological parameters on the Antarctic
continent, excluding the Antarctic Peninsular. It seems that the original observations site is
still in use. However with the encroachment of new, bigger buildings, the anemometer mast
was relocated 300 metres to the north when the AWS was installed on 15 January 1994.
Davis
Regular surface weather observations and upper air soundings have occurred since
1957, except between November 1964 and February 1969 when the station was closed.
Observation enclosure has relocated only once, appoximately 70 metres to the northeast of
the original site in March 1992. Dual observations commenced at the new site from 19 March
1992 and continued until 1 December 1992. From this time the new site became the official
site.
EC/WGAM-VIII, INF-4, Appendix, p. 9
Casey
Wilkes Base was established by the USA in 1957, but it was taken over by ANARE
from 1960 to 1969, before being abandoned due to encroaching snow. The original Casey
station, was established 2 km to the SSE of Wilkes in 1969. A new Casey station was
constructed and the Meteorological Observations site was relocated in February 1989
approximately 500m to the southwest of the old site. The first observation from the new site
was on 1 February 1989. Duel observations were performed throughout 1989.
Use of the Nipher shield
The Nipher shield is a form of screen, which is fitted to a snow-gauge, for the
purpose of eliminating, as far as possible, wind eddies in the mouth of the gauge, and so
enabling a truer catch of snow to be made. The Nipher shield has been used at Davis and
Casey.
EC/WGAM-VIII, INF-4, Appendix, p. 10
Some notes on time series plots of various weather elements
for Australian Antarctic stations
Doug Shepherd
Tasmania and Antarctica Region
Bureau of Meteorology
December 2002
Introduction
Attached are time series plots of various weather elements observed at Australian
Antarctic stations. These notes briefly describe the data used to create the plots and
provide some comments on the plots, including apparent variability and trends.
Data
Observational data have been extracted from the Australian Data Archive for
Meteorology (ADAM) for Wilkes/Casey, Davis and Mawson. The data for Wilkes/Casey
are a composite of Wilkes [to about Jan 1969], Casey (The Tunnel) [to about Jan 1989]
and the current Casey site.
Quality control of the data is limited to the gross error checks used on input to ADAM. It is
highly likely that some apparent variations over time are the result of changes in site location,
site exposure, instrumentation or observing practices.
The measurement of precipitation amounts is extremely difficult in the Antarctic environment
and these data need to be treated with caution. Precipitation data are limited to Casey and
Davis. Distinguishing falling snow from blowing snow is also difficult at times, so the weather
code data for these elements need to be treated with care as well.
Plots for annual precipitation, monthly precipitation, annual mean daily maximum
temperature, and annual mean daily minimum temperature are based on monthly data in
ADAM. Plots for mean temperature, mean dewpoint, mean MSL pressure, percentage
occurrence of selected weather codes, and percentage occurrence of temperatures above
0C are based on synoptic data in ADAM ( 3-hourly).
Annual means or totals derived from monthly data are shown for a given year only if data
exist for all 12 months. Annual percentages for weather codes are shown for a given year
only if at least 112 observations are available for each month (this equates to about 4
observations per day). Annual means for temperature, dewpoint and pressure, and
percentages for temperature above 0C, are shown for a given year only if at least 196
observations are available for each month (this equates to about 7 observations per day).
For each figure a common scale is used for the plots, so that comparison between sites can
be made readily.
EC/WGAM-VIII, INF-4, Appendix, p. 11
Precipitation
Annual (see figure 1)
 Mean annual precipitation is much higher at Casey (190mm) than at Davis (75mm).
 There has been an increase at Casey, but this may be related to the site move around
1989.
 There appears to have been an increase at Davis through the 1980s and 1990s.
Monthly (see figure 2)
 The plots of monthly precipitation show large variability from month to month.
 Precipitation tends to be higher during autumn/winter at both locations.
Weather Codes
Codes 70 to 75 – Snowfall (see figure 3)
 On average these are reported more frequently at Wilkes/Casey (15%) and Davis
(11%) than at Mawson (4%).
 The plot for Wilkes/Casey shows an increase around the time of the move from Wilkes to
Casey. There has been a decrease over recent years.
 Davis shows a slight increase over time. A peak is evident during the 1970s.
 Mawson shows little change. The high value for 1958 seems doubtful.
Codes 70 & 71 - Slight snowfall (see figure 4)
 Most of the overall snowfall occurs as slight snow. On average, more is experienced at
Wilkes/Casey than at Davis than at Mawson.
 Trends are similar to the overall snowfall trends, but the 1958 outlier at Mawson is
absent.
Codes 72 to 75 - Moderate/heavy snowfall (see figure 5)
 Only a small part of overall snowfall occurs as moderate or heavy snow.
 The mean frequency of occurrence at Wilkes/Casey is roughly double that at Davis,
which in turn is roughly double that at Mawson.
 Wilkes/Casey shows a peak during the 1970s and a tendency to have increased again in
recent years.
 There have been some fluctuations at Davis, but there appears to have been a decrease
in frequency overall.
 Mawson shows little trend. The 1958 value seems spurious.
Codes 36 to 39 - Drifting/blowing snow (see figure 6)
 The mean frequency is higher at Mawson (16%) than at Wilkes/Casey (13%). Both of
these are much greater than at Davis (7%).
EC/WGAM-VIII, INF-4, Appendix, p. 12


Little change is evident at Wilkes/Casey and Davis over time.
The smoothed fit at Mawson shows a peak around 1970 and a secondary peak around
1990.
Temperature
Maximum (see figure 7)
 The mean maximum for Wilkes/Casey is roughly 2C higher than for Davis or Mawson.
 Wilkes/Casey appears to have increased around the time of the change from Wilkes to
Casey and decreased with the change of site at Casey around 1989. The latter change
is supported by overlapping data for 1989 that show the new site to be about 0.5C
lower.
 Davis shows an increase over time, while Mawson shows a negligible decrease.
Minimum (see figure 8)
 The mean minimum for Wilkes/Casey is slightly greater than for Davis, which is about
1C higher than for Mawson.
 Wilkes/Casey seems to have increased around the time of transition from Wilkes to
Casey and decreased with the Casey site change. However, overlapping data for 1989
indicate that the new site is only about 0.2C lower.
 There seems to be little evidence of trend at Davis.
 The plot for Mawson shows a general decrease over time.
Mean (see figure 9)
 On average, Wilkes/Casey is about 1C higher than Davis, which is about 1C higher
than Mawson.
 At Wilkes/Casey there is a suggestion of an increase around the time of the move from
Wilkes to Casey and a decrease around the change of Casey site.
 There is little trend at Davis.
 Mawson shows a slight decrease.
Temperature above 0C (see figure 10)
 Wilkes/Casey has the lowest mean occurrence. At first glance this seems to contradict
the previously mentioned averages for maximum, minimum and mean temperatures, but
results from the smaller seasonal variation at Wilkes/Casey.
 There was an increase in occurrence at Wilkes/Casey till the late 1980s, then a marked
decline. Site changes probably contributed to these trends.
 After an early increase, Davis has been fairly stable since about 1970.
 Mawson was generally steady through the 1970s and 1980s, but decreased during the
1990s.
Dewpoint (see figure 11)
 The mean values indicate that Wilkes/Casey is moistest while Mawson is driest.
EC/WGAM-VIII, INF-4, Appendix, p. 13


Decreases in dewpoint at all sites around 1980 may be related to changes from hair
hygrographs to humicaps.
The introduction of AWS around 1994 may have contributed to the increases since the
1980s.
MSL Pressure (see figure 12)



Mean values are similar for the three locations, with Wilkes/Casey being less than Davis
and Davis being slightly less than Mawson.
The smoothed values for Wilkes/Casey show a peak around 1970 with little change after
1980.
Davis and Mawson also have a peak around 1970 but they appear to have been
decreasing steadily since that time.
EC/WGAM-VIII, INF-4, Appendix, p.14
Figure 12
Annual precipitation
Solid line shows mean. Broken line shows smoothed fit.
mm
Casey
300
250
200
150
100
50
0
1960
1970
mm
1980
1990
2000
Davis
300
250
200
150
100
50
0
1960
Data from file: antprcp
1970
1980
1990
2000
14:41 Mon 11 Nov 2002
EC/WGAM-VIII, INF-4, Appendix, p.15
Figure 12
Monthly precipitation
Solid line shows mean.
mm
Casey
80
60
40
20
0
1960
1970
mm
1980
1990
2000
1990
2000
Davis
80
60
40
20
0
1960
Data from file: antprcp
1970
1980
14:43 Mon 11 Nov 2002
EC/WGAM-VIII, INF-4, Appendix, p.16
Figure 12
Annual percentage occurrence of weather codes: 70, 71, 72, 73, 74, 75
Solid line shows mean. Broken line shows smoothed fit.
%
Wilkes Casey
20
15
10
5
0
1960
1970
%
1980
1990
2000
1990
2000
Davis
20
15
10
5
0
1960
1970
%
1980
Mawson
20
15
10
5
0
Data from file: anttdw
1960
1970
1980
1990
2000
14:47 Mon 11 Nov 2002
EC/WGAM-VIII, INF-4, Appendix, p.17
Figure 12
Annual percentage occurrence of weather codes: 70, 71
Solid line shows mean. Broken line shows smoothed fit.
%
Wilkes Casey
20
15
10
5
0
1960
1970
%
1980
1990
2000
1990
2000
Davis
20
15
10
5
0
1960
1970
%
1980
Mawson
20
15
10
5
0
Data from file: anttdw
1960
1970
1980
1990
2000
14:51 Mon 11 Nov 2002
EC/WGAM-VIII, INF-4, Appendix, p.18
Figure 12
Annual percentage occurrence of weather codes: 72, 73, 74, 75
Solid line shows mean. Broken line shows smoothed fit.
%
Wilkes Casey
6
5
4
3
2
1
0
1960
1970
%
1980
1990
2000
1990
2000
Davis
6
5
4
3
2
1
0
1960
1970
%
1980
Mawson
6
5
4
3
2
1
0
Data from file: anttdw
1960
1970
1980
1990
2000
14:54 Mon 11 Nov 2002
EC/WGAM-VIII, INF-4, Appendix, p.19
Figure 12
Annual percentage occurrence of weather codes: 36, 37, 38, 39
Solid line shows mean. Broken line shows smoothed fit.
%
Wilkes Casey
20
15
10
5
0
1960
1970
%
1980
1990
2000
1990
2000
Davis
20
15
10
5
0
1960
1970
%
1980
Mawson
20
15
10
5
0
Data from file: anttdw
1960
1970
1980
1990
2000
14:59 Mon 11 Nov 2002
EC/WGAM-VIII, INF-4, Appendix, p.20
Figure 12
Annual mean daily maximum temperature
Solid line shows mean. Broken line shows smoothed fit.
°C
Wilkes Casey
-4
-6
-8
-10
1960
1970
°C
1980
1990
2000
1990
2000
Davis
-4
-6
-8
-10
1960
1970
°C
1980
Mawson
-4
-6
-8
-10
1960
Data from file: anttxtn
1970
1980
1990
2000
15:01 Mon 11 Nov 2002
EC/WGAM-VIII, INF-4, Appendix, p.21
Figure 12
Annual mean daily minimum temperature
Solid line shows mean. Broken line shows smoothed fit.
°C
Wilkes Casey
-10
-12
-14
-16
1960
1970
°C
1980
1990
2000
1990
2000
Davis
-10
-12
-14
-16
1960
1970
°C
1980
Mawson
-10
-12
-14
-16
1960
Data from file: anttxtn
1970
1980
1990
2000
15:01 Mon 11 Nov 2002
EC/WGAM-VIII, INF-4, Appendix, p.22
Figure 12
Annual mean temperature
Solid line shows mean. Broken line shows smoothed fit.
°C
Wilkes Casey
-6
-8
-10
-12
-14
1960
1970
°C
1980
1990
2000
1990
2000
Davis
-6
-8
-10
-12
-14
1960
1970
°C
1980
Mawson
-6
-8
-10
-12
-14
Data from file: anttdw
1960
1970
1980
1990
2000
15:04 Mon 11 Nov 2002
EC/WGAM-VIII, INF-4, Appendix, p.23
Figure 12
Annual percentage occurrence of temperature above 0°C
Solid line shows mean. Broken line shows smoothed fit.
%
Wilkes Casey
15
10
5
0
1960
1970
%
1980
1990
2000
1990
2000
Davis
15
10
5
0
1960
1970
%
1980
Mawson
15
10
5
0
Data from file: anttdw
1960
1970
1980
1990
2000
15:06 Mon 11 Nov 2002
EC/WGAM-VIII, INF-4, Appendix, p.24
Figure 12
Annual mean dewpoint
Solid line shows mean. Broken line shows smoothed fit.
°C
Wilkes Casey
-10
-12
-14
-16
-18
-20
-22
-24
1960
1970
°C
1980
1990
2000
1990
2000
Davis
-10
-12
-14
-16
-18
-20
-22
-24
1960
1970
°C
1980
Mawson
-10
-12
-14
-16
-18
-20
-22
-24
Data from file: anttdw
1960
1970
1980
1990
2000
15:08 Mon 11 Nov 2002
EC/WGAM-VIII, INF-4, Appendix, p.25
Figure 12
Annual mean MSL pressure
Solid line shows mean. Broken line shows smoothed fit.
hPa
Wilkes Casey
995
990
985
980
975
1960
1970
hPa
1980
1990
2000
1990
2000
Davis
995
990
985
980
975
1960
1970
hPa
1980
Mawson
995
990
985
980
975
Data from file: anttdwp
1960
1970
1980
1990
2000
13:49 Wed 04 Dec 2002