Introduction
Antarctic tropospheric clouds have been the subject of many studies,
including relevant reviews by and
. Detailed ground or airborne observation campaigns
e.g. are difficult, expensive
to conduct and rare in this region ; however,
satellite measurements have made a number of useful insights possible
. The properties
of snow- and ice-covered ground – namely being white, highly reflective, and
very cold – pose challenges to the use of passive satellite sensors for
cloud identification . These challenges are largely
circumvented by the active instruments on the CloudSat
and CALIPSO satellites whose
data we use in this study. While detailed atmospheric models potentially
allow further studies over far greater regional and temporal scales
, cloud
is difficult to model and accurately forecast, particularly over Antarctica
and the Southern Ocean , with the paucity of
observations a contributing factor.
The Antarctic coastal region is one of the most active areas of
synoptic-scale cyclonic storms in the Southern Hemisphere
, with suggesting that
these lows are associated with deep and high-level clouds and precipitation.
Additionally, identified that the meridional
moisture flux is dominated by motions at synoptic scales and reveal that the
Amundsen Sea sector experiences the highest variability around the Antarctic,
a potential driver of the variability observed in the region. This study
focuses on cloud properties over the Ross Sea and the Ross Ice Shelf (RIS)
because these regions are of particular interest in understanding the
controls of cloud properties around Antarctica. For example, it has been
reported that the largest seasonal variations in cloud occurrence across the
Antarctic are observed in these regions, with close to 60 % during winter
and 90 % in the summer . A number of recent
studies have also identified
unique cloud properties in these regions, and case studies detailed in
suggest a strong dependence on the meteorological
scenario.
The RIS is a largely flat expanse of permanent ice fed by both the West
Antarctic Ice Sheet (WAIS) and East Antarctic Ice Sheet (EAIS). The western
edge of the shelf is bounded by the 2 km high barrier of the
Transantarctic Mountains (TAM), with the EAIS behind. The surface meteorology
of the region is dominated by katabatic winds from the ice sheets
, and low-pressure systems
over the Ross Sea. The Ross Sea is located along the northern boundary of the
RIS and frequently experiences large low-pressure systems originating off the
coast of Adélie Land located well to the north-west. These are known to
advect moist marine air from the ocean/sea ice onto the RIS, often via the
WAIS and Siple Coast .
also highlighted the importance of marine air
intrusions for cloud fraction over the West Antarctic Ice Sheet driven by
cyclonic activity in the Ross and western Amundsen seas. This combination of
cyclones, the barrier presented by the TAM, and katabatic drainage helps to
feed a southerly wind regime that dominates the climatology of the RIS known
as the Ross Ice Shelf airstream (RAS) .
discussed a case study where a cyclone off Marie
Byrd Land transported moisture across the WAIS to the southern base of the
RIS which formed into cloud due to both low-level convergence and lifting
caused by a “knob flow”. A distinct extended thermal infrared signature
hypothesized to be associated with low-level cloud was observed along
the corridor of high winds linked to this RAS event. Recent work by
has developed a synoptic classification scheme by
applying the k-means clustering method to 33 years of ERA-Interim
surface wind data. This has been useful in understanding the range,
frequency, and influence of the different phenomena around the RIS (see
Sect. for details). More recent work by
demonstrated how the position and depth of the
Amundsen Sea Low influences the frequency and form of these different weather
regimes over the Ross Sea and RIS. This study aims to quantify cloud
occurrence over the RIS and southern Ross Sea using the CloudSat/CALIPSO
2B-CLDCLASS-LIDAR product , both spatially and
vertically. We also examine the occurrence, phase, and type of cloud with
a focus on whether synoptic drivers, identified via the synoptic regimes
developed by , provide a coherent pattern.
Clouds over the Southern Ocean and Antarctica can consist of liquid water,
mixed phases (i.e. consisting of supercooled liquid water droplets and ice
crystals), or ice crystals
.
Cloud phase is important to determine because ice crystals and water droplets
have different radiative properties and therefore reflect and absorb
different levels of incoming shortwave radiation
. Cloud composition over Antarctica
and the Southern Ocean is currently not well understood or modelled; however,
have shown that the radiative budget in this
area is highly sensitive to changes in cloud phase.
have shown that different types of clouds have
distinctive microphysical properties, resulting in different radiative
forcings
. It
is therefore clear that classification is an important task. The
International Satellite Cloud Climatology Project (ISCCP) uses passive
measurements to classify clouds into nine different types based on
their cloud top pressure and cloud optical thickness
. Later work by
developed an approach to classify clouds into eight types by combining
radiometer observations with “active” measurements from ground-based
lidar and radar. This classification scheme was modified for CloudSat
and CALIPSO observations to provide cloud type distributions globally
which are available in the 2B-CLDCLASS-LIDAR R04 (2BCL4) product used
in this study .
A recent study by investigated clouds over McMurdo
station, located at the north-western corner of the RIS, using
spectroradiometer measurements as well as observations from the NASA A-Train
satellites. They identified two major sources of moisture: marine air
intrusions originating over the WAIS which then cross the RIS (predominantly
ice-based), and moist air advection from the Ross Sea (more likely to contain
liquid). Large cyclones in the Ross Sea did not contribute significant levels
of moisture at Ross Island. In a follow-up study,
extended this work to show a link between high ice content and increased
vertical motion of the air parcel prior to observation.
used vertical profiles of cloud occurrence from
a pre-R05 2B-GEOPROF-LIDAR product .
They found a pronounced seasonal cycle in cloudiness over Antarctica and the
Southern Ocean with higher cloud occurrences during the winter. They also
found a nearly discontinuous drop-off in cloudiness near 8 km over
much of the continent. However, they and the review by
have questioned whether this is an artefact in the
data because this discontinuity corresponds to a change in the horizontal and
vertical resolutions of the CALIPSO data. also
highlighted that their vertical profiles revealed two distinct maxima, with
one near the surface level and the other near the top of the troposphere.
The increase in cloud during winter is contrary to the findings of
, who calculated seasonal variations spatially and
found that summer and autumn featured higher cloud occurrence than winter and
spring over most of Antarctica and the Southern Ocean, but particularly over
the RIS. Sea ice was suggested as a contributing factor, blocking evaporation
that occurs over open water, along with the extremely low temperatures.
Low-level cloud featured the highest inter-seasonal variability, with low
occurrence during winter and reduced occurrence during spring relative to
summer and autumn. also examined clouds over the
Southern Ocean using a combination of active and passive satellite data. They
separated the clouds in this region into eight regimes, but identified that
all of these regimes contained a relatively high occurrence of low cloud,
with 79 % of all cloud layers observed featuring tops below 3 km
in altitude. Multi-layered cloud systems were observed in approximately
34 % of cloud profiles. also found that cloud
systems are geometrically thicker during the austral winter and that all of
the eight regimes show enhanced low-level cloud fraction in the summer but
that the seasonal variation at higher levels is more complex. Those regimes
found to be most closely associated with mid-latitude cyclones also produced
precipitation more frequently.
Datasets and methods
CloudSat/CALIPSO data
CloudSat and CALIPSO
are two satellites that exist within the NASA A-Train,
a constellation of satellites with identical orbits that pass over the same
parts of the earth within a narrow time window (less than 1 km apart
90 % for the time period used in this study .
CloudSat carries a millimetre-wavelength (94 GHz) cloud profiling
radar (CPR) with a vertical resolution of 240 m and a sea-level
footprint of 1.4km×1.7km. It detects tiny water
droplets within clouds while also penetrating through optically dense upper
layers to detect further layers at lower altitudes; however, studies have
shown that it struggles to resolve cloud below 1 km above ground
level due to ground clutter . The Cloud-Aerosol Lidar
with Orthogonal Polarization (CALIOP) instrument carried by the CALIPSO
satellite provides vertical resolution of the order of 30 to 60 m
with a roughly circular sea-level footprint of 100 m in diameter. It
is able to accurately detect cloud down to ground level, but has reduced
sensitivity during daylight operations and cannot penetrate thick cloud. In
particular, this study uses the 2B-GEOPROF-LIDAR R04 (2BGL4) and R05 (2BGL5)
products which combine the CALIOP and
CPR observations to examine the vertical distribution of cloud occurrence. We
also use the 2BCL4 product which provides cloud occurrence, phase, and cloud
type information using a combination of CPR, CALIOP, and MODIS output with
ancillary temperature information from the European Centre for Medium-Range
Weather Forecasts (ECMWF). Analysis presented in Sect. shows
that the pre-R04 2B-GEOPROF-LIDAR products amongst
others display
a discontinuity at 8.2 km which appears to be limited to the poles in
both regions. indicates that the CALIOP data have
a centre-to-centre pacing of 333 m between profiles in the horizontal
and a 30 m vertical resolution below 8.2 km. Above
8.2 km, further averaging is applied to create a 1 km
along-track resolution and a 60 m resolution in the vertical. Thus,
we believe that the observed discontinuity is related to this change. We
therefore focus our analysis on the use of the 2BCL4 product.
The 2BCL4 product classifies clouds by examining the vertical profiles and
horizontal extent of clouds derived from the CPR and CALIOP measurements, the
presence of precipitation, cloud temperature from ancillary ECMWF
predictions, and upward radiances from MODIS measurements
and is consistent with the previous ISCCP classification
. The clustering algorithm uses a combination of
rule-based and fuzzy logic classification schemes to achieve this end. The
cloud types identified by the 2BCL4 product and their main defining
characteristics are identified in Table . Factors taken
into account in the classifier include cloud top and base height and
temperature, as well as cloud phase, thickness, horizontal extent and cover.
Different thresholds for cloud top/base heights are chosen for polar regions,
tropics, and mid-latitudes. Reported cloud phase is restricted by cloud base
and cloud top temperature. For cloud base temperature below
-38.5 ∘C, only ice cloud is permitted. For cloud base temperature
between -38.5 and 1 ∘C, all phases are permitted (liquid, ice and
mixed). For cloud base temperature above 1 ∘C, the cloud is
classified as liquid when cloud top temperature is above -7 ∘C,
liquid or mixed when the cloud top temperature is between -38.5 and
-7 ∘C or mixed for cloud top temperature below -38.5 ∘C
. Although CALIOP provides the depolarization ratio to
identify cloud phase, it is not reliable alone due to multiple scattering and
the fact that the CALIOP signal is quickly attenuated in multi-layer and
thick clouds. Instead, it is used in combination with the attenuated
backscatter coefficient and radar reflectivity, and exploits differences in
the number concentration, vertical distribution, and radiative properties of
ice particles and water droplets to distinguish different phase clouds when
this cannot be uniquely determined by cloud top/base temperature alone. In
this study, the stratus (St) and stratocumulus (Sc) cloud types have been
agglomerated based on advice released on the CloudSat website
.
Cloud types identified by the 2BCL4 cloud classification algorithm
and some of the properties upon which the algorithm is
based. Abbreviations: cloud base (CB), horizontal extent (HE),
vertical extent (VE), liquid water content (LWP).
Adapted from Table 3 in .
Cloud type
CB (km)
HE (km)
VE (km)
LWP (kgm-2)
Rain
High cloud (Ci)
7–
1–1000
1–7
= 0
none
Altostratus (As)
2–7
1000
1–7
≈0
none
Altocumulus (Ac)
2–7
1000
0–7
>0
virga possible
Stratus (St)
0–2
100
0–1
>0
none or slight
Stratocumulus (Sc)
0–2
1000
0–1
>0
drizzle or snow possible
Cumulus (Cu)
0–3
1–
0–7
>0
drizzle or snow possible
Deep convective (DC)
0–3
10–
7–
>0
intense shower of rain or hail possible
Nimbostratus (Ns)
0–4
50–1000
7–
>0
prolonged rain or snow
Topographic map of the RIS and Ross Sea including the
boundaries of the study area (thick blue line) and border between
the RIS and Ross Sea sectors. (Map derived from the SCAR Antarctic
Digital Database.)
The area of interest in this study covers as much of the RIS as possible and
extends into the southern Ross Sea. Defined by the edges of the ice shelf
(160 to -150∘ E), it extends from the bottom of the A-Train track
at 82∘ S north to 75∘ S. The area is divided into two
sectors (RIS and the Ross Sea) along the 78∘ S circle of latitude,
each of which are further divided into east/west sectors to form four
quadrants. Figure identifies the study region and its bounds.
Note that despite the larger area of the Ross Sea compared to the RIS defined
in this study, the number of vertical profiles linked to the RIS region is
far higher than that for the Ross Sea (4.1 vs. 1.8 million). This disparity
is associated with a strong latitudinal variation in the sampling density
associated with the satellite orbits. This study uses observations made
between 1 January 2007 and 31 December 2010 when both the CloudSat and
CALIPSO satellites were fully operational and aligned.
In Sect. we inspect cloud occurrence as a function of altitude
for different cloud phases and the cloud fraction. The cloud occurrence is
derived by counting the occurrence of cloud at a particular altitude using
the CloudLayerBase and CloudLayerTop fields of the 2BCL4 product. This is in
contrast to the methodology used in , which used a
threshold of 50 % for the CloudFraction field (fraction of lidar volumes
in a radar resolution volume that contains hydrometeors) for determination of
cloud occurrence. In this study we calculate the cloud fraction (not related
to the CloudFraction field) as the complement of the clear sky fraction,
which is the number of clear sky profiles divided by the total number of
profiles. As such, it is independent of altitude.
Synoptic climatology
To provide context on atmospheric circulation over the
duration of this study, classifications and regimes developed in the work of
and are used. Five broad
synoptic-scale regimes, hereafter referred to as “Coggins regimes”,
encompass 20 classes created by applying the k means clustering technique
to 10 m winds from 33 years of ERA-Interim reanalysis
over the RIS/Ross Sea region. The 20 classes grouped into
five regimes were found to be representative of conditions in the area and
span the entire time period of available cloud observations, and so are an
obvious choice for this analysis. The first two Coggins regimes are the weak
northern cyclonic (WNC) and strong northern cyclonic (SNC) regimes which
feature cyclones to the north of the RIS, with the “weak” and “strong”
ratings referring to their effect on the winds over the RIS; WNC generally
provides weak forcing and low wind speeds, while SNC features a strong
synoptic pressure gradient force and high wind speeds over the RIS. The Ross
Ice Shelf airstream (RAS) Coggins regime covers the strongest winds over the
RIS and typically features a strong cyclone to the north and east that
provides a large pressure gradient over the ice shelf which forms RAS-like
signatures , while the weak southern cyclonic (WSC)
regime is associated with relatively weak cyclones and mesocyclones
positioned over the RIS with medium wind speeds. Finally, the weak synoptic
(WS) Coggins regime covers periods where a very weak pressure gradient and
very low winds are present over the RIS.
Table shows the relative frequency of occurrence
of the regimes over the entire observational period examined and a normalized
seasonal frequency. Examination of the “all” column shows that the WSC
regime is relatively rare (9 % annual frequency), while the WS regime is
observed frequently (29 %). The WNC and RAS regimes are also quite common
(25 and 23 % respectively), while SNC is less common (14 %). Seasonal
analysis of the frequency of occurrence shows the WNC regime occurs most
frequently during austral autumn (34 %) but much less frequently during
winter (19 %), while the SNC regime is more uniform across all four
seasons – this likely reflects the ubiquitous nature of synoptic-scale
cyclonic storms around Antarctica . The RAS
regime is seen much more frequently in winter (37 %) than summer
(10 %), while the WSC and WS regimes alternately favour autumn
(WSC 36 %) and summer (WS 36 %) at the expense of summer
(WSC 12 %) and autumn (WS 17 %). It must be noted that
Table is structured for seasonal analysis of
individual regimes (rows sum to 100 %) and does not provide a comparable
statistic of regime frequency in each season (season columns do not sum to
100 %).
Relative frequency of occurrence of the Coggins regimes
annually (all) and seasonally (DJF–SON) in the ERA-Interim
reanalysis (%). DJF/MAM/JJA/SON correspond to austral
summer/autumn/winter/spring respectively. Values for seasons are
normalized so that rows sum to 100 % (not including “all”).
all
DJF
MAM
JJA
SON
WNC
23
24
34
19
23
SNC
14
22
27
26
25
RAS
25
10
28
37
25
WSC
9
12
36
30
23
WS
29
36
17
21
27
Results
Discontinuity in the 2B-GEOPROF-LIDAR R04 product
As an initial point of comparison with the previous Antarctic-wide studies of
and we display seasonal mean
cloud occurrence statistics in Fig. derived from the 2BGL4,
2BGL5 and 2BCL4 products for the Ross Sea and the RIS. A visual comparison
between the three products generally shows good agreement (within a few
percentage points), though the 2BGL4 values of cloud occurrence are a little
above the values derived from the other two products everywhere below
8 km. A step change in the cloud occurrence can also be observed at
8.2 km in all seasons and over both the Ross Sea and the RIS in the
2BGL4 product. It should be noted that this step change is particularly large
in the winter and spring and is also significantly larger over the RIS than
the Ross Sea. The 2BGL5 and 2BCL4 values do not display this discontinuity
and are much more similar to each other, though it is noticeable that the
2BCL4 values of cloud occurrence are always smaller than the other two
products below 1 km. Interestingly the temporal average cloud
occurrence for the 2BGL4 product is always larger than that for the 2BCL4
product, which in turn is always greater than the 2BGL5 product.
Mean vertical profiles of cloud occurrence derived from the
2BGL4, 2BGL5 and 2BCL4 data for the Ross Sea (a–d) and RIS
sectors (e–h) for different seasons. Total sector cloud
fraction (temporal average cloud occurrence independent of altitude)
is annotated at the top of each sub-figure. L, M, and H labels
indicate the low, medium, and high cloud regions, respectively, as
discussed in the text.
To further examine the extent of this issue, Fig. displays the
zonal mean value of the ratio of the cloud occurrence at 8.3 km to
the cloud occurrence at 8.0 km derived from the three products. The
two altitude bins are in consecutive height bins, but are linked to different
vertical and horizontal resolutions in the 2BGL4 processing scheme according
to . Inspection of Fig. shows that the
ratio varies between 0.9 and 1.1 for all three products near the Equator and
at mid-latitudes. However, the 2BGL4 value of the ratio deviates
significantly from that derived from the other two products at latitudes
poleward of 75∘ in both hemispheres. The deviation between the 2BGL5
product and the 2BCL4 product is also relatively large in the Northern
Hemisphere above 60∘ N. Previous studies
have highlighted
this discontinuity near 8 km, but have questioned whether it is an
instrumental artefact or a physical feature. The analysis in
Fig. clearly suggests that this is an instrumental artefact
specific to both polar regions. The larger discontinuity observed in
Fig. in winter may suggest a temperature-dependent issue. But,
further analysis is beyond the scope of this study given the good
correspondence between the 2BCL4 and 2BGL5 products.
Latitudinal variation of the ratio of cloud occurrence at
8.3 km to cloud occurrence at 8.0 km derived from
the 2B-GEOPROF-LIDAR products and the 2B-CLDCLASS-LIDAR
products. The envelopes represent the interquartile ranges of the
ratio observed at that latitude. Note that consistently anomalous
values ≪1 are confined to the polar latitudes.
Cloud occurrence and phase by season
Given the uncertainty identified within the 2BGL4 (GEOPROF) product, we
choose to deviate from previous studies and use the 2BCL4 (CLD-CLASS)
product. We consider this preferable to the 2BGL5 product, despite the
apparent resolution of the uncertainties in 2BGL4, as it provides information
on cloud phase and type, which are particularly interesting in this region.
We initially examine cloud occurrence as a function of cloud phase.
reported that the largest seasonal variations in
cloud occurrence were observed over the RIS and sea ice region in the
surrounding Ross Sea, suggesting this region may be of particular interest in
understanding the controls of cloud in the region. Cloud occurrence is
separated into two sectors: the Ross Sea and the RIS (see
Fig. ). For the purpose of this analysis we separate clouds into
three vertical ranges: low-level clouds (0–2 km), mid-level clouds
(2–6 km) and high-level clouds (6– km) identified by horizontal
lines in Figs. , and .
Mean vertical profiles of cumulative cloud occurrence for
different cloud phases derived from 2BCL4 data for the Ross Sea
(a–d) and RIS sectors (e–h) for different
seasons. Total sector cloud fraction (temporal average cloud
occurrence independent of altitude) is annotated at the top of each
sub-figure, along with values for the western (W) and eastern (E)
halves of each sector. L, M, and H labels indicate the low, medium,
and high cloud regions, respectively, as discussed in the text.
Figure a–d display the cloud occurrence for the Ross Sea region
in each season broken into different cloud phases (shaded areas). The maximum
cloud fraction (82 %) is observed during summer, with the minimum cloud
fraction (70 %) observed during winter. The largest cloud fraction is
observed over the eastern portion of the Ross Sea in every season, with the
greatest seasonal cloud fraction in summer (86 %). The smallest cloud
fraction (66 %) was observed in winter over the western portion of the
Ross Sea. Cloud occurrence as a function of altitude shows the same pattern,
with the maximum (about 40 %) occurring in summer and the minimum (about
27 %) during winter. Though all maxima occur between 1.5 and 2 km
above sea level (a.s.l.), the winter maximum is noticeably weaker. Mixed
phase clouds are predominant near the cloud occurrence altitudinal maximum,
with water and ice cloud contributing roughly equally to the remainder.
Changes in the occurrence of mixed phase cloud appear to constitute the
majority of the change in the cloud occurrence at that altitude.
Cloud occurrence reduces uniformly at increasing altitude from the maxima in
summer and autumn, while in winter the cloud occurrence reduces rapidly
between the peak and 3.5 kma.s.l., and then remains relatively
uniform before a more rapid reduction at higher altitudes (8 km in
winter and 6 km in spring). Previous work detailed in
highlighted a discontinuity in cloudiness near
8 kma.s.l. over much of the continent which appears to be linked to
the processing artefact identified previously.
Unlike the study of we do not observe two distinct
maxima in the vertical profiles of the cloud occurrence; however, this
feature was relatively weak over the WAIS see Fig. 5
in, which may hint at the specific drivers of the cloud
environment in this region. In particular, the absence of a secondary peak in
mid- and high-level cloud occurrence is interesting given the ubiquitous
nature of cyclones in the region , and the muted
seasonal signal in cloud occurrence above 2 kma.s.l. could be
explained by the lack of a strong seasonal signal in cyclone frequency in
this region (supported by the small seasonal signal in the frequency of the
SNC regime displayed in Table ). The difference
in our cloud occurrence calculation methodology to that used by
may have some impact; however, it is unlikely to
explain all of the difference.
As might be expected, the liquid water phase occurs predominantly in
low-level clouds with a local maximum between 300 and 900 ma.s.l.
in all seasons, the largest enhancement occurring during summer. The
difficulty of detecting cloud within 1 km of the ground using
CloudSat due to ground clutter may bias
low-level cloud detection in favour of periods of reduced attenuation of the
CALIOP lidar instrument (clear sky or optically thin mid- to high-level
cloud). Mid-level (between 2 and 6 kma.s.l.) cloud occurrence
varies little between seasons at the upper limit (6 km) at close to
20 %, but mid-level cloud fraction is greatest in summer and lowest in
winter. More high-level cloud (above 8 kma.s.l.) is observed during
winter than summer, which matches with the results identified in
. We also note that clouds were not observed in this
study above 10 kma.s.l. in both the summer and autumn, or
12 km in spring, but were seen above 14 km in the winter.
suggest that the maximum cloud height over the Southern
Ocean will be impacted by the seasonal variations in tropopause depth likely
explains this pattern, which interestingly shows a similar seasonal
progression to that for polar stratospheric cloud occurrence
.
The mean seasonal cloud occurrence vertical profiles for the RIS are
displayed in Fig. e–h. The eastern portion of the RIS has
slightly greater cloud occurrence than the western portion of the RIS in all
seasons apart from autumn. The cloud fraction for the RIS area is 70 % in
summer and between 63 and 65 % in all other seasons. The greatest cloud
fraction by sector is observed over the eastern RIS during summer (72 %),
while the lowest is observed in winter over the western RIS (61 %).
Inspection of the cloud occurrence as a function of altitude shows a maximum
at approximately 2 kma.s.l. in every season, slightly higher than
the altitude of the peak observed over the Ross Sea. This peak in cloud
occurrence matches with a similar peak in ice water content (IWC) and liquid
water content (LWC) values discussed in over Ross
Island (located at the south-western corner of the Ross Sea region in this
study). Summer experiences the greatest cloud occurrence as a function of
altitude at this peak, but the seasonal variability is rather muted (just
under 5 % variation across all seasons) relative to that observed over
the Ross Sea (approximately 12 %), with the minimum in winter. The mixed
phase class is a contributor to this peak in all seasons, but is dominant in
summer. The cloud occurrence linked to the ice phase is slightly larger than
that for mixed phase cloud in autumn.
Again, the water phase occurs predominantly for low-level clouds
(below 2 kma.s.l.) with maxima below 600 m observed
in every season (this is particularly clear for summer). The ice phase
is effectively the only contributor for high-level clouds (above
6 km) and is the largest contributor to cloud occurrence in
every season, though mixed phase cloud is dominant up to approximately
3 km in summer. The quantity of mixed and water phase cloud as
a proportion of total cloud occurrence is substantially lower than
that observed over the Ross Sea, possibly suggesting a lack of
moisture in this region and the impact of colder temperatures.
The seasonal variation in cloud occurrence at mid-levels (between 2 and
6 kma.s.l.) is approximately 5–10 %, which is smaller than the
variations observed at the peak occurrence level. Within this altitude range,
cloud occurrence is more constant in winter and spring and reduces with
altitude in spring and summer. The cumulative occurrence of mid-level clouds
is marginally higher in summer than other seasons, with a minimum value in
winter. High-level clouds (above 6 km) are distinctly more common in
autumn and winter than spring and summer. Similar to the Ross Sea case, the
majority of clouds are limited to below 10 kma.s.l. in both the
summer and autumn, with maximum high-cloud occurrence in winter. Over the
entire vertical profile, the seasonal variation over the RIS is smaller than
that observed over the Ross Sea (cf. Fig. a–d and e–h).
Cloud occurrence and phase by synoptic regime
We now examine composites of the vertical distribution of cloud occurrence
and cloud fraction for these two regions based on the synoptic-scale Coggins
regimes. Figure a–e display the cloud occurrence profiles over
the Ross Sea for the five different Coggins regimes. Comparison of
Fig. a–e shows that the combined cloud occurrence (associated
with the three different cloud phase classes) in every regime again maximizes
just below 2 kma.s.l. The greatest occurrence at that peak is
observed in the SNC regime and the smallest occurrence in the WSC regime. At
middle altitudes (4 to 6 km), the difference in cloud occurrence
between regimes is very large, with the SNC regime again displaying the
largest cloud occurrences
(between 28 and 35 % in this altitude range) and the WSC regime the least
(between 7 and 17 %). The WSC and WS regimes have rather similar vertical
profiles, with the vast majority of cloud occurrence linked to low- to
mid-level clouds below 4 kma.s.l. The SNC type on the other hand
has high cloud occurrence at nearly all levels compared to the other regimes
and has signs of a secondary maximum at 5 km. The RAS and WNC have
intermediate levels of cloud occurrence between the SNC and WSC/WS regimes,
with shallower rates of reduction in the cloud occurrence above the peak.
Inspection of the distribution of phases linked to the SNC regime
(linked to strong cyclonic activity in the north of the Ross Sea)
suggests that this regime is dominated by ice cloud at all levels down
to about 2 kma.s.l. (i.e. mid- to high-level clouds are
predominately comprised of ice). A small amount of water phase cloud
is observed very close to the surface and a peak in mixed phase cloud
is observed just below the peak in the combined cloud occurrence. The
quantity of mixed phase cloud being larger than the ice phase only
occurs in the WS and WNC types near the low-level maxima. The SNC and
RAS regimes feature the largest proportion of ice cloud overall. The
cloud fraction (see labels in Fig. ) varies from 82 %
for the SNC regime to 58 % for the WSC regime, with the other
regimes having values between 72 and 77 %. This variation is
significantly larger than that observed when observations are
composited based on season. This result suggests that cloud fraction
is strongly impacted by synoptic situation. In particular, the SNC
regime has high occurrence frequencies in the mid- to high-level cloud
region above 2 kma.s.l. Additionally, we note that the
variation between the western and eastern portions of the Ross Sea is
larger in the SNC, RAS, and WSC regimes (10–13 %) than over the
seasons (7–8 %), while the WNC regime shows little variation
longitudinally. This suggests that synoptic forcing is a more
important control on longitudinal differences than season, though this
should be expected because the strength and position of cyclonic
centres is a principal determinant of the Coggins regimes. While
overall the synoptic typing seems to be important, we note that the
proportion of liquid and mixed phase cloud varies relatively little
over the Ross Sea as a function of the Coggins regimes (between
approximately 15 and 20 % at the altitude of maximum occurrence),
while the seasonal variation is substantially larger (between
approximately 9 and 30 % at the altitude of maximum
occurrence). This result supports the view that temperature is
a strong driver of the occurrence of ice cloud as previously
identified by , though the variability observed
between synoptic types suggests that temperature anomalies associated
with specific synoptic types also have some influence.
Figure f–j display composites for the RIS for the Coggins
regimes. Cloud occurrence is significantly smaller in every regime relative
to the profiles over the Ross Sea (cf. Fig. a–e). Examination
of the cloud occurrence profiles as a function of altitude for each regime
suggests that the WNC, WSC, and WS regimes have similar forms, as do the SNC
and RAS regimes (to each other). Interestingly, cloud occurrence is higher in
the RAS regime between 2 and 6 km than SNC. This likely suggests that
the impact of cyclones in the northern Ross Sea is not as strong an influence
on cloud over the RIS. At mid to high levels (above 4 km), RAS and
the SNC have the largest cloud occurrences, with ice cloud dominating in this
region. The variation at upper levels is also noticeably larger between the
various synoptic regimes in Fig. than between the seasons
displayed in Fig. . This seems to suggest that the synoptic
state is a stronger driver of mid- to high-level cloud than seasonal
variations, this being particularly clear when we consider that the regime
with most high-level cloud (the SNC regime) displays almost no seasonality
(see Table ). Figure therefore
shows an advantage in using a classification scheme based on synoptic states
relative to one using seasons in this region. Previous work by
also suggested that seasonality might not be a strong
influence on the Southern Ocean, with two exceptions, these being the
quantity of ice cloud and the height at which the maximum cloud fraction
occurs in the upper troposphere.
Examination of the cloud fraction over the entire RIS and the western and
eastern sectors shows less variability than over the Ross Sea. The cloud
fraction varies only between 55 and 68 % and the differences in cloud
fraction between the western and eastern sectors are only sizeable (9 %)
for the WSC regime. The difference between the cloud fraction between the
western and eastern sectors is 5 % or less in all other regimes. Given
that the WNC and SNC regimes are dominated by the positions of cyclones over
the Ross Sea, this may not be surprising in those cases. However, the lack of
longitudinal variation associated with the RAS which is traditionally linked
to flow near the TAM is a surprise.
Mean vertical profiles of cumulative cloud occurrence for
different cloud phases derived from 2BCL4 data for the Ross Sea
(a–e) and RIS regions (f–j) for the Coggins
regimes. L, M, and H labels indicate the low, medium, and high cloud
regions, respectively, as discussed in the text.
Multilayer cloud by season and regime
While useful, the mean vertical profiles of cloud occurrence displayed in
Figs. and do not fully represent the individual
profiles composited in that season or regime. For example, two states
associated with a distinct high and low cloud type might be combined in the
averaging process to form the mean cloud occurrence observed. Alternatively,
multi-layered cloud might be present and contribute to the mean cloud
occurrence profiles. In an effort to display this variability,
Fig. shows the quantity of clear skies, single-layer cloud, and
multi-layer cloud for the Coggins regimes and seasons over the Ross Sea and
RIS. Inspection of Fig. a for the Ross Sea region suggests that
clear skies are observed 26 % of the time on average. However, when the
cloud occurrence information is composited based on the Coggins regime, the
frequency of occurrence of clear skies varies between 18 % for the SNC
regime and 42 % for the WSC regime. Seasonal variations in clear sky
occurrence are considerably smaller at 18 to 30 %. Again, this highlights
that clouds are observed preferentially in the Ross Sea when strong cyclonic
centres are observed in the northern Ross Sea. Changes in the frequency of
multi-layer clouds are also notable, with the frequency varying from 15 %
for the WSC regime to 33 % for the SNC regime. The occurrence of
multi-layer cloud is considerably more constant as a function of season,
varying between 21 and 25 %, which highlights that the quantity of
multi-layer cloud is also strongly impacted by synoptic conditions.
Figure b displays the occurrence of clear skies, single-layer
cloud and multi-layer cloud for the RIS region. The variability as a function
of both Coggins regime and season is again muted relative to the Ross Sea
region. The occurrence of clear skies varies from 32 % for the SNC regime
to 45 % for WSC, with the other three regimes having frequencies between
34 and 35 %, which is similar to that for the SNC regime. This suggests
that only the synoptic conditions linked to the WSC regime are strongly
linked to clear skies. When clear sky occurrence is examined as a function of
season, a very small seasonal variability is observed (values fall between 30
and 37 %). This reinforces our previous conclusion for the Ross Sea
region: that clear skies are not strongly influenced by season and therefore
surface temperatures. Examination of multi-layer cloud values shows
a variation between 15 % for WSC and 23 % for both the RAS and SNC
types. This suggests that the RAS regime is also preferentially related to
multi-layer cloud over the RIS. Work by has
previously suggested that the RAS regime might be linked to the occurrence of
low- and mid-level cloud, the latter being associated with vertical ascent
generated by low-level convergence as the RAS decelerates downstream of wind
speed maximum along the TAM. This relationship also appears to be observed
based on our statistical analysis. The high occurrence of cloud at mid-levels
(between 2 and 6 kma.s.l.) displayed in Fig. g
therefore suggests that the RAS and possibly the marine air intrusions
identified in have a noticeable climatological
impact on cloud occurrence over the RIS. In addition, the seasonal
progression and variation linked to synoptic typing display very different
impacts over the Ross Sea and RIS. This perhaps highlights the stronger
influence of cyclones on cloud occurrence over the Ross Sea than the RIS. The
higher frequency of occurrence of multi-layer cloud linked to the SNC type
over the Ross Sea than the RIS also suggests the position of the cyclone
centre plays an important role in cloud distributions.
Distribution of the number of cloud layers over the Ross Sea
and RIS for all cases, the Coggins regimes and season.
Cloud type by season and regime
Figure displays the fractional occurrence of the various cloud
types over the Ross Sea and RIS composited based on Coggins regime and
season. For the sake of conciseness, we will only discuss the types which
have substantial fractional occurrence rates (above 15 % in any class).
The frequency of nimbostratus (Nb) is so small over these regions that this
type is not included in Fig. . For the Ross Sea, the most
commonly occurring cloud type is deep convective (DC), which varies from
32 % for the WS regime to 43 % for the RAS regime, with the other
regimes ranging between 32 and 39 % (see Fig. a). This cloud
type is likely identified due to large horizontal and vertical extents of the
cloud rather than the presence of deep convection in the polar region. The
seasonal variation has a maximum in autumn of 42 % with a minimum of
36 % in winter. Thus, in this high-level cloud type, more variation
between classes is associated with synoptic classification than season.
Interestingly the maximum occurrence of this type over the Ross Sea is
associated with the RAS regime rather than the SNC regime, previously
identified as the regime associated with the highest cloud occurrence. When
we additionally include the impact of clear skies (values displayed in
Fig. ), this conclusion remains unchanged.
The next most common cloud type over the Ross Sea is the altostratus (As)
type, which varies between 30 and 36 % based on Coggins regimes and
between 29 and 36 % based on season. In particular, the fractional
occurrence of this cloud type maximizes in winter and minimizes in summer
over the Ross Sea. However, when the frequency of clear skies (see
Fig. ) is also considered, the seasonal variation becomes very
small (23 to 25 %), while the regime variation is enhanced to 17 to
30 %. Thus, in this case close inspection also suggests that synoptic
forcing is a driver of the occurrence of this cloud type, with the highest
occurrence in the SNC regime and lowest occurrence in the WSC regime. Note
that the As type is predominantly a mid- to high-level cloud dominated by ice
and thus may not be strongly impacted by seasonally varying quantities, such
as sea ice cover and surface temperature.
Low-level clouds (combined stratus/stratocumulus or
St/Sc types) are observed relatively frequently in the WS
(21 %) and WSC (19 %) regimes over the Ross Sea. Both these regimes
are associated with weaker synoptic forcing and observed less frequently than
the regimes with stronger synoptic forcing. Seasonal variation in these types
changes from 17 % in summer to 12 % in winter. Thus, it seems that
the occurrence of this class is more associated with periods of weak synoptic
forcing; note that these conditions occur more often in summer (see
Table ), which in turn might suggest that local
factors are important. Inclusion of information on clear sky rates does not
change this result.
Figure b displays cloud type fractional frequency information
for the RIS region. Over the RIS, the As cloud type is most prevalent,
varying between 38 and 46 % based on synoptic regime and 25 and 43 %
based on season. However, when clear sky occurrence is considered these
values reduce to 24 to 32 % for the regimes and 25 to 27 % for the
seasons. This suggests that the quantity of altostratus remains nearly
constant seasonally. The highest occurrence of the As type when clear skies
are taken into consideration is linked to the SNC and RAS regimes, with very
similar low occurrences (24 to 25 %) for the other regimes.
The next most prevalent cloud type over the RIS is the DC type, which changes
between occurrence rates of 23 and 33 % for the various Coggins regimes
and 24 and 30 % based on seasons. The highest fractional occurrence of
the DC type occurs for the RAS regime and the minimum is, surprisingly,
linked to the SNC type. Thus, two regimes which are related to strong
synoptic forcing in the region have very different impacts on this cloud
type. This latter result might be explained by the position of the cyclonic
centres, preferentially in the north-eastern Ross Sea for the SNC type,
relative to the RIS. When the frequency of clear skies is included in our
analysis, a larger variation in this cloud type is linked to synoptic forcing
than seasonal changes.
The combined St and Sc cloud types also have an appreciable occurrence
rate over the RIS (13 %). When the Coggins regimes are considered
this type has a minimum occurrence of 7 % linked to the RAS regime
and a maximum occurrence of 20 % linked to the WNC regime. It
should be noted that there is an obvious change in the fractional
frequency of this type between strong synoptic forcing regimes (RAS
and SNC) and weaker synoptic forcing regimes (WNC, WSC, and WS). This
separation again suggests that these clouds are linked to periods of
weak synoptic forcing. The range of the fractional occurrence rates
associated with the different seasons is again smaller than that
associated with the synoptic types; summer displays the highest
occurrence rate. When clear sky frequencies are included in
calculations this result is unchanged.
The fraction of the cirrus (Ci) cloud type is also appreciable over the RIS
and has the same frequency of fractional occurrence as the combined St and Sc
cloud type (13 %). The fraction of this cloud type maximizes at 17 %
for the SNC regime and has a minimum occurrence of 9 % for the WNC
regime. The Ci type is observed most frequently in winter (20 %) and
least in summer (6 %). Thus, synoptic variations do not appear to be
a very strong control on this cloud type. This conclusion is unchanged when
the occurrence of clear skies is included in our analysis. We note that the
larger fractional occurrence of Ci over the RIS compared to the Ross Sea
could be associated with the proximity of the TAM to the RIS and the
influence of isolated cirrus generated by orographically forced waves, this
conjecture being supported by and
.
Percent fraction of cloud types over the Ross Sea and RIS for
all cases, the Coggins regimes and seasons. The cloud types are
identified in Table .
Cloud height and thickness
Figure a and f display joint histograms of the cloud top height
vs. the geometric cloud thickness over the Ross Sea and RIS respectively.
Figure a shows that low-level (below 2 kma.s.l.) thin
cloud (thicknesses below 3 km) has a high occurrence over the Ross
Sea, an observation previously identified by .
However, thin clouds are observed relatively frequently for cloud top heights
between the surface and approximately 8 km over the Ross Sea. Clouds
that effectively cover nearly the complete vertical column to cloud top (i.e.
that have similar thicknesses to their cloud top height) are also observed
frequently. We also note that clouds with high cloud tops (above
6 km) are relatively rare. The logarithmic scale associated with
Fig. highlights that thin low-level cloud is very common. A
similar pattern is observed over the RIS region overall (see
Fig. f), though cloud occurrence is higher in general for the
Ross Sea, particularly at lower levels. This is to be expected, given the
differences in solar heating of the surface, sea ice concentration changes
and sea surface temperatures. In particular, the greater availability of
moisture over the Ross Sea associated with open water in summer would likely
be an important contributor. The overall pattern is similar to the joint
histograms identified by over the Southern Ocean.
To further understand the distribution of clouds over the two regions,
Fig. b–e and g–j display anomalies from the annual means for
each season for the Ross Sea and RIS respectively. Examination of
Fig. b–e suggests that the anomaly patterns are rather similar
in the summer and autumn, with higher cloud occurrence observed for low-level
(below 2 km) and mid-level (2 to 6 km) cloud which covers the
majority of the vertical column up to the cloud top height. Lower cloud
occurrence is associated with thin high top clouds for summer and autumn. The
anomaly patterns in winter and spring are near mirror images of those in
summer and autumn, with higher cloud occurrence for thin high top cloud and lower occurrence (relative to
the annual mean) for low-level and mid-level cloud covering the vertical
column. High top clouds (above 8 km) with a range of thicknesses are
also enhanced in winter and spring, with the enhancement being more
noticeable for thick clouds in winter. previously
identified that the increase in thicker clouds in winter over the Southern
Ocean may be associated with storm track activity.
also suggest that the maximum cloud height might vary seasonally based on the
tropopause height; therefore, this also seems like a reasonable explanation
for the enhanced occurrence of high-level cloud above 8 km in the
winter relative to the summer. The anomaly patterns for each season are
generally rather similar over the Ross Sea and the RIS. This is surprising
since this may imply that moisture availability is not a large driver of
cloud.
Joint histogram of the cloud top height vs. geometric cloud
thickness over the Ross Sea and RIS for the entire year on
a logarithmic scale (a, f) and the difference from the
annual mean over the respective region (RS and RIS) for each season
on a linear scale (b–e, g–j).
Figure b–f and h–l display the anomalies from the mean
associated with the Coggins regimes for the Ross Sea and RIS respectively.
Figure a and g display histograms of the frequency of occurrence
as a function of thickness and cloud top height for the Ross Sea and RIS
respectively, are exact reproductions of Fig. a and f and are
included to aid in interpretation. Inspection of the anomalies from the mean
linked to different synoptic regimes shows some interesting structures. The
SNC regime is linked to a dearth of low-level cloud relative to the mean,
particularly over the Ross Sea, while the WSC regime is linked to
a considerable enhancement in the frequency of low- and mid-level cloud which
covers the majority of the vertical column up to the cloud top height. These
variations may also explain the small quantity of multi-layer cloud in this
regime over the Ross Sea. The enhancement in the WSC regime is also observed
in the WS regime, but that regime is also related to a stronger reduction in
clouds covering the vertical column above 4 km. For the SNC regime,
the dearth in low-level cloud over the Ross Sea is counter-balanced by an
increase in thick (greater than 7 km deep) high-level cloud relative
to the mean, which also accounts for the increased cumulative cloud
occurrence for the SNC regime identified in Figs. b
and a. While some aspects of the anomaly for the RAS regime (see
Fig. d) are similar to the SNC regime pattern (see
Fig. c), notably a reduction in low-level cloud relative to the
mean, differences can be observed. For example, the clouds with high cloud
tops (above 8 kma.s.l.) are under- rather than over-represented
relative to the mean for the entire thickness range for RAS compared to SNC.
Enhanced cloud occurrence in the RAS regime is primarily limited to mid-level
clouds, most notably linked to clouds with thicknesses below 2 km and
the region identifying that the cloud covers nearly the full atmospheric
column.
To put the anomaly patterns identified for the Ross Sea into context, it is
also worthwhile considering the patterns over the RIS
(Fig. h–l). Unlike the seasonal analysis presented in
Fig. , which displayed large similarities for the anomaly
patterns over the Ross Sea and RIS, the patterns show more variability for
the WNC, SNC, and RAS regimes between the two regions. In particular, the WNC
regime displays a strong enhancement in the quantity of low- and mid-level
cloud below 4 kma.s.l., with thin cloud and cloud covering the
majority of the atmospheric column up to the cloud top height being enhanced.
The SNC type shows a similar pattern to that over the Ross Sea, but the
strong enhancement of deep, high cloud top height cloud is not observed in
this case. The RAS regime joint histogram shows a stronger reduction in low-
and mid-level cloud below 4 km over the RIS than the Ross Sea and the
enhanced cloud occurrence region now occurs for high-level cloud (cloud top
height above 6 kma.s.l.). Thus, the vertical extent of the RAS
regime changes considerably between the Ross Sea and the RIS. Based on
previous work detailed in this may be associated
with vertical ascent over the RIS.
Joint histogram of the cloud top height vs. geometric cloud
thickness over the Ross Sea and RIS for the entire year on
a logarithmic scale (a, f) and the difference from the
annual mean over the respective region (RS and RIS) for each Coggins
regime on a linear scale (b–e, g–j).
Conclusions and discussion
This study has quantified the vertical distribution of cloud fraction, phase,
and type over the Ross Ice Shelf and southern Ross Sea using 4 years of data
from the 2B-CLDCLASS-LIDAR R04 product composited
using seasons and synoptic regimes . The
following results highlight the usefulness of incorporating a synoptic
classification scheme into the climatological analysis of clouds in this
region.
Large differences exist between the cloud occurrence as
a function of altitude for synoptic regimes relative to those for
seasonal variation (cf. Figs. and ).
There is strong variation in clear sky and multi-layer cloud
occurrence as a function of synoptic regime as opposed to season
(see Fig. ).
There is higher variance in all cloud type occurrences, apart
from the cirrus type, over both the Ross Sea and RIS associated with
synoptic type compared to seasonal composites (see
Fig. ), which remains true when the frequency of clear
skies is taken into account or discounted from our analysis.
Anomalies from the mean joint histogram of cloud top height
against thickness display significant differences over the Ross Sea
and RIS sectors as a function of synoptic regime, but are near
identical over these two regions when a seasonal analysis is
completed (see Figs. and ).
Clouds are observed preferentially in the Ross Sea when strong
cyclonic centres are observed in the northern Ross Sea.
The cumulative cloud occurrence observed in the western and
eastern portions of the Ross Sea and RIS display larger differences
for composites based on the synoptic regimes than seasons. This
again suggests a significant influence of the position of cyclonic
centres. However, the lack of longitudinal variation associated
with the RAS which is traditionally linked to flow near the TAM is
unexpected.
We have therefore proven that an analysis based on synoptic regimes
explains more of the variation in overall cloud occurrence and
specific cloud types than a simple seasonal analysis. This complements
previous studies which have inferred these relationships
or used a case study
approach . It is however
important that the seasonal component of this analysis is not
disregarded; it can more effectively capture variations in temperature
and subsequently moisture availability via both the water holding
capacity of the air and the presence/absence of open ocean due to
seasonal sea ice. For example, seasonal analysis identified that the
occurrence of mixed phase and liquid water cloud varies more strongly
as a function of season than regime, suggesting seasonal variability
in mean temperature is a strong driver of ice cloud as previously
identified by .
We also examined the 2BGL4 and 2BGL5 data products. The 2BGL4, used in
previous studies in this region, displays a discontinuity at 8.2 km
which is not observable in the other products and appears to correspond to
a change in the horizontal and vertical resolutions of the CALIPSO dataset
used above this level . This discontinuity appears to
occur at latitudes poleward of 75∘ in both hemispheres. The 2BGL5
product appears to have addressed this issue.
Figures and identify that cloud occurrence as
a function of altitude is dominated by low-level cloud, peak cloud occurrence
occurring below 2 km in every season and synoptic regime. This
supports previous work by and
which indicated that there is a relatively high occurrence of low-level cloud
above the Southern Ocean and Antarctica respectively.
also suggested that low-level cloud constitutes the major cloud type in
Antarctica and is more frequent during summer than winter. Our analysis also
suggests that stratus and stratocumulus are more common in summer than winter
(see Fig. ) over both the Ross Sea and RIS. Separation into
different synoptic classes also implies that periods of weak synoptic forcing
(WNC, WSC, and WS Coggins regimes) are important for the formation of these
clouds. The greater prevalence of these types over the Ross Sea and RIS also
suggests that sea ice state and temperature could be important factors.
also identified that high-level and deep clouds are
more frequent in winter and spring than summer. The deep convective (DC)
cloud type is observed to have a maximum in winter and spring and lowest
occurrence rates in summer consistent with the result indicated in
. However, examination of the variations in the
frequency of this cloud type with synoptic regime also suggest that this is
most often observed during periods linked to strong cyclonic activity (the
SNC regime) as hypothesized by . Our synoptic
classification additionally identifies that the cloud fraction appears to
largely be controlled by the SNC regime which is linked to strong cyclones in
the northern Ross Sea; however, RAS events also seem to be a strong
controlling factor during winter over the RIS.
The results of this synoptic classification also strongly support the
representative nature of the case studies detailed in
which identified significantly contrasting
cloud properties above Ross Island associated with different
meteorological regimes. For example, they identified that warm, moist
air moving directly over Ross Island from the north brought low clouds
which were likely predominantly liquid phase. Our analysis shows that
there is far more liquid water cloud (and also mixed phase cloud) over
the Ross Sea than the RIS in every season and for every synoptic
type. Thus, any southward flow is likely to have this impact.
In contrast, clouds within marine air masses arriving from the WAIS,
and descending onto the Ross Ice Shelf before reaching Ross Island,
show strong ice phase signatures based on the study of
. Our analysis also shows that the RAS regime
displays large quantities of ice cloud at all levels over the RIS. The
SNC regime is also predominately linked to ice clouds at all levels
down to about 2 kma.s.l. The fact that the SNC and RAS
regimes were dominated by ice phase cloud is likely associated with
the strong vertical motions linked to these synoptic types. This
result is inferred from the discussion in
which identified that cloud ice water content is strongly impacted by
vertical motion.
The highest cloud occurrence was found over the eastern Ross Sea
quadrant during the summer, while the lowest cloud occurrence is
observed over the western halves of both the RIS and Ross Sea sectors
during the winter. We observe a link between strong synoptic forcing
(as judged by wind speeds over the RIS and Ross Sea) and greater
occurrence of high-level cloud (above 6 kma.s.l.), while
regimes linked to reduced synoptic forcing seem to be related to
a greater occurrence of low-level cloud.
The strong changes in cloud occurrence vertical distribution, cloud fraction
and cloud type associated with specific synoptic types allows us to make some
wider inferences based on analysis of the Coggins regimes. For example,
demonstrated that the depth and location of the
Amundsen Sea Low have significant impacts over the Ross Sea and RIS. Thus, we
can infer that changes in the depth of the Amundsen Sea Low will likely have
caused significant changes in the cloud environment over the Ross Sea and
RIS. The variability in cloud types for different synoptic conditions and the
importance of some types for precipitation also suggest that changes in
synoptic forcing over the region related to the Amundsen Sea Low may well
have impacted snow accumulation in the region. In particular, the high
frequency of occurrence of the DC cloud type, a type linked to intense
precipitation events statistically (see Table ), during
the RAS regime suggests that snow accumulation in this region may be strongly
modulated by the occurrence rate of this synoptic regime. This will be an
area of further work.