Theor. Appl. Climatol. 60, 63±76 (1998)
General Department of Mathematics, Technological Education Institute of Piraeus, Greece
Lower Tropospheric Structure and Synoptic Scale Circulation
Patterns During Prolonged Temperature Inversions over
Athens, Greece
N. G. Prezerakos
With 7 Figures
Received March 11, 1997
Revised October 6, 1997
Summary
In this paper an attempt is made to detect prolonged (of
more than 24 hours duration) temperature inversions in the
planetary boundary layer over Athens, to study their main
characteristics and to ®nd out the synoptic situations with
which the inversions are associated. Given the close
relationship between the synoptic-scale atmospheric circulation and the occurrence, maintainance and decay of
temperature inversions, a simultaneous three category
classi®cation of presented inversions and their respective
synoptic situations is presented. The classi®cation relies
mainly on the similarities and differences in the formation
and the maintenance of prolonged temperature inversions.
To provide a record of the structure of the lower
troposphere and the synoptic conditions favourable to the
formation of inversions, mean ascents of temperature and
dew-point temperature and mean wind pro®les for the years
1980±1994 were calculated for each category into which a
total of 297 cases fell. The main element of this structure
which strongly affects the pollution of the lower troposphere is the prolonged temperature inversion. Also, for
each category, mean 500 and 850 hPa heights and
temperature charts, 500 hPa height anomaly charts, mean
sea level (MSL) pressure charts and MSL pressure anomaly
charts were drawn.
1. Introduction
Temperature inversion is a meteorological phenomenon which has been of great interest to
Athenians during the last 22 years. This has
been due to the close connection between the
planetary boundary layer over Athens (henceforth APBL), temperature inversions (henceforth
TI) and air pollution. This is a major problem
since it reaches alarming levels many times
not only in Athens but in the whole of Attica,
which is the larger region to which Athens
belongs.
Many scientists (Zambakas, 1973; Lalas et al.,
1982, 1983, 1987; Kambezidis et al., 1986, 1988;
Katsoulis, 1988; Gusten et al., 1988; Melas et al.,
1992, 1996; Kallos et al., 1993; Moussiopoulos
et al., 1993; Varvagianni et al., 1993; Varotsos
and Kondratyev, 1995) have dealt with the
problem of atmospheric pollution in Athens but
few of them have studied TI extensively
(Dikaiakos, 1972, 1974; Tselepidaki et al.,
1984; Prezerakos, 1984a). Almost all of the
aforementioned papers showed a strong linkage
of air pollution episodes to TI. The TI associated
directly with these episodes were not the type
which appear at night and early in the morning
and are destroyed rapidly during the course of the
morning heating, but the type of inversions
which remain over the whole day and night and
even longer. Therefore speci®c research is
needed for these prolonged temperature inversions (henceforth PTI).
64
N. G. Prezerakos
The occurrence of this sort of TI is favoured
by certain synoptic-scale tropospheric circulation
systems whose prediction by weather forecasters
has improved enormously due to the rapid
evolution of computers and numerical atmospheric models. These forecasts lead automatically to the accurate prediction of the PTI
appearances one, two or even three days in
advance.
This last process plays an important role for
the Athenians' health and prosperity because the
accurate prediction of PTI in the APBL provides
the Greek authorities with the possibility of
taking suitable measures for pollutant emission
restriction in advance-not just after the ground
level concentrations have exceeded the maximum permitted values.
An attempt is made here to determine and
study the structure of the APBL associated with
PTIs, that is, inversions which remain for more
than 24 hours, appearing in the lower troposphere. We also study the synoptic-scale tropospheric circulation systems with which these
PTIs are associated. The main aim of this and an
earlier related paper (Prezerakos, 1984a) is to
contribute to the accurate prediction of the
occurrence of PTIs in the APBL without
associating the PTI with air pollution.
2. Data and Method
To study the structure of the APBL and the
synoptic tropospheric circulation systems which
are associated with the PTI the upper air
observations at the Helliniko meteorological
station (WMO No 16716) were used for selecting
the cases to be considered. Helliniko is about
10 km south of the centre of Athens and about
300 m from the coastline, at an elavation of 10 m.
The geography, climate and pollution sources in
Attica are extensively described in other papers
(Kambezidis et al., 1986; Prezerakos, 1986;
Katsoulis, 1988) and so there is no need to
reiterate them here.
Upper air observations at Helliniko are made
by Vaissala radiosondes twice daily at 0000 and
1200 UTC and have been detailed up to 700 hPa
level with a resolution of 10 hPa during the last
22 years. Thus every TI which has occurred in
from the surface up to 700 hPa would be
certainly have been recorded.
A total of 297 PTIs in the APBL were detected
in the 15-year period, 1980-1994. The number of
PTI events during a 15-year period is suf®cient to
thoroughly study the APBL structure and the
synoptic-scale tropospheric circulation patterns
with which air pollution events are associated
and is also suf®cient to obtain statistically
signi®cant results. Although air pollution is
closely associated with all kinds of stable layers
in the lower troposphere, only the layers which
showed a real TI, that is, a layer for which the top
temperature was higher than the base temperature, were selected. For each layer beneath the
700 hPa level in which such a TI occurred (in
cases where there were multiple inversion layers,
the lowest one was considered) its date, duration
and other characteristics were recorded. Such a
TI is said to be of 12-hour duration when it is
reported in two successive 0000 and 1200 UTC
soundings at Helliniko. If the TI appears in the
next 0000 UTC sounding than its duration is
said to be 24 hours, if the same TI appears in
the next 1200 UTC sounding then its duration is
said to be 36 hours. Continuing in the same
manner we de®ne 48, 60, 72, hour durations et
cetera.
The way the duration of the TI was de®ned
does not guarantee that the inversion had not
been destroyed temporarily in the time interval
between two successive observations. Nevertheless, our experience from observing atmospheric conditions for many years in the APBL
pollution watch supports the view that these TIs
are not destroyed temporarily, especialy in the
time interval between the 0000 and 1200 UTC
observations.
The TIs with a duration longer than 24 hours
were identi®ed. Consequently the height, the top
and base temperature, dew-point temperature and
wind were registered for every inversion. Simultaneously it was realised that the group of
inversions selected could be classi®ed into
categories according to the manner of their
formation. This led to three categories named
A, B and C. Category A includes PTIs in the
APBL forming by a combination of surface
radiation cooling and synoptic-scale subsidence
above. These inversions are features mainly of
early-morning soundings, but under certain
weather conditions they are not destroyed during
the course of the day-time heating and thus are
Lower Tropospheric Structure and Synoptic Scale Circulation Patterns During Prolonged Temperature Inversions
maintained for 24 hours or more. Category B
includes PTIs in the APBL forming by advection
of warm air aloft and or cold air below, either
alone or in combination. For this differential
horizontal advection, to be more effective, large
scale vertical wind shear and large scale
horizontal temperature gradients are required
(Saucier, 1955). The observed frontal inversions,
when fronts were near Attica, are attributed to
this cause but their durations are seldom 24 hours
and so they are not included in this investigation.
Category C, includes TIs forming by both surface
radiation cooling and differential horizontal
advection with synoptic-scale subsidence above.
These latter inversions start as early-morning
radiation inversions mostly combined with
synoptic-scale subsidence and are retained during day-time because of a sea breeze at the lower
half of the planetary boundary layer. This
combination of early-morning radiation TI
strengthened by synoptic-scale subsidence, followed by differential horizontal advection inversions caused by sea breeze create the APBL TI
with longest duration. This category seems to be
quite similar to category A and thus can be
considered as a subcategory of category A.
Table 1 shows a complete classi®cation of the
297 PTI events in the APBL that occurred in the
15-year period 1980±1994. Table 1 separates the
A, B and C categories into months in order to
separate the inversions of every category by
season. Since the height, depth and intensity of
PTIs in each category appeared to be very similar
within each month it was not considered necessary to make a subclassi®cation of these inversion characteristics. Table 1 shows that category
C only appears during summer, whereas category
A appears in cold seasons. The appearance of
category A is pronounced during spring and
especially during May with 36 events. Category
B appears only during spring and especially
during March and category C appears during the
65
warm period of the year but mainly in June.
Table 1 also shows that the number of events
within each category and season is suf®cient for
statistically signi®cant results in relation to mean
meteorological ®elds and their anomalies.
To study better the APBL structure and the
synoptic scale circulation systems under consideration, mean soundings up to the 700 hPa
level for Helliniko at 0000 and 1200 UTC have
been calculated. These have been plotted on
tephigrams for each category, for months with
the largest number of PTI cases separately and
for the ®rst day (F-day) of a group of successive
days during which the PTIs occurred. Mean
monthly charts of 500 and 850 hPa height and
temperature, 500 hPa height anomaly and MSL
pressure and its anomaly were made for all
categories of the same months of F days at 1200
UTC. By anomalies we mean the difference from
the long-term (1950±1975) average as published
by Deutscher Wetterdienst (Barry and Perry,
1973). In addition, the student's t test (at the 0.01
statistical level) has been applied (Brooks and
Carruthers, 1953) at each grid point of the
anomaly ®elds in order to test whether the
anomaly values have a probability of less than
1 per cent of being random.
The geographical region considered in this
investigation is large enough to include all
synoptic patterns and possible centres of actions
favouring the occurrence of PTIs in the APBL.
This geographical region is covered by a grid
point regime with a grid distance of 150 km. The
calculations were performed by the computer
facilities of the Greek Meteorological Service,
using as input the European Centre of Medium
range Weather Forecast (ECMWF) data as
disseminated to the member states and the
Deutscher Wetterdienst 500- hPa height and
MSL pressure long-term monthly averages,
gridded by Bessel's interpolation method in the
same format as the ECMWF data.
Table 1. Number of PTI per Month and Category within the 1980±1994 Period
Month
Categ.
Jan
Feb
Mar
Apr
May
A
B
C
Total
7
12
18
15
26
12
7
12
33
38
36
5
15
56
Jun
60
60
Jul
15
15
Aug
15
15
Sep
30
30
Oct
Nov
Dec
Total
23
5
3
23
5
3
130
32
135
297
66
N. G. Prezerakos
3. Analysis and Discussion
3.1 Category A
As Table 1 shows, there were 130 events of PTI
belonging to category A during the 15-year
period under consideration. In this table we can
see that spring and, in particular May, includes
the maximum number of events. Winter is
second in number of events, while in the warm
period, from June to September, PTIs of this
category do not occur. As the air pollution
problem of Athens becomes severe, with cloudless, windless weather regimes and high solar
angles which enhance photochemical pollution,
we preferred to analyse and study the spring and
in particular the May PTIs. There is no need for
the analysis and study to be extended to the
winter or even the autumn events since the APBL
structure and tropospheric circulation during
category A PTIs resemble the respective ones
of spring. Also they are much fewer because the
frequent appearance of synoptic-scale circulation
systems with strong winds and greater cloudiness
with precipitation destroys the inversions before
they become prolonged.
Figure 1 shows the mean structure of the lower
troposphere (up to 700 hPa level) on F-days in
May. At 0000 UTC (Fig. 1a) the inversion depth
is about 760 m, the inversion base at ground level
and the inversion intensity (temperature increase)
is about 4.5 C. The average degree of stability,
=z, is about 2 C/100 m which is somewhat
greater than the values found by Katsoulis (1988)
in cases of pollution episodes in Athens. This is
resonable as this paper deals with inversions with
exceptional intensity lasting longer than 36
hours, whereas Katsoulis (1988) deals with all
nocturnal inversions when SO2 was more than
250 mg/m3 and/or smoke more than 200 mg/m3.
The mean wind vector near the inversion layer is
calm whereas it does not exceed 2.5 msÿ1 above
up to 700 hPa. The variation of wind direction
with height shows some warm advection above
the inversion layer which cannot be strong due to
the very low wind speed. Humidity at the
surface decreases rapidly with height which is a
well-known characteristic of stable air masses
near the ground with synoptic-scale subsidence
above a site near the sea, as at Helliniko
meteorological station. These ascents also sug-
Fig. 1. Mean May sounding for Hellinico upper air station
during category A prolonged temperature inversions (PTI)
at 0000 UTC (a) and 1200 UTC (b) of F days (®rst day of a
group of successive days with a PTI over Athens) during
1980±1994 (36 cases). The continuous line is air temperature and the dashed line is dew point temperature. Direction
(degrees) and speed (kn) (1 kn 0.5 msÿ1 ) of the vector
mean wind are given for various levels
gest that there are no clouds at all up to 700 hPa
(3000 m).
During the course of the morning, as the days
are cloudless, the solar radiation heats the
underlying surface which creates a convective
lower planetary boundary layer capped by an
inversion. This layer grows with time and reaches
a maximum height at around midday (Tennekes,
1973; Yordanov and Kolarova, 1988). Although
the diurnal solar heating is suf®cient to destroy
nocturnal surface inversions in Athens in May,
inversions of category A, forming mainly by
subsidence and enhanced by surface cooling in
early morning, retain their identity lifting only
their base. Something similar occurs in Hemsby,
U.K. (Milionis and Davies, 1992). As a consequence of the above process the 1200 UTC mean
ascent (Fig. 1b) shows the temperature inversion
base to be at about 320 m and its intensity is
reduced to about 1.5 C. The wind speed at the
surface and at inversion height is about 2.5 msÿ1
causing mixing near the surface which together
Lower Tropospheric Structure and Synoptic Scale Circulation Patterns During Prolonged Temperature Inversions
with diurnal heating (eddy convection), leads to a
dry adiabatic lapse rate up to 350 m height,
where the inversion base occurs. Also, the
average mixing depth as de®ned by Katsoulis
(1988) (extending from the surface to the height
where the potential temperature exceeds its
surface value by 2 C) is on average 450 m. Also
the degree of stability, =z, of the inversion
layer is on average 2 C/100 m which, compared
with morning one, indicates that stability remains
strong enough to retain the inversion longer,
which indicates the existence of strong synopticscale subsidence.
Figure 2 shows the mean tropospheric circulation of F days at 1200 UTC in May. This
synoptic situation in the vicinity of Greece
remains almost unchanged during the time period
that a PTI occurs in the APBL. So the mean May
circulation at 500 hPa isobaric surface is dominated by a synoptic-scale wave with the ridge
over the central Mediterranean and central
Europe (Fig. 2a). This ridge progresses slowly
eastward and creates an advection of negative
relative vorticity over Greece and in particular
over Athens. This is also suggested by the 500
hPa height anomaly ®eld at ridge vicinity (for the
similarity between 500 hPa height anomaly and
the relative geostrophic vorticity and its advection see Prezerakos, 1990). The anomaly values
over Italy surmount 180 gpm and are statistically
signi®cant at the 0.01 level, a fact which
indicates that similar ridges over the region are
frequent. This kind of circulation must play an
important role in the creation of the atmospheric
conditions that favour the occurrence of PTIs in
the APBL. This is highlighted by the close
association of synoptic-scale subsidence and
anticyclonic circulation or advection of negative
relative vorticity over Greece. This veri®ed in
Fig. 3 where the vertical velocity ®elds at
Fig. 2. Mean monthly charts of (a) 500 hPa and (b) 850 hPa
height and (c) Mean Sea Level Pressure (MSL) during
category A prolonged temperature inversions over Athens
at 1200UTC on F days in May during 1980 to 1994 (36
cases). Continuous thick lines height contours every 60 m in
(a), (b) and Isobars every 4 hPa in (c). Continuous thin lines
show isotherms every 5 C in (a) and 2 C in (b). Thin
dotted lines show anomaly isopleths of the height ®eld in
(a) and the pressure ®eld in (c). Shaded areas delineate
height anomaly values (a) and pressure anomaly values
(c), statistically signi®cant at the 0.01 level
67
68
N. G. Prezerakos
500 hPa(a), 700 hPa(b) and 850 hPa(c) are
presented. These vertical velocities have been
calculated by integration of the continuity
equation (Wiin Nielsen, 1973):
ÿ
@!
ˆ rp ~
V
@p
1†
(! ˆ dp=dt : a measure of vertical velocity, p:
atmospheric pressure, ~
V: wind vector, rp :
gradient on an isobaric surface).
Utilizing the ECMWF's initialized data Fig. 3
shows clearly that the air in the troposphere at the
500 hPa level and below, subsides with mean
vertical velocity of about 4 hPa/h and a bit less at
the 850 hPa level. This differential vertical
motion results in a subsidence type TI.
The surface pressure map in Fig. 2c depicts an
extended rather smooth ®eld of high atmospheric
pressure with three large anticyclones covering
most part of Europe (the Balkans included),
central-north Africa and the central Mediterranean. Taking into account the recent synoptic
evolution of the high barometric ®eld in association with the 500 hPa ¯ow (Fig. 2a) and the
thermal and height ®elds at the 850 hPa isobaric
surface (Fig. 2b) we can conclude that the
northern anticyclone over the Balkans is a mobile
cold type, which had moved in the rear of a cold
front towards the Balkans. This large anticyclone
is in the stage of changing into a deep warm
anticyclone (except for a cold shallow surface
layer which retains its identity longer). In so
doing it slows down and becomes stationary over
the Balkans and Greece. The large anticyclone to
the south is a warm-core deep anticyclone of
subtropical origin. Over time the two large
anticyclones show a tendency to form a single
continuously decaying anticyclone. The smooth
high pressure ®eld over the Balkans guarantees
the absence of a signi®cant synoptic scale
pressure gradient. Positive pressure anomalies
over the Balkans, including the Athens region,
have values greater than 3 hPa and are signi®cant
at the 0.01 level, a fact which underlines the
dominant role of this stationary anticyclone with
strong subsidence in the lower troposphere for
the appearance of a prolonged inversion of
category A.
The 850 isobaric surface shows an anticyclonic circulation over Attica. This circulation
Fig. 3. Mean monthly vertical velocities in hPa at 500 hPa
(a), 700 hPa (b) and 850 hPa (c) during category A prolonged temperature inversions over Athens at 1200UTC of
F days in May during 1980 to 1994 (36 cases). Isopleths
every 2 h Pa/h, " upward, # downward
is in vertical consistency with the surface anticyclone. A somewhat cold advection shown in
the Athens region combined with anticyclone
¯ow favours synoptic-scale subsidence in the
Lower Tropospheric Structure and Synoptic Scale Circulation Patterns During Prolonged Temperature Inversions
lower troposphere. This appears on F day
because a small displacement of the anticyclone
eastwards on the next day results in Attica being
under the central part of anticyclone where no
advection occurs. Atmospheric subsidence maintains stability conditions although sometimes
it is accompanied by cold advection which tends
to destroy the TI. It is worth mentioning,
however, that the differential downward motion
(as Fig. 3 shows) in the lower part of the
troposphere results mostly in an increase in
stability. In other words the meteorological
quantity @=@z > 0 increases. Examination of
the ascents show that the stronger the inversion
the greater the daily maximum temperature and
the lower the height of the inversion base at 1200
UTC.
The PTIs in the APBL are destroyed when the
atmospheric circulation becomes cyclonic over
Greece with a corresponding advection of cold
air. This kind of circulation arises from the
presence of cold fronts, which cross Greece
coming from the west, northwest or even sometimes directly from the north. These latter fronts
are the well-known ``Balkan fonts'' which are
closely associated with cold air outbreaks
towards Greece (Metaxas, 1978; Prezerakos and
Angouridakis, 1984). The 500 hPa circulation
associated with the ``Balkan fronts'' is due to the
intensive strengthening and displacement northwards or northeastwards of the large scale ridge
being in central Europe. This is a consequence of
strong warm air advection occurring over the
north-eastern Atlantic ahead of the low pressure
system shown in Fig. 2a. The extension of the
ridge north or northeastwards adopting mostly
the shape of a meridional blocking, results in the
establishment of an intense cyclonic circulation
at the eastern ¯ank of this ridge (Bjerknes, 1951;
Prezerakos and Flocas, 1996). A signi®cant
maximum of relative vorticity then moves southwards, accompanied by cold air masses. In
most of the events this maximum of vorticity
results in cyclogenesis when it meets suitable
conditions at the surface ahead of a cold front
moving southwards pushed by an extension of a
wedge of a cold anticyclone growing up at the
northern part of the existing anticyclone (Fig.
2c). During this time the rest of the high pressure
®eld weakens signi®cantly until it loses its
identity.
69
3.2 Category B
Category B types, as Table 1 shows, appear only
in spring and especially in March. Figure 4
depicts schematically the mean structure of the
lower troposphere, when inversions of horizontal
differential thermal advection appear over Attica
in March of F days. Figure 4 shows clearly that
this kind of inversion forms by the advection of
warm air aloft. Early in the morning (Fig. 4a), the
combined effect of surface cooling and distinct
warm advection above the 900 hPa level results
in a ground inversion with a mean depth of about
1600 m. The inversion intensity is, on average,
4 C. Also the average degree of stability, @=@z,
is about 1.2 C/100 m which, despite the large
vertical extent, is smaller than the corresponding
average degree of stability (2 C/100 m) of
category A inversions. Radiation and subsidence,
responsible for the inversions in May (category
A), create stronger stability in the lower troposphere.
The origin of the category B type of TI can be
better understood by inspecting the 1200 UTC
ascents depicted in Fig. 4b. The distinct warm
advection starts at about 900 hPa, where the wind
reaches signi®cant speeds from the southwest.
Above this level the wind direction veers and the
Fig. 4. As Fig. 1 but for category B in March (15 cases)
70
N. G. Prezerakos
speed increases rapidly with height, increasing
the warm and relatively dry air advection as the
dew-point depression suggests. Despite the
warming from below due to solar radiation at
this time that TI is not destroyed but its base is
lifted by about 350 m, with a mixing layer depth
(Katsoulis, 1988) of about 480 m. The average
degree of stability of the inversion at this time
(1200 UTC) is @=@z = 1 C/100 m, which is
somewhat less than at night. The persistence of
category B type PTIs even to midday is an
occurrence which deserves to be emphasized.
The advected warm air shows a tendency for
large scale upward motion, which leads to a
temperature drop. This process becomes very
signi®cant when various small scale disturbances
moving within the southwesterly tropospheric
¯ow approach Attica. On average this upwardmotion cooling is not suf®cient to compensate
for the temperature rise due to warm air
advection at the APBL's top. This is veri®ed by
calculating the average thermal advection at the
850 hPa level over Athens;
@T
~
ÿV rp T ˆ ÿV
2†
ˆ 4 10ÿ5 Csÿ1
@S
(symbols as in Eq. (1), @T
@S : temperature gradient
along the ¯ow).
This average warm air advection assisted by
surface heating during day time should raise the
850 hPa temperature during a twelve-hour interval (0000 to 1200 UTC) by about 2 C. Instead,
a drop of about 1 C occurred on average. This
fact indicates the contribution by the upward
motion at the 850 hPa level and below. This
upward motion ! < 0 in a stable strati®ed
atmosphere …ÿ ÿ ÿ† > 0 contributes to a temperature decrease according to the second term
of the right hand side of the thermodynamic
Eq. (3) (Wiin Nielsen, 1973) for temperature
change in the atmosphere. But in the third term
of the right hand, the diabatic effect must
contribute positively to the temperature rise only
in the second half of the time interval, close to
1200 UTC, when the surface is heated suf®ciently
@T
1
q
~
ˆ ÿV rp T ‡ !…ÿa ÿ ÿ† ‡
@t p
cp
dt
3†
(symbols as in equation (1) and (2), ÿ ˆ
@T=@p: adiabatic lapse rate moist or dry
depending upon whether the air is saturated,
ÿ ˆ @T=@p : actual lapse rate, cp : speci®c heat
of moist air at constant pressure, q=dt heat
energy change per unit mass caused by processes
other than condensation).
Although local temperature decreases slightly
with time at the 850 hPa level over Helliniko the
inversion is maintained. This inversion can be
reinforced in cases when the tropospheric ¯ow
over and near Attica is not perturbed but it is
purely anticyclonic, favouring large scale subsidence. Also, the large vertical wind shear in the
APBL (calm at surface and more than 12.5 msÿ1
at 850 hPa) (Fig. 4) associated with pronounced
stability caused by TI could, in turn, increase the
surface wind speed by downward convection of
momentum occasionally (especially when the
mobile relatively small atmospheric perturbations are approaching Attica) and decrease
signi®cantly the stable structure of the APBL.
In such time intervals a horizontal escape of
pollutants usually occurs from Attica.
Figure 5 shows the mean tropospheric circulation which favours the occurrence of the category
B PTI in the APBL in March on F days. The
mean 500 hPa (Fig. 5a) is dominated by an
extended trough from northern France to south of
the Atlas mountains in north Africa. This trough
is associated, in perfect vertical consistency, with
a developing depression at the surface in the
region of the Atlas mountains, moving northeastwards and another one in central Europe
(Fig. 5a). The mean 500 hPa ¯ow over Greece is
southwesterly and mostly anticyclonic. Because
the 500 hPa height ®eld is an average one, the
aforementioned small scale pertubations moving
in this ¯ow have been smoothed out. The
anticyclonic ridge has just passed Greece, lying
over Western Turkey and the eastern Mediterranean. Over this region the 500 hPa height
anomaly values exceed 120 gpm and are significant at the 0.01 level. Also within the trough
region height anomaly values are smaller than
ÿ120 gpm in eastern Europe and ÿ60 gpm in the
Atlas mountains region. The surface pressure
®eld is smooth over Balkans and relatively low as
the pressure anomaly values suggest (less than
ÿ6 hPa). This smooth relatively low pressure
®eld suggests that there is no considerable
Lower Tropospheric Structure and Synoptic Scale Circulation Patterns During Prolonged Temperature Inversions
Fig. 5. As Fig. 2 but for category B in March (15 cases)
71
synoptic-scale pressure gradient over Attica
and in Balkans. This results in low surface
wind speeds extending upwards, obstructing
the warm air advection below the 950 hPa
level. The warm air advection becomes signi®cant above the 950 hPa level, as the 850 hPa
map shows (Fig. 5b), setting up the regime of
differential warm air advection which, in turn,
builds up and maintains the TI in the PBL.
This regime is more pronounced over the
Mediterranean and the coastal regions. This is
a consequence of the southern ¯ow shown
clearly in the 500 and 850 hPa maps (Fig. 5a,b)
of dry and warm air advected directly from
the Sahara desert. This air is usually much
warmer than the sea surface during this season
(spring) and so is cooled from below so that a
surface TI builts up. This inversion opposes the
vertical eddy ¯ux of water vapor and makes
atmospheric air unable to be enriched with
humidity, apart from a very shallow layer, as
the APBL's structure shows (Fig. 4). This
structure can be destroyed when the air ¯ow
reaches physical obstacles (i.e. high land),
where the air leaves its low tropospheric wet
shallow layer at the windward side of the
obstacle. The leeward air ¯ow has a low dewpoint and high dry-bulb temperature and is
warmed additionally by vertical shrinking as it
moves from the top of the obstacle to the surface,
becoming very warm and dry with gusty winds.
Such events have frequently been observed
during spring in north Crete (a major island
south of the Greek mainland) (Prezerakos, 1991;
1994).
Occurrences of category B type PTIs in the
APBL are usually precursors of severe weather
phenomena coming into Greece. These are due to
developing depressions which approach Greece
from the southwest following the maximum of
warm advection at the 850 hPa level and the
maximum of advection of relative vorticity at
500 hPa. These sorts of depressions are some
times rejuvenated in the region south of Sicily
(Prezerakos and Michaelides, 1989) resulting in
vigorous surface lows which pass through the
Greek mainland or nearby Crete causing very
strong surface winds and other severe weather
phenomena with the simultaneous destruction of
any temperature inversion in the Greek planetary
boundary layer.
72
N. G. Prezerakos
3.3 Category C
The category C PTIs (nocturnal inversions with
subsidence in the lower troposphere elongated by
the sea breeze) in the APBL occur during the
warm period of the year (May to September)
especially in June, (as Table 1 shows) when clear
skies and calm winds favour nocturnal temperature inversions and the sea breeze competes with
etesians (the prevailing northeasterlies in eastern
Greece during summer).
The major geographic axis of the Athens basin
is along a NE-SW direction, with the sea in the
west and south (Prezerakos, 1986; Katsoulis,
1988). The pure sea breeze on days with very
weak synoptic-scale pressure gradient, and the
sea breeze on days with a synoptic-scale pressure
gradient favouring off-land winds, are identi®ed.
The sea breeze on days with a synoptic-scale
pressure gradient causing on shore winds is more
dif®cult to identify. The latter cases are usually
associated with cyclonic synoptic-scale tropospheric circulation (upward synoptic-scale vertical motion) with surface winds of suf®cient
intensity (more than 7.5 msÿ1 ). These meteorological conditions appear very seldom during
summer since the polar jet stream is far to the
north of Greece (Reiter, 1975). Furthermore,
when such synoptic conditions are associated
with TIs in the APBL we can easily ®nd out that
they are similar to the initial conditions of
category B inversions. This kind of inversion
rarely lasts more than 24 hours because the
moderate surface wind, in combination with the
rough land surface, result in turbulence which
tends to destroy any inversions.
As the Athens sea breeze penetrates as far as
mount Parnitha, 30 km inland from the Saronicos
gulf coast (Prezerakos, 1986), persistence of
meteorological conditions associated with category C PTIs usually leads to an increase of air
pollution near the southern foot of mount
Parnitha during day-time, with a likely displacement of the polluted air back towards the
city during night-time forced by land breeze or
weak etesians (Lalas et al., 1983; Katsoulis,
1988).
Category C PTIs are the most questionable
ones concerning their likely interruption in the
time interval between the end of the sea-breeze
and the radiosonde release (0000 UTC). To ®ll
Fig. 6. As Fig. 1 but for category C in June (60 cases)
this gap boundary-layer observations are needed
during this time interval.
The 60 cases of category C PTIs in June, have
a mean structure of the lowest troposphere of F
days as shown schematically in Fig 6. The
morning temperature ascent (Fig. 6a) is typical
for a nocturnal surface temperature inversion
with synoptic-scale subsidence above as the large
dew-point departure shows. This ascent is quite
similar to that category A which indicates that
Category C could be considered as a subcategory
of A. The wind pro®le shows a weak northeasterly ¯ow of about 1.5 msÿ1 force inside the
inversion layer, while above the inversion the
wind increases slightly with height, and its
direction backs indicating some cold air advection, but not enough to destroy the inversion. The
depth of the inversion is on average 450 m,
connected with a stable layer of about 400 m
depth above the inversion layer. The inversion
intensity is about 3 C on average and the
average degree of stability, =z, is 2 C/
100m, strong enough to characterize the inversion as a nocturnal surface-radiation cooling one.
Figure 6b illustrates the mean structure of the
Athenean troposphere below 700 hPa level at
1200 UTC on F days for category C PTIs.
Comparison of this midday structure with the
Lower Tropospheric Structure and Synoptic Scale Circulation Patterns During Prolonged Temperature Inversions
previous midnight one shows that the top and the
base of the inversion were lifted and the whole
layer was warmed. This is due to the midday
surface heating which created a convective
mixed layer, with a superadiabatic lapse rate
between the surface and 160 m to where the base
of the inversion was lifted. This surface heating,
which is extraordinarily strong during summer,
very often destroys the inversion except for the
occasions on which sea-breeze conditions have
been well established, as the Fig. 6b indicates,
and when the tropospheric subsidence is strong
enough. Although the mean monthly midday
temperature ascent on pure sea-breeze cases is
smoothed and it is not able to show the residual
radiative inversion retained by the sea-breeze
(Prezerakos, 1986), this inversion is clearly
visible in Fig. 6b. This last fact leads to the
conclusion that the basic characteristics of the
inversions of category C are mostly similar,
being retained thus at the average ascent.
The dominant characteristic of an onshore
¯ow is the growth of an internal boundary layer
with a temperature inversion at the vicinity of the
interface. This inversion caps the internal boundary layer and its intensity is a function mainly of
the boundary layer height and the atmospheric
stability (Gryning and Batcharova, 1990; Varvagianni et al., 1993; Melas et al., 1996). As the
¯ow proceeds inland over the warmer ground, at
midday the inversion is gradually destroyed from
below. Unfortunately there are no data available
to check the distance inland from Helliniko over
which the inversion shown in Fig. 6b maintains
its identity. However, observations made at
Athens observatory (8 km distance from Helliniko) in May 1990, proved that the sea breeze
temperature inversion appears at this site (personal communication with Dr. Kambezidis),
which is close to Parthenon (2 km from the
centre of Athens) and 300 m from the built up
area. The distance inland over which the seabreeze inversion is maintained plays an important role in determining the extension of the area
with high air pollution caused by the sea breeze.
Figure 6 also clearly shows the midday wind
pro®le, which is fairly typical for a sea-breeze
regime during its period of maximum development at a coastal station (Prezerakos, 1986). The
intensity of the temperature inversion has
reduced to 1.5 C and the degree of stability,
73
=z, is on average 1 C/100m, justifying
fairly well its role as a trap of air pollutants by
obstructing their vertical escape.
Figure 7 shows schematically the lower tropospheric mean circulation in the European and
Mediterranean area at 1200 UTC on F days of
category C PTIs in June. The main feature of the
500 hPa map (Fig. 7a) is a synoptic-scale wave
with a ridge just west and northwest of Greece, a
closed low off the west coast of France, and a
vigorous high over the northeastern Atlantic. The
axis of the ridge is almost identical with the 19
E meridian, whereas the Atlantic high shows a
northeastward extension. This extension must
have caused the occurrance of the low height
®eld over western Europe with its centre in
western France (Bjerknes, 1951; Prezerakos and
Flocas, 1996). Height anomaly values exceed
180 gpm over the Balkans, (signi®cant at the 0.01
level). The highest values of height anomalies,
(signi®cant at the 0.01 level) appear in the
Atlantic high, indicating the persistence of the
system as would be expected since it is well
known Azores anticyclone. The position of the
ridge axis close to Greece is typical for the
weakening of the etesian winds over the eastern
Greek mainland and the Aegean Sea (Reiter,
1971; Prezerakos, 1984b).
At the 850 hPa level, (Fig. 7b) most of the
Balkans, southern italy, Libya and the Mediterranean in between are covered by an anticyclone,
which is consistent with the 500 hPa ¯ow. In the
eastern Balkans, the eastern Greek mainland and
the Aegean Sea there is a northerly ¯ow
maintaining a weak cold air advection, for which
there is also evidence in the wind pro®le at
Helliniko (Fig. 6). These northeasterlies also
appear at the surface, as Fig. 7c shows. The
isobars present a similar picture to that of the
850 hPa contours, at least, for southern Italy and
the Balkans. The high pressure in the Balkans
expressed by pressure anomalies with values
around 6 hPa, (signi®cant at the 0.01 level)
indicate that the persistence of the high over
the Balkans is mainly due to anticyclones
moving in the rear of the cold fronts, passing
through north Greece towards northwestern
Turkey. The combination of high pressure over
the Balkans with the permanent (during summer)
low pressure over the eastern Mediterranean still
exists, resulting in maintainance of the etesians.
74
N. G. Prezerakos
This synoptic-scale regime is stronger than the
mesoscale sea breeze during the initial days of an
etesian period (etesians last several days, see
Reiter, 1971). During the initial two to three days
of such etesian period (etesians episode) the
gradient winds are very strong and any seperate
mesoscale circulation (e.g. sea-breeze) is overshadowed (Varvagianni et al., 1993). But on the
last days of the episode the synoptic-scale
pressure gradient weakens signi®cantly allowing
the sea-breeze to overcome the etesians. As Fig.
7 shows, the PTIs occur during such periods
when the sea breeze dominates over the etesians,
at least at Helliniko. The return ¯ow of the sea
breeze coincides with the etesians in the layer
from about 850 to 900 hPa (Prezerakos, 1986).
4. Conclusions
Fig. 7. As Fig. 2 but for category C in June (60 cases)
During the 15-year period from 1980 to 1994 297
events consisting of the lower troposphere
prolonged temperature inversions (PTIs), persisting at least over 24 hours (as derived from 0000
and 1200 UTC radiosondes) occurred over the
Helliniko upper air station. Based on the
common characteristics which these inversions
possessed, with respect to formation and maintenance, they were classi®ed into three categories: A, B and C, for each month of the year.
Category A appears in almost all months of the
year, except for summer and September. The
month with the greatest number (36) of PTIs is
May. From the mean monthly ascents for May
and for the ®rst day of a group of successive days
on which a PTI of the same category occurred it
is found that category A types start as surface
radiation cooling inversion, with strong atmospheric stability near the ground, and with no low
and middle level clouds at all. Additionally,
strong synoptic-scale subsidence contributes to
the inversion layer. The wind is weak from the
surface to the inversion layer but increases above
it, up to 700 hPa level, although it still remains
weak. Hence no signi®cant thermal advection
occurs. Of course the base of the inversion is
lifted and its depth and intensity decrease during
the day, especially at midday, because of the
warming from below, which is strong in May in
the study region.
At 1200 UTC in May with inversions of
category A type, anticyclonic circulation over the
Lower Tropospheric Structure and Synoptic Scale Circulation Patterns During Prolonged Temperature Inversions
Balkans causes advection of negative relative
vorticity at 500 hPa level resulting in large scale
subsidence in the lower troposphere. These
conditions, in turn, result in an even surface air
pressure ®eld over the Balkans with cloudless
skies. Strong and positive 500 hPa height and
MSL pressure anomalies (signi®cant at the 0.01
level) over the Balkans region suggest that a
persistent synoptic-scale atmospheric circulation
over the Balkans causes the inversions in the
planetary boundary layer over Athens.
Inversions of category B only appear in spring
and especially in March (15 cases). They form
mainly by advection of warm air aloft. Warm air
advection, in combination with the surface
cooling early in the morning, results in a strong
surface temperature inversion. However the
degree of stability, =z, of category B types
is smaller than for category A types where it
reaches 2 C/100 m. This is due to the presence
of large scale subsidence in category A types.
Warm air advection in category B types usually
results in large scale upward motion that tends to
decrease stability and sometimes may even
destroy the inversion.
The main characteristics of atmospheric circulation associated with category B type inversions is the smooth low surface air pressure ®eld
causing very weak winds and the strong vertical
wind shear starting just above the 900 hPa level.
Such conditions are associated with strong
depressions with fronts approaching Greece.
For that reason category B inversions are usually
precursors of severe weather phenomena in
Greece.
Prolonged inversions of category C type
appear mainly during the warm months of the
year and especially in June (60 cases). They start
usually as nocturnal surface radiation cooling
inversions with synoptic-scale subsidence in the
lower troposphere. During the course of the
morning the developing sea breeze competes
with the existing weak etesians and usually
penetrates inland as far as mount Parnitha. The
sea-breeze temperature inversion replaces the
nocturnal inversion and so extends the duration
of the temperature inversion. For inversions of
this type it is not known how far inland the
sea-breeze temperature inversion extends and
whether the inversion is interrupted between
1200 and 0000 UTC.
75
The synoptic-scale tropospheric circulation,
which is associated with category C types
resembles most the circulation which occurs
when a period of etesians is almost at its terminal
stage, i.e., when the etesians have weakened
enough to be overcome by the sea breeze. The
main characteristics of this circulation are the
occurrence of a 500 hPa ridge close to Greece
and a rather smooth ®eld of high air pressure at
the surface and at the 850 hPa level. This high
pressure ®eld combined with permanent low of
the eastern Mediterranean during summer, creates the synoptic-scale gradient for the etesians in
eastern Greece.
This paper has attempted to relate the occurrence of persistant temperature inversions that
cause air pollution in the Athens area with
certain types of tropospheric circulation. Since
fairly accurate forecasts of circulation patterns
are now available, even more than 72 hours in
advance, the results of this study will allow
improved pollution forecasts and therefore pollution abatement measures.
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Author's address: Prof. Nicholas Perzerakos, General
Department of Mathematics, TEI of Pireaus, 250, Thivon
and P. Ralli. Aigaleo GR-12244 Athens, Greece.