1072
MONTHLY WEATHER REVIEW
VOLUME 127
A Diagnostic Study of the Flateyri Avalanche Cyclone, 24–26 October 1995,
Using Potential Vorticity Inversion
SIGURDUR THORSTEINSSON
Icelandic Meteorological Office, Reykjavik, Iceland
JÓN EGILL KRISTJÁNSSON
Department of Geophysics, University of Oslo, Oslo, Norway
BJøRN RøSTING
The Norwegian Meteorological Institute, Oslo, Norway
VIDAR ERLINGSSON
Icelandic Meteorological Office, Reykjavik, Iceland
GUDMUNDUR FREYR ULFARSSON
Department of Civil and Environmental Engineering, University of Washington, Seattle, Washington
(Manuscript received 11 December 1997, in final form 23 June 1998)
ABSTRACT
The evolution of a deep North Atlantic cyclone, which caused devastating avalanches in northwest (NW)
Iceland in October 1995, was investigated. As the main tool for this investigation, potential vorticity analysis
was used. This allows the quantification and comparison of the roles of different processes that contribute to
the cyclone deepening at different stages.
Interpretation of potential vorticity inversions and isentropic air trajectories yields the following picture of
the cyclone development. The thermal field over the North Atlantic had acquired strong west–east gradients due
to a combination of advection of cold air southeastward from a cold cyclonic gyre south of Iceland and advection
of warm air northward on the westward flank of a warm anticyclonic ridge over central Europe. A low-level
baroclinic wave forming just south of Ireland was rapidly reinforced due to interaction with a descending, highvalue, upper-level potential vorticity anomaly and was isentropically advected from the low south of Iceland.
As the wave deepened, diabatic heating in association with the frontal systems became a major source of cyclonic
vorticity. Cross sections of the height fields associated with potential vorticity anomalies reveal the baroclinic
nature of some of the anomalies.
The isentropic trajectory analysis shows strong ascent of warm air taking place over Iceland and thereby
explaining the heavy precipitation in NW Iceland. The advection of rather warm, humid air overlying very cold
air from a persistent high over Greenland, together with orographic lifting, seems to be responsible for the
snowfall that together with heavy winds produced the unusual avalanches in Iceland.
1. Introduction
Cyclones developing over the North Atlantic are often
at their most intense as they approach Iceland and are
therefore an important issue in weather forecasting
there. This study is one of a series of studies that seek
Corresponding author address: Dr. Sigurdur Thorsteinsson, Icelandic Meteorological Office, Bústadavegi 9, IS-150 Reykjavı́k, Iceland.
E-mail: siggi@vedur.is
q 1999 American Meteorological Society
insight into the roles of physical, dynamic, and topographic factors in determining the behavior of these cyclone life cycles. In the first of these studies, the socalled Greenhouse low of 2–3 February 1991 was investigated through a combination of model simulations,
reanalysis of surface observations, and satellite data (see
Kristjánsson and Thorsteinsson 1995). In a follow-up
paper (Kristjánsson et al. 1999) we returned to the same
cyclone, this time investigating the cyclone evolution
from the potential vorticity perspective.
In this paper, we investigate the explosive cyclone of
24–26 October 1995, which evolved in the region be-
JUNE 1999
1073
THORSTEINSSON ET AL.
tween the British Isles and Iceland. It caused a series
of devastating avalanches in the West Fjord area, northwest (NW) Iceland, including a fatal one at Flateyri,
where 20 people died. While this part of the country is
frequently hit by avalanches, usually in connection with
violent winds from the north (N) or northeast (NE), the
timing of this avalanche makes it very exceptional, since
in late October there is usually little preexisting snow
in the hills. It has been estimated by Jóhannesson and
Jónsson (1996) that the return period for such a combination of high winds and heavy precipitation sustained
over several days, together with the unusual timing, is
of the order of decades to a century. There are reports
of a similar cyclone hitting NW Iceland on 26–28 October 1934 (Jónsson 1993). One cause for the enormous
snowfall that took place seems to be the extended time
that a combination of warm and humid air masses overlying colder northerly winds in lower layers remained
over Iceland as the cyclone developed to the southeast
(SE) of Iceland and pivoted NW.
We have examined the avalanche cyclone using the
following potential vorticity (PV) framework. Piecewise
inversions of the Ertel potential vorticity (EPV) are performed under nonlinear balance conditions to get a
quantitative picture of the role of different mechanisms
in the evolution of the cyclone. The method has been
described by Davis and Emanuel (1991) and Davis
(1992). We first conduct some tests where we show how
the total height field can, to a good approximation, be
retained by summing up all the portions of the EPV
field. Then EPV anomalies are subjectively selected for
inversion. In addition, we have investigated the baroclinic nature of the perturbations by studying the threedimensional structure of the perturbation heights obtained by inverting different portions of the EPV field.
We carry out three-dimensional isentropic air trajectories to identify what role a preexisting, deep, quasibarotropic low south of Iceland played, that is, how and
to what extent it interacted with the baroclinic waves
to the east of it, one of which was directly responsible
for the avalanche.
The EPV inversion methodology of Davis and Emanuel (1991) and Davis (1992) has previously been applied
by Davis et al. (1993) and Stoelinga (1996) to extratropical cyclones and by Wu and Emanuel (1995a,b) to
tropical cyclone evolution. Both Davis et al. (1993) and
Stoelinga (1996) emphasized the crucial role played by
latent heating in accounting for the cyclone deepening.
In the latter study a careful comparison was carried out
of the contributions from different parts of the physical
parameterizations as well as dry dynamics.
Among questions that we wish to address with the
aid of PV inversion, isentropic analysis of PV, and of
parcel trajectories are the following. What role did the
cyclonic gyre south of Iceland play in preconditioning
the environment over the North Atlantic, allowing for
the intense cyclonic development that took place? How
important was latent heating in the bent-back warm front
FIG. 1. Analyzed 300-hPa geopotential height field from Deutscher
Wetterdienst at 1200 UTC 23 Oct 1995.
in deepening the low and, hence, in producing very
strong winds over NW Iceland on 25–26 October?
The diagnostic methods used in this study are described in section 2. Section 3 describes the results of
applying these methods to the case chosen, giving the
time evolution of the various EPV features during the
rapid deepening phase of the storm. The conclusions
are stated in section 4.
2. Diagnostics using Ertel’s potential vorticity
a. Basic equations
Here we briefly outline the basic equations of the EPV
diagnostic method. More details are given by Davis and
Emanuel (1991), Davis (1992), and Kristjánsson et al.
(1999). We start by defining Ertel’s potential vorticity
(q) as
1
q 5 h · =u,
r
(1)
where r is density, h denotes the absolute vorticity vector, and =u is the three-dimensional gradient of potential
temperature. In spherical coordinates, using the Exner
function (p) as a vertical coordinate, this becomes
q52
1
2
gkp ]u
1
]y ]u
1 ]u ]u
h
2
1
.
p
]p
a cosf ]p ]l
a ]p ]f
(2)
Here k 5 R/C p 5 0.286, a is the earth’s radius, while
l, u denote longitude and latitude, respectively. We now
rewrite (2) by introducing a nondivergent streamfunction, C, and geopotential, F, performing a scale analysis, yielding
1074
MONTHLY WEATHER REVIEW
VOLUME 127
FIG. 2. ECMWF analysis of 500-hPa geopotential height (solid)
and 1000–500-hPa thickness (dashed) at (a) 1200 UTC 24 Oct 1995
and (b) 1200 UTC 25 Oct 1995.
q5
[
gkp
]2 F
1
]2 C ]2 F
( f 1 ¹ 2 C) 2 2 2
2
p
]p
a cos f ]l]p ]l]p
2
]
1 ]2 C ]2 F
.
a 2 ]f ]p ]f ]p
(3)
In order to exploit the invertibility principle of EPV
(e.g., Hoskins et al. 1985) the nonlinear balance equation is used, that is,
1
2
2
]
]C ]C
¹ 2 F 5 = · ( f =C) 1 4
,
.
2
a cos f ](l, f ) ]l ]f
(4)
To solve the last two equations, appropriate initial and
boundary conditions are needed. The horizontal boundary conditions are given by ]F/]p 5 2u and ]C/]p
5 2u /f. The vertical boundary conditions are given by
FIG. 3. Analyzed mean sea level pressure (solid lines) at (a) 1200
UTC on 24 Oct 1995, (b) 0600 UTC on 25 Oct 1995, and (c) 0000
UTC on 26 Oct 1995. The frontal analyses are based on Deutscher
Wetterdienst. The shaded areas correspond to EPV $ 15 dPVU on
400 hPa; 1 dPVU 5 0.1 PVU.
JUNE 1999
1075
THORSTEINSSON ET AL.
FIG. 4. The map shows locations in NW Iceland referred to in the text. The table shows weather conditions at two weather
stations. Standard WMO notation is used.
the geopotential and the streamfunction. As a first guess
we use the geostrophic relation
C0 5
F
.
f
(5)
We have defined the average as the time mean over the
60 h between 0000 UTC 24 October and 1200 UTC 26
October 1995. The perturbation field is, in turn, decomposed into a sum of n anomalies, through
O q.
n
q9 5
b. EPV anomalies
i
(8)
i51
The potential temperature and potential vorticity
fields are decomposed into average fields and perturbation fields as follows:
qtot 5 q(l, f, p, t) 5 q(l, f, p) 1 q9(l, f, p, t)
(6)
utot 5 u (l, f, p, t) 5 u (l, f, p) 1 u9(l, f, p, t).
(7)
In practice n may be a large number. Most of these
anomalies will be small and have little effect. We will
look at a few selected anomalies that we believe to be
of importance. To take into account the cumulative effect from the rest of the perturbation field we define a
residue EPV field as the sum of the anomalies that are
left out:
O
n
qres 5
qi ,
(9)
i5N11
FIG. 5. The surface cyclone track (the bold dot gives position every
6 h) and mean sea level pressure (hPa) from 1200 UTC 24 Oct to
1200 UTC 26 Oct 1995.
where N is the number of selected anomalies and n is
the total number of anomalies in the perturbation field.
We now perform piecewise EPV inversions, to obtain
geopotential height fields corresponding to the different
EPV and u9 anomalies. We have chosen to use an average of the methods, subtraction from total (ST) and
addition to the mean (AM), defined by Davis (1992).
According to Davis this should give similar results to
the fully linearized method (FL). This procedure overcomes the mathematical disadvantages of using the nonlinear EPV, while maintaining the advantage of being
more general than the linear quasigeostrophic PV (e.g.,
Hakim et al. 1996). Hence, we have the following approximate relation:
1076
MONTHLY WEATHER REVIEW
VOLUME 127
TABLE 1. Contributions from different control test perturbation fields at 6-h intervals, based on model analysis, to geopotential height (in
m) at 900 hPa at the location of the surface cyclone. A 60-h average from 0000 UTC 24 Oct to 1200 UTC 26 Oct was used.
Date
Time (UTC)
24 Oct
25 Oct
26 Oct
0000
0600
1200
1800
0000
0600
1200
1800
0000
0600
1200
951
994
937
1004
917
1011
886
981
826
936
693
879
653
868
700
895
756
897
767
873
814
854
UPVT
LPVT
Surface u9T
UuPVT
2103
33
11
6
2115
30
18
5
293
26
6
213
239
216
253
6
23
266
237
28
216
2117
235
213
248
2121
246
26
269
283
258
5
273
226
246
27
259
28
238
237
0
29
27
225
Perturbation
Error
253
210
262
25
2105
11
2102
6
2114
4
2180
26
2221
6
2205
10
2152
11
2106
1
240
0
Total
Mean
O F.
n
Ftot ø Fmean 1
(10)
i
i51
We invert the residue in (9) to get the residue field:
O
n
Fres 5
Fi .
(11)
i5N11
We introduce an error field, Ferr , that captures the
nonlinearity that is still left in the average of the ST
and AM methods. Then we write (10) as
OF 1F
N
Ftot 5 Fmean 1
i
res
1 Ferr .
(12)
i51
In section 3b(1) we will test the validity of relation
(10) in our case by examining if the error field in (12)
is small. In section 3b(2) we will describe the partition
of the EPV and u fields into interesting anomalies and
residue. For further analysis we will there partition the
residue vertically.
c. Grid configuration and data
The computations just described have been carried
out using data from analyses carried out by the European Centre for Medium Range Weather Forecasts
(ECMWF), interpolated onto a High-Resolution Limited-Area Model (HIRLAM) grid (Källén 1996) having
16 vertical levels and a horizontal resolution of 0.58
in a rotated Gaussian grid. The grid area covers most
of the North Atlantic and most of western (W) Europe,
as well as the NE corner of Canada.
These data are interpolated to pressure levels, corresponding to the World Meteorological Organization
(WMO) mandatory levels, ranging from 100 to 1000
hPa. EPV is calculated by finite differences at mandatory levels, ranging from 150 to 900 hPa. Potential temperature at 125 hPa (150–100 hPa average) and 950 hPa
(1000–900 hPa average) is used for upper and lower
boundary conditions, respectively.
3. Piecewise EPV inversions applied to the
24–26 October 1995 cyclone
a. Synoptic description
During 21–23 October 1995 a cold cyclone was established south of Iceland, as clearly seen on the 300hPa analysis (Fig. 1). The cyclone originally formed in
connection with a deep quasi-barotropic low that came
to rest near Iceland on 21 October. Figures 2a, b show
the 500-hPa height and 1000–500 hPa thickness analyzed for 1200 UTC 24 October 1995 and at 1200 UTC
25 October 1995, while Figs. 3a–c show the analyzed
mean sea level pressure field at 18-h intervals starting
at 1200 UTC 24 October 1995, together with upperlevel (400 hPa) potential vorticity. Early on, the most
important feature is the deep quasi-barotropic low southsouthwest (SSW) of Iceland (Fig. 2a) described above.
Due to thermal advection from this low and the high
pressure ridge over central Europe, warm and cold air
masses were brought close to each other along a zone
that stretched from SE Iceland, across and west of Ireland, and toward the NW corner of Portugal, then turning west. We note, for instance, how the 5360-m thickness line (bold) was brought as far south as 458N at
208W, while over Scandinavia that same isoline lay at
678N. Along the strong thermal gradient between the
two air masses, there were several baroclinic waves at
this point, but only one of them grew into a major cyclone, which eventually caused the avalanche in Iceland.
At 1200 UTC 24 October 1995 we estimate the position
of the baroclinic wave to be just SW of Ireland (Fig.
3a). Twenty-four hours later, this wave had developed
significantly, as it had moved some 2000 km northward
(see Fig. 2b). As the wave intensified the thermal advection associated with it was reinforced, and, as seen
in Fig. 3b, the cold front swept across Ireland and the
British Isles, while the warm air had now reached the
east coast of Iceland, with the associated frontal systems
already causing heavy precipitation over northern Iceland. The precipitation fell as snow (Fig. 4) over high
terrain, and subsequently all the way down to sea level
(the elevation of both Bolungarvı́k and Flateyri is only
JUNE 1999
1077
THORSTEINSSON ET AL.
a few meters above sea level while Thverfjall has an
elevation of 752 m). We also note that the high over
Greenland contributed to the northeasterly winds over
Iceland.
Over the subsequent 12 h the intense low SE of Iceland moved slowly northwestward, while the associated
frontal systems continued their cyclonic motion, due to
the strong vorticity associated with the low (Figs. 3b,
c). At 0000 UTC 26 October the cold front had moved
well into Scandinavia, while the warm front was now
aligned N–S, just north of Iceland. At the same time
the high over Greenland was still firm. As a result, very
strong northerly winds raged over NW Iceland, at the
same time dumping large amounts of snow, due to a
combination of frontal and orographic precipitation.
The avalanche fell on the town of Flateyri in the West
Fjord area, NW Iceland (Fig. 4) at 0300 UTC 26 October. Lowland temperatures in the West Fjords were
between 258 and 228C during the period. Sustained
mean wind speeds of up to 90 kt were measured on
Mount Thverfjall, shown in Fig. 4. The wind and weather conditions shown from Bolungarvı́k are probably
characteristic of the lowland weather conditions in this
period.
We also note from Fig. 5 that the surface cyclone was
deepest at 1200 UTC 25 October, that is, about 24–36
h after the formation of the cyclonic wave, which is the
same duration as was found for the 2–3 February 1991
cyclone by Kristjánsson and Thorsteinsson (1995). In
both cases the cyclonic wave was initiated in a highly
baroclinic flow over the North Atlantic and then moved
rapidly northward, and later northwestward as it intensified.
b. Results from the piecewise EPV inversions
1) CONTROL
FIG. 6. The 60-h mean: (a) EPV (in dPVU) at 400 hPa, (b) EPV
(in dPVU) at 900 hPa, and (c) lower boundary u (K) at 950 hPa.
The shaded areas correspond to (a) EPV $ 15 dPVU and (b) EPV
$ 5 dPVU; 1 dPVU 5 0.1 PVU.
TEST
Before we can use diagnostics from piecewise EPV
inversions, we must first show that the total height field
can be decomposed into contributions from different
portions of the EPV field. For this purpose we run a
simple test of the ‘‘null hypothesis’’ that the contributions from the piecewise inversions add up to give the
total contribution to the mean. Stated explicitly, we desire to test the null hypothesis H 0 that the error field
Ferr is negligible when calculating (12).
We define the N selected anomalies such that they
cover the whole integration area and then we have by
definition that the residue field Fres is zero in (12). We
divide the atmosphere into four layers that cover the
whole horizontal integration area: the upper-level EPV
total field (UPVT) covering 500–250 hPa, the low level
EPV total field (LPVT) covering 900–600 hPa, the surface u9 total field (surface u9T) covering 950-hPa u
anomalies, and finally, the EPV total field covering 250–
150 hPa and the 125-hPa u total field are combined
(UuPVT).
1078
MONTHLY WEATHER REVIEW
VOLUME 127
FIG. 7. Perturbation fields associated with 400-hPa EPV (in dPVU, top row), 900-hPa EPV (in dPVU; middle row), and lower boundary
u (in K; bottom row). Anomalies are defined as positive values within each rectangle. We have (top row) the anomalies UPV P1 (solid) and
UPV P2 (dotted), (middle row) LPV P (solid), and (bottom row) surface u9 P (solid). The shaded areas (top and middle rows) correspond
to q9 . 2 dPVU and (bottom row) indicate u9 . 2 K; (left column) 1200 UTC 24 Oct, (middle column) 0600 UTC 25 Oct, and (right
column) 0000 UTC 26 Oct.
The results are shown in Table 1. The error term is
small, indicating the correctness of the piecewise calculations. The error term stems from the nonlinearity
of the EPV equation, which despite the ingenious method of Davis (1992), described in the previous section,
still affects the results slightly.
2) EPV
STRUCTURE AND EVOLUTION
Figure 6 shows the time-averaged q and u fields over
the 60-h period 0000 UTC 24 October–1200 UTC 26
October 1995. Figure 6a depicts a spiralling tongue of
high-EPV air at 900 hPa associated with the cyclonic
gyre south of Iceland. With values well in excess of 2.0
potential vorticity units (PVU) in some areas in a 60-h
mean, there is strong evidence of a stratospheric intrusion, as is often found to the rear of intense extratropical
cyclones (e.g., Shapiro and Keyser 1990). Figure 6b
shows the lower-tropospheric time-averaged EPV field,
which has less-distinct features, due to the rapid movement of the frontal system, with which this anomaly is
mainly associated. Finally, Fig. 6c shows the surface u
field which, among other features, clearly illustrates the
surface baroclinic zone to the W and NW of the British
Isles and the dome of cold air over Greenland.
In Fig. 7 we show the evolution of the 400- and 900-
JUNE 1999
THORSTEINSSON ET AL.
1079
FIG. 8. EPV cross sections from analysis at (a) 0600 UTC 25 Oct 1995, (b) 1200 UTC 25 Oct 1995, (c) 1200 UTC 25 October 1995,
and (d) at 1800 UTC 25 Oct 1995. The line PVU 5 1.5 has been enhanced for clarity. (Locations of cross sections are shown in Figs.
6a,b).
hPa EPV perturbation fields and the u9 perturbation
field. The EPV anomalies of UPV P1 and UPV P2, LPV
P and surface u9 P, are defined as the positive values
within the rectangles. The anomalies have been subjectively chosen as the most outstanding features in the
EPV and u9 perturbation fields. The main reason that
only positive anomalies were selected is that the negative anomalies are typically less intense and less temporally coherent than their positive counterparts.
This means that a ‘‘residue field’’ mainly consisting
of negative EPV values remains. To further help the
analysis we have partitioned the residue into vertical
layers that correspond to the layers chosen for the main
anomalies. UPV residue is the residue from the 500–
250-hPa EPV perturbation field, LPV residue is the residue from the 900–600-hPa EPV perturbation field, and
surface u9 residue is the residue from the 950-hPa u
perturbation field. UuPV residue is the combined residue from the 125-hPa u perturbation field and the 250–
150-hPa EPV perturbation field. Because there is no
anomaly selected in this upper boundary area the UuPV
residue is the contribution from the total perturbation
field in these layers.
The UPV P1 anomaly (Fig. 7, top row) is always
located close to the surface low (see Fig. 5), while UPV
P2 is located farther to the south and west and might
therefore be expected to exert a smaller influence on the
low. The LPV P anomaly (middle row) is mostly aligned
with the frontal systems. This anomaly has a small amplitude at the initial time (Fig. 7d), but then grows in
the developing phase of the cyclone (Fig. 7e) and has
a magnitude of more than 1 PVU at 0600 UTC 25 October (Fig. 7e). Subsequently, this anomaly weakens and
has become quite insignificant at 0000 UTC 26 October
(Fig. 7f), probably due to the fact that the advection of
rather warm, moist air has now been cut off due to the
occlusion process. Instead, cold, much-drier air now enters from the north, producing less latent heating. The
fact that the observations (Fig. 4) indicate heavy precipitation over NW Iceland at this time suggests that
this precipitation is largely orographic and, hence, less
organized than in the frontal system seen to the eastnortheast (ENE) and SE of Iceland in Fig. 7e. This
explanation is supported by the fact that IR satellite
images (not shown) do not indicate very cold cloud tops
at this point. Finally, the surface u9 P anomaly (bottom
row) represents the tongue of warm air in the warm
sector of the cyclone. We note how this air is advected
cyclonically in accordance with the movement of the
frontal systems (Figs. 3a–c) and how the associated
anomaly increases slowly in magnitude from 4 K at
1200 UTC 24 October (Fig. 7g) to 6 K at 1200 UTC
26 October (Fig. 7i).
Cross sections through the low center at different
1080
MONTHLY WEATHER REVIEW
VOLUME 127
FIG. 10. Trajectories on the 310 K isentropic surface starting at
0000 UTC 24 October and ending in the PV anomaly UPV P2 at
0000 UTC 25 Oct. Height (hPa) of the 310 K surface and winds on
the 310 K surface at 0000 UTC 24 Oct are also presented.
FIG. 9. Infrared (National Oceanic and Atmospheric Administration
channel 4) satellite picture at 1258 UTC 25 Oct 1995. (Obtained from
the U.K. University of Dundee.)
times (Figs. 8a–d) show many interesting features not
revealed by horizontally projected maps. For instance,
in the central part of Fig. 8a, we see a lowering of the
tropopause in the cold air behind the cold front. At the
same time, there is an EPV anomaly of more than 2
PVU at 700–1000 hPa, between 58 and 108W in the
same figure, associated with the bent-back frontal system seen over Iceland in Fig. 3b. Six hours later (Fig.
8b) we see indications of a merging between those two
anomalies at 88W. The high tropopause to the far right
in this figure is associated with the warm air mass over
western Europe, while the high tropopause to the far
left is related to the relatively warm air north of the
occlusion (see Fig. 9).
Figure 8c shows the same features from a slightly
different angle. The coupling to the upper levels is very
distinct in the far right of the figure, but now we have
to the left of that (138–208W) a situation with a strong
EPV anomaly near 700 hPa and a high tropopause above
with low EPV values in the upper troposphere. This is
because in this cross section the front is intersected
twice, due to its spiral shape (Fig. 9), and the hightropopause air in the warm sector air is undergoing an
upslope ascent.
One of the most important properties of EPV is its
conservation, following an air parcel, in adiabatic, inviscid flow. In the mid- and upper troposphere the flow
can often be assumed to satisfy these criteria. With this
principle in mind, we have investigated the possible role
of EPV advection in the initial stages of the cyclone
development. We do this mainly by studying horizontal
transport on isentropic surfaces. This corresponds to the
first term on the right in the continuity equation for EPV,
namely,
12
]q
] u̇
5 2 v · =u q 1 q 2
.
]t
]u q
(13)
The above conservation principle is equivalent to setting
the last term of (13) to zero. Figure 10 shows trajectories
along the u 5 310 K surface from 0000 UTC 24 October
until 0000 UTC 25 October. These trajectories were
FIG. 11. Trajectories on the 295 K isentropic surface starting at
0600 UTC 25 October and ending over Iceland at 1200 UTC 25 Oct.
Height (hPa) of the 295 K surface and winds on the 295 K surface
at 0600 UTC 25 Oct are also presented.
JUNE 1999
THORSTEINSSON ET AL.
1081
FIG. 12. (a) The PV anomaly LPV P and winds on the 295 K surface at 0600 UTC 25 Oct. (b) Same as
in (a) but now including 12-h trajectories on the 295 K surface ending at 1800 UTC 25 Oct.
chosen in such a way that their arrival points are close
to the center of the developing cyclone wave. It is seen
that the air arriving at this level originates in the deep
trough (‘‘cyclonic gyre’’) to the S and SW of Iceland.
We also note that the air trajectories are descending from
an area of high-EPV values (3–4 PVU) at the 350-hPa
level to an area of much lower EPV values (,1 PVU)
near the 600-hPa level, suggesting a large positive advection of EPV through the first term on the right of
(13). This strongly suggests that the trough S and SW
of Iceland may have played a crucial role in initiating
the cyclone of interest.
The spatial structure of the PV distribution at 1200
UTC 25 October shows a dipole structure W of the
surface cyclone center (Fig. 8c). The lower part of the
dipole is a pronounced low-level EPV anomaly, its
strength being about 2.5 PVU. This anomaly is probably
due to the release of latent heat within the ascending
low-level flow, which is shown in Fig. 11. This figure
presents air trajectories on the 295 K surface and shows
the lower-level conveyor belt that rises from 800 hPa,
100 km E of Iceland, and reaches about 600–700 hPa
over Iceland 6 h later, as seen in Fig. 11. The ascent
mainly takes place over Iceland, explaining the heavy
precipitation in NW Iceland.
Figure 12a shows the pronounced low-level EPV
anomaly on the 295 K surface, near the surface cyclone
at 0600 UTC 25 October. Figure 12b shows that the
low-level EPV anomaly is advected cyclonically around
the cyclone and is located over NE Iceland 12 h later.
The trajectories show that such advection of EPV anomalies, probably diabatically produced, takes place. This
low-level EPV anomaly is at 1200 UTC 25 October in
phase with the upper EPV anomaly, and the associated
wind fields are adding to produce very strong winds.
Above the region of maximum release of latent heat
there is a sink of EPV, which partly explains the low
values of EPV aloft in the middle of the cross section
shown in Fig. 8c. However, the conveyor belt, which
starts at midtropospheric levels, contains low-EPV air.
Figure 13 shows trajectories originating over the regions
west of the Iberian Peninsula near 408N, 208W, at 0000
UTC 24 October. The flow is depicted on the 310 K
surface and starts at 550–600-hPa levels, rising to near
FIG. 13. (a) Trajectory starting at 0000 UTC 24 Oct and ending at 1200 UTC 26 Oct; wind on the 310
K surface at 0000 UTC 24 Oct. (b) Trajectory starting at 0000 UTC 25 Oct and ending over Iceland at
0600 UTC 25 Oct; wind on the 310 K surface and the height (hPa) of the 310 K surface at 0000 UTC 25
Oct.
1082
MONTHLY WEATHER REVIEW
VOLUME 127
FIG. 14. Contributions to perturbation heights (gpm) at 900 hPa from UPV P1 and UPV P2 (q at 500 to 250 hPa; top row and uppermiddle row, respectively), LPV P (q at 900 to 600 hPa; lower-middle row), and surface u9 P (bottom row). The position of the surface low
center is denoted by L; (left column) 1200 UTC 24 Oct, (middle column) 0600 UTC 25 Oct, and (right column) 0000 UTC 26 Oct 1995.
JUNE 1999
1083
THORSTEINSSON ET AL.
FIG. 15. Contributions to perturbation heights (gpm) at 900 hPa from UPV residual (q at 500 to 250 hPa; top row), LPV residual (q at
900 to 600 hPa; middle row), and lower boundary u residual (bottom row). The position of the surface low center is denoted by L; (left
column) 1200 UTC 24 Oct, (middle column) 0600 UTC 25 Oct, and (right column) 0000 UTC 26 Oct 1995.
the 300-hPa level N of Iceland, from where it turns
eastward as seen in Fig. 13. As for the low-level conveyor belt, the strongest ascent appears to take place
over Iceland during the most intense stage of cyclogenesis.
Finally, at 1800 UTC 25 October (Fig. 8d), a similar
pattern to that of Fig. 8b is seen, except that the lowertropospheric EPV anomaly does not penetrate as far
downward, signifying the filling of the low. Another
interesting feature here is the low tropopause in the right
part of the figure, associated with the cold air that has
been advected cyclonically around the low and gradually catching up with the warm air farther to the north
and west as the cyclone occludes.
3) CONTRIBUTIONS
TO CYCLONE DEEPENING
We shall now look at contributions to the 900-hPa
geopotential height fields from the four main positive
EPV anomalies and the residue in each layer (Figs. 14,
15). Somewhat surprisingly, it is found that UPV P2
has a larger influence on the geopotential field than does
UPV P1. In fact we see from Table 2 that by far the
largest contribution during the initiation phase of the
baroclinic wave (0000 UTC 24 October) comes from
UPV P2. This is in contrast to the initiation phase found
by Kristjánsson et al. (1999) for the February 1991 cyclone, where the UPV anomaly played a marginal role
in the early stages but became more and more important
1084
225
240
0
2104
22
236
26
2150
9
2204
9
6
26
2222
7
229
22
254
16
257
11
259
1
251
4
268
59
248
76
282
56
2146
63
2186
65
29
2292
302
220
2302
264
227
2297
252
229
2290
250
239
2256
247
814
854
767
873
756
897
700
895
653
868
1200
0600
26 Oct
0000
1800
1200
2178
28
212
27
2113
3
2100
5
7
213
2104
10
257
22
259
22
264
11
228
34
2188
71
2136
70
286
70
267
62
234
2224
244
239
2190
226
247
2170
179
220
2205
133
917
1011
886
981
826
936
693
879
25 Oct
0600
0000
1800
1200
6
260
27
6
252
9
222
33
Surface u9 P
Surface u9 residue
Perturbation
Error
230
60
225
58
LPV P
LPV residue
UuPV residue
221
2240
147
214
2210
122
UPV P1
UPV P2
UPV residue
219
37
937
1004
Total
Mean
951
994
24 Oct
0600
0000
Date
Time (UTC)
TABLE 2. Contributions from different anomalies at 6-h intervals, based on model analysis, to geopotential height (in m) at 900 hPa at the location of the surface cyclone. A 60-h average
from 0000 UTC 24 Oct to 1200 UTC 26 Oct was used.
MONTHLY WEATHER REVIEW
VOLUME 127
as the baroclinic low deepened. We see also that, while
UPV P1 has only a very small impact in the area where
the surface cyclone is located, all the other three anomalies are significant. The UPV P2 anomaly typically has
the largest impact slightly to the rear of the 900-hPa
cyclone, while LPV P is the anomaly most nearly in
phase with it, although we note a tendency for a larger
contribution along the warm front than at the cyclone
center, especially at the final time (Fig. 14i). The surface
u9 P anomaly contributes significantly to deepening the
900-hPa low in the warm sector. On the other hand, the
UPV P1 anomaly seems to be connected with the cyclonic gyre south of Iceland.
Looking back at the flow field due to the UPV P1
anomaly (Fig. 14a) we note now that the wind field
from this anomaly acts precisely in such a way as to
advect the UPV P2 anomaly (Fig. 7a) toward the area
of interest. Hence, it appears that the UPV P1 anomaly
is largely responsible for the positive EPV advection
discussed in the previous paragraph. As mentioned before, this anomaly has little direct impact on the lowlevel flow of the developing cyclone over the British
Isles. The interaction between the UPV P1 and P2 anomalies discussed here is an important reminder that care
must be taken in inferring simple causal relationships
based on instantaneous forcings.
Figure 15 shows the contributions from the UPV,
LPV, and surface u9 residue fields at 900 hPa. The residue fields contribute significantly to the weakening of
the low over the whole time period near the low center
(see Table 2). The impact of the UPV residue is largest
ahead of the surface cyclone (Figs. 15a–c), which coincides with the position of negative values of UPV
(Figs. 7a–c). We suggest that this negative anomaly is
caused by warm, relatively low-EPV air rising ahead of
the low, in connection with frontal ascent ahead of the
cold front [cf. the ‘‘warm-conveyor belt’’ of Browning
(1990, Fig. 11)]. Latent heating in the ascending air also
contributes to the negative anomaly aloft, that is, above
the level of heating.
The LPV residue has its largest contribution a few
hundred kilometers behind the occlusion (Figs. 15d–f)
but also contributes significantly to filling near the low
center (Figs. 15d, e). This coincides roughly with the
position of the negative anomaly itself, seen in Figs.
7d–f. One factor that presumably contributes to this negative anomaly is shallow convection with associated
stratocumulus clouds, since in that case latent heating
creates potential vorticity below 900 hPa, while between
900 and 600 hPa, which is where LPV is defined, a
negative EPV tendency would occur.
The surface u9 residue contributes significantly to the
filling in the cold air behind the cold front (Figs. 15g–
i). This is due to intense cold advection in this area,
which generates a negative surface u9 anomaly.
Figure 16 compares the evolution of the UPV, LPV,
and surface u9 total fields, also treated in Table 1. We
note that the contribution from UPVT is large before
JUNE 1999
1085
THORSTEINSSON ET AL.
FIG. 16. Time evolution of the UPV, LPV, and surface u9 total fields at the cyclone center.
1200 UTC 24 October but becomes quite small on 25
October, when the surface cyclone is at its deepest. This
is because, as shown in Table 2, the UPV residue becomes very influential at this time, hence cancelling the
effect of UPV P2 (and UPV P1). The same is not the
case with the lower-tropospheric or surface perturbation
fields. The contribution from the LPV residue is only
about half that of the LPV P anomaly on 25 October,
and the surface u9 residue is quite small when compared
to the surface u9 P anomaly in this period. The LPV P
anomaly contributes significantly at the time when the
cyclone is most intense. It follows from Fig. 16 and
Table 2 that the main cause of the cyclone deepening
is the rapid intensification of LPVT and the increased
deepening due to UPVT at 1200 UTC 25 October 1995.
The latter may be caused either by vertical propagation
of EPV from below, that is, in connection with the
‘‘wrapped-up’’ frontal spiral (see Fig. 9), or by horizontal advection of the UPV P2 anomaly, which, due
to the slow movement of the cyclone at this time, allows
this anomaly to catch up with the cyclone. Contributions
from UPVT and surface u9 P prolong the lifetime of
the cyclone, since they are still significant on 26 October, at a time when the contribution from LPVT has
all but vanished, as seen in Fig. 7f.
4) BAROCLINIC
NATURE OF THE ANOMALIES
In order to get a better view of the three-dimensional
aspects of the perturbations we shall now study EPV
cross sections through the cyclone, perpendicular to the
thermal wind. Even though we have relied heavily on
PV diagnostics so far in this study, there is no doubt
that many aspects of the cyclone development could
have been explained quite well using the more traditional, quasigeostrophic theory, including the tendency
and omega equations. We have investigated baroclinic
aspects of cross sections through the cyclone center that
show geopotential height contributions from the different EPV and surface u9 anomalies. In Figs. 17a–f we
show NW–SE cross sections near 508N obtained at 1200
UTC 24 October (for locations of cross sections see Fig.
3). First in Fig. 17a the perturbation geopotential height
is displayed, showing a westward tilt of the trough with
height, as expected in a developing baroclinic wave.
Figures 17b–f then show a decomposition of the perturbation height into contributions from different portions of the EPV field. At this time the UPV P2 anomaly
is by far the most important one, as was seen in Table
2. Its influence is largest at 300–400 hPa, and from that
region a ‘‘trough’’ extends downward and southeastward in the section. Smaller contributions are obtained
from the LPV P and surface u9 P anomalies, and both
of them are mainly confined to the lower troposphere.
Figure 17f displays the sum of the contributions from
the upper, lower, and surface residue terms.
In Fig. 18a the baroclinic tilt with height is even more
distinct than (in section A-B) 18 h earlier (Fig. 17a).
As in the previous figure, UPV P1 is negligible, and
UPV P2 yields the strongest contribution, but by now
the contribution from LPV P has become very prominent, while the surface u9 P anomaly also has a noticeable, though smaller, effect near the surface. This is not
surprising, since by now the frontal system has become
well developed and the associated warm sector air is
also more prominent than before.
These features can be explained by referring to the
expression for the Rossby penetration height H, which
is given by
H5
fL
.
N
(14)
Here L is the characteristic horizontal dimension of the
1086
MONTHLY WEATHER REVIEW
VOLUME 127
FIG. 17. Cross section from A to B in Fig. 3, displaying geopotential height for (a) departure of analyzed values from the mean; EPV
anomaly contributions from (b) UPV P1, (c) UPV P2, (d) LPV P, and (e) surface u9 P; and (f ) the upper, lower, and surface residual fields
for 1200 UTC 24 Oct 1995. Units: 10 m.
system, while N denotes the Brunt–Väisälä frequency.
The parameter H is a measure of the vertical scale of
the response and of the communication between, for
example, upper forcing and low-level response. Thus,
in our case we have an upper-level anomaly of large
horizontal scale that will therefore produce large effects
at the surface, while for our lower-level anomalies,
which are of small horizontal scales, the responses are
confined close to the lower troposphere as seen in Fig. 18.
4. Conclusions
A deep North Atlantic cyclone that caused devastating avalanches in NW Iceland in October 1995 has been
investigated, using potential vorticity inversion, following Davis and Emanuel (1991). For the sake of generality, Ertel’s potential vorticity has been used. Due to
its nonlinearity it is not clear a priori that contributions
to the height field from different sources will add up.
However, we have carried out tests (null hypothesis),
showing only a small error when all terms have been
added.
Our investigation suggests that cyclone development
was initiated when a preexisting upper-level EPV anomaly began interacting with a baroclinic surface wave
south of Ireland on 24 October 1995. This upper-level
anomaly was shown to be associated with the descent
and isentropic advection of high-valued PV air in con-
JUNE 1999
THORSTEINSSON ET AL.
1087
FIG. 18. As in Fig. 17 but cross section from C to D at 0600 UTC 25 Oct 1995.
nection with a cold-core, quasi-stationary, quasi-barotropic low south of Iceland.
The rapid deepening that followed, as the cyclone
moved northward, and later northwestward, was aided
by strong latent heating along the occluding warm front.
This latent heating has been found to produce low-level
EPV along the front and, hence, to contribute strongly
to deepening the cyclone.
By far the largest negative EPV contributions came
from the upper troposphere, partly associated with ascending warm, low-EPV air ahead of the cold front.
Analysis of isentropic trajectories show strong warm
air ascent over Iceland. The conditions leading to the
avalanches can be explained by noting that in addition
to frontal precipitation associated with the deep cyclone
there was low-level advection of very cold air from a
persistent cold high over Greenland. This high contributed to the low temperatures and strong wind field over
NW Iceland on 26 October 1995.
The results indicate a markedly different evolution
from that of the February 1991 cyclone studied by Kristjánsson et al. (1999). In that case low-level baroclinicity and latent heating were the main cyclogenetic processes early on, while the upper-level EPV anomaly
became significant in the final deepening phase. However, in the present case, the upper-level EPV feature
already contributed strongly at the initial time and was
quite crucial for the rapid development that took place
on 24 October 1995. There is little doubt that preconditioning was crucial in advecting this air southeastward
1088
MONTHLY WEATHER REVIEW
around the cyclonic gyre south of Iceland, hence allowing it to interact with the intense low-level baroclinic
zone near Ireland. It seems that this cyclone development has many similarities with that of the ‘‘October
cyclone’’ that hit the British Isles in October 1987 (see
Hoskins and Berrisford 1988), although in that case the
cyclone track lay farther to the south and east than in
this case.
Acknowledgments. This research was supported by
the Students’ Innovation Fund of the University of Iceland and the Science Fund of the Icelandic Research
Council. We wish to thank Dr. Christopher A. Davis of
NCAR for kindly supplying some of the software for
computing the EPV inversions, and Mr. Anstein Foss
for providing software for computing trajectories on isentropic surfaces.
APPENDIX
Glossary
EPV Ertel’s potential vorticity.
Error Error field. The difference between the total
geopotential height field and the sum of the mean
geopotential height field and the contributions from
all selected anomalies and the residue [see (12)].
LPV P Lower-level EPV positive anomaly. One of
the main anomalies used for analysis [see section
3b(2)].
LPV residue Lower-level EPV residue field. Combination of all lower positive and negative EPV
anomalies that are not selected for analysis in layers
900–600 hPa.
LPVT Lower-level EPV total field, 900–600 hPa.
Used for the control test.
n
Perturbation The total perturbation field Si51
Fi .
The sum of the contributions from all anomaly and
residue fields.
Surface u9T Surface (lower) boundary u9 total field,
at 950 hPa. Used for the control test.
Surface u9 Surface level u9 anomaly. One of the
main anomalies used for analysis [see section
3b(2)].
Surface u9 residue Surface level u9 residue field.
Combination of all lower positive and negative u
anomalies that are not selected for analysis in the
950-hPa layer.
UPV P1 Upper-level EPV positive anomaly 1. One
of the main anomalies used for analysis in layers
500–250 hPa [see section 3b(2)].
UPV P2 Upper-level EPV positive anomaly 2. One
of the main anomalies used for analysis [see section
3b(2)].
UPV residue Upper-level EPV residue field. Combination of all upper positive and negative EPV
anomalies that are not selected for analysis.
VOLUME 127
UPVT Upper-level EPV total field, 500–250 hPa.
Used for the control test.
UuPV residue Upper boundary u and uppermost
EPV residue field. The combination of all positive
and negative EPV anomalies that are not selected
for analysis in layers 250–150 hPa and all positive
and negative u anomalies that are not selected for
analysis in the 125-hPa layer. In our case this field
equals the total field because no anomalies were
selected in these layers.
UuPVT Upper-boundary u and uppermost EPV total field. Combination of upper-boundary u (125
hPa) total field and the uppermost PV total field
(250–150 hPa). Used for the control test.
REFERENCES
Browning, K. A., 1990: Organization of clouds and precipitation in
extratropical cyclones. Extratropical Cyclones: The Erik Palmén
Memorial Volume, C. W. Newton and E. O. Holopainen, Eds.,
Amer. Meteor. Soc., 129–153.
Davis, C. A., 1992: Piecewise potential vorticity inversion. J. Atmos.
Sci., 49, 1397–1411.
, and K. A. Emanuel, 1991: Potential vorticity diagnostics of
cyclogenesis. Mon. Wea. Rev., 119, 1929–1953.
, M. T. Stoelinga, and Y.-H. Kuo, 1993: The integrated effect of
condensation in numerical simulations of extratropical cyclogenesis. Mon. Wea. Rev., 121, 2309–2330.
Hakim, G. J., D. Keyser, and L. F. Bosart, 1996: The Ohio Valley
wave-merger cyclogenesis event of 25–26 January 1978. Part
II: Diagnosis using quasigeostrophic potential vorticity inversion. Mon. Wea. Rev., 124, 2176–2205.
Hoskins, B. J., and P. Berrisford, 1988: A potential vorticity perspective of the storm of 15–16 October 1987. Weather, 43, 122–
129.
, M. E. McIntyre, and A. W. Robertson, 1985: On the use and
significance of isentropic potential vorticity maps. Quart. J. Roy.
Meteor. Soc., 111, 877–946.
Jóhannesson, T., and T. Jónsson, 1996: Weather in Vestfirdir before
and during several avalanche cycles in the period 1949 to 1995.
Vedurstofa Íslands Internal Rep. VÍ-G96015-Úr15, 8 pp. [Available from Icelandic Meteorological Office, Bústadavegi 9, IS150 Reykjavik, Iceland.]
Jónsson, T., 1993: Vedur á Íslandi ı́ 100 ár. Ísafold, 237 pp.
Källén, E., Ed., 1996: HIRLAM documentation manual. System 2.5.
[Available from SMHI, S-60176 Norrköping, Sweden.]
Kristjánsson, J. E., and S. Thorsteinsson, 1995: The structure and
evolution of an explosive cyclone near Iceland. Tellus, 47A,
656–670.
,
, and G. F. Ulfarsson, 1999: Potential vorticity-based interpretation of the evolution of the Greenhouse Low, 2–3 February 1991. Tellus, 51A, 233-248.
Shapiro, M. A., and D. Keyser, 1990: Fronts, jet streams and the
tropopause. Extratropical Cyclones: The Erik Palmén Memorial
Volume, C. W. Newton and E. O. Holopainen, Eds., Amer. Meteor. Soc., 167–191.
Stoelinga, M. T., 1996: A potential vorticity–based study on the role
of diabatic heating and friction in a numerically simulated baroclinic cyclone. Mon. Wea. Rev., 124, 849–874.
Wu, C.-C., and K. A. Emanuel, 1995a: Potential vorticity diagnostics
of hurricane movement. Part I: A case study of Hurricane Bob
(1991). Mon. Wea. Rev., 123, 69–92.
, and
, 1995b: Potential vorticity diagnostics of hurricane
movement. Part II: Tropical Storm Ana (1991) and Hurricane
Andrew (1992). Mon. Wea. Rev., 123, 93–109.