NHESSNatural Hazards and Earth System SciencesNHESSNat. Hazards Earth Syst. Sci.1684-9981Copernicus PublicationsGöttingen, Germany10.5194/nhess-17-1653-2017Brief communication: Characteristic properties of extreme wave events observed in the northern Baltic Proper, Baltic SeaBjörkqvistJan-Victorjan-victor.bjorkqvist@fmi.fihttps://orcid.org/0000-0001-8981-2758TuomiLaurahttps://orcid.org/0000-0003-2471-6815TollmanNikoKangasAnttiPetterssonHeidihttps://orcid.org/0000-0002-5055-4664MarjamaaRiikkaJokinenHannuForteliusCarlFinnish Meteorological Institute, P.O. Box 503, 00101 Helsinki,
FinlandJan-Victor Björkqvist (jan-victor.bjorkqvist@fmi.fi)25September20171791653165827March201726April201725August2017This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/This article is available from https://nhess.copernicus.org/articles/17/1653/2017/nhess-17-1653-2017.htmlThe full text article is available as a PDF file from https://nhess.copernicus.org/articles/17/1653/2017/nhess-17-1653-2017.pdf
A significant wave height of 7 m has been measured five times by the
northern Baltic Proper wave buoy in the Baltic Sea, exceeding 8 m
twice (2004 and 2017). We classified these storms into two groups by duration
and wave steepness. Interestingly, the two highest events exhibited opposite
properties, with the 2017 event being the longest storm on record. This storm
is also the first where the harshest wave conditions were modelled to occur
in the western part of the Baltic Proper. The metrics quantifying the storm's
duration and steepness might aid in issuing warnings for extreme wave
conditions.
Introduction
Extreme wave conditions impact the transport and safety at sea. They slow
down larger vessels and can threaten the safety of smaller ships. Both
operational wave measurements and wave forecasting models are needed to issue
warnings and provide accurate estimates of the conditions seafarers will face
along their routes.
The wind conditions play a key part in the formation of the extreme wave
conditions at sea. Because of the small size and geometry of the Baltic Sea,
both the fetch and the duration of the wind event limit the wave growth. The
Baltic Proper has the largest fetch and the harshest wave conditions of all
the Baltic Sea sub-basins . Earlier studies have shown that
the highest waves are typically in the north- and southeastern part of the
domain e.g.. The highest wave event on
record happened when the northern Baltic Proper (NBP) wave buoy measured a
significant wave height of 8.2 m in December 2004. Recently, in January
2017, an 8 m significant wave height was recorded for the second time in the
20-year-long measurement history at the same location.
Even higher waves have been estimated to have occurred in the northern Baltic
Proper during the wind storm Gudrun in January 2005. The highest waves were
evaluate to be slightly south-southeast from the location of the NBP wave
buoy, where a significant wave height of 7.2 m was measured. Wave experts
who reviewed the results of three validated wave forecast models and the wind
conditions during the storm concluded that the highest significant wave
height was in the order of 9.5 m .
The ship routes from Stockholm to Helsinki and Tallinn – the capitals of
Sweden, Finland and Estonia respectively – cross the area where the highest waves occur.
Furthermore, the wave direction in this area is typically from south or
southwest during storms, thus propagating perpendicular to the shipping
routes. The most disastrous accident along these routes was the sinking of MS
Estonia in 1994; 852 lives were lost. The significant wave height
was estimated to be between 4 and 5 m during the accident .
While the wave conditions were evaluated to not be the primary reason behind
this accident, they caused damage to the vessel and complicated the rescue
missions.
In this paper we evaluate the characteristic properties of five extreme wave
events in the Baltic Sea measurement records. Special attention is given to
two storms: “Rafael” in 2004 and “Toini” in 2017. The accuracy of the
wave forecast of the Finnish Meteorological Institute (FMI) is evaluated, and
the issuing of warnings for extreme events is briefly discussed based on the
findings.
Description of the area and the available data
The Baltic Sea is a semi-enclosed water body with several sub-basins
(Fig. a). Since this paper focuses on extreme wave
events, we will limit our study to the largest basin – the Baltic Proper.
To analyse the storms, we use wave measurements from the operational wave buoy
of the FMI that is moored at a depth of 100 m in the NBP. The data from another
Directional Waverider moored on the eastern side of the Swedish island of
Gotland provide some spatial information about the wave conditions. To
evaluate the wind conditions, we use 10 min average wind data from FMI's
weather station at Bogskär. An overview of the locations can be found in
Fig. a.
We use the parameters significant wave height (Hs=Hm0) and
peak wave period (Tp) in the analysis. The mean
inverse wave steepness 〈λp/Hs〉 serves
as an indicator of the steepness conditions, where the peak wavelength
λp is estimated from the peak period using linear wave
theory, and the brackets denote the temporal average.
We analyse the spatial attributes of the wave field using FMI's operational
wave forecast model WAM cycle 4 . WAM is a
third-generation phase-averaged spectral wave model that solves the action
balance equation to simulate the wave energy at each grid point. This wave
model has been successfully implemented in the Baltic Sea
e.g.. In 2004 FMI's operational
wave forecast model had a spatial resolution of 22 km and output time
interval of 3 h. The spatial resolution has been increased and is
currently 4 nmi (∼ 7.4 km), while the output temporal
resolution is 1 h.
The surface wind field at 10 m height from FMI's operational numerical
weather prediction system HIRLAM (High-Resolution Limited Area Model)
functions as the meteorological forcing for the wave
model. The present FMI-HIRLAM has 0.068∘ horizontal resolution and 64
vertical terrain-following hybrid levels. The 54 h forecasts are run four
times a day (00:00, 06:00, 12:00 and 18:00 UTC) using boundary conditions
from the Boundary Condition Optional Project of the ECMWF (European Centre
for Medium-Range Weather Forecasts). In 2004 FMI's NWP (Numerical Weather
Prediction) system HIRLAM had 0.2∘ horizontal resolution and 40
vertical levels. The physics and the parameterisations in HIRLAM have also
improved over the years, which has increased the accuracy of the forecast
surface winds .
The modelled significant wave height at
22:00 UTC (a) and the meteorological conditions at
21:00 UTC (b) during storm Toini in 11 January 2017. The locations
of the NBP wave buoy (circle), the Gotland wave buoy (star) and Bogskär
wind measurements (plus) are also shown in (a).
Characteristic storm properties
For the purpose of this paper a storm is defined as an event when the
significant wave height exceeds 7 m at least once. We further define
the duration of a storm as the time during which the significant wave height exceeds
6 m. In the measurement history of the NBP wave buoy (1996–2017)
this amounts to five storm events: two in December 1999, one in
December 2004, one in January 2005 and one in January 2017.
The NBP wave buoy has measured a significant wave height of 8 m only
twice (2004 and 2017). During the other three storms the measured maximum has
been under 7.5 m (Table ). The measured maximum
values of the peak wave period Tp in four of the five storms were
13 s. The observed peak period during the first storm in 1999
(henceforth 1999a) did not exceed 12 s. However, the peak period was
still growing at the start of an unfortunate 3 h gap in the
measurements.
Based on a 6-year model hindcast (November 2001 to October 2007)
found the statistical exceedance time for a significant
wave height of 6 m to be 8.8 hyr-1 at the NBP wave buoy.
The analysis of the storms reveals that the true duration of the storms was slightly longer, typically around 10–15 h
(Table ).
The maximum values of the wave parameters during the storms. The
exceedance time for the significant wave height over 6 m and mean
inverse significant steepness for that exceedance time are also
given.
Timemax Hsmax TpHs≥6m〈λp/Hs〉6 December 19997.4 m12.0 s< 7 h2517 December 19997.4 m12.5 s13.5 h3022 December 20048.2 m12.7 s9.0 h279 January 20057.2 m12.8 s13.5 h2911 January 20178.0 m12.5 s15.5 h30
A comparison of the two most severe storms (Rafael in 2004 and Toini in 2017)
reveals several characterising differences. Rafael was short, with a
6 m exceedance time of only 9 h, while Toini lasted
6.5 h longer. The mean inverse significant steepness were 27 and 30
for Rafael and Toini respectively, meaning that the waves were steeper during
Rafael. The difference in steepness is partially explained by the behaviour
of the peak period. It reached its maximum value during the storm in 2017,
while the maximum peak period in 2004 was observed after the significant wave
height had decayed to under 6 m (not shown).
Also other storms from 1999 and 2005 can be classified into one of the two
groups set by the 2004 and 2017 events. One group is identified by a short
duration, late occurrence of the maximum peak period and steeper wave
conditions (1999a, 2004). The second group consists of longer storms that
reach their maximum peak period during the 6 m exceedance time,
resulting in less steep wave conditions (1999b, 2005, 2017).
The wave observations from the NBP cannot be considered entirely
representable for the entire Baltic Proper. The highest modelled wave events
were located south-southeast of the wave buoy during Gudrun in
2005 , slightly west of the wave buoy during Toini in 2017
(Fig. ) and slightly east of the wave buoy during
Rafael in 2004 (not shown). High waves have also been modelled in the
southern Baltic Sea e.g., which is an area suffering
from an acute lack of wave measurements. However, the sparsity of remotely
sensed wave data and the uncertainties related to modelling the wave extremes
(Fig. ) underline the usability of the reliable long-term
wave buoy measurements presented in this paper.
ForecastingToini 2017
On 10–12 January a vast low pressure was situated over the Norwegian Sea,
while a deepening secondary low formed over southern Scandinavia (see
Fig. b). The secondary low moved northwards along the
east coast of Sweden. This weather pattern created circumstances where
southerly wind was in gale or strong gale force for approximately 20 h in the
entire Baltic Proper, while the variation in wind direction was
insignificant.
Forecasts for the significant wave height
Hs for storms Toini (a) and Rafael (b). The
notation “12 h” means that the forecast was available 12 h prior to the
observed maximum. The continuous black line is the values measured by the NBP
wave buoy.
Toini was forecasted quite well already 24 h before the observed
maximum (Fig. a). The biggest difference is that the
forecasts available 18 and 24 h prior to the storm predicted the
maximum significant wave height to take place at 02:00 UTC, while the
forecasts available 6 h before and during the storm predicted the
maximum at 23:00 UTC and 22:00 UTC respectively. The observed maximum
occurred at 22:30 UTC. The storm duration was also predicted more correctly
closer to the storm, with a 9 h duration 24 h before the
storm compared to a 13 h duration 6 h prior the the event.
The maximum significant wave height was nevertheless underestimated in all
forecasts. The model bias for the 6 m exceedance time ranged from
-0.5 to -0.8 m in the different forecasts.
The predicted mean inverse significant steepness for the 6 m
exceedance time was 29 for all the lead times, providing an accurate
description of the steepness conditions. The peak period was predicted
correctly in the sense that it reached its maximum value during the storm
period, just as observed. The values of the peak period were underestimated
by roughly 1 s (not shown). The modelled peak period did not exceed
12 s anywhere in the Baltic Proper.
In the forecast available 24 h prior to the storm the highest
significant wave height was 7.0 m slightly southwest of the wave
buoy at 22:00 UTC. In the forecast available during the storm the maximum
(7.4 m) was located west of the wave buoy. The most extreme wave
events have, up until now, been modelled to take place in the eastern part of
the Baltic Proper . The exceptional wave conditions in the
western part of the Baltic Proper during Toini were also captured by the
Gotland wave buoy, which measured its highest significant wave height to
date (5.6 m).
Rafael 2004
On 21–22 December 2004 two low-pressure centres over the Arctic Ocean and
the North Atlantic joined together to form a strong low-pressure system with
a single centre northwest of Norway. The south to southwest wind speed
increased to storm force in the northern Baltic Proper. Compared to Toini the
duration of strong gale winds was much shorter in Rafael, lasting roughly
8 h. The wind direction was also more southwesterly compared to the more
southerly wind during Toini.
The forecast of Rafael underpredicted the significant wave height by up to
0.9 m and the peak period by about 2 s in all forecasts
available less than 24 h before the storm. The model bias for the
6 m exceedance time ranged from -0.7 to -1.1 m in the
different forecasts. As for Toini, the predicted time of the extreme values
differs between the forecasts (Fig. b). The maximum
significant wave height was predicted correctly at 21:00 UTC in the
forecasts available less than 12 h prior to the event. The observed
maximum occurred at 20:00 UTC.
The length of the storm in the forecasts was between 3 and 6 h, which
is shorter than the observed 9 h duration. We can conclude that the
duration was underestimated in the forecast (Fig. b), even
though the coarse 3 h time resolution of the model output makes it
challenging to quantify the exact duration. The forecasted steepness values
between 24 and 27 were in good accord with the observed value of 27, which
was also exactly the value for the forecast available 24 h before the
storm. The maximum modelled significant wave height for the entire Baltic
Proper basin was 7.5 m in the latest forecast.
Warning of extreme waves
Warnings for severe and extreme wave conditions were launched at FMI in 2011.
The wave warnings are issued for ships and boats together with other
meteorological warnings regularly seven times a day, or as needed in the case of an
unexpected situation. The thresholds for the warnings are 2.5, 4 and
7 m in significant wave height. The first 2.5 m limit is
important for smaller boats, especially during the leisure boating season,
while the 4 m limit represents wave conditions that might impact even
larger vessels. The 7 m significant wave height is considered to be
potentially dangerous for all ships.
Since 2011, Toini is the first storm in the Baltic Sea when the significant
wave height has exceeded 7 m. However, for Toini the warning was
given only for severe wave conditions, with a more specific estimate of
6–7 m for the northern Baltic Sea. Although an extreme wave warning
was considered, the wave forecast 24 h before the storm predicted the
highest significant wave height to be 6.95 m. The expert estimate for
the significant wave height based on an analysis of meteorological and
oceanographic forecasts and statistics was 6.9 m. The warning was
updated to extreme wave conditions during the storm as the observed
significant wave height exceeded 7 m.
The accuracy of the wave forecasts is constantly evaluated against the wave
buoy measurements. The verification results show that FMI's wave forecast
system has good accuracy at the NBP buoy location with a slight tendency to
underestimate the largest values of significant wave height. However, both
the NWP systems and wave forecast models are regularly upgraded, resulting in
new combinations of, for example, spatial and temporal resolutions, physics, and
parametrisations . Although the less extreme
values are known to be modelled well , it is
challenging to obtain a comprehensive understanding of how the operational
models behave in extreme circumstances, since significant wave heights of
over 7 m occur very rarely.
A discussion concerning the issuing of wave warnings for the Baltic Sea
should be initiated between the relevant institutes and end users. In
addition to re-establishing and harmonising the thresholds of significant
wave heights, the use of other parameters (e.g. duration) should also be
explored in light of the difficulties of predicting a single maximum value
for the wave height. Any decision to include new parameters should be based
on the needs of the seafarers. On a more general note, the use of ensemble
forecasts might prove useful when issuing wave warnings. An in-depth study is
nevertheless needed in order to quantify to which extent the added
information warrants the increased computational cost.
Summary
We analysed the five wave events in the northern Baltic Proper that have
exceeded a significant wave height of 7 m during 1996–2017. In addition to
the maximum wave height we calculated the duration (Hs>6m) and the mean inverse wave steepness for the storm. On the basis
of our analysis we classify the extreme wave events into two groups. One
category is characterised by a long duration (>10 h) and a high
mean inverse significant steepness (>28). The other group consists of
shorter and steeper storm events (see Table ).
The two storms with the highest significant wave heights (8.2 m in
2004 and 8.0 m in 2017) exhibited different characteristics. Toini in
2017 had the longest duration (15.5 h) to date, but it is also the
first storm where the wave model places the most extreme wave conditions in
the western part of the northern Baltic Proper. Rafael in 2004 remains the
most extreme event if classified solely based on the maximum observed
significant wave height.
The duration and steepness characteristics of Toini were fairly well resolved
by the wave forecasts. These metrics may therefore provide an additional tool
to aid in deciding when to issue warnings for extreme wave conditions in the
future.
The measurement time series for all storm events and the
time series for the forecasts from 2004 and 2017 are available as the
Supplement to this article.
The Supplement related to this article is available online at https://doi.org/10.5194/nhess-17-1653-2017-supplement.
The paper was initiated by JVB and LT.
The wave measurements were analysed by JVB, HP and HJ, while the wave model
data were reviewed by JVB, RM, AK and LT. NT, AK and CF were responsible for
the analysis of the meteorological conditions. The manuscript was prepared by
JVB and LT with contributions from all co-authors.
The authors declare that they have no conflict of
interest.
Acknowledgements
We would like to thank the reviewers Alvaro Semedo and David Moncoulon for
their comments and suggestions, which enabled us to improve our article. This
work was partly funded by the SmartSea project of the Strategic Research
Council of the Academy of Finland, grant no. 292 985. We gratefully
acknowledge the valuable comments provided by Havu Pellikka.
Edited by: Rosa Lasaponara Reviewed by: Alvaro Semedo and David
Moncoulon
ReferencesDatawell Waverider Reference Manual, Datawell BV, Voltastraat 3, 1704 RP
Heerhugowaard, The Netherlands,
http://www.datawell.nl/Portals/0/Documents/Manuals/datawell_manual_dwr-mk3_dwr-g_wr-sg_2017-01-01.pdf
(last access: 2 March 2017), 2017.Eerola, K.: Twenty-One 5 Years of Verification from the HIRLAM NWP System,
Weather Forecast., 28, 270–285, 10.1175/WAFD-12-00068.1, 2013.HIRLAM-B: System documentation, http://hirlam.org/ (last access: 22
September 2017), 2017.
Joint Accident Investigation Commission of Estonia, F. and Sweden: Final
Report on the Capsizing on 28 September 1994 in the Baltic Sea of the Ro-ro
Passenger Vessel MV Estonia, Edita Limited, 1997.Jönsson, A., Broman, B., and Rahm, L.: Variations in the Baltic Sea wave
fields, Ocean Eng., 30, 107–126, 10.1016/S0029-8018(01)00103-2, 2003.
Komen, G. J., Cavaleri, L., Donelan, M., Hasselmann, K., Hasselmann, S., and
Janssen, P. A. E. M.: Dynamics and modelling of ocean waves, Cambridge
University Press, Cambridge, 1994.Räämet, A. and Soomere, T.: The wave climate and its seasonal
variability in the northeastern Baltic Sea, Estonian J. Earth Sci., 59,
100–113, 10.3176/earth.2010.1.08, 2010.Soomere, T., Behrens, A., Tuomi, L., and Nielsen, J. W.: Wave conditions in
the Baltic Proper and in the Gulf of Finland during windstorm Gudrun, Nat.
Hazards Earth Syst. Sci., 8, 37-46, 10.5194/nhess-8-37-2008,
2008.Tuomi, L.: The accuracy of FIMR wave forecasts in 2002–2005, MERI – Report
Series of the Finnish Institute of Marine Research, 63, 7–17, available at:
https://helda.helsinki.fi/handle/10138/157960 (last access: 22
September 2017), 2008.Tuomi, L., Kahma, K. K., and Pettersson, H.: Wave hindcast statistics in the
seasonally ice-covered Baltic Sea, Boreal Environ. Res., 16, 451–472,
available at: http://www.borenv.net/BER/pdfs/ber16/ber16-451.pdf, (last
access: 22 September 2017), 2011.WAMDIG: The WAM model – A third generation ocean wave prediction model, J.
Phys. Oceanogr., 18, 1775–1810, 10.1175/1520-0485(1988)018<1775:TWMTGO>2.0.CO;2, 1988.