Fourth Study Conference on BALTEX Scala Cinema Gudhjem

3. Methods
Circulation indices and circulation weather types are
determined for every storm day in the catalogue of storms at
Vilsandi. In the first stage, weather types corresponding to
storm days are summed up by months and analyzed. The
stormiest types are detected. The circulation indices related
to storms are analyzed. Mean circulation indices for storms
of different wind direction are calculated.
Special attention is paid to extremely stormy periods that
may last even several months. These are determined
following fixed criteria as listed in Orviku et al. (2003). It is
assumed that the most severe coastal damages occur during
these periods.
Storms measured in Vilsandi are classified using principal
component analysis. Characteristics of storms as well as the
circulation indices are involved into the classification
procedure.
Relying on the knowledge of relationship between
circulation and storms, possible severe storms in the past are
reconstructed. Extreme circulation indices refer to storm
days. A comparative analysis of wind speed data at Vilsandi
and circulation indices in 1881-1940 allow verifying the
relationships detected.
4. Preliminary results and discussion
Preliminary results of coupled analysis of storm and
circulation data demonstrate a significant relationship.
Total number of stormy days during 50 years (1948-1997) in
Vilsandi was 1180, resulting in 23.6 as annual mean value.
The highest storminess was observed in 1992 (36 days). The
largest part of storms has occurred during two circulation
weather types: the cyclonic (C) – 263 days, and the northern
(N) – 236 days. The monthly distribution of these storms is
presented in Figure 2.
days
50
45
40
35
30
25
20
15
10
5
0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Figure 2. Monthly numbers of storm days for circulation
weather types C and N (total for 1948-1997).
Storms during cyclonic circulation are typical for the warm
period when the northern type has brought no storms at all.
But the major part of storms for both these weather types has
been observed during the storm season, i.e. between
September and March. Thereby, a very high occurrence of
storms with northerly flow has taken place in October,
November and March.
More than 100 storms have been observed also with
southwestern (SW) and western (W) circulation types, 164
and 115 days, respectively. The contribution of the other
types is much lower. We also note that the total frequency of
the hybrid circulation types is many times lower than of the
main types. Wind obstacles located east of the observation
- 128 -
N
C
site can explain very low occurrence of storms with
easterly circulation types.
Circulation indices are rather clearly related to storms of
the same direction. Thereby, the influence of wind shear is
evident. For example, high positive values of the zonal
circulation index are followed by southwesterly storms
and high negative values of the meridional circulation
index correspond to northwesterly storms.
It should also be mentioned that not all severe storms
concur with extremes of the circulation indices. Many of
them happened when a deep low is centered in the study
area. In that case, the circulation indices cannot describe
atmospheric circulation adequately. Therefore, sea level
pressure data are involved into principal component
analysis for classification of storms.
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