Atmosphere 2022, 13, 1634 
50f 21 
and Hydrographic Agency (BSH), the automatic classification method developed by Jenk- 
inson and Collison [41] to objectify the “Lamb Weather Types” (hereafter LW'Ts) is used 
operationally and as part of the analysis of the state of the North Sea region (e.g., [42]). 
This classification method is also used for this study. It uses daily means of the SLP fields 
at 16 grid points in the extended North Sea region (see Figure la) to calculate the vector 
components of the geostrophic wind—the wind that blows parallel to the isobars—and 
the vorticity. The geostrophic wind is a good approximation of the large-scale wind con- 
ditions in the free atmosphere. The LWTs are derived from the relationships between 
the geostrophic wind components and the vorticity. While the classification procedure 
originally allowed for 27 different weather types, a classification procedure reduced to 
6 characteristic weather types is used here to assure more reliable and robust statistics by 
reclassifying the hybrid weather types which appear only 5—6 times a year, and by evenly 
distributing the rather undistinctive types into rotational and directional types [42,43] 
These six characteristic weather types are anticyclonic (A), cyclonic (C), north-east (NE), 
north-west (NW), south-east (SE), south-west (SW). 
(a) Sn 
60° NM 
1 
Denmark 
50° A: 
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In 
b) 
1. chen 
_Elbe 
; „Drachten SS En 
Netherlands 
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denbury 
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LUNC DU) 
wine: Germany 
Anker 
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33 
75911.5 15 18 
78 30 
„Km 
10 k.. 
Figure 1. (a) Grid points (red dots) used to perform the LWT classification based on SLP data and 
domain of hydrodynamic modelling (blue rectangle); (b) Domain of the hydrodynamic-numerical 
model with depth information (m above mean sea level); the Elbe estuary is highlighted as red 
rectangle and the stations Cuxhaven and St. Pauli are marked with an orange and magenta star, 
respectively. The map in the background is provided by [44]. 
2.2.3. Gale Strength 
Another variable that is relevant for ELWs is the gale strength. A gale index G*, which 
ıSs representative for the North Sea region as a whole, is calculated, based on an elliptical 
relationship between a wind index and a vorticity index [details of the calculation available