Aldenhoff and Holfort

out at the end of the season. The history of official ice observations
dates back to the winter of 1896/1897.
Remote sensing data from optical, microwave, and infrared sensors
are the prime information source for larger-scale information. Optical
:mages provide high-resolution information that is easy to assess for
aumans but suffer from cloud cover and require solar illumination.
Sources include Copernicus Sentinel-2 and Sentinel-3, MODIS from
Aqua and Terra and Landsat-8/9. Lower-resolution optical data is
obtained from MetOP satellites. SAR imagery provides high-resolution
information independent from weather and solar illumination.
Sentinel-1 is the main source for this kind of data, but other
providers are used if data is available to the ice service.
The operational ice products are discussed in more detail in the
zollowing subsections. The products target different customers and
herefore vary in detail and language. All products are available in
digital form from the official website (www.bsh.de/ice), free of charge.

2.1.1 Ice report (Eisbericht, Amtsblatt des
Bundesamtes für Seeschifffahrt
und Hydrographie)
The main product throughout the winter season in the Baltic
Sea and German Bight is the ice report (Eisbericht, Amtsblatt des
Bundesamtes für Seeschifffahrt und Hydrographie) (BSH, 1928).
The report informs in text form about the current ice conditions in
the Baltic Sea and the German Bight. It was first published in the
winter of 1928/1929. The report is divided into sections
corresponding to sea areas with ice occurrence. Each section
consists of a brief description of sea ice conditions and a short
outlook of the expected development of the ice conditions in the
next 24 h or 72 h on weekdays and Fridays, respectively.
Furthermore, the ice report contains information about current
and upcoming traffic restrictions as well as icebreaker activity
published by national ice services. The report is concluded by a
list of international ice observing stations and the respective
observations coded by the four digit Baltic Sea Ice Code. The
teport is issued on weekdays and is written in English to reach a
broader range of customers. Publication starts with the first
persistent ice in the Baltic Sea and ends when the last ice has
vanished or the last restrictions are retracted and only ice remnants
are present.

2.1.2 Ice charts
There are three types of ice charts regularly published by the
German ice service: an overview ice chart of the Baltic Sea, a
regional ice chart of the German Baltic Sea, and a regional ice
chart of the western Baltic and North Sea. Generally, each ice chart
consist of two pages: the first shows sea ice concentration and the
second sea ice stage of development. The ice charts follow the
WMO Ice Chart Colour Code Standard (WMO, 2004). Color codes
‚or rotten fast ice and hatching for different new ice concentrations
are added to better reflect the ice conditions in the Baltic Sea.
Furthermore, the ice charts contain contour lines of modeled sea
surface temperature and meteorological information from coastal
stations. All ice charts are, at present, manually drawn by an ice
analyst using the geographic information system software QGIS

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10.3389/fmars.2024.1339610

(QGIS.org, 2022). The ice analyst draws polygons of similar ice
types and assigns the ice concentration and ice thickness as the
main parameters. Furthermore, information about the melt status,
e.g., rotten ice, as well as point symbols for features like ridged ice
areas, leads too small to outline, or brash ice barriers can be added.
The decisions are mostly based on information from remote sensing
‘'magery as well as auxiliary information from ice observers and
other ice services. The ice charts are projected into WGS 84/World
Mercator (EPSG:3395). The standard output file format of the
published ice charts is PDF, but since the past several years, the
charts are also delivered in S-411 format for Electronic Chart
Display and Information Systems (ECDIS) (JCOMM, 2014). The
original polygonal data is stored as shapefiles as well as in a
geographical database for further use, processing, and analysis.

2.1.2.1 Overview ice chart of the Baltic Sea
The overview ice chart of the Baltic covers an area from
approximately 53.5° N, 9.5° E to 66° N, 30.2° E. The page with
the ice stage of development contains a table with actual restrictions
to navigation. Furthermore, active icebreakers are annotated in the
ice chart. Figure 2 shows an example of the ice chart from 2022-02-03.
 The overview ice chart is published weekly on the day with best
data coverage. The focus is to produce an ice chart with a high level
of detail and as accurate as possible at the time of data acquisition.

2.1.2.2 Regional ice charts of the German Baltic Sea and
the western Baltic Sea and North Sea
Regional ice charts of the German Baltic Sea and the western
Baltic Sea and North Sea are produced daily in case of ice
occurrence as operational ice charts for the German coastal
waters. Publication time is around noon, local time. The ice chart
of the German Baltic Sea covers an area from approximately 53.5°
N, 8° E and 55.8°, N 15°E. Figure 3 shows an example from 2021-02-17.
 The ice chart for the western Baltic Sea and North Sea covers an
area from approximately 53.5° N, 7° E to 60° N, 15° E. Figure 4
presents an example from 2021-02-17.

2.1.3 German ice report and NAVTEX messages
With ice occurrence along the German coasts, mariners are
informed daily about sea ice conditions via the GTS and the
NAVTEX service. The German ice report with information about
the ice conditions in German coastal waters is transmitted via GTS.
[t contains the Baltic Sea Ice Code of the German fairway stations
and a brief description of the ice situation in areas of interest for
navigation in English. NAVTEX messages are very brief summaries
of the ice conditions in German waters. They are published
separately for the German Bight and the Baltic Sea in German
and English before 10:30 a.m.

2.1.4 North Sea report (Nordseebericht) and
Baltic Sea report (Ostseebericht)
North Sea report (Nordseebericht) and Baltic Sea report
(Ostseebericht) are two publications that describe the ice situation
along the German coasts. The reports are issued Monday to Friday
when ice occurs at the respective coasts and are written in German. They

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