How thick can Baltic sea ice get? Mikko Lensu Finnish Meteorological Institute

In the Baltic ice research is closely related to winter navigation there are about 25 000 port calls to Finnish ports during the ice season. Two main obstacles of ship progress are ice compression and deformed ice (ridges) FMI seeks to provide reliable information and forecasts on compression and deformed ice thickness.

Baltic level ice thicknesses outside fast ice zone typically 20-60 cm, maximally 1 m. Larger thicknesses are due to ice deformation Ridging is the most prominent deformation process. Baltic ridges typically 1-2 m high and 5-10 m deep Largest Baltic ridges 4-5 high and 20 m deep So one possible answer to the question is 25 m sail keel

Continued deformation creates local rubble fields or ridge clusters Another possible answer: The average thickness over 200 meters can be more than 12 meters 2600 meters of 0.6 m thick level ice compressed into 200 m wide formation

Deformation processes followed in detail using coastal radar imagery grabbed by a radar server Stores 1000 images per hour 30 images/hour sent to FMI for real time viewing and display Similar to SAR data: can be used to SAR retrieval algorithm development Radar image

Tankar ice observatory Radar image area IPS Stress buoy Radar Drifters

Animation of ice deformation event during 8-9 February 2012 20 km Fast ice Ridges and old ship channels criscrossing the moving pack ice RADAR Ships Archipelago

Ice thickness and ice thickness distribution are scale dependent concepts 1 meter scale resolves ridge keel maximum depth and ridge keel shape variation 100 meter scale averages over keel shapes 1 km scale averages over most ridge clusters The scale of Downscaling surface profiling is 1 meter airborne EM thickness mapping is 30 meters ship-ice interaction is 100 meters CryoSat altimetry is 300 meters Baltic ice models is 1 km Arctic ice models is 10 km Upscaling

27 ridge sails within 250 meters Surface laser profile EM thickness sounding Data averaged over 100 m

Ice chart 6 March 2011 Level ice thickness There is no deformed ice thickness or average ice thickness Ridging indicated symbolically or using three degrees

SAR-based thickness product of FMI 6 March2011 Extrapolation of observed level ice thickness using SAR signatures

FMI ice forecast model HELMI 6 March 2011 Multicategory thickness description, includes ridged ice Resolution 1 NM

HELMI ridged ice fields 6 March 2011 Ridged ice thickness up to 5 meters Ridged ice concentration up to 35 % Ridged ice volume fraction up to 80 %

Helicopterborne Electromagnetic (HEM) thickness mapping campaing 27 March 2011 Almost 3000 km of profile

Another possible answer: The average thickness can be more than 2 meters in a scale of 20 kilometers

Comparison of EM data (scale 30 m) with bottom mounted sonar data (scale 1 m) EM averages keel shapes and slightly underestimates thickness

100 m thickness averages 100 m thickness averages 5000 8 4500 All HEM data, averaged to 100 m 3500 6 log(instances) Instances from 27989 4000 Semilog plot: exponential slope 7 3000 2500 2000 1500 5 4 3 2 1000 1 500 0 0 0 1 2 3 4 Thickness [m] 5 6 0 1 Mean 0.95 m, mode 0.45 The fraction for 100-m segments thicker than 2 m = 8 %, thicker than 4 m = 1% 2 3 4 Thickness [m] 5

600 7 6 All HEM data, averaged to 1000 m 500 400 300 200 Semilog plot: exponential slope 5 log(instances) Instances from 2765 1000 m thickness averages 1000 m thickness averages 4 3 2 100 1 0-1 0 1 2 3 Thickness [m] 4 5 0-1 0 Mean 0.95 m, mode 0.6 m The fraction of 1-km segments thicker than 2 m is 6%. For 1-NM segments = 5% 1 2 3 Thickness [m] 4 5

1x1 NM HELMI model thickness 4000 HELMI modeled thickness distribution Instances from 16173 3500 3000 Scale 1 NM 2500 Underestimates thicknesses and the observed very thick values do not appear 2000 1500 1000 Possible reason for underestimation: snow parameterisation 500 0-0.5 0 0.5 1 1.5 Thickness [m] Mean 0.73 m, mode 0.65 2 2.5

Comparison of thickness fields HEM campaign, all data HELMI model 6 March 2011

Comparison of thickness fields, Tankar intensive area Very thick ice is found in coastal ridge fields which are not well captured by the model HEM campaign, all data HELMI model 6 March 2011

So how thick can Baltic sea ice get? It depends on scale. But it is much thicker than we have been thinking.

64.4 64.3 64.2 64.1 64 63.9 63.8 63.7 63.6 63.5 21.5 22 22.5 23 23.5