Matti Leppäranta

Physics of seasonally ice-covered lakes: a review

Recently, the attention to the ice season in lakes has been growing remarkably amongst limnological communities, in particular, due to interest in the response of mid- and high-latitude lakes to global warming. We review the present advances in understanding the governing physical processes in seasonally ice-covered lakes. Emphasis is placed on the general description of the main physical mechanisms that distinguish the ice-covered season from open water conditions. Physical properties of both ice cover and ice-covered water column are considered. For the former, growth and decay of the seasonal ice, its structure, mechanical and optical properties are discussed. The latter subject deals with circulation and mixing under ice. The relative contribution of the two major circulation drivers, namely heat release from sediment and solar heating, is used for classifying the typical circulation and mixing patterns under ice. In order to provide a physical basis for lake ice phenology, the heat transfer processes related to formation and melting of the seasonal ice cover are discussed in a separate section. Since the ice-covered period in lakes remains poorly investigated to date, this review aims at elaborating an effective strategy for future research based on modern field and modeling methods.

The life story of a first-year sea ice ridge

Abstract Field data are described and analyzed from all-winter monitoring of the structure and temperature of one sea ice ridge in the northern Baltic Sea in the winter of 1991. The ridge aged to 3.5 months and experienced substantial structural evolution: the consolidated layer grew to 1 m, average porosity decreased from 0.28 to 0.18, keel thickness decreased by 1 m, and the ridge geometry became smoother. The porosity decreased due to freezing and showed a persistent minimum of 0.20–0.23 in the midkeel region; the void distribution changed due to packing rearrangements of ice blocks. Ice volume changed due to thermodynamic growth and decay. Within the sail and consolidated layer the heat flow was mainly vertical varying with time according to the surface forcing; the corresponding total ice production estimated from the temperature data would be 0.14 m, a bit more than the measured ice production 0.10 m. Predictions of the consolidated layer growth, based on (a) ice surface temperature or (b) air temperature or (c) local undeformed ice growth, gave good results. In spring the ice blocks throughout the keel beneath the consolidated layer melted uniformly.

On applying granular flow theory to a deforming broken ice field

SummaryThe prediction of arctic ocean ice dynamics relies on a correct modelling of the stresses acting on the ice field, including the Coriolis effect, wind and current stresses and the ice interaction. Observations made in the past decade show significant ice interaction and flow patterns which can be consistently modeled with a plastic rheology. The main local physical process considered in these rheologies is the pressure ridging phenomenon. However recent arctic field work carried out in the marginal ice zone (less than 100 km from the edge of the ice field) shows the ice very near the edge to consist of a large number of discrete floes. While a plastic rheology may well have application under compact conditions in this region, under dispersed conditions the rheology may be different. In order to address this issue, in this work the ice floe collisions induced by ice deformation are analyzed. The internal kinematics as represented by the ice floe fluctuations is derived. Comparisons between the results and field data show excellent correlation. However, the theoretically predicted floe fluctuations are about one order of magnitude lower than the field measurements. Possibilities for this discrepancy are proposed and discussed.

Comparisons of sea-ice velocity fields from ERS-1 SAR and a dynamic model

Numerical models for sea-ice thickness distribution and velocity are used for ice-dynamics research and ice forecasting. In the modeling work, ERS-1 SAR is an excellent tool, in particular by providing spatial ice-velocity fields as described in the present Baltic Sea study. Ice velocities were extracted from SAR data with 3 and 6 day time intervals using the optical-flow method. A considerable stiffening of the ice pack was observed due to the change in the character of ice deformation under compression from rafting to ridging as the minimum ice thickness increased from 10 to 3O cm. The coastal alignment was strong in the ice motion and the coastal boundary layer width was 20-30 km. An analysis of the SAR data with an ice-dynamics model showed that the observed overall ice-velocity field could be produred using the Hibler viscous-plastic ice rheology. The compressive strength of the ice (over 10 km sc ales) was 2.5x 10 4 N m −2 ±50% for ridging and negligible for rafting of very thin ice. The shear strength was significant and the normal yield ellipse aspect ratio of 2 was valid. The 3 day time interval is val id for updating an ice model but for detailed ice-dynamics investigations a data frequency of 1 d −1 or higher would be preferable.

An airborne electromagnetic system on a fixed wing aircraft for sea ice thickness mapping

Abstract An experimental campaign has been performed in 1991–1994 in the Baltic Sea, a brackish water basin, to test the ability of an airborne electromagnetic (AEM) system, mounted to a fixed wing aircraft (Twin Otter), to map sea ice thickness. Measurements are made with a vertical coplanar configuration of coils mounted on wing tips. The EM frequency is 3.1 kHz. A laser profilometer was integrated into the system for high frequency surface topography mapping. Test profiles were performed along 1–2 km long lines calibrated for ground truth. In good mapping conditions the thickness accuracy is ± 0.2 m for undeformed ice but worse for deformed ice with variable geometry. The raw horizontal resolution is 100 m. Analytical half-space EM models described well the overall ice thickness distribution. A 3D model was used for extraction of more detailed ice geometry. For unconsolidated deformed ice a nonzero electric conductivity must be assumed. The penetration length, which depends on the ice conductivity distribution, gives up to 70 m for fully resistive ice. The system is feasible for polar seas since the conductivity contrast between sea ice and sea water is much higher there than in the Baltic. The Twin Otter offers a wide operation area because the measurement ground speed is 90–120 knots and maximum flight duration is 6–8 hours.

Numerical modelling of snow and ice thicknesses in Lake Vanajavesi, Finland

Abstract Snow and ice thermodynamics was simulated applying a one-dimensional model for an individual ice season 2008–2009 and for the climatological normal period 1971–2000. Meteorological data were used as the model input. The novel model features were advanced treatment of superimposed ice and turbulent heat fluxes, coupling of snow and ice layers and snow modelled from precipitation. The simulated snow, snow–ice and ice thickness showed good agreement with observations for 2008–2009. Modelled ice climatology was also reasonable, with 0.5 cm d−1 growth in December–March and 2 cm d−1 melting in April. Tuned heat flux from water to ice was 0.5 W m−2. The diurnal weather cycle gave significant impact on ice thickness in spring. Ice climatology was highly sensitive to snow conditions. Surface temperature showed strong dependency on thickness of thin ice (<0.5 m), supporting the feasibility of thermal remote sensing and showing the importance of lake ice in numerical weather prediction. The lake ice season responded strongly to air temperature: a level increase by 1 or 5°C decreased the mean length of the ice season by 13 or 78 d (from 152 d) and the thickness of ice by 6 or 22 cm (from 50 cm), respectively.

Effects of water clarity on lake stratification and lake‐atmosphere heat exchange

Recent progress of including lake subroutines in numerical weather prediction (NWP) models has led to more accurate forecasts. In lake models, one essential parameter is water clarity, parameterized via the light extinction coefficient, Kd, for which a global constant value is usually used. We used direct eddy covariance fluxes and basic meteorological measurements coupled with lake water temperature and clarity measurements from a boreal lake to estimate the performance of two lake models, LAKE and FLake. These models represent two 1D modeling frameworks broadly used in NWP. The results show that the lake models are very sensitive to changes in Kd when it is lower than 0.5 m−1. The progress of thermal stratification depended strongly on Kd. In dark water simulations the mixed layer was shallower, longwave and turbulent heat losses higher and therefore the average water column temperatures lower than in clear water simulations. Thus, changes in water clarity can also affect the onset of ice cover. The more complex LAKE modeled the seasonal thermocline deepening whereas it remained virtually constant during summer in the FLake model. Both models overestimated the surface water temperatures by about 1°C and latent heat flux by >30%, but the variation in heat storage and sensible heat flux were adequately simulated. Our results suggest that, at least for humic lakes, a lake-specific, but not time-depending, constant value for Kd can be used and that a global mapping of Kd would be most beneficial in regions with relatively clear lakes, e.g. in lakes at high altitudes.

Transmission of solar radiation through the snow cover on floating ice

Spectral measurements of solar radiation in the band 400–900nm were performed above and inside the snowpack in two locations in Finland, using a spectroradiometer. The transmittance and extinction coefficient were estimated for different snow layers. Four small candle-shaped photosynthetically active radiation (PAR) sensors were also used to measure the transfer of PAR inside the snowpack. In addition to the light measurements, physical characterization of snow stratigraphy was done, including the thickness, density, hardness (hand test), salinity, and grain size and shape (photographs of crystals). The transmittance varied from <1% (0–12 cm layer) to 80% (0–4 cm layer), and the extinction coefficient was between 0.03 cm (4–8 cm layer) and 0.8 cm (0–4 cm layer). The physical properties of the snow varied considerably between locations and days. The density of the snow varied between 140 and 480 kgm.

Physical properties of the seasonal snow cover in Dronning Maud Land, East Antarctica

Abstract Snow stratigraphy was analyzed in the Maudheimvidda area of western Dronning Maud Land, East Antarctica, during austral summer 1999/2000 as a part of the Finnish Antarctic Research Programme (FINNARP). Measurements were made in shallow (1–2m) snow pits along a 350 km transect from the coast to the polar plateau, covering at least one annual cycle and an elevation range from sea level to about 2500 m. The aim of the study is to document spatial and temporal variations in snow-cover properties, with the further aim of relating these variations to environmental factors and to patterns observable by remote sensing. The measurements suggest five principal snow zones: (i) sea ice, (ii) the seaward edge zone of the ice shelf, (iii) the inner parts of the ice shelf, (iv) the snow cover above the grounding line and (v) the local topographic highs. Local topographic highs such as ice domes and ice rises differ from other snow environments: the snow is less densely packed, possibly an indication of locally reduced speed of the katabatic outflow. Fewer and thinner crusts on the topographic highs are consistent with RADARSAT backscatter variations.

The ice cover on small and large lakes: scaling analysis and mathematical modelling

Lake ice cover is described by its thickness, temperature, stratigraphy and overlying snow layer. When the ratio of ice thickness to lake size is above ∼10−5, the ice cover is stable; otherwise, mechanical forcing breaks the ice cover, and ice drifting takes place with lead-opening and ridging. This transition enables a convenient distinction to be made between small and large lakes. The evolution of the ice cover on small lakes is solved by a wholly thermodynamic model, but a coupled mechanical–thermodynamic model is needed for large lakes. The latter indicates a wide distribution of ice thickness, and frazil ice may be formed in openings. Ecological conditions in large lakes differ markedly from those in small lakes because vertical mixing and oxygen renewal may take place during the ice season, and the euphotic zone penetrates well into the water column in thin ice regions. Mesoscale sea ice models are applicable to large lakes with only minor tuning of the key parameters. These model systems are presented and analysed using Lake Peipsi as an example. As the climate changes, the transition size between small and large lake ice cover will change.