Jouko Launiainen

Derivation of the relationship between the Obukhov stability parameter and the bulk Richardson number for flux-profile studies

Using the relationship between the bulk Richardson numberRz and the Obukhov stability parameterz/L (L is the Obukhov length), formally obtained from the flux-profile relationships, methods to estimatez/L are discussed. Generally,z/L can not be uniquely solved analytically from flux-profile relationships, and it may be defined using routine observations only by iteration. In this paper, relationships ofz/L in terms ofRz obtained semianalytically were corrected for variable aerodynamic roughnessz0 and for aerodynamic-to-temperature roughness ratiosz0/zT, using the flux-profile iteration procedure. Assuming the so-called log-linear profiles to be valid for the nearneutral and moderately stable region (z/L<1), a simple relationship is obtained. For the extension to strong stability, a simple series expansion, based on utilisation of specified universal functions, is derived.For the unstable region, a simple form based on utilisation of the Businger-Dyer type universal functions, is derived. The formulae yield good estimates for surfaces having an aerodynamic roughness of 10−5 to 10−1 m, and an aerodynamic-to-temperature roughness ratio ofz0/zT=0.5 to 7.3. When applied to the universal functions, the formulae yield transfer coefficients and fluxes which are almost identical with those from the iteration procedure.

Modelling of ice thermodynamics in natural water bodies

Abstract A thermodynamic synoptic-scale sea-ice (freshwater lake) model was constructed for simulating ice thickness, ice temperature and air–ice interaction. The model includes calculation of the air–ice interface temperature and surface fluxes, as well as heat conduction in the snow and ice and heat flux and ice thickness variations at the ice–ocean boundary. Attention was given to the parameterization of the various fluxes, especially the radiative fluxes and in the calculation of the turbulent surface heat fluxes the effect of atmospheric stratification is taken into account. An iterative air–ice interface temperature serves as the key parameter controlling the surface heat balance, heat conduction and ice thickness variation at the upper surface. A conservative integral difference scheme is used to solve the heat conduction equation for an ice column consisting of 10 to 30 layers, normally in the vertical. In addition to thickness variations, the model yields the time development of the ice surface and in-ice temperatures and air–ice fluxes. The model is also able to generate the near-surface atmospheric profiles of wind, temperature and moisture. Model-calculated ice thickness and in-ice temperature variations were compared with observational data from the Bohai Sea and the Baltic Sea as a first step in verifying the model. Additionally, phenomenological process studies of subsurface melting and of the role of new snow above ice in spring is reported. The results indicate the atmospheric boundary layer (ABL) dynamically coupled with the ice and heat conduction in the ice as factors responsible for controlling ice growth, except under conditions of remarkable heat fluxes from the ocean. In the spring, the role played by shortwave radiation in the surface heat balance and extinction in ice are of primary importance and control the melting. After validation, the model should be applicable and suitable for ice thermodynamics studies, for coupled air–ice–ocean models and even for climatic scenarios.

Hydrographic observations in Denmark Strait in fall 1997, and their implications for the entrainment into the overflow plume

Denmark Strait is the most important exit for water masses formed in the Arctic Mediterranean Sea and supplies a substantial fraction of the North Atlantic Deep Water. Observations obtained from RV Aranda in August-September 1997 indicate that the water crossing the 620m deep sill is mainly drawn from the intermediate waters of the East Greenland Current. The overflow plume was stratified and capped by a less saline layer as it descended beyond 2000m. The presence of a low salinity lid implies that entrainment of ambient water is small and that the downstream evolution of the plume characteristics is due to mixing, within the plume, between the initial overflow waters. Low salinity, but dense, water from the East Greenland Current flowing over the shelf may cross the shelf break south of the sill and add a less dense fraction to the overflow.

Surface heat budget over the Weddell Sea: Buoy results and model comparisons

[1]xa0The surface heat budget over the Weddell Sea ice cover in 1996 was studied on the basis of data from Argos buoys equipped with meteorological sensors. In addition, a thermodynamic sea ice model, satellite-based data on the sea ice concentration, sonar results on ice thickness distribution, and output from large-scale meteorological models were all utilized. Applying the buoy data, the sensible heat flux over sea ice was calculated by Monin-Obukhov theory using the gradient method, and the latent heat flux was obtained by the bulk method. A second estimate for the surface fluxes was obtained from the thermodynamic sea ice model, which was forced by the buoy observations. The results showed a reasonable agreement. The dominating component in the heat budget over ice was the net longwave radiation, which had a mean annual cooling effect of −28 W m−2. This was balanced by the net shortwave radiation (annual mean 13 W m−2), the sensible (13 W m−2) and latent (−3 W m−2) heat fluxes, and the conductive heat flux through the ice (5 W m−2). The regional surface fluxes over the fractured ice cover were estimated using the buoy data and Special Sensor Microwave Imager (SSMI)-derived ice concentrations. In winter the regional surface sensible heat flux was sensitive to the ice concentration and thickness distribution. The estimate for the area-averaged formation rate of new ice in leads in winter varies from 0.05 to 0.21 m per month depending on the SSMI processing algorithm applied. Countergradient fluxes occurred 8–10% of the time. The buoy observations were compared with the operational analyses of the European Centre for Medium-Range Weather Forecasts (ECMWF) and the reanalyses of the National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR). The 2 m air temperature and surface temperature were 3.5° and 4.4°C too high, respectively, in the ECMWF and 3.2° and 3.0°C too low in the NCEP/NCAR fields, but the models reproduced the synoptic-scale temperature variations well. The errors seem to be related to the cloud cover and the surface boundary conditions. Neither of the models recognizes leads in the ice pack, and the ice and snow thicknesses are often far from reality. The distribution of the cloud cover in the both models differed a lot from observations.

Weddell Sea ice drift: Kinematics and wind forcing

Ice drift in the Weddell Sea was studied on the basis of positional and meteorological data from Argos buoys drifting in 1990–1992 and surface pressure analyses from the European Centre for Medium Range Weather Forecasts (ECMWF). The drift kinematics showed differences between the eastern and western parts of the Weddell Sea. Close to the Antarctic Peninsula, the ice drifted as an almost nonrotating uniform field at a low speed, having reduced small-scale motions with little meandering, compared to regions further to the east. Inertial motion was detected from the ice drift in areas east of 35°W and in the region of the Antarctic Circumpolar Current. On timescales of days, wind was the primary forcing factor for the drift. A linear model between the wind and ice drift explained 40–80% of the drift velocity variance. The degree of explanation was higher in the central Weddell Sea (around 40°W) and lower closer to the Antarctic Peninsula. The geostrophic wind was found to provide almost as good a basis for the general drift estimation as the surface wind observed by the buoys, although strong cyclones were not well detected by the ECMWF analyses. The data suggest a dependency upon atmospheric stability such that stable stratification reduces the wind forcing on the drift. For 60–80% of the time the direction of the drift deviated less than 45° from the geostrophic wind and for 45–70% of the time less than 45° from the ocean current. Ice transport through a transect crossing the Weddell Sea from the Antarctic Peninsula tip to Kapp Norwegia was estimated on the basis of the geostrophic winds, the drifts observed response to the wind, and literature-based information on ice concentration and thickness. The estimated annual mean net export in 1992–1994 varied from 8000 to 22,000 m3/s. Most of the net export took place in winter and spring, export prevailing west of 35°W and import east of it.

Response of the Weddell Sea pack ice to wind forcing

Sea ice drift in the Weddell Sea was studied on the basis of data from seven buoys deployed on ice floes in January–February 1996. Six of the buoys formed an array with initial mutual distances of 50 km, which by October 1996 were stretched to ∼400 km in an east–west direction. The differential kinematical parameters i.e., divergence, shearing rate, and vorticity, were estimated from the buoy array positions by applying a time-dependent method. The large-scale drift of the array was divergent until August 1996, but after the array came under the influence of the Antarctic Circumpolar Current the deformation was mainly shearing. Meteorological data from European Centre for Medium-Range Weather Forecasts analyses and Special Sensor Microwave Imager derived ice concentrations were interpolated at the buoy sites. The drift velocity was highly wind-dependent; it was also wind-dependent in winter when the momentum balance was mainly between the internal ice stress and the air drag. The drift divergence and shearing rate were related to the wind forcing, while the array vorticity correlated better with the air pressure itself. We estimated the air-ice drag coefficient to be 1.8×10−3 at a height of 10 m over the ice and the geostrophic drag coefficient to be 6.1 × 10−4. The stability effects on the coefficients were mostly felt in conditions of light winds. The average ice-water drag coefficient based on 5 day periods was 2.1×10−3. The ice transport through a transect crossing the Weddell Sea was computed on the basis of the geostrophic winds, observed wind-drift dependence, SSM/I-derived ice concentrations, and the literature on ice draft statistics. The resulting annual mean net export in 1996 was 1600 km3 yr−1.

Radiative and turbulent surface heat fluxes over sea ice in the western Weddell Sea in early summer

[1]xa0The radiative and turbulent heat fluxes between the snow-covered sea ice and the atmosphere were analyzed on the basis of observations during the Ice Station Polarstern (ISPOL) in the western Weddell Sea from 28 November 2004 to 2 January 2005. The net heat flux to the snowpack was 3 ± 2 W m−2 (mean ± standard deviation; defined positive toward snow), consisting of the net shortwave radiation (52 ± 8 W m−2), net longwave radiation (−29 ± 4 W m−2), latent heat flux (−14 ± 5 W m−2), and sensible heat flux (−6 ± 5 W m−2). The snowpack receives heat at daytime while releases heat every night. Snow thinning was due to approximately equal contributions of the increase of snow density, melt, and evaporation. The surface albedo only decreased from 0.9 to 0.8. During a case of cold air advection, the sensible heat flux was even below −50 W m−2. At night, the snow surface temperature was strongly controlled by the incoming longwave radiation. The diurnal cycle in the downward solar radiation drove diurnal cycles in 14 other variables. Comparisons against observations from the Arctic sea ice in summer indicated that at ISPOL the air was colder, surface albedo was higher, and a larger portion of the absorbed solar radiation was returned to the atmosphere via turbulent heat fluxes. The limited melt allowed larger diurnal cycles. Due to regional differences in atmospheric circulation and ice conditions, the ISPOL results cannot be fully generalized for the entire Antarctic sea ice zone.

Ice drift in the Weddell Sea in 1990–1991 as tracked by a satellite buoy

Ice drift in the Weddell Sea has been studied by using a satellite buoy deployed on an ice floe. The buoy survived for a 20-month period, indicating a drift trajectory of 10,000 km and yielding 13 months of marine meteorological data. The drift of the ice floe was studied with respect to the winds measured by the buoy. In the central Weddell Sea, the mean drift speed of the ice floe was 0.15 m/s, which was about 3% of the wind speed. More specifically, the drift ratio was 3.4% in the marginal ice zone and 2.4% in the inner pack ice field. On average, the drift was directed 36° left of the wind direction, but the turning angle was larger during the austral summer and smaller during the winter. On time scales of days the drift was primarily wind-dependent, except for cases during winter periods of high ice concentration and internal ice resistance. For time scales of several months, purely wind-based simulations of the drift resulted in a discrepancy between the observed and simulated trajectories, but the inclusion of a slow (0.02 m/s) residual current made the simulations significantly better. The geostrophic wind based on European Centre for Medium-Range Weather Forecasts pressure analyses was estimated for a 1-month period, and the ice floe was found to drift almost parallel to the geostrophic wind with a speed of 2% of the geostrophic wind speed. Inertial-type motion superimposed on the wind-induced drift was found to be a characteristic feature in the marginal ice zone during the austral summer, but it could not be found from the drift in winter when kinetic energy was transferred to larger scales of motion and dissipated into the ice field.

Temporal and spatial variability of surface fluxes over the ice edge zone in the northern Baltic Sea

Three land-fast ice stations (one of them was the Finnish research ice breaker Aranda) nand the German research aircraft Falcon were applied to measure the turbulent and radiation nfluxes over the ice edge zone in the northern Baltic Sea during the Baltic Air-Sea-Ice Study n(BASIS) field experiment from 16 February to 6 March 1998. The temporal and spatial nvariability of the surface fluxes is discussed. Synoptic weather systems passed the nexperimental area in a rapid sequence and dominated the conditions (wind speed, airsurface ntemperature difference, cloud field) for the variability of the turbulent and radiation nfluxes. At the ice stations, the largest upward sensible heat fluxes of about 100 Wm�2 were nmeasured during the passage of a cold front when the air cooled faster (�5 K per hour) than nthe surface. The largest downward flux of about �200 Wm�2 occurred during warm air nadvection when the air temperature reached +10�C but the surface temperature remained at n0�C. Spatial variability of fluxes was observed from the small scale (scale of ice floes and nopen water spots) to the mesoscale (width of the ice edge zone). The degree of spatial nvariability depends on the synoptic situation: during melting conditions downward heat nfluxes were the same over ice and open water, whereas during strong cold-air advection nupward heat fluxes differed by more than 100 Wm�2. A remarkable amount of grey ice nwith intermediate surface temperature was observed. The ice in the Baltic Sea cannot be ndescribed by one ice type only.

Turbulent surface fluxes and air–ice coupling in the Baltic Air–Sea–Ice Study (BASIS)

Abstract Turbulent surface fluxes were studied using observations taken over sea ice in the Baltic Sea in March 1998. The fluxes of momentum and sensible heat were measured by a sonic anemometer and compared with fluxes derived from wind velocity and air-temperature profiles. The neutral 10 m drag coefficient showed no apparent dependence on wind speed (in the range 2–20 m s–1), resulting in a mean value of 1.0 × 10–3 for smooth snow-covered ice and 1.5 × 10−3 for deformed ice. The overall mean value was 1.28 × 10–3. The roughness length for temperature revealed a greater apparent dependence on wind speed and was slightly larger than the aerodynamic roughness for low wind speeds, and vice versa for moderate and high winds. We give an empirical expression that predicts how the scalar roughness depends on the aerodynamic roughness (drag coefficient) and wind speed. Agreement of the gradient-method results with the eddy-flux results supports the validity of the Monin-Obukhov similarity theory. Fluxes modelled by a coupled air-ice-sea model compared well with the eddy-flux and gradient methods. Surface temperature estimates by the three methods also agreed well. Tests and sensitivity analysis emphasize the need for especially accurate sensor calibration and strict information about the sensor heights for the gradient method.