bluuebunny/crossref_metadata_2025 huggingface dataset:
Geoid and topography over subduction zones: The effect of phase transformations
Authors: King Scott D.
Published: October 2002

The association between local maxima in the geoid and subduction zones is examined. While it is well known that subduction zones are associated with broad local maxima in the geoid at spherical harmonic degrees 4–9, there is an impressive correlation between back arc geoid maxima at the 5000 km length scale (i.e., spherical harmonic degrees >9) and subduction zones with the exception of the Scotia and Caribbean arcs. Geoid maxima are also observed in the forearc regions of the Aleutian and Central American subduction zones. Numerical modeling of compressible convection with multiple phase transformations indicates that an increase in viscosity at or near the 660 km phase transformation is necessary to explain the pattern of geoid maxima associated with subduction zones. The addition of an endothermic phase transformation at 660 km depth does not provide sufficient resistance to the slab to eliminate the need for a viscosity increase at or near 660 km. Short‐wavelength geoid and topography profiles might provide an additional constraint on the deformation at subduction zones; however, it may be necessary to incorporate geologic history into subduction calculations to further improve the results presented here.
Diagnostic and prognostic model studies of the Adriatic Sea general circulation: Seasonal variability
Authors: Zavatarelli M. , Pinardi N. , Kourafalou V. H. , Maggiore A.
Published: October 2002

The climatological seasonal variability of the Adriatic Sea general circulation is studied by carrying out diagnostic and prognostic numerical experiments. Two different sets of European Centre for Medium‐Range Weather Forecasts‐derived forcing functions were used to drive the model, because of the uncertainties in the heat and momentum flux determination, and the results are compared. The diagnostic simulations allowed a comparison of the model solutions with the observed baroclinic general circulation, and the results, in agreement with recent observations, suggest that during winter the Western Adriatic Coastal Current has a significant barotropic component, while during summer all the general circulation features are largely baroclinic. The prognostic simulations indicated that the circulation has a large seasonal variability, whose amplitude may be affected by the strength of the wind‐forcing field.
Lagrangian analysis of a convective mixed layer
Authors: D'Asaro Eric A. , Winters Kraig B. , Lien Ren‐Chieh
Published: October 2002

We characterize and quantify the transport of heat (Boussinesq density) in a highly idealized entraining convective mixed layer based on simulations of Lagrangian measurements in a two‐dimensional model. The primary objectives are to assess and explore the merits and difficulties in estimating the heat budget from perfect and imperfect Lagrangian floats. A significant advantage of Lagrangian measurements is that the time derivative of temperature along these trajectories gives a direct measure of the diffusive heat flux. Using simulated perfect Lagrangian floats, estimates of the surface buoyancy flux, the depth of the mixed layer, vertical profiles of advective and diffusive heat flux, and the overall rate of cooling are shown to agree accurately with the known results extracted from the Eulerian simulations. The Lagrangian nature of the data is exploited to reveal the structure of the flow within the convective layer and to quantify the heat fluxes associated with the different types of eddies. Phase plots of Lagrangian trajectories in density‐depth space reveal three distinct classes of motions: (1) plumes, which develop in the cold, heavy near‐surface thermal boundary layer and plunge into the mixed layer interior carrying heavy water downward; (2) interior turbulence, comprising random motions between the base of the thermal boundary layer and the base of the surface mixed layer; and (3) entrainment of interior water into plumes below the thermal boundary layer, i.e., a transition from class 2 to class 1. Plumes dominate the heat transport. Simulations were also made using slightly buoyant floats; these are not perfectly Lagrangian. Buoyancy concentrates the floats near the surface resulting in an oversampling of the stronger plumes. Making the same heat budget calculations as with the perfect floats results in a nonzero estimated Lagrangian heating rate in the interior and a curved profile of vertical heat flux that is up to 3 times too large. The local time rate change of density is not significantly affected. A correction of the heat transport for oversampling of the plumes removes most of the error. This technique allows the correct heat budget to be measured for this flow using weakly buoyant floats.
Buoyancy‐ and eddy‐driven circulation in the Atlantic layer of the Canada Basin
Authors: Nishino Shigeto
Published: October 2002

The circulation driven by buoyancy and mesoscale eddy effects in the Atlantic layer of the Canada Basin is obtained diagnostically from a climatological density field. The buoyancy forcing is caused by the divergence of diapycnal flow. The diapycnal flow is estimated from a density balance between the diapycnal advection and diffusion based on the density field. On the other hand, the eddy forcing is formulated as the curl of vertical friction. The vertical friction is estimated from geostrophic shear, which is obtained from the density field. Both the flow fields caused by the buoyancy forcing and by the eddy forcing similarly suggest a new flow structure characterized by a transbasin westward flow, which appears from the eastern central basin to the southwestern basin. The transbasin westward flow is also shown in geostrophic circulation at 500 m relative to reference levels in a wide range (1000–3000 m). The significant part of the flow is consistent with a temperature distribution at 500 m.
An experimental study of turbulent flow over a low‐angle dune
Authors: Best Jim , Kostaschuk Ray
Published: October 2002

Many large, sand bed alluvial channels are dominated by dunes that possess low‐angle lee sides, often <10°, which play a critical role in the transportation of sediment and generation of significant bed form roughness. Despite the fact that these low‐angle dunes are very common in such channels many current models of dune flow dynamics are based on bed forms with an angle of repose slip face that generates a zone of permanent separated flow in the dune lee. Study of flow associated with low‐angle dunes in the field is inherently difficult since it is usually both hard to measure very near the bed and hard to quantify the nature of turbulence over these bed forms. Results from a detailed scale model experimental study of flow over a low‐angle dune, which is based on a prototype dune from the Fraser River, Canada, present a necessary link between flume and field studies and document the origins of macroturbulence associated with these bed forms. Two‐dimensional laser Doppler anemometer measurements over a low‐angle dune (maximum lower lee side slope = 14°) show that dune morphology exerts a dominant control on the turbulent flow, causing flow deceleration in the lower lee and development of an intermittent layer of shear at the interface with the higher velocity flow above. The scale model confirms that permanent flow separation does not occur over low‐angle dunes but, instead, is replaced by a small region (here ∼7% of the dune wavelength in length) of intermittent flow reversal, which may be present for up to 4% of the time. Shear layers generated along this small zone of decelerated and/or separated flow in the lower lee have a much smaller velocity differential than is characteristic of shear layers generated by flow separation in the lee of angle of repose dunes. Turbulence production associated with low‐angle dunes is dominated by eddies generated along this shear layer, which produce highly variable horizontal and vertical velocities and large Reynolds stresses in this region. These results show that macroturbulence associated with low‐angle dunes is generated by intermittent separation or shear layer generation due to velocity gradients established in the zone of lee side flow expansion. Velocity profiles and maps of turbulence structure from the scale model are in reasonable agreement with field measurements from low‐angle dunes in natural sand bed rivers. These results highlight the need to consider the temporal evolution and intermittency of shear layer behavior, often very near the bed, when interpreting the generation of macroturbulence and dispersal of sediment associated with low‐angle dunes.
Indian Ocean: Validation of the Miami Isopycnic Coordinate Ocean Model and ENSO events during 1958–1998
Authors: Haugen Vibeke E. , Johannessen Ola M. , Evensen Geir
Published: October 2002

In the Indian Ocean, in situ data are sparse both in time and space. Therefore, numerical models are one of the major tools for further understanding of the ocean circulation. We have implemented, validated, and done a 40 year simulation experiment forced by synoptic atmospheric data by using the Miami Isopycnic Coordinate Ocean Model (MICOM) not previously used for the Indian Ocean. The simulation results compare well to available observations including an extensive altimeter data set from ERS and TOPEX/Poseidon. The model simulation discovered an anticyclonic gyre in the southern Bay of Bengal, confirmed by altimeter data and previously unknown. This gyre is clearly influenced by the strength of the Indian Monsoon Current. From the 40 year interannual investigation, abnormal cooling as high as 4°C was simulated off Indonesia, in the eastern part of the Indian Ocean, and warming in the west, off Somalia, during years which coincide with negative Southern Oscillation Index (SOI). These years also coincide with Pacific Ocean El Niño years, except for 1961. The cooling off Indonesia is normally followed by a warming the following year. We also observed a reduction in upwelling off the southwest coast of India, which is one of the major fishing areas along the continental shelf, which also coincide with El Niño–Southern Oscillation (ENSO) years. We conclude that El Niño events occur very clearly in the Indian Ocean.
Importance of the Gulf of Aqaba for the formation of bottom water in the Red Sea
Authors: Plähn Olaf , Baschek Burkard , Badewien Thomas H. , Walter Maren , Rhein Monika
Published: October 2002

Conductivity‐temperature‐depth tracer and direct current measurements collected in the northern Red Sea in February and March 1999 are used to study the formation of deep and bottom water in that region. Historical data showed that open ocean convection in the Red Sea can contribute to the renewal of intermediate or deep water but cannot ventilate the bottom water. The observations in 1999 showed no evidence for open ocean convection in the Red Sea during the winter 1998/1999. The overflow water from the Gulf of Aqaba was found to be the densest water mass in the northern Red Sea. An anomaly of the chlorofluorocarbon component CFC‐12 observed in the Gulf of Aqaba and at the bottom of the Red Sea suggests a strong contribution of this water mass to the renewal of bottom water in the Red Sea. The CFC data obtained during this cruise are the first available for this region. Because of the new signal, it is possible for the first time to subdivide the deep water column into deep and bottom water in the northern Red Sea. The available data set also shows that the outflow water from the Gulf of Suez is not dense enough to reach down to the bottom of the Red Sea but was found about 250 m above the bottom.
Surface heat budget over the Weddell Sea: Buoy results and model comparisons
Authors: Vihma Timo , Uotila Juha , Cheng Bin , Launiainen Jouko
Published: October 2002

The 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.
Observations of pCO2 in the coastal upwelling off Chile: Spatial and temporal extrapolation using satellite data
Authors: Lefèvre Nathalie , Aiken Jim , Rutllant Jose , Daneri Giovanni , Lavender Samantha , Smyth Tim
Published: October 2002

Atmospheric and oceanic partial pressures of carbon dioxide and fluorescence were measured underway off the coast of northern Chile in January 1997. Seawater samples were taken for the analysis of nutrient and chlorophyll concentrations at 31 stations. The objectives were to improve the understanding of the biologically induced responses of upwelling processes off the coast of Chile and to assess the air‐sea exchange of CO2 during the austral summer. The cruise sampled within a grid from 22°40S to 24°S extending offshore to 71°52′W during 10–16 January 1997 (survey 1) and 22–27 January 1997 (survey 2). Survey 1 measurements showed relatively uniform hydrographic and chemical properties, except south of the Mejillones peninsula, where colder and fresher water was observed. During the second survey, concurrent satellite imagery showed colder water in a narrow coastal band. The relatively cold and fresh water recorded there suggested that upwelled water originated mainly from subantarctic water rather than from the nutrient‐rich equatorial subsurface water. Onshore‐offshore variability dominated over longshore variability. The offshore subtropical water was characterized by a temperature of 23°C, a salinity of 34.9, and relatively uniform pCO2 around 400 μatm that was mainly temperature driven. Although strong CO2 drawdowns (pCO2 as low as 200 μatm) were located close to the coast following the upwelling events, overall, the region was a source of CO2 to the atmosphere. A pCO2 algorithm was constructed using the cruise data of sea surface temperature (SST) and salinity (S). Synthetic salinity was calculated from S‐SST relationships determined for survey 2 and used with advanced very high resolution radiometer SST to calculate pCO2 over the cruise area, weekly, from January to March. European Centre for Medium‐Range Weather Forecasts wind speeds were used to calculate the CO2 flux in the upwelling off Chile (22°S–29°S, 68°W–73°W). The region was a source of CO2 flux of 1.6 mmol m−2 d−1 from 1 January to 25 March 1997.
Preferential sinking export of biogenic silica during the spring and summer in the North Water Polynya (northern Baffin Bay): Temperature or biological control?
Authors: Michel Christine , Gosselin Michel , Nozais Christian
Published: October 2002

Short‐term free‐drifting particle interceptor traps were deployed at 28 stations from April to July 1998 in the North Water Polynya (northern Baffin Bay). The amount, composition, and vertical transformation of the organic material sinking out of the euphotic zone were assessed. Clear seasonal sedimentation patterns were apparent throughout the Polynya. Maximum sedimentation occurred during the month of June, at which time high sedimentation of intact diatom cells and empty frustules was observed. In July, abundant resting spores and zooplankton feces were sinking out of the euphotic zone. Vertical transformation of the sinking material, between 50 m and 100 m, revealed a consistent loss of ∼30–35% for particulate organic carbon and nitrogen and a loss of ∼10% for biogenic silica. A temperature model of silica dissolution was used to assess the role of temperature in controlling the low loss of biogenic silica observed in the upper water column. Model results show divergence of modeled rates from in situ loss rates at times when the biosiliceous fraction of the sinking material was high. This indicates that biological factors played a key role in reducing biogenic silica dissolution in the North Water Polynya. Sedimentation of intact cells, abundant resting spores, and feces all contributed to enhance preservation of silica in the sinking material. These results suggest that the North Water Polynya is a sink for biogenic silica and emphasize the significance of biological processes in controlling the silica pump in the marine environment.
Near‐surface water vapor over polar sea ice is always near ice saturation
Authors: Andreas Edgar L , Guest Peter S. , Persson P. Ola G. , Fairall Christopher W. , Horst Thomas W. , Moritz Richard E. , Semmer Steven R.
Published: October 2002

During our yearlong participation in the Surface Heat Budget of the Arctic Ocean experiment (SHEBA), we found the measured relative humidity, figured for saturation with respect to ice, to almost always be near 100%. Often, multiple humidity sensors even showed supersaturation. Four months of observations over sea ice in the Antarctic showed the same behavior. These frequent, ubiquitous, and reproducible measurements are too compelling to discount. We hypothesize that the high relative humidity is a consequence of plentiful water vapor given up by leads and polynyas. We thus develop a simple time‐dependent vapor budget model that we solve analytically to assess the role of leads in supplying water vapor to the polar atmospheric boundary layer. The solution to that model shows that (1) because the polar marine boundary layer is generally thin, its timescale for reaching moisture equilibrium is much shorter than the timescale of the synoptic processes that tend to disrupt equilibrium, and (2) because they have relatively warm surfaces, open leads and polynyas supply water vapor more rapidly than the surrounding sea ice surface can remove it, despite an open water fractional area that may be only 5%. In concert, the two processes commonly lead to water vapor densities in the boundary layer over sea ice that are near the value for ice saturation.
Spatial distribution and radiative effects of soot in the snow and sea ice during the SHEBA experiment
Authors: Grenfell Thomas C. , Light Bonnie , Sturm Matthew
Published: October 2002

Soot observations around the periphery of the Arctic Ocean indicate snowpack concentrations ranging from about 1 to more than 200 ng carbon/g snow (ngC/g), with typical values being near 40–50 ngC/g. Values of this magnitude would significantly affect not only the albedo and transmissivity of the ice cover but also surface melt rates and internal heat storage in the ice. During the Surface Heat Budget of the Arctic Ocean (SHEBA) drift, there was concern that soot emitted from the ship could adversely impact the heat and mass balance measurements, producing results that would not be representative of the region as a whole. To investigate this possibility, a series of soot measurements was carried out starting in the spring of 1998 during the time of maximum snowpack thickness. On the upwind side of the ship, where the heat and mass balance program was carried out, soot concentrations averaged over the depth of the snowpack spanned a range from 1 to 7 ngC/g, with average values of 4–5 ngC/g. On the downwind side, concentrations increased to 35 ngC/g and above. Measurements made up to 16 km from the ship yielded average background soot levels of approximately 4.4 ngC/g, with a standard deviation of 2.9 ngC/g evenly distributed throughout the different snow layers. These concentrations were not statistically distinguishable from the values measured in the observing areas on the upwind side of the ship. This indicates that soot concentrations in the central Arctic Basin are substantially lower than those reported for the coastal regions and are not sufficient to produce a significant decrease in the albedo. Although measurements of sea ice samples gave similarly low values, parameter studies show that the snow soot levels could be significant if the summer melt caused all the soot to be concentrated at the ice surface.
An annual cycle of Arctic cloud characteristics observed by radar and lidar at SHEBA
Authors: Intrieri J. M. , Shupe M. D. , Uttal T. , McCarty B. J.
Published: October 2002

The temporal distributions of cloudiness, vertical distribution of cloud boundary heights, and occurrence of liquid phase in clouds are determined from radar and lidar data sets collected from October 1997 to October 1998 during the Surface Heat Budget of the Arctic Ocean (SHEBA) project. The radar/lidar combination was necessary for comprehensive cloud detection over a variety of physical conditions and is significantly more detailed (5–9 s temporal resolution, 30–40 m vertical resolution) than measurements made by surface observers or satellites. The combined measurements revealed that clouds were almost continuously present, with an annual average occurrence of 85%, and displayed an overall annual trend of a cloudier summer and clearer winter. A monthly averaged cloud occurrence maximum of 97% was observed in September and a minimum of 63% was observed in February. Monthly averaged lowest cloud base heights were between 0.25 and 1.0 km above ground level (agl) and monthly averaged highest cloud top heights were between 2.5 and 5.5 km agl, and displayed no significant seasonal variation. The number of cloud layers was typically 1 or 2, with the summer months tending to be multilayered. The lidar utilized depolarization ratios to detect liquid water; the percentage of lidar‐observed clouds containing liquid was 73% for the year. The least amount of liquid water phase was observed during December in 25% of the lidar‐detected clouds and the maximum was observed during July in 95% of the lidar‐detected clouds. Liquid was distributed in a combination of all‐liquid and mixed phase clouds, and was detected at altitudes as high as 6.5 km agl and at temperatures as low as −34°C.
Evaluating the principles of cloud remote sensing with AVHRR and MAS imagery over SHEBA
Authors: Xiong Xiaozhen , Li Wei , Lubin Dan , Stamnes Knut
Published: October 2002

A rigorous discrete ordinates radiative transfer formulation has been applied to two Advanced Very High Resolution Radiometer (AVHRR) images extracted from telemetry collected by the CCGS Des Groseilliers satellite tracking system during SHEBA to estimate cloud optical depth and effective radius of the cloud droplet size distribution. The two cases, from 2 and 3 June 1998, were chosen for analysis because (1) the images contained mostly liquid water clouds and (2) contemporaneous MODIS Airborne Simulator (MAS) overflight imagery was available for these AVHRR overpasses. The objective is to apply the same detailed radiative transfer formulation to both the MAS and AVHRR data so that the quality of the retrievals from the latter can be evaluated. Retrievals of cloud optical properties from MAS are assumed to be more reliable, because (1) all MAS channels have direct radiometric calibration, (2) the higher spatial resolution of MAS (50 m nadir versus 1.1 km nadir with AVHRR) should yield smaller uncertainties related to partially cloudy pixels in a given study area, and (3) effective droplet radius can be retrieved directly from the MAS 1.62‐μm channel without additional uncertainties involved with subtracting a thermal radiance component. Examination of the retrievals from both sensors in these two cases reveals considerable spatial variability (more than a factor of 2) in cloud optical depth, on a variety of scales ranging from tens of meters to tens of kilometers, even for relatively uniform liquid water clouds. Retrievals of cloud effective droplet radius from AVHRR are generally consistent with those from MAS, suggesting that AVHRR can be reliably used to estimate this quantity. However, AVHRR‐based retrievals of cloud optical depth are subject to large errors that result from small uncertainties in the absolute radiometric calibration of AVHRR channel 2. Using recalibration coefficients from one of the more robust AVHRR postlaunch calibration efforts, the cloud optical depths that we retrieved from NOAA 14 AVHRR channel 2 are consistently 30–50% larger than those obtained from MAS. The intercomparison of MAS and AVHRR retrievals of cloud optical depth also revealed errors with AVHRR due to partial cloud cover, and these errors are not immediately apparent when examining the AVHRR retrievals alone. If the AVHRR retrievals are averaged to spatial resolutions of order 10–30 km, they appear to become more stable for use in applications such as atmospheric energy budget calculations.
Mesoscale simulation of surface fluxes and boundary layer clouds associated with a Beaufort Sea polynya
Authors: Mailhot J. , Tremblay A. , Bélair S. , Gultepe I. , Isaac G. A.
Published: October 2002

Measurements with the Canadian Convair‐580 aircraft over a large polynya in the Beaufort Sea provided detailed observations of turbulent heat fluxes and cloud properties during the First ISCCP Regional Experiment (FIRE) Arctic Cloud Experiment (FIRE.ACE). On 25 April 1998, cold air advection resulted in strong surface heat fluxes over the polynya and in the formation, despite the low temperatures (−19°C), of mixed‐phase clouds at the top of the Arctic boundary layer. The Canadian Mesoscale Compressible Community model (MC2) has been used to simulate this case at 2‐km resolution, with a detailed treatment of surface processes and the actual observed structure of the large polynya. The evolution of the Arctic boundary layer, together with most of the cloud features, compares favorably with in situ aircraft observations. The sensitivity of the Arctic boundary layer clouds to various surface and cloud microphysical processes has been examined. Aircraft observations and model simulations confirm that the generation of clouds associated with polynyas depends critically on the air‐sea temperature contrast controlling the magnitude of the heat fluxes. The crucial role of leads and polynyas for cloud formation is highlighted in a sensitivity run with surface evaporation turned off. Though it does not affect significantly the structure of the Arctic boundary layer, evaporation from the open waters provides the small moisture excess needed to trigger the generation of low‐level clouds when cold air advects over the polynya. Sensitivity runs with two cloud microphysical schemes revealed the importance of the turbulent transport of moisture in producing supercooled liquid clouds in this case. It is also found that these unusual boundary layer clouds particular to the Arctic conditions can be reasonably well reproduced with a cloud microphysical scheme of intermediate complexity that accounts for mixed phases.
An annual cycle of Arctic surface cloud forcing at SHEBA
Authors: Intrieri J. M. , Fairall C. W. , Shupe M. D. , Persson P. O. G. , Andreas E. L. , Guest P. S. , Moritz R. E.
Published: October 2002

We present an analysis of surface fluxes and cloud forcing from data obtained during the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment, conducted in the Beaufort and Chuchki Seas and the Arctic Ocean from November 1997 to October 1998. The measurements used as part of this study include fluxes from optical radiometer sets, turbulent fluxes from an instrumented tower, cloud fraction from a depolarization lidar and ceilometer, and atmospheric temperature and humidity profiles from radiosondes. Clear‐sky radiative fluxes were modeled in order to estimate the cloud radiative forcing since direct observation of fluxes in cloud‐free conditions created large statistical sampling errors. This was particularly true during summer when cloud fractions were typically very high. A yearlong data set of measurements, obtained on a multiyear ice floe at the SHEBA camp, was processed in 20‐day blocks to produce the annual evolution of the surface cloud forcing components: upward, downward, and net longwave and shortwave radiative fluxes and turbulent (sensible and latent heat) fluxes. We found that clouds act to warm the Arctic surface for most of the annual cycle with a brief period of cooling in the middle of summer. Our best estimates for the annual average surface cloud forcings are −10 W m−2 for shortwave, 38 W m−2 for longwave, and −6 W m−2 for turbulent fluxes. Total cloud forcing (the sum of all components) is about 30 W m−2 for the fall, winter, and spring, dipping to a minimum of −4 W m−2 in early July. We compare the results of this study with satellite, model, and drifting station data.
Effects of air mass origin on Arctic cloud microphysical parameters for April 1998 during FIRE.ACE
Authors: Gultepe I. , Isaac G. A.
Published: October 2002

Observations collected in April 1998 using the Canadian Convair‐580 during the First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment‐Arctic Cloud Experiment (FIRE.ACE) are used to study cloud microphysics over the Arctic Ocean. Cloud microphysical parameters in climate models are specified as either constants or specific relationships based on cloud systems originating from either the ocean or land. The Arctic Ocean during winter and spring is mainly covered with ice. Because of this condition, the influence of the Arctic Ocean on cloud systems can be very different as compared to that of the midlatitude ocean. Air mass back‐trajectories calculated from the Canadian Meteorological Center (CMC) model outputs were used to define the origin of air masses as either the Pacific Ocean (PO) or the Arctic Ocean (AO). The uncertainty in specifying the origin of the air mass was less than 20%. The PO mean aerosol number concentration (Na) from the aircraft measurements was larger (108 cm−3) than for the AO cases (41 cm−3). The PO mean droplet number concentration (Nd) was 48 cm−3 in comparison to 77 cm−3 for the AO cases. The droplet effective radii (reff) for the AO and PO cases were 9.3 and 5.6 μm, respectively. Liquid water content (ice water content) changed from 0.05 (0.01) g m−3 for the AO cases to 0.13 (0.03) g m−3 for the PO cases. The averaged ice crystal number concentration was 20 L−1 for the PO cases and 10 L−1 for the AO cases. For April 1998, a statistical significance test on mean values at the 85% confidence level showed that the AO and PO cases had distinct microphysical and aerosol characteristics.
Ice deformation near SHEBA
Authors: Lindsay R. W.
Published: October 2002

The deformation rate of sea ice is a key parameter for determining the evolution of the ice thickness distribution. It determines the rate of new ice formation through opening and the rate of ridging through closing and shear. An extensive suite of ground‐based and satellite‐based measurements of ice motion is used to construct a daily time series of the ice velocity and deformation in the vicinity of the Surface Heat Budget of the Arctic Ocean (SHEBA) ice camp that is suitable for forcing a model of the ice thickness distribution. The velocity is interpolated to a square grid that remains centered on the camp, has a spacing of 25 km, is 400 km on a side, and is determined for a 371‐day period from 2 October 1997 to 7 October 1998. Velocity measurements from buoys, Advanced Very High Resolution Radiometer (AVHRR), Special Sensor Microwave/Imager (SSMI), and Radarsat Geophysical Processing System (RGPS) are merged using optimal interpolation and a Kalman filter approach. The deformation rate is taken directly from the RGPS measurements when available. The daily total deformation rate measured on a scale of 100 km near the camp averaged 2.21% d−1, and the standard deviation was 1.78% d−1. The divergence was positive in the early winter and negative through most of the spring and summer. There were two major opening/closing events, one in January and one at the end of July. The net divergence over the year was very near zero. The vorticity indicated a net rotation of 87° over the year, with the winter showing strong anticyclonic turning and the summer showing strong cyclonic turning.
Role of surface fluxes in ocean general circulation models using satellite sea surface temperature: Validation of and sensitivity to the forcing frequency of the Mediterranean thermohaline circulation
Authors: Artale Vincenzo , Iudicone Daniele , Santoleri Rosalia , Rupolo Volfango , Marullo Salvatore , D'Ortenzio Fabrizio
Published: October 2002

In this article we study the effect of high‐frequency surface momentum and heat fluxes in the numerical simulation of some key ocean processes of the Mediterranean thermohaline circulation. The lack of synoptic and reliable heat and freshwater flux data sets is bypassed using the relaxation approach both for the salinity and temperature surface fields. We propose a parameterization of the heat fluxes in which the temperature‐restoring coefficient depends on wind intensity and regime and in which the use of simuoultaneous satellite daily sea surface temperature (SST) estimates as a restoring field is required. The consistency of the proposed parameterization and of its numerical implementation with the previous oceanic boundary layer studies has been verified trough the analysis of the Saunders’ proportionality constant. This parameterization coupling simultaneous surface heat fluxes and wind trough the skin‐bulk temperature difference, recovers the high variability of the air‐sea exchanges of the extreme events in the Mediterranean Sea. The effect of high‐frequency surface momentum and heat fluxes is studied comparing results from two different experiments forced with monthly and daily surface wind and satellite SST data sets. These comparisons show the relevance of high‐frequency forcing in the representation of the dynamical processes relative to the intermediate water mass transformation and horizontal advection as well as in the deep water formation in the northwestern Mediterranean Sea.
Validation of TOVS Path‐P data during SHEBA
Authors: Schweiger Axel J. , Lindsay Ronald W. , Francis Jennifer A. , Key Jeff , Intrieri Janet M. , Shupe Matthew D.
Published: October 2002

Products from the TIROS‐N Operational Vertical Sounder (TOVS) Polar Pathfinder (Path‐P) data set are compared with surface measurements and other satellite remote sensing retrievals during the Surface Heat Balance of the Arctic Ocean (SHEBA) field program (October 1997 to September 1998). The comparison provides estimates of Path‐P retrieval uncertainties. Results are placed in the context of the natural variability and timescales of variability to allow potential users to judge the applicability of the data set for their purpose. Results show temperature profiles to be accurate within 3 K, total column precipitable water within 2 mm annually, and surface temperature within 3 K. Uncertainties in temperature retrieval are below “within‐season” variability during all times of the year. Uncertainties in water vapor retrieval during winter and summer are slightly below observed variability in those seasons but are well below during spring. Uncertainty in retrieved cloud fraction is highly dependent on the timescale of observations. Cloud fractions from the surface and satellite are well correlated (correlation coefficient > 0.7) at timescales greater than 4 days but show weaker correlation at shorter timescales. Uncertainty in TOVS‐retrieved cloud fraction is less than 20% for 5‐day averages. In winter, TOVS‐retrieved cloud fractions are higher than those reported in standard meteorological observations but match those derived from lidar data. This supports the notion that standard meteorological observations may underestimate cloudiness in winter. Cloud‐top temperatures measured from the surface (lidar/radar) are significantly different from those estimated using TOVS and Advanced Very High Resolution Radiometer (AVHRR) radiances, which highlights the fundamental and inherent dissimilarity between these two measurement techniques.
Evaluation of data sets used to force sea ice models in the Arctic Ocean
Authors: Curry J. A. , Schramm J. L. , Alam A. , Reeder R. , Arbetter T. E. , Guest P.
Published: October 2002

Basin‐scale sea ice models are often run uncoupled to either an atmosphere or ocean model to evaluate the sea ice model, to compare different models, and to test changes in physical parameterizations. Such simulations require that the boundary forcing be specified. The specification of atmospheric forcing associated with the surface heat and freshwater fluxes has been done in various sea ice simulations using climatology, numerical weather prediction analyses, or and satellite data. However, the errors in the boundary forcing may be so large that it is difficult to determine whether discrepancies between simulated and observed properties of sea ice should be attributed to deficiencies in the sea ice model or to the boundary forcing. To assess the errors in boundary forcing, we use data from the Surface Heat Budget of the Arctic Ocean (SHEBA) to evaluate various data sets that have been used to provide boundary forcing for sea ice models that are associated with the surface heat and freshwater fluxes. The impact of errors in these data sets on a sea ice model is assessed by using a single‐column ice thickness distribution model, which is alternately forced with in situ measurements from SHEBA and output from large‐scale analyses. Substantial discrepancies are found among the data sets. The response of the sea ice model to the different forcing data sets was considerable.
Long‐term evolution and morphodynamic equilibrium of tidal channels
Authors: Lanzoni Stefano , Seminara Giovanni
Published: October 2002

This contribution investigates the morphodynamic equilibrium of funnel‐shaped well‐mixed estuaries and/or tidal channels. The one‐dimensional de Saint Venant and Exner equations are solved numerically for the ideal case of a frictionally dominated estuary consisting of noncohesive sediment and with insignificant intertidal storage of water in tidal flats and salt marshes. This class of estuaries turns out to be invariably flood dominated. The resulting asymmetries in surface elevations and tidal currents lead to a net sediment flux within a tidal cycle which is directed landward. As a consequence, sediments are trapped within the estuary and the bottom profile evolves asymptotically toward an equilibrium configuration, allowing a vanishing net sediment flux everywhere and, in accordance with field observations, a nearly constant value of the maximum flood/ebb speed. Such an equilibrium bed profile is characterized by a concavity increasing as the estuary convergence increases and by a uniquely determined value of the depth at the inlet section. The final length of the estuary is fixed by the longitudinal extension of the very shallow area which tends to form in the landward portion of the estuary. Note that sediment advection is neglected in the analysis, an assumption appropriate to the case of not too fine sediment.
Seasonal ice area and volume production of the Arctic Ocean: November 1996 through April 1997
Authors: Kwok R. , Cunningham G. F.
Published: October 2002

The RADARSAT Geophysical Processor System (RGPS) produces estimates of sea ice motion, deformation, and thickness of the Arctic Ocean sea ice cover from time‐sequential synthetic aperture radar (SAR) imagery acquired by RADARSAT. Based on these products, we summarize the evolution of a region of the Arctic Ocean ice cover in terms of its area change, ice volume production, and multiyear ice coverage over a 6‐month period between 7 November 1996 through 30 April 1997. This region encompasses a large part of the Western Arctic Ocean. The initial ice area covers ∼2.52 × 106 km2. At the end of April, this region covers ∼2.59 × 106 km2, a net divergence of ∼3%. Over the period, thin ice (0–20 cm) typically covers <2% of this area. The RGPS estimates only the thickness distribution of sea ice volume produced by openings and closings of the ice cover since the initial observation. Thus, only the ice volume of the seasonal ice cover can be determined. Between November 1996 and April 1997, ∼1000 km3 (∼0.4 m) of sea ice is produced. The amount stored in undeformed ice and ridged ice is dependent on the character of the redistribution function. The RGPS products also provide estimates of multiyear ice coverage. From the time‐series of multiyear ice coverage, we estimate that nearly 83% of the area is covered by multiyear ice. For more detailed examination of the regional dependence of ice‐cover deformation, volume production, and MY ice coverage, the larger region is divided into five subregions. Where possible, we provide an analysis of the errors in the above estimates. In cases where quantification of the uncertainty is not feasible, we list the potential error sources. There are remaining sources of uncertainty in these calculations that remain unquantified, but at this time, there are no adequate in situ or remote‐sensing data for comprehensive evaluation of the above estimates.
Relating arctic pack ice stress and deformation under winter conditions
Authors: Richter‐Menge Jacqueline A. , McNutt S. Lyn , Overland James E. , Kwok Ronald
Published: October 2002

Together, thermodynamic and dynamic processes determine the thickness distribution of the ice cover on polar oceans, which governs the exchange of energy between the atmosphere and the ocean. Key to the dynamic processes is the mechanical behavior of the ice cover. During the Surface Heat Budget of the Arctic Ocean (SHEBA) field experiment, we deployed sensors to measure the internal ice stress at several locations within a 15 × 15 km area. These measurements are combined with satellite‐derived ice motion and imagery products. The objective is to make a first step toward using these data sources for evaluating sea ice dynamics models by assessing whether the stress signal can be qualitatively linked to the regional‐scale (10–100 km) deformation activity. Four case studies are presented, each with distinguishing characteristics: consolidation of the seasonal ice zone against the Alaskan coast (5–7 December 1997); advancement of the consolidation zone into the perennial ice pack (11–13 December); extreme divergence (14–17 January); and consolidation of the pack against Wrangel Island and the Siberian coast (20–23 February). The results of this analysis (1) demonstrate that stress measurements are related to the regional deformation behavior of the ice cover, (2) confirm that regional‐scale ice dynamics is primarily a function of coastal geometry and sustained, large‐scale wind direction and magnitude, (3) provide continued evidence that the ice pack behaves as a granular hardening plastic, and (4) encourage pursuit of efforts to use direct measurements of ice stress and deformation in the formulation and development of sea ice dynamics models.
Flow, water mass changes, and hydraulics in the Bosphorus
Authors: Gregg Michael C. , Özsoy Emin
Published: October 2002

Using average sections along the Bosphorus taken in September 1994 with a loosely tethered profiler and an acoustic Doppler current profiler, Gregg et al. [1999] found the exchange flow between the Sea of Marmara and the Black Sea to be quasi‐steady but far from satisfying the hydraulic control conditions for two‐layered flows. Here we examine synoptic sections and use images from an acoustic backscatter system to provide the first detailed look at the flow and water mass changes in the Bosphorus and to assess how well the flow satisfies the hydraulic assumptions. Thirty kilometers long, 28–100 m deep, and 0.75–3 km wide, the Bosphorus has bathymetry for more complex than that used in analytic or numerical models of exchange flows. The particulars affect dynamics in important ways. For instance, owing to changes in channel shape, the narrowest section, known as the contraction, is not also the minimum in cross‐sectional area. Rather, it is a transition between the wider northern half and the narrower southern half of the strait, and some places south of the contraction have slightly smaller areas. Sharp bends occur throughout the strait and often produce flow separations as well as directing upper and lower flows to opposite sides of the channel. Never <28% of the water column, the interface thickens to 75% in the southern half of the strait as a result of intense mixing downstream of the contraction. As a consequence of the strong mixing and numerous flow separations, we conclude that the exchange flow may be at least partly controlled by friction instead of being a simple hydraulic flow.