J. L. Schramm

Overview of Arctic Cloud and Radiation Characteristics

Abstract To provide a background for ARMs activities at the North Slope of Alaska/Adjacent Arctic Ocean sites, an overview is given of our current state of knowledge of Arctic cloud and radiation properties and processes. The authors describe the Arctic temperature and humidity characteristics, cloud properties and processes, radiative characteristics of the atmosphere and surface, direct and indirect radiative effects of aerosols, and the modeling and satellite remote sensing of cloud and radiative characteristics. An assessment is given of the current performance of satellite remote sensing and climate modeling in the Arctic as related to cloud and radiation issues. Radiation-climate feedback processes are discussed, and estimates are made of the sign and magnitude of the individual feedback components. Future plans to address these issues are described.

Sea ice-albedo climate feedback mechanism

Abstract The sea ice-albedo feedback mechanism over the Arctic Ocean multiyear sea ice is investigated by conducting a series of experiments using several one-dimensional models of the coupled sea ice-atmosphere system. In its simplest form, ice-albedo feedback is thought to be associated with a decrease in the areal cover of snow and ice and a corresponding increase in the surface temperature, further decreasing the areal cover of snow and ice. It is shown that the sea ice-albedo feedback can operate even in multiyear pack ice, without the disappearance of this ice, associated with internal processes occurring within the multiyear ice pack (e.g., duration of the snow cover, ice thickness, ice distribution, lead fraction, and melt pond characteristics). The strength of the ice-albedo feedback mechanism is compared for several different thermodynamic sea ice models: a new model that includes ice thickness distribution, the Ebert and Curry model, the Maykut and Untersteiner model, and the Semtner level-3 an...

Influence of the Sea Ice Thickness Distribution on Polar Climate in CCSM3

Abstract The sea ice simulation of the Community Climate System Model version 3 (CCSM3) T42-gx1 and T85-gx1 control simulations is presented and the influence of the parameterized sea ice thickness distribution (ITD) on polar climate conditions is examined. This includes an analysis of the change in mean climate conditions and simulated sea ice feedbacks when an ITD is included. It is found that including a representation of the subgrid-scale ITD results in larger ice growth rates and thicker sea ice. These larger growth rates represent a higher heat loss from the ocean ice column to the atmosphere, resulting in warmer surface conditions. Ocean circulation, most notably in the Southern Hemisphere, is also modified by the ITD because of the influence of enhanced high-latitude ice formation on the ocean buoyancy flux and resulting deep water formation. Changes in atmospheric circulation also result, again most notably in the Southern Hemisphere. There are indications that the ITD also modifies simulated sea...

Applications of SHEBA/FIRE data to evaluation of snow/ice albedo parameterizations

Climate models use a wide variety of parameterizations for surface albedos of the ice-covered ocean. These range from simple broadband albedo parameterizations that distinguish among snow-covered and bare ice to more sophisticated parameterizations that include dependence on ice and snow depth, solar zenith angle, and spectral resolution. Several sophisticated parameterizations have also been developed for thermodynamic sea ice models that additionally include dependence on ice and snow age, and melt pond characteristics. Observations obtained in the Arctic Ocean during 1997–1998 in conjunction with the Surface Heat Budget of the Arctic Ocean (SHEBA) and FIRE Arctic Clouds Experiment provide a unique data set against which to evaluate parameterizations of sea ice surface albedo. We apply eight different surface albedo parameterizations to the SHEBA/FIRE data set and evaluate the parameterized albedos against the observed albedos. Results show that these parameterizations yield very different representations of the annual cycle of sea ice albedo. The importance of details and functional relationships of the albedo parameterizations is assessed by incorporating into a single-column sea ice model two different albedo parameterizations, one complex and one simple, that have the same annually averaged surface albedo. The baseline sea ice characteristics and strength of the ice-albedo feedback are compared for the simulations of the different surface albedos.

Impact of clouds on the surface radiation balance of the Arctic Ocean

SummaryThe relationship between clouds and the surface radiative fluxes over the Arctic Ocean are explored by conducting a series of modelling experiments using a one-dimensional thermodynamic sea ice model. The sensitivity of radiative flux to perturbations in cloud fraction and cloud optical depth are determined. These experiments illustrate the substantial effect that clouds have on the state of the sea ice and on the surface radiative fluxes. The effect of clouds on the net flux of radiation at the surface is very complex over the Arctic Ocean particularly due to the presence of the underlying sea ice. Owing to changes in surface albedo and temperature associated with changing cloud properties, there is a strong non-linearity between cloud properties and surface radiative fluxes. The model results are evaluated in three different contexts: 1) the sensitivity of the arctic surface radiation balance to uncertainties in cloud properties; 2) the impact of interannual variability in cloud characteristics on surface radiation fluxes and sea ice surface characteristics; and 3) the impact of climate change and the resulting changes in cloud properties on the surface radiation fluxes and sea ice characteristics.

Modeling the thermodynamics of a sea ice thickness distribution: 1. Sensitivity to ice thickness resolution

A one-dimensional ice thickness distribution model is presented to determine the minimum number of ice thicknesses necessary to resolve the area-averaged annual cycles of ice thickness and turbulent fluxes. The baseline case includes 40 ice thickness categories; ice thickness and area, meltwater ponds, ice salinity and age, snow cover, and surface albedo evolve independently for each ice category. A ridging and ice export parameterization, and a coupled one-dimensional ocean mixed layer model are also included. Sensitivity studies indicate that 16 ice thickness categories can accurately resolve the baseline annual cycles of area-averaged ice thickness and the summertime turbulent fluxes in this model, provided that one third of the thickness categories represent ice thinner than 0.8 m. Resolving the distribution of ice in this thickness range is important in simulating the area-averaged ice characteristics. Wintertime values of the turbulent fluxes differ from the baseline by up to 1 W m−2 for fewer than 40 ice thickness categories. The large difference in turbulent fluxes between open water and ice thicker than 1.6 m makes the area-averaged value sensitive to the number of ice thickness categories, since this number can affect the categories that are merged, the categories that melt completely, ice that is ridged, and the resolution of the ice thickness distribution. This makes an accurate simulation of the baseline wintertime turbulent fluxes difficult. Model simulations with fewer than 40 categories provide a reasonable estimate of the baseline response to a surface longwave heat flux perturbation of greater than 5 W m−2. The annually area-averaged ice thickness is within 10 cm. The model response to a heat flux perturbation of less than 5 W m−2 is similar for a wide range of ice thickness categories, since the thickness distribution of ice thinner than 1 m is not affected.

Disposition of solar radiation in sea ice and the upper ocean

A one-dimensional sea ice model with an ice thickness distribution is presented to examine the disposition of the incoming surface shortwave radiation within the sea ice and the upper ocean. The sea ice model consists of 15 different ice thickness categories and an open water (leads) category. Ice growth, melting on horizontal and vertical surfaces, meltwater pond growth and drainage, and snow accumulation evolve independently for each of the ice categories. Leads, melt ponds, and thinner ice categories are of particular interest, as these features account for nearly all of the solar energy that is transmitted into the upper ocean beneath the ice pack. Also examined is the surface reflection and absorption, internal ice absorption, and lateral melting for ponded and pond-free ice. Area-averaged results show that 69% of the total annual solar energy is reflected, 15% is absorbed by the snow cover, 12% is absorbed by the ice, and 4% is transmitted to the ocean mixed layer through thin ice and leads.

Water vapor feedback over the Arctic Ocean

Previous studies of the clear sky greenhouse effect and water vapor feedback have focused on subpolar regions. In view of modeled amplification of greenhouse warming in the Arctic we investigate the humidity characteristics, clear sky greenhouse effect and water vapor feedback in the Arctic by using 10 years of radiosonde data obtained from the Russian drifting ice island stations along with a radiative transfer model. By taking advantage of the natural variability associated with seasonal and interannual variations, we can infer the water vapor feedback from the data. Results of this study indicate that water vapor feedback over the Arctic Ocean is substantially more complex than in other regions because of the relative lack of convective coupling between the surface and the atmosphere and the different thermodynamic and radiative environment in the Arctic. In particular, the effect of water vapor on the net flux of radiation is complicated by low temperatures, low amounts of water vapor, and the presence of temperature and humidity inversions. During winter a “hyper” water vapor feedback arises from the control of ice saturation on the lower tropospheric humidity and a water vapor “window” in the rotation band at low atmospheric humidities. Implications for global warming are discussed.

Modeling the thermodynamics of a sea ice thickness distribution. 2. Sea ice/ocean interactions

In this paper we examine the coupling of an ice thickness distribution model with an ocean mixed layer model. The annual cycle of the modeled mixed layer temperature, salinity, and depth are compared to observations from the drifting ice stations of the Arctic Ice Dynamics Joint Experiment (AIDJEX). The role of the ice thickness distribution in determining the ice/ocean coupling is examined. We find that the ice thickness distribution is important for the exchange of heat, salt, and fresh water between the ice and the ocean, especially with regard to the effects of thin, first-year ice. Several different parameterizations of the ice/ocean turbulent heat exchange are compared. The results indicate that the storage of heat in the ocean mixed layer is an important physical process for determining the annual average ice thickness. Double diffusion and the stabilizing effects of meltwater also impact the exchange of heat between the ocean and ice cover. Questions remain as to the relative importance of these processes and their accurate parameterization in ice/ocean coupled models.

Toward the modeling of enhanced basal melting in ridge keels

Observations from sonar data have suggested enhanced melting of thick, ridged ice relative to level ice. There are several mechanisms that may account for this intensified melting. In this paper, we examine the effects of two-dimensional (2-D) heat conduction and enlarged basal surface area due to the sloping sides of the keel on heat conduction and melt rates. The cross section of the 2-D ridge is taken to be an isosceles triangle with a rounded crest. This is roughly the shape observed and allows a convenient numerical representation in polar coordinates. For comparison, ridges of similar shape are also represented as a collection of 1-D columns of varying thickness, similar to what is implicit in typical ice thickness distribution models. The results show that 2-D ridges inhibit the heat conduction compared to 1-D ridges owing to the dominating effect of weaker temperature gradients. The slope of the keel is the dominant factor in determining the temperature gradient. A size distribution of 2-D ridges reduces heat transfer to the atmosphere by 3 W m−2 compared to a similar distribution of 1-D ridges. Over an annual cycle, basal ablation along the keel is insignificant for 2-D ridges with small slopes, whereas ridges with large slopes show ablation rates determined by the ice-ocean heat flux. These melt rates imply a transition from a triangular to a more rounded shape. The 1-D ridge geometry is not adequate to simulate the net melting at the keel base over an annual cycle. Melt rates are calculated along the ridge keel and for level ice over a 40 day period for comparison with observations. For 1-D ridges, all ice thicker than 5 m melts more slowly than the corresponding level ice. The inclusion of 2-D heat conduction increases the amount of ablation in the thicker ice relative to the 5 m ice, especially for ridges with larger slopes. However, this increase explains only a small fraction of the enhanced basal melting seen in the observations. These results suggest that other mechanisms are important in determining the mass loss from thicker ice.