Energy Budget and Heat Transfer
Arctic spring landscapes are usually characterized by a mosaic of coexisting snow-covered and bare ground patches. This phenomenon has major implications for hydrological processes, including meltwater production and runoff. Furthermore, as indicated by aircraft observations, it affects land-surface-atmosphere exchanges, leading to a high degree of variability in surface energy terms during melt. During the warm seasons in the Arctic solar heating melts the upper layers of the ice sheet and of the snow cover releasing fresh meltwater over the surface. These areas of snowmelt have a temperature of 0° C and interact with the atmosphere, the ocean and the ice surfaces absorbing more solar radiation than the nearby surfaces thus leading to differential solar radiation partition. Snowmelt processes can be considered the result of both effects due to the surface absorption of radiation and turbulent transport. Because of the paramount importance of the surface albedo in regulating the components of the climatic system, melting processes appear to have a key role in governing the global radiation balance. They also have effects on the water vapor content of the atmosphere, originating feed-back mechanisms between the surface and the cloud cover.
The principal aim of our research is to understand the crucial melting phenomena that occurs in Ny-Ålesund during the spring time period, throughout the computation of a simple parameterization of snowmelt events taking into the account previous study already made in this polar region. In particular, scientific objectives of this research activity are:
- To measure sensible and latent heat fluxes through the eddy covariance technique in order to monitor continuously the surface energy balance and its season and annual trends.
- To study the influence of spring melting process on each single surface energy balance component to obtain realistic parameterization of this phenomena.
- To compute, taking into account both radiative and conductivity term, the subsurface heat flux in order to have a good evaluation of the surface energy balance also during the melting period and on order to give fundamental information for the glaciological studies.
- Investigate connection between the surface energetic partitioning and the cloud coverage.
Sensible and latent heat fluxes are measured by a 3-D micrometeorological system suitable to apply the well-known eddy-covariance technique. These measurements allow to continuously monitor the surface energy balance and its seasonal and annual trends. Simple parameterization models will be then used to simulate/reproduce the measured features. In particular, taking into account both radiation and conductivity the subsurface heat flux is monitored in order to have a good evaluation of the surface energy balance also during the melting period also to provide fundamental information for the glaciological studies. Thanks to the vertical profile of snow temperature and radiation measurements it is possible to determine relative contribution of both heat conduction and short-wave penetration to the overall heat transfer process in the terrain and, in particular, to investigate in detail snow melting in spring and the important role played in this process by cloud coverage. The influence of spring melting process on each single surface energy balance component is also investigated and realistically parameterized.
Radiation Budget
Short wave and long wave radiation fluxes at the surface and in the atmosphere could be significantly modified by changes in cloud amount and characteristics as well as by changes in several long and short-lived pollutants (SLPs). Arctic aerosol presents a large inter-annual as well as seasonal variation, with a maximum in spring (Arctic Haze) and minimum in fall. Pollution transport can occur also in summer, main sources being Asian dust and smoke from boreal forest fires. As a consequence of the specific conditions in the polar regions, tropospheric aerosols can significantly modify the overall albedo of the surface-atmosphere system. Radiative forcing can change sign depending on chemical species, surface properties and solar geometry. Moreover, even slight changes in cloud microphysical or physical properties resulting from interactions with aerosols will profoundly impact the radiation balance and Arctic climate. Evidence indicates that strong couplings exist between the surface and clouds, however, the magnitudes, and in some cases the sign, of the cloud-radiation feedback mechanisms are still unknown and appear to be a complicated function of cloud height, thickness, phase and particle size. Our understanding of Arctic cloud properties and their impact on radiation fluxes is limited by the fact that little observational data exist on Arctic clouds, especially during the dark winter season.
Surface radiation balance, its components, and their seasonal and inter-annual variation are accurately determined at the top of the CCT. The disposability of a high tower will largely improve significance of up-welling flux measurements and albedo evaluations. Direct radiative effects of Arctic Haze and polluted layers will be determined. Closure studies will be performed, comparing results from radiation measurements with model evaluations, in which aerosol properties are realistically described on the basis of LIDAR vertical profiles and both in-situ and sun-photometric measurements. Effects of cloudiness on both short-wave and long-wave fluxes will be evaluated continuously from down-welling radiation measurements, and suitable cloud cover indexes and cloud classification schemes developed. Comparison with synoptic human observations and all-sky camera data will allow to improve methodologies for automatic detection of cloudiness in the Arctic region. Overall cloud forcing will be determined and its seasonal and inter-annual behavior (amplitude, sign) investigated. Optical indexes and parameterization schemes developed on the basis of radiometric measurements and visual observation will be related not only to the variation of incoming flux, but also to the redistribution of the solar energy in direct and diffuse components. Influence of cloudiness on the snow-melting process will be deeply investigate.
All components of the radiation budget and their temporal variations will be determined and investigated through measurements performed at the top of the CCT at 32 m of height. Redundancy of measurements as well as the use of top class instrumentation will allow the improvement of accuracy. Technical solutions will derive from the large experience acquired from similar measurements on the Antarctic Plateau. The measurement of all components of the radiative balance at the surface, will allow the evaluation of the net flux, and the determination of seasonal and inter-annual variations of this very important climatological parameter. Concerning clouds, measurements of global and diffuse shortwave components will allow to fix procedures as those proposed by Long and other authors to investigate their effects on shortwave and longwave components of the radiative balance and determine cloud cover indexes. These procedures will be, if necessary, adjusted for the analysis of data collected in the Arctic region. Procedures based on longwave measurements will allow to provide evaluations of cloud coverage and radiative effects when the sun is below the horizon. A total sky imager will be installed and maintained to obtain objective information on cloudiness conditions. To enlarge the information obtained from these visual measurements to the infrared range, a second system will be developed and installed.
Snow Spectral Reflectance
One of the effects of global warming is the reduction of the surface extension of snow cover planet wide. Spectral reflectance of snow is therefore a crucial climatic and hydrological variable, as it determines the energy balance of the snow covered areas and the temperature profile in the snow and in the lower atmosphere. Snow optical properties in the visible and near-infrared wavelengths depend on grain shape and size, density and thickness of the snowpack, occurrence of impurities (dust, soot, pollen and other plant materials) and liquid water content. In particular, that snow reflectance is higher in the visible part of the electromagnetic spectrum, while decreases rapidly at longer wavelengths. The increase of grain size determines a decrease of reflectance all over the spectral range from visible to short wave infrared (350-2500 nm); analysis of field data has also provided evidence of the great role of surface roughness in controlling snow spectral behaviour. Snow is a highly unstable target and structural changes in grain size and shape may occur quite rapidly so that snow albedo decreases with time: this behaviour should be considered in climate models in order to describe adequately the energy balance of the surface. With climate changes, the variables that determine snow metamorphism, such as air temperature and the temperature gradient in the snowpack, can be altered and the time evolution of snow albedo can thus be modified, representing a potentially important feedback on global warming. The rate of metamorphism of snow crystal are still under investigation as well as the environmental variables involved in the process.
Snow reflectance is due to its light scattering properties, that are function of the size and shape of snow grains, of the absorption by the ice medium, of the presence of impurities and of the surface roughness. In order to recognize the single contribution of these parameters, field spectroradiometric and nivologic measurements are acquired in different site in the Ny-Ålesund area, close to the CCT. This area is particularly suitable for this type of studies because during spring time the snow cover does not undergo to intense metamorphic variations, thus remaining unaltered during the entire period of the spectroradiometrical surveys. Snow surfaces with the same morphologic characteristics have also an areal extension sufficient to be recognized in the satellite images. By means of prior surveys data elaboration it will also be possible to estimate the specific surface area (SSA) of the snow, defined as the surface area of snow crystals that is accessible to gases per unit mass. It has been verified that during snow metamorphism as the grain size increases, the SSA decreases, thus supplying an important contribution to atmosphere-snow interaction models. The scientific challenge of these research activity is the understanding of the relationships between the spectral behaviour of snow and its physical characteristic in order to detect the subtle differences in the snow grains and types not only with field measurements but also using satellite images that can easily allow regional observations.
Field activities were carried out in different sites close to the CCT, along the coast near Ny-Ålesund and also in the inner area, at higher elevation. Reflectance data were acquired by a field spectroradiometer in the wavelength range 350-2500 nm, selecting snow surfaces with different macro- and microscopic morphological characteristics and different snow grains associations. Surface snow observations as well as local climatological data were collected during every measurements session, in order to associate the spectral data collected to a specific snow association type. These measures will be integrated by an accurate morphologic profile of the surface acquired with a field laser scanner. The integration of spectroradiometric and structural snow data with high precision surface profiles will contribute to the definition of a precise reflectance model also when surfaces cannot be assumed to be perfect Lambertian reflector due to roughness effects. All the investigated sites will be recognised on the satellite images in order to integrate the field measurements with satellite spectral data.
