The albedo of snow for partially cloudy skies

The albedo of snow for different cloudiness conditions is an important parameter in the Earth's radiation budget analysis and in the study of snowpack's thermal conditions. In this study an efficient approximate method is derived to calculate the incident spectral solar flux and snow-cover albedo in terms of different atmospheric, cloud, and snow parameters. The global flux under partially cloudy skies is expressed in terms of the clear sky flux and a coefficient which models the effect of scattering and absorption by cloud patches and multiple reflections between the cloud base and snowcover. The direct and the diffuse components of the clear sky flux are obtained using the spectral flux outside the atmosphere and the spectral transmission coefficients for absorption and scattering by molecules and aerosols.The spectral snow reflectance model considers both specular surface reflection and volumetric multiple scattering. The surface reflection is calculated by using a crystal-shape-dependent bidirectional reflectance distribution function; the volumetric multiple scattering is calculated by using a crystal-size-dependent approximate solution in the radiative transfer equation. The input parameters to the model are atmospheric precipitable water, ozone content, turbidity, cloud optical thickness, the size and shape of ice crystals of snow and surface pressure. The model yields spectral and integrated solar flux and snow reflectance as a function of solar elevation and fractional cloudcover.The model is illustrated using representative parameters for the Antarctic coastal regions. The albedo for a clear sky depends inversely on the solar elevation. At high elevations the albedo depends primarily upon the grain size; at low elevation the albedo depends on grain size and shape. The gradient of the albedo-elevation curve increases as the grains become larger and faceted. The albedo for a densely overcast sky is a few percent higher than the clear-sky albedo at high elevations. A simple relationship between grain size and the overcast albedo is obtained. For a set of grain size and shape, the albedo as a function of solar elevation and fractional cloud cover is tabulated.     

Comparison of weather station snowfall with winter snow accumulation in high arctic basins

Abstract

Most water balance studies in the High Arctic indicate that the weather stations underestimate annual precipitation, but the magnitude of such error is unknown. Based on up to seven years of field measurements, this study provides a comparison of snowfall at weather stations with the winter snow accumulation in their nearby drainage basins.

Snowfall is the major form of precipitation in the polar region for nine months every year. Without vegetation, snowdrift is controlled by the local terrain. By establishing the snow characteristics for different terrain types, total basin snow storage can be obtained by areally weighting the snow cover for various terrain units in the basin. Such a method was successfully employed to compute total winter snowfall in the drainage basins near Resolute, Eureka and Mould Bay. Results show that the basins had 130 to 300per cent more snow than the weather stations recorded. Using revised snowfall values that are reinforced by Koerner's snow core measurements from ice‐caps, it is hoped that a more realistic precipitation map can be provided for the High Arctic.     

A note on albedo variations of the frozen surface of Lake Simcoe during march, 1978: Research note

Abstract

The relationships between monthly anomalies of sea surface temperature (SST) and monthly anomalies of several surface wind parameters are examined using ten years of data from the mid‐latitude North Pacific Ocean. The wind parameters involve both u³ _* and curl τ, where u_* is the atmospheric friction velocity and τ the surface stress. These quantities are calculated from surface wind components analysed on synoptic (6‐hourly) maps. In order to examine the effect of synoptic disturbances, the time series of surface wind components at each grid point is high‐pass filtered (passing periods less than 10 days) and the above wind parameters are calculated from both filtered and unfiltered wind components.

Two statistically significant relationships are found between monthly anomalies of SST and those of the various wind parameters. The first is a large coherent negative correlation between monthly anomalies of u³ _* calculated from the high‐pass filtered wind components and month‐to‐month changes in the SST anomalies in the Central Pacific. This relationship is attributed to the production of turbulent vertical mixing in the ocean by synoptic disturbances in the atmosphere. The second relationship is a large positive correlation between curl τ calculated from the unfiltered wind components and SST anomaly changes in the Eastern Pacific. This relationship, which is opposite to that expected from Ekman pumping, is attributed to a negative association between the wind stress curl and the meridional advection of heat by the eastern boundary current system. It is shown that these atmospheric forcing mechanisms explain up to 10 per cent of the variance of monthly SST anomalies in a large part of the mid‐latitude North Pacific Ocean. This amount is in addition to, but certainly less than, that which can be explained by anomalous horizontal advection through statistical relationships with sea‐level pressure anomalies (Davis, 1976).     

Assessment of GCM simulated snow albedo using direct observations

 Annual cycles of monthly albedos simulated with a general circulation model (GCM) are compared with surface observations. The data observed at 35 stations are retrieved from the Global Energy Balance Archive (GEBA) and drawn from the soil moisture and meteorological observations in the former Soviet Union. The model data are obtained with the ECHAM4 GCM in a ten-year simulation of the present-day climate at T106 resolution. The model calculated albedo values are modified before they are compared with the surface observations: They are interpolated to the stations and adjusted to account for altitude differences and fractional forest area. During the snow-free period, the model underestimates the albedo by up to 0.05 at the stations (with values between 0.2 and 0.25 measured over short grass) because the albedo for grassland is too low in the model. During the period with seasonal snow cover, the model underestimates the albedo by up to 0.2 at stations in Russia, Scandinavia and Canada, which experience severe winters. This underestimation is due to an oversimplified parameterization of the snow covered grid fraction and an inadequate linear relation between snow albedo and temperature. The derivative of albedo with respect to the forest fraction implemented in ECHAM is in line with the observations, although a small overestimation of the model’s gradient has been detected. Received: 3 July 1998 / Accepted: 24 December 1998     

An improved snow hydrology for GCMs. Part 1: snow cover fraction,albedo, grain size,and age

A new, physically-based snow hydrology has been implemented into the NCAR CCM1. The snow albedo is based on snow depth, solar zenith angle, snow cover pollutants, cloudiness, and a new parameter, the snow grain size. Snow grain size in turn depends on temperature and snow age. An improved expression is used for fractional snow cover which relates it to surface roughness and to snow depth. Each component of the new snow hydrology was implemented separately and then combined to make a new control run integrated for ten seasonal cycles. With the new snow hydrology, springtime snow melt occurs more rapidly, leading to a more reasonable late spring and summer distribution of snow cover. Little impact is seen on winter snow cover, since the new hydrology affects snow melt directly, but snowfall only indirectly, if at all. The influence of the variable grain size appears more important when snow packs are relatively deep while variable fractional snow cover becomes increasingly important as the snow pack thins. Variable surface roughness affects the snow cover fraction directly, but shows little effect on the seasonal cycle of the snow line. As an applicaion of the new snow hydrology, we have rerun simulations involving Antarctic and Northern Hemisphere glaciation; these simulations were previously made with CCM1 and the old snow hydrology. Relatively little difference is seen for Antarctica, but a profound difference occurs for the Northern Hemisphere. In particular, ice sheets computed using net snow accumulations from the GCM are more numerous and larger in extent with the new snow hydrology. The new snow hydrology leads to a better simulation of the seasonal cycle of snow cover, however, our primary goal in implementing it into the GCM is to improve the predictive capabilities of the model. Since the snow hydrology is based on fundamental physical processes, and has well-defined parameters, it should enable model simulations of climatic change in which we have increased confidence.This paper was presented at the Second International Conference on Modelling of Global Climate Variability, held in Hamburg 7–11 September 1992 under the auspices of the Max Planck Institute for Meteorology. Guest Editor for these papers is L. Dümenil     

Energy and water balance studies of a snow cover during snowmelt period at a high arctic site

Summary  The predicted global warming is supposed to have an enhanced effect on the arctic regions. How this will influence the water, carbon dioxide and methane balances in the European arctic tundra is the objective of the EU-funded project “Understanding Land Surface Physical Processes in the Arctic” (LAPP), to which where SINTEF is one of several contributors. The snow cover is one of the limiting factors for these exchange processes and knowledge of how it behaves and will behave under a different climate is important. Data collected for water and energy balance studies in an area close to Ny-?lesund at 79°N at Svalbard are the basis of this study. Measurements during the ablation periods since 1992 show an average air temperature for the periods of 2.1 °C, an average incoming shorwave radiation of 230 W/m² and an average measured runoff intensity of 14 mm/day with a maximum of 68 mm/day. Three models of different complexity are tested in order to simulate the water and energy balance of a snow cover on the arctic tundra. The three models are: a complex numerical model (CROCUS), a simple energy balance model and a temperature index model. The simulations were carried out for the melt periods in 1992 and 1996 as these two periods represent very different meteorological conditions. The results of these simulations exposed weaknesses in all the models. The energy balance model lacks calculation of cold content in the snowpack. This influences both the outgoing longwave radiation and the timing of the melt. Due to the effect of compensating errors in the simulations, CROCUS performed better than the simple energy balance model but also this model has problems with the simulation of outgoing longwave radiation. The temperature index model does not perform well for snowmelt studies in regions were radiation is the main driving energy source for the melt. Received September 28, 1999 Revised September 18, 2000     

Intercomparison and validation of snow albedo parameterization schemes in climate models

Snow albedo is known to be crucial for heat exchange at high latitudes and high altitudes, and is also an important parameter in General Circulation Models (GCMs) because of its strong positive feedback properties. In this study, seven GCM snow albedo schemes and a multiple linear regression model were intercompared and validated against 59 years of in situ data from Svalbard, the French Alps and six stations in the former Soviet Union. For each site, the significant meteorological parameters for modeling the snow albedo were identified by constructing the 95% confidence intervals. The significant parameters were found to be: temperature, snow depth, positive degree day and a dummy of snow depth, and the multiple linear regression model was constructed to include these. Overall, the intercomparison showed that the modeled snow albedo varied more than the observed albedo for all models, and that the albedo was often underestimated. In addition, for several of the models, the snow albedo decreased at a faster rate or by a greater magnitude during the winter snow metamorphosis than the observed albedo. Both the temperature dependent schemes and the prognostic schemes showed shortcomings.     

Influence of horizontal inhomogeneity on albedo and absorptivity of snow cover

The variations of albedo and absorptivity of the snow cover are considered caused by the presence of the snow roughness in the form of sastrugi. The numerical modeling is carried out within the framework of statistical approach based on the analytic averaging of the radiative transfer equation and statistically homogeneous model on the basis of Poisson flows of points at the straight lines. The estimates of the influence of 3D-effects of the rough surface are represented depending on optical and geometrical characteristics of sastrugi and on the illumination conditions. It is demonstrated that if the absorption by the snow particles is weak (the single scattering albedo w = 0.9999) the reflection of radiation by snow decreases by ∼ 2–3% when the sastrugi appear. This effect is more significant in near infrared spectral region where w is below 0.99.     

Impacts of snow cover on vegetation phenology in the arctic from satellite data

The dynamics of snow cover is considered an essential factor in phenological changes in Arctic tundra and other northern biomes. The Moderate Resolution Imaging Spectroradiometer (MODIS)/Terra satellite data were selected to monitor the spatial and temporal heterogeneity of vegetation phenology and the timing of snow cover in western Arctic Russia (the Yamal Peninsula) during the period 2000-10. The magnitude of changes in vegetation phenology and the timing of snow cover were highly heterogeneous across latitudinal gradients and vegetation types in western Arctic Russia. There were identical latitudinal gradients for "start of season" (SOS) (r2 = 0.982, p<0.0001), "end of season" (EOS) (r2 = 0.938, p<0.0001), and "last day of snow cover" (LSC) (r2 = 0.984, p<0.0001), while slightly weaker relationships between latitudinal gradients and "first day of snow cover" (FSC) were observed (r2 = 0.48, p<0.0042). Delayed SOS and FSC, and advanced EOS and LSC were found in the south of the region, while there were completely different shifts in the north. SOS for the various land cover features responded to snow cover differently, while EOS among different vegetation types responded to snowfall almost the same. The timing of snow cover is likely a key driving factor behind the dynamics of vegetation phenology over the Arctic tundra. The present study suggests that snow cover urgently needs more attention to advance understanding of vegetation phenology in the future.     

Sensitivity of glaciation to initial snow cover,CO2, snow albedo,and oceanic roughness in the NCAR CCM

We present results from numerical experiments made with a GCM, the NCAR CCM1, that were designed to estimate the annual balance between snow-fall accumulation and ablation for geographically important land regions for a variety of conditions. We also attempt to assess the reliability of these results by investigating model sensitivity to changes in prescribed physical parameters. Experiments were run with an initial imposition of 1 m of (midwinter) snowcover over all northern hemisphere land points. Over Alaska, western Canada, Siberia, and the Tibetan Plateau the model tended to retain this snow cover through the summer and in some cases increase its depth as well. We define these regions as glaciation sensitive and note some correspondence between them and source regions for the Pleistocene ice sheets. An experiment with greatly reduced CO₂ (100 ppm) showed a tendency towards spontaneous glaciation, i.e., the model remained snow-covered throughout the summer over the same geographic regions noted above. With 200 ppm CO₂ (roughly equal to values at the last glacial maximum), snow cover over these regions did not quite survive the summer on a consistent basis. Combining 200 ppm CO₂ and 1 m of initial northern hemisphere snow cover yielded glaciation-sensitive conditions, agreeing remarkably well with locations undergoing glaciation during the Pleistocene. To assess the reliability of these results, we have determined minimal model uncertainty by varying two of the empirical coefficients in the model within physically plausible ranges. In one case surface roughness of all ocean gridpoints was reduced by an order of magnitude, leading to local 10% reductions in precipitation (snowfall), a change hard to distinguish from inherent model variability. In the other case, the fraction of a land grid square assumed to be occupied by snow cover for albedo purposes was varied from one-half to unity. Large changes occurred in the degree of summer melting, and in some cases the sign of the net balance changed as fractional snow cover was changed. We conclude that the model may be able to reveal regions sensitive to glaciation, but that it cannot yield a reliable quantitative computation of the magnitude of the net snow accumulation that can be implicitly or explicitly integrated through time.This paper was presented at the International Conference on Modelling of Global Climate Change and Variability, held in Hamburg 11–15 September 1989 under the auspices of the Meteorological Institute of the University of Hamburg and the Max Planck Institute for Meteorology. Guest Editor for these papers is Dr. L. Dilmenil     

Modeling of aerosol induced snow albedo feedbacks over the Himalayas and its implications on regional climate

Climate Dynamics - Regional climate model (RegCM-4.6.0) coupled with Community Land Model (CLM4.5), which includes SNow, ICe and Aerosol Radiation (SNICAR) model was used to investigate the effect...     

An improved method for determining the degree of snow crystal riming by vertical Doppler radar

An improved method is described to quantitatively determine the degree of snow crystal riming in stratiform precipitation using vertically pointing Doppler radar. Using this method, an automatic determination of the degree of riming at any height above the melting layer is possible. The quantitative statement of snow crystal riming with a limited uncertainty is obtained by a strict separation of stratiform and convective precipitation. For stratiform precipitation, the uncertainty of the degree of riming determined by this remote sensing technique typically remains below one unit on a six step riming scale with somewhat larger uncertainties for very weak riming. A criterion is given for when the situation is stratiform enough to use the proposed method for determining the degree of snow crystal riming. This method is usefull in all studies where direct crystal observation is not possible and the degree of riming is needed with good time resolution over a longer time period and in different altitudes.     

A note on the effects of virtual temperature: Research note

Abstract

The effects of virtual temperature on parcel buoyancy are investigated and the following results obtained. For descent or unsaturated ascent, the buoyancy acceleration is enhanced if the mixing ratio increases with pressure. However, for saturated ascent, the buoyancy acceleration is inhibited unless the lapse rate of mixing ratio exceeds a value slightly greater than that along a moist adiabat. This criterion is evaluated for several mean soundings. For the case of Oklahoma supercells, the mixing ratio lapse rate is clearly large enough. However, for other cases it often is not. Horizontal gradients in water vapour and condensed water can have a significant influence on the thermal wind.     

Impact of absorbing aerosol deposition on snow albedo reduction over the southern Tibetan plateau based on satellite observations

We investigate the snow albedo variation in spring over the southern Tibetan Plateau induced by the deposition of light-absorbing aerosols using remote sensing data from moderate resolution imaging spectroradiometer (MODIS) aboard Terra satellite during 2001–2012. We have selected pixels with 100 % snow cover for the entire period in March and April to avoid albedo contamination by other types of land surfaces. A model simulation using GEOS-Chem shows that aerosol optical depth (AOD) is a good indicator for black carbon and dust deposition on snow over the southern Tibetan Plateau. The monthly means of satellite-retrieved land surface temperature (LST) and AOD over 100 % snow-covered pixels during the 12 years are used in multiple linear regression analysis to derive the empirical relationship between snow albedo and these variables. Along with the LST effect, AOD is shown to be an important factor contributing to snow albedo reduction. We illustrate through statistical analysis that a 1-K increase in LST and a 0.1 increase in AOD indicate decreases in snow albedo by 0.75 and 2.1 % in the southern Tibetan Plateau, corresponding to local shortwave radiative forcing of 1.5 and 4.2 W m⁻², respectively.

     

Chemical components of lower tropospheric aerosols in the high arctic: Six years of observations

Six years of observations (1980 to 1986) of the composition of lower tropospheric aerosols at Alert on northern Ellesmere Island in the Canadian high Arctic yield insight into the seasonal variation of Arctic air pollutants as well as of substances of natural origin. A principal component analysis of 138 observations of 21 aerosol constituents (major ions, metals, nonmetallic trace elements) for the most polluted period of December to April identified not only a soil, sea salt and anthropogenic aerosol component, but also one associated with photochemical reactions in the atmosphere that occur at polar sunrise. Depending on the source of their gaseous precursors, elements in the photochemical component can be natural or anthropogenic in origin. For instance, SO₄ ^2-, existing mostly as H₂SO₄, originates probably from both anthropogenic and natural sources while Br^– is likely of marine origin. In contrast, SO₄ ^2- in the anthropogenic component has the stoichiometry of NH₄HSO₄. In the winter months, over 90% of Arctic SO₄ ^2- is in the anthropogenic and photochemical components.In winter, a substantial portion (11 to 35%) of Na⁺ is associated with the anthropogenic aerosol component suggesting either that marine aerosols have been physically or chemically modified by interactions with air pollution or that there are anthropogenic sources of Na⁺.The aerosol soil component is controlled by both local and distant dust sources. During a year, it has two peaks at Alert, one in April/May coinciding with the Asian dust storm season and one in September.There is a marked difference in the seasonal variation of particulate Br^– and iodine concentrations in the air. Both have a peak in April/May associated with polar sunrise and, hence, photochemical reactions in the atmosphere. However, iodine also peaks in early fall. This may be a product of biogenic iodine emissions to the atmosphere during secondary blooms in northern oceans in late summer.Presented at the Second Conference on Baseline Observations in Atmospheric Chemistry (SABOAC II) in Melbourne, Australia, November 1988     

A note on the comparison of theoretical and empirical quantiles for monthly rainfall totals: Research note

Abstract

For a moderately large number of years of observations the quantiles of rainfall totals for individual months can be estimated from ranked values. With only a few years of observations it is necessary to estimate the quantiles from a distribution fitted to the data for all months.

The Compound‐Poisson distribution can utilise the information available in rainfall totals for all months to give reasonably precise parameter estimates for individual months. Representative parameters can be obtained from as few as 10 years of data. Quantiles are calculated from, and statistically qualified by, the estimated mean and covariance structure of the fitted parameters.     

Prairie tornadoes: Research note

Abstract

The frequency of tornadoes on the Canadian Prairies is a subject of much current debate. The author questions the traditional view that tornado frequency on the Prairies decreases westwards from southern Manitoba. A study of various sources of information on severe thunderstorm events on both sides of the 49th parallel leads to the conclusion that prairie tornadoes often are not isolated singular cases but rather are members of tornado outbreaks. Tornadoes in the northern Great Plains and Prairies are more frequent than has previously been thought, and are likely to be under‐reported compared with those of “tornado alley” in the United States. The Prairie Region can validly be regarded as an outlying section of the North American tornado belt.