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Our study

At present our study is supported by following reserch funds: In the following part of this page some results from our recent study are presented.

Snow impurity concentrations and the relevant albedo change

The study "Monitoring of snow impurity concentrations and the relevant albedo change by ground based measurements and satellite remote sensing" was supported by the Experimental Research Fund for Global Environment Conservation, the Ministry of the Environment of Japan during FY2009-FY2013.

1. Research Objective
The light-absorbing aerosol depositions on snow/ice surface and the relevant albedo variations are monitored from ground-based measurements and satellite remote sensing. Using these data, a physically based snow albedo model, which can calculate the broadband snow albedos from snow parameters including snow impurity concentrations, is developed and incorporated in general circulation model to improve the estimation of climate impact by light-absorbing aerosol in snowpack. Snow impurity concentration and broadband albedo of snow/ice surface in broad areas are monitored by satellite remote sensing.

2. Research Method
To monitor the light-absorbing aerosol concentrations in snow/ice (snow impurities) and their effect on the albedo variation, the continuous radiation budget and snow pit work measurements are carried out in Hokkaido, Japan. Snow samples collected from snow pit work are analyzed to obtain the concentrations of mineral dust, elemental carbon (EC ~ black carbon (BC)) and organic carbon (OC). Using these data, a physically based snow albedo model and snow grain size calculation scheme are developed and incorporated in an earth system model to estimate the climate impact of light-absorbing aerosols in snowpack. Through this process, snow metamorphism and albedo process (SMAP) model, which is a one dimensional snowpack model, is developed and validated with the data measured in Hokkaido. Using the satellite remote sensing algorithms to retrieve snow impurity concentration and broadband albedo of snow/ice surface, those snow parameters in Greenland, where drastic snow and ice melting is presently undergoing, are monitored with (Moderate Resolution Imaging Spectroradiometer) MODIS data.

3. Result
(i) In-situ measurements of snow impurities at domestic sites
The mass concentrations of EC (presumably equivalent to BC), OC and dust in snow at Sapporo, Japan were analyzed from 2007 to 2013 (Fig. 1). These snow impurity concentrations were relatively low during the accumulation season from December to February, while gradually increased in the melting season from March to April. On the other hand, the remarkable long-term trend was not confirmed during the past six winters.

Mass concentrations of elemental carbon (EC), organic carbon (OC) and mineral dust in snow layer
Figure 1. Mass concentrations of elemental carbon (EC), organic carbon (OC) and mineral dust in snow layer 0-2 cm deep at Sapporo from December to April during 6 winters from 2007 to 2013.


(ii) Improvement of a physically based snow albedo model
A physically based albedo model was developed to calculate the broadband albedos and solar heating profile from any layer structure of snow physical parameters including snow grain size and snow impurity concentrations (Aoki et al., 2011). It was confirmed that albedos simulated by the model with data mentioned in (i) agreed well with the in-situ measurements.

(iii) Development of Snow Metamorphism and Albedo Process model (SMAP)
A 1-dimendional multilayered physical snowpack model SMAP that incorporates a physically based snow albedo model of (ii), was developed to calculate temporal evolution of energy and mass balances of snowpack by taking snow settlement, phase changes, water percolation, and snow metamorphism into account (Niwano et al., 2012). The model evaluation was performed during two winters from 2007 to 2009 at Sapporo, Japan. It was found that accuracy of SMAP was reasonable under dry snow conditions, however, inadequate performance under wet snow conditions highlighted the necessity of further model development. We also investigated the effects of snow impurities on snowmelt with SMAP at Sapporo during the two winters. It was found that snowpack durations at Sapporo were shortened by 19 days during the 2007-2008 winter and by 16 days during the 2008-2009 winter due to radiative forcings caused by snow impurities.



4. Discussion
Light absorbing aerosol in snowpack (snow impurities) has an effect to amplify global warming by reducing the albedo (positive feedback). It is important to clarify the following three issues: how high the impurity concentrations are, how about their trends are, and how much their contributions to albedo reduction and snow melting are.
   The mass concentrations of light-absorbing snow impurities such as EC (presumably equivalent to BC), OC and dust in snow were monitored at Sapporo and Memuro, Japan by an analysis of snow samples and using a ground-based spectral radiometer (Kuchiki et al., 2009). The result was that there was not a remarkable long-term trend at Sapporo from 2007 to 2013. Additionally, the retrieval result of BC concentrations using the spectral radiometer at Sapporo and Memuro suggested that the mixing state of snow and BC particles was different depending on the site and season. This is the important factor in a modeling of the effect of snow impurities on snow albedo.
   A vertical profile of solar heating is investigated using a physically based albedo model. The result showed that most of solar radiation at the wavelengths longer than 1.4 μm is absorbed near the snow surface and that at the wavelengths less than 1.4 μm is absorbed in the thicker layers which depend strongly on snow grain size and impurity concentrations. This could be relevant to the fact that snow grain size in melting period increases within a short period and overall snow layers can melt efficiently.
   In order to assess effects of snow impurities on snow-atmosphere interaction, numerical sensitivity tests with SMAP were performed. Figure 2 indicates snow depths simulated by the default settings that input mass concentrations of BC and dust (CTL), and pure snow experiment that assumed no snow impurities (PURE) during 2007-2009 winters at Sapporo. By comparing snowpack durations between CTL and PURE scenarios, we found that snowpack durations at Sapporo were shortened by more than 2 weeks due to radiative forcings caused by snow impurities. Snow impurity effect on snow melting was significant in melting period for the colder first year and continued since January for the warmer second year.

Effects of snow impurities on snowpack durations at Sapporo
Figure 2. Half-hourly snow depths simulated with SMAP by default settings (CTL: gray solid curve) and pure snow experiment (PURE: gray dots), and the observed snow depths (black solid curve) at Sapporo during two winters: (a) 2007-2008 and (b) 2008-2009.





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