College of Oceanic and Atmospheric Sciences, Ocean University of China, Qingdao 266100, China
Key Laboratory of Physical Oceanography, Ministry of Education, Qingdao 266100, China
The National Major Research High Resolution Sea Ice Model Development Program of China under contract No. 2018YFA0605903; the National Natural Science Foundation of China under contract No. 41776192; the Fundamental Research Funds for the Central Universities under contract No. 202165005.
Over the past decades, sea ice in the polar regions has been significantly affecting local and even hemispheric climate through a positive ice albedo feedback mechanism. The role of fast ice, as opposed to drift ice, has not been well-studied due to its relatively small coverage over the earth. In this paper, the optical properties and surface energy balance of land fast ice in spring are studied using in situ observations in Barrow, Alaska. The results show that the albedo of the fast ice varied between 0.57 and 0.85 while the transmittance increased from 1.3×10−3 to 4.1×10−3 during the observation period. Snowfall and air temperature affected the albedo and absorbance of sea ice, but the transmittance had no obvious relationship with precipitation or snow cover. Net solar shortwave radiation contributes to the surface energy balance with a positive 99.2% of the incident flux, with sensible heat flux for the remaining 0.8%. Meanwhile, the ice surface loses energy through the net longwave radiation by 18.7% of the total emission, while the latent heat flux accounts for only 0.1%. Heat conduction is also an important factor in the overall energy budget of sea ice, contributing 81.2% of the energy loss. Results of the radiative transfer model reveal that the spectral transmittance of the fast ice is determined by the thickness of snow and sea ice as well as the amount of inclusions. As major inclusions, the ice biota and particulates have a significant influence on the magnitude and distribution of the spectral transmittance. Based on the radiative transfer model, concentrations of chlorophyll and particulate in the fast ice are estimated at 5.51 mg/m2 and 95.79 g/m2, which are typical values in the spring in Barrow.
Figure 2. Radiometers used in the observation. a. CNR4, b. Ramses ACC-VIS, and c. PRR800/810.
Figure 3. In situ instrumentation on the site.
Figure 4. Time series for air temperature (AT) (a), relative humidity (RH) (b), pressure (P) (c), wind speed (WS) (d), and cloudness (e) during the study period.
Figure 5. Time series of the incident radiation (a), reflected radiation (b), spectral albedo (c), and integral albedo (d), measured by Ramses ACC-VIS in Barrow.
Figure 6. Time series of the incident radiation (a), transmitted radiation (b), spectral transmittance (c), and integral transmittance (d), measured by PRR800/810 in Barrow.
Figure 7. Time series of the incident radiation (a), absorbed radiation (b), spectral absorbance (c), and integral absorbance (d), derived from interpolated data from Ramses ACC-VIS and PRR800/810 in Barrow.
Figure 8. Variations in radiation and heat flux on the surface of the fast ice, including the incident shortwave Rsd, reflected shortwave Rsu, incident longwave Rld, reflected longwave Rlu, solar radiation at top of the atmosphere S, and albedo (a), net shortwave and longwave radiation (Ns and Nl) (b), and turbulent heat flux (c).
Figure 9. Energy budget of fast ice in the spring. From left to right: net shortwave radiation, net longwave radiation, sensible heat flux, latent heat flux, and heat conduction, with their values marked.
Figure 10. Spectrum-independence of the albedo (a) and transmittance (b) from in situ observations and simulations with and without inclusions in the ice. The y-axis on the right in panel b is for the curve without interior inclusions.
Figure 11. Upper part of the ice core sampled at the site in Barrow.