Comparisons of passive microwave remote sensing sea ice concentrations with ship-based visual observations during the CHINARE Arctic summer cruises of 2010–2018

In order to apply satellite data to guiding navigation in the Arctic more effectively,the sea ice concentrations(SIC)derived from passive microwave(PM)products were compared with ship-based visual observations(OBS)collected during the Chinese National Arctic Research Expeditions(CHINARE).A total of 3667 observations were collected in the Arctic summers of 2010,2012,2014,2016,and 2018.PM SIC were derived from the NASA-Team(NT),Bootstrap(BT)and Climate Data Record(CDR)algorithms based on the SSMIS sensor,as well as the BT,enhanced NASA-Team(NT2)and ARTIST Sea Ice(ASI)algorithms based on AMSR-E/AMSR-2 sensors.The daily arithmetic average of PM SIC values and the daily weighted average of OBS SIC values were used for the comparisons.The correlation coefficients(CC),biases and root mean square deviations(RMSD)between PM SIC and OBS SIC were compared in terms of the overall trend,and under mild/normal/severe ice conditions.Using the OBS data,the influences of floe size and ice thickness on the SIC retrieval of different PM products were evaluated by calculating the daily weighted average of floe size code and ice thickness.Our results show that CC values range from 0.89(AMSR-E/AMSR-2 NT2)to 0.95(SSMIS NT),biases range from−3.96%(SSMIS NT)to 12.05%(AMSR-E/AMSR-2 NT2),and RMSD values range from 10.81%(SSMIS NT)to 20.15%(AMSR-E/AMSR-2 NT2).Floe size has a significant influence on the SIC retrievals of the PM products,and most of the PM products tend to underestimate SIC under smaller floe size conditions and overestimate SIC under larger floe size conditions.Ice thickness thicker than 30 cm does not have a significant influence on the SIC retrieval of PM products.Overall,the best(worst)agreement occurs between OBS SIC and SSMIS NT(AMSR-E/AMSR-2 NT2)SIC in the Arctic summer.

Sea ice conditions and navigability through the Northeast Passage in the past 40 years based on remote-sensing data

Sea ice conditions and navigability along four typical routes of the Northeast Passage(NEP)are analysed using remote-sensing data from 1979 to 2019.The influence of air temperature(T_(air))and surface wind on the sea ice concentration(SIC)and the navigability of routes is determined.It is found that the annually averaged SICs of the different routes have decreased over the past 41 years.The fastest rate of decrease occurred in the Kara Sea(∼−1%per year),while the slowest rates of decrease occurred in the Laptev/East Siberian Sea(∼−0.42%per year).The number of navigable days for the Kara Sea has become∼1–2 months longer than the Laptev/East Siberian Sea route as a result.The effect of T_(air) on SIC,quantified byΔSIC/ΔT_(air) in the routes through the eastern Kara Sea and Laptev/East Siberian Sea in 2010s was∼−0.04/℃,two to three times that seen during the 1980s.Air temperature is becoming a significant driving force of melting ice in these routes.Surface winds are also a crucial factor for the navigability of the Vilkitsky Strait and Long Strait,as they drive ice drift,and affect the navigability of the Kara Strait by introducing warm air.