南极海冰问题评述:观测

海冰通过其对地面反照率的作用以及对大气和海洋之间热交换的局地障碍和对世界海洋环流的作用在全球热平衡和气候变化中起着重要的作用。南极海冰又因其极为显著的时空变化引起越来越多的关注。海冰的观测是海冰研究的重要内容 ,本文综述了南极海冰观测的发展过程 ,着重介绍了卫星在探测南极海冰方面的重要进展 .

A REVIEW OF ANTARCTIC SEA ICE PROBLEMS:OBSERVATIONS

Sea ice is widely recognized as having a strong influence on global environment system.Its high albedo lessens the absorption of solar radiation;its strong insulation effect restricts exchange of heat,mass and momentum between ocean and atmosphere;and its rejection of salt during freezing and aging alters the salinity content of the upper ocean.This survey is limited to discuss the history of Antarctic sea ice observation,especially emphasized on the developments since satellite era. Scientists had realized Antarctic sea ice even before they set foot on the continent itself.But until now,Antarctic sea ice zone around 55-65°S remains the most data sparse latitude range over the globe for conventional meteorological observations.Changes in the Antarctic sea ice zone may be among the most important of the global changes taking place. Visible and infrared imagery has been available from the NOAA satellite series since the launch of NOAA 2 in October 1972.The NOAA 2-5 satellites carried two-channel Very High Resolution Radiometers(VHRRs),which have since been succeeded by four or five-channel Advanced Very High Resolution Radiometers(AVHRRs).The resolution of AVHRR image is approximately 1.1km.The usefulness of the AVHRR imagery for sea ice studies is hindered by the obscuring effect of cloud cover. The first Electrically Scanning Microwave Radiometer(ESMR) aboard the Nimbus 5,which launched in December 1972 provided global,microwave coverage.It has several decided advantages over the observation in the visible and infrared wavelenths.Specially the microwave data are not obscured by cloud.The value of microwave observation for sea ice studies derives from the sharp contrast between the microwave emissivities of sea ice and open water.The Scanning Multichannel Microwave Radiometer(SMMR) allows the possibility of determining not only sea ice concentration but also additional parameters,such as ice temperature,snow cover,multiyear ice fraction and so on.Like the SMMR instruments,the active-microwave Synthetic Aperture Rader(SAR) has large potential for sea ice studies. The in situ observation for Antarctic sea ice is not negligible.It is the only way to know the truth,which is vital for calibrating satellite data. Weekly ice chart for the Antarctic have been produced in near real time by the Navy-NOAA Joint Ice Center(JIC) since January 1973.Commonly based on JIC ice chart,different Antarctic sea ice data sets give the ice areas vary significantly.Attention must be paied when these data sets are used.