Antarctic snow and sea ice processes: Effects on passive microwave emissions and AMSR-E sea ice products
In this research, passive microwave remote sensing products generated for the Antarctic sea ice zone from the Advance Microwave Scanning Radiometer-Earth Observing System (AMSR-E) sensor were compared with various in situ field measurements, both from previous Antarctic campaigns in the published literature and as obtained during the Sea Ice Mass Balance in the Antarctic (SIMBA) project during the International Polar Year (IPY) 2007--2008. Data gathered during the SIMBA project was used to understand the geophysical processes occurring in the sea ice and snow cover of the Bellingshausen Sea and to provide a physical basis for modeling of microwave emissions.
In Chapter 2, the AMSR-E sea ice temperature product was compared with AMSR-E snow depth product and previous in situ field measurements. The comparisons were not intended to provide a strict validation of remote sensing products, but to evaluate the physical context of the remotely sensed data and examine potential trends. From examination of the data, it was found that the AMSR-E sea ice temperature product conflicted with several generally observed sea ice properties. The apparent contradictory behavior of the satellite data product is indicative of radiative temperature behavior related to changes in emissivity within the ice pack. Further comparisons of the AMSR-E sea ice temperature product with in situ temperature data from Ice Mass-balance Buoys (IMB) from two Antarctic field programs showed no correlation. However, apparent response of sea ice temperature product to snow/ice interface flooding events was noted.
In Chapter 3, an important sea ice process related to the formation of "gap layers" within Antarctic sea ice was examined and modeled. Gap layers are horizontal voids that develop internally within the sea ice structure, often filled with decaying sea ice, saline slush, and a microbial biological community that thrives on the available nutrients. Gap layers are commonly observed in summer melt conditions in Antarctic sea ice, but are not widely observed in the Arctic. A thermodynamic model was developed based on a typical summer temperature gradient reversal in the snow pack and sea ice, typical salinity profile and heat flux to explain the internal melting of sea ice and formation of gap layers. The modeled rates of gap layer formation generally agreed with published field observations.
In Chapter 4, an overview of the Sea Ice Mass Balance in the Antarctic (SIMBA) experiment is provided detailing various geophysical measurements and the observed snow and sea ice processes occurring during the winter-spring transition in the Bellingshausen Sea. Time series measurements were obtained for snow and sea ice conditions during a 27-day drift station through a number of atmospheric cycles of warming and cooling that are typical of the season for this region. Characteristic sites representing the range of snow and ice conditions on the drifting floe (Ice Station Belgica) were sampled at regular intervals to understand changing conditions in response to the atmospheric events. Detailed snow and ice properties and structure, including high resolution time-series records of snow and ice temperature were obtained from ice mass-balance buoys (IMBs) and other sources to record the changes.
Chapter 5 presents the results of microwave emission modeling performed using the SIMBA field data, specifically processes that are commonly observed in the Antarctic sea ice zone that are considered to have an impact on passive microwave retrievals from space. In several model cases of varying snow cover thickness, the flooding of the snow-ice interface with sea water to form a saline slush layer in the snow cover was simulated. Additionally, a model case including brine wicking at the surface of first year sea ice with thin snow cover was simulated. These processes (related to Chapter 2) have been attributed to anomalous behavior in the AMSR-E sea ice temperature product and were identified as sources of error in other passive microwave sea ice products. The modeling results indicated that brightness temperature at low frequencies (6.9 and 10.7 GHz) showed a large decrease (on the order of 15 to 30 °K) and are consistent with previous laboratory experiments. Further time-series examination of microwave emissions from space, cross frequency and polarization responses, has potential to indicate areas with widespread snow/ice interface flooding.