Summer and early-fall Sea-ice concentration in the Ross Sea: comparison of in Situ ASPeCt observations and satellite passive microwave estimates

Abstract

Sea-ice conditions were observed using the AsPeCt observation protocol on three cruises in the Ross Sea spanning the Antarctic Summer Season (APIs, December 1999–February 2000; Anslope 1, March–April 2003; Anslope 2, February–April 2004). An additional dataset was analyzed from helicopter video Surveys taken during the APIs cruise. The helicopter video was analyzed using two techniques: first, as an AsPeCt dataset where it was Sampled visually for ice concentration, floe Sizes and ice type on a point basis at 11 km intervals; Second, computerized image processing on a Subset of nine helicopter flights to obtain ice concentration on a continuous basis (1 S intervals) for the entire flight. This continuous Sampling was used to validate the point-sampling methods to characterize the ice cover; the ‘AsPeCt Sampling’ on the helicopter video and the use of the AsPeCt protocol on the Ship Surveys. The estimates for average ice concentration agreed within 5% for the continuous digitized data and point Sampling at 11 km intervals in this comparison. The Ship and video in Situ datasets were then compared with ice concentrations from SsM/I passive microwave Satellite data derived using the Bootstrap and NAsA-Team algorithms. Less than 50% of the variance in Summer ice concentration observed in Situ was explainable by Satellite microwave data. The Satellite data were also inconsistent in measurement, both underestimating and overestimating the concentration for Summer conditions, but improved in the fall period when conditions were colder. This improvement was in the explainable variance of >70%, although in Situ concentration was underestimated (albeit consistently) by the Satellite imagery in fall.

Antarctic summer sea ice concentration and extent: comparison of ODEN 2006 ship observations, satellite passive microwave and NIC sea ice charts

“…Worby et al (2008) have suggested that ship avoidance of higher concentrations may be a factor in these satellite to ship comparisons in the interior pack ice. Knuth and Ackley (2006) found for example, in comparing satellite passive microwave estimates to helicopter digital imagery, that the satellite imagery consistently underestimated ice concentration for high concentrations rather than the overestimate implied here for ship observations only. Since the helicopter data was derived from straight line tracks with no bias for ice navigation in lower concentration, unlike the Oden ship track, this airborne comparison is probably more accurate for ice concentration.…”

“…Passive microwave sensors, unhampered by cloud cover and darkness are particularly well suited to 2 B. Ozsoy-Cicek et al: Antarctic summer sea ice concentration and extent: comparison obtain a large spatial and long temporal record of sea ice concentration and extent (Comiso and Nishio, 2008). However, ice characteristics from space have shown weaker correlation with ship-based observations in summer than in winter, both for ice concentration (Knuth and Ackley, 2006) and ice extent (Worby and Comiso, 2004). These summer studies showed differences between ship observations and satellite passive microwave (SSM/I) of, typically, ±20% in ice concentration and up to 1-2 degrees latitude (>100 km) in ice extent.…”

“…These summer studies showed differences between ship observations and satellite passive microwave (SSM/I) of, typically, ±20% in ice concentration and up to 1-2 degrees latitude (>100 km) in ice extent. While the discrepancies in ice concentration tend to average out over large numbers of measurements (Knuth and Ackley, 2006), the bias in ice extent is one-sided, where ship observations found ice edges further north than observed by satellite microwave (Worby and Comiso, 2004). For the presatellite era (prior to 1960s), de la Mare (1997) used whale catch data as a proxy for summer ice edge location.…”

A physical algorithm to measure sea ice concentration from passive microwave remote sensing data

“…In periods of summer melt and autumn freeze-up the errors rise dramatically, sometimes up to 50% (Agnew and Howell, 2003;Andersen et al, 2007;Ivanova et al, 2014;Knuth and Ackley, 2006;Meier et al, 2001;Spreen et al, 2008). The majority of authors note the following reasons for the errors in ice concentration retrieval by the algorithms from satellite microwave radiometry data:…”

“…-inability to separate emission from more than two ice types (see, for example, Teleti and Luis, 2013); -seasonal variability of sea ice and snow emissivity (Agnew and Howell, 2003;Ivanova et al, 2015;Knuth and Ackley, 2006;Spreen et al, 2008); -non-seasonal regional variability of snow and ice surface emissivity (Agnew and Howell, 2003;Ivanova et al, 2015;Knuth and Ackley, 2006;Spreen et al, 2008); -surface effects, such as surface roughness, snow cover, melting snow, and melt ponds (Andersen et al, 2007;Hewison et al, 2002;Knuth and Ackley, 2006); -weather effects, such as precipitation (rain, snow, snowstorm, etc.) (Andersen et al, 2006(Andersen et al, , 2007Cho and Nishiura, 2010).…”