Snow load on sea ice is one of the main source of uncertainty in deriving sea ice thickness from altimetry freeboard measurements, like those from ESA’s CryoSat-2 radar altimeter and NASA’s ICESat-2 laser altimeter. Inaccurate snow depths will bias sea ice thickness from freeboard altimetry measurements; for example: a 20 cm error in snow depth leads to ~1 m error in sea ice thickness.

Lack of reliable snow-thickness observations throughout the year and suitable snow climatological models are preventing us to decrease the error associated to snow load on sea ice. Thus both NASA and ESA are showing great interest in reducing the snow uncertainty and in producing the most accurate Arctic-wide snow pack for use in obtaining Arctic sea ice thickness and volume from CryoSat-2 and ICESat2 altimetry measurements.

Combining modelers, observation and remote sensing experts, the main goal of the first NESOSI meeting is to share our progress on modeling snow on sea ice thus far, address uncertainties that are holding us back from producing the most accurate product, discuss current and future observations which will be particularly useful for improving snow processes in modeling the snow pack, identify a road map and how to best collaborate amongst agencies.

Area D - Board 471

Reliably profiling sea ice thickness from space is a key objective of the sea ice research community as this provides invaluable information regarding the state of polar sea ice missing from the more routine measurements of sea ice coverage. Active altimetry missions have been launched by both NASA and ESA within the last two decades, e.g. NASA's ICESat (2003-2008), ESA's CryoSat-2 (2010 onwards) and NASA's ICESat-2 (2018 onwards), which have driven much of the recent attempts to estimate basin-scale sea ice thickness to-date. However, such efforts are hindered by the challenge of obtaining accurate altimetry measurements of ice freeboard (its extension above sea level), along with the ancillary data needed (e.g. snow depth, ice/snow density) to infer ice thickness.

To help calibrate and validate these satellite altimetry ice thickness estimates, several different airborne and in-situ campaigns have been launched by both agencies. NASA's Operation IceBridge has provided annual multi-sensor airborne profiling of western Arctic and Southern Ocean sea ice since 2009, while ESA's CryoVEx has collected airborne and ground-based data in various campaigns since 2002, including several coordinated activities with Operation IceBridge. These efforts have provided invaluable data for cross-calibration and validation, with future campaigns expected to continue to provide the necessary data needed to improve our understanding of CryoSat-2 radar returns and calibrate/validate the new ICESat-2 laser altimeter (ATLAS). Several other ground-based sea ice campaigns have been undertaken and are yet to be fully exploited.

In this talk we present a summary of these joint NASA-ESA efforts, provide examples of recent attempts to improve and reconcile the sea ice thickness record from these various altimetry missions, and look ahead to upcoming cal/val opportunities.

Area D - Board 472