A multidecade polar climate record from radar scatterometer data: the Scatterometer Climate Record Pathfinder project

Radar scatterometers were originally designed to measure near-surface winds from space by measuring the normalized radar backscatter of the ocean's surface. However, the backscatter data is also collected over the polar ice-covered oceans and land regions. Scatterometers have been providing continuous synoptic microwave coverage of the Earth for over a decade and the earliest dataset is nearly three decades old. The scatterometer backscatter measurements have proven useful in a wide variety of ocean, ice, and terrestrial applications. The long time series enables extensive studies of seasonal and interannual variability as well as surface changes related to climate change.

In this paper data produced and distributed by the NASA-sponsored Scatterometer Climate Record Pathfinder (SCP) project (http://www.scp.byu.edu/) is described and its use illustrated in several polar climate-related studies, including long-term sea ice extent and Antarctic tabular iceberg counts. The goal of the paper is to promote the use of use of the long historical time series of scatterometer backscatter data for climate studies.

The SCP provides two basic forms of gridded data products: 1) calibrated browse backscatter images at the intrinsic sensor resolution; and 2) enhanced spatial resolution backscatter images, which are based on combining multiple overlapping passes. Images are derived from the current SeaWinds scatterometer onboard QuikSCAT (QSCAT) which launched in 1999, the NASA scatterometer (NSCAT) which flew on NASDA's ADEOS-1 platform during 1996-97, European Space Agency's (ESA) scatterometer (ESCAT) carried onboard both the ERS-1 and ERS-2 satellites (from 1992 until 2001), and the Seasat-A scatterometer system (SASS) which flew in 1978. The radar backscatter in the images is a function of surface roughness and dielectric properties of the imaged area. For ice and snow, the roughness factors include surface geometry and roughness, laying, liquid water content, density, and alignment of scatterers. The dielectric properties are related to physical characteristics of the effective scattering layer, including snow grain size, brine concentration and inclusion size in sea ice, as well as by the phase state of water. The backscatter values are very precisely calibrated to a few tenths of a dB and include different frequencies, polarizations and observational geometries (incidence and azimuth angles) that permit detailed studies of a variety of physical phenomena. Backscatter values are very sensitive to the presence of liquid water and so are useful in discriminating between freeze and thaw conditions.

An important application of scatterometer data is sea ice monitoring where the scatterometer observations complement radiometer observations in measuring sea ice extent and motion. For example, in studying sea ice motion, merged scatterometer and radiometer observations provide better estimates then either alone. Scatterometer data is less sensitive to atmospheric effects than radiometer data and can offer high resolution, though it has less sensitivity to sea ice concentration. Several recent sea ice studies have used combined data from one or more scatterometers and other sensors to isolate scattering mechanisms for different ice types and thereby improve discrimination.