Relating micro ave backscatter azimuth modulation to surface properties of the greenland ice sheet

Abstract

Azimuth modulation of the normalized radar cross- section in satellite data sets over Greenland is investigated. Data sets from the NASA Scatterometer (NSCAT) and from the European Remote Sensing Advanced Microwave Instrument (ERS) are employed. Azimuth dependence is clearly observed. The largest azimuth dependence occurs in the C-band ERS data with peak-to-peak azimuth modulations up to 3.0 dB. The Ku- band NSCAT data exhibits slightly smaller modulations of upto 2.0 dB. Azimuth modulation is largest in the lower dry snow zone for ERS and in the dry to percolation transition zone for NSCAT. The incidence angle dependence of the azimuth modulation is parameterized over the ice sheet. In general, the azimuth modulation is found to either decrease with increasing incidence angles, or be relatively independent of incidence angle. Regions of large incidence angle dependence for the azimuth modulation include the western dry snow zone for ERS and the northeast dry snow to percolation transition zone for NSCAT. The second order azimuth modulation orientation is highly correlated with wind direction. A new simple surface model is introduced to relate azimuth modulation to surface properties. Using this model, the size and orientation of surface sastrugi are estimated. I. INTRODUCTION The Greenland ice sheet is a critical area of study in esti- mating effects of global climate change. With only a limited number of in situ measurements due to the considerable effort associated with on site studies, remote sensing is an essential tool for studying the dynamics of this region. While satellite based normalized radar cross-section (σ o ) data have been used in a variety of successful studies over Greenland, past studies have generally ignored microwave measurement dependency on azimuth angle. This work is an extension of previous work found in (1). Data from the C-band European Remote Sensing Advanced Microwave Instrument in wind scatterometer mode (hereafter ERS) and the Ku-band NASA Scatterometer (NSCAT) are employed. Both sensors cover a large range of incidence angles. For NSCAT, only the vertical polarization (v-pol) measurements have sufficient azimuth coverage to be used in the study. The ERS measurements are also v-pol. The data shown herein is from the time interval Julian Day (JD) 330 to 360, 1996. This is during the winter when the Greenland surface is relatively constant. Although scarcely studied in Greenland, azimuth modula- tion is common in microwave remote sensing studies over other areas of the Earth. The primary application of azimuth modulation is in using scatterometers to measure vector wind speeds over the ocean surface. The azimuth modulation over Greenland is attributed to a complex snow surface structure created by wind driven snow deposition, aeolian transport, and the formation of wind slabs and hoar layers. The scale of the surface roughness varies from dunes on the km scale to meter scale erosional features known as sastrugi. First, the location and properties of the azimuth modulation are discussed. In particular, the incidence angle dependence of the azimuth modulation is addressed and the locality of the maximum modulation with respect to the different ice facies is addressed. Then the orientation of the azimuth modulation is presented to compare with Greenland wind models. Finally, a simple surface model is presented which relates the azimuth modulation to the geophysical properties of the snow surface.