Toward a Better Modeling of Surface Emissivity to Improve AMSU Data Assimilation Over Antarctica

Abstract

This work is in direct line with the Concordiasi international project. It aims to better constrain atmospheric analyses by improving the assimilation of low-level Advanced Microwave Sounding Unit (AMSU)-A and AMSU-B microwave observations over Antarctica. So far, a very small amount of available AMSU observations is effectively assimilated over Antarctica. To assimilate more observations, different issues have to be dealt with. In this work, the surface emissivity issue over Antarctica is examined. In a first step, a thorough review of the use of a specular assumption to calculate emissivity from AMSU-A measurements has been undertaken. The effect of five different assumptions about the surface on retrieved AMSU emissivities has then been evaluated using a one-year database: specular, Lambertian, and three intermediate assumptions. Simulations of brightness temperatures at AMSU sounding frequencies have been produced using a radiative transfer model. The emissivities obtained using the five assumptions have been found very useful in improving these simulations. The most successful schemes are found to be the Lambertian scheme during the winter season and a specular or an intermediate scheme (50% specular, 50% Lambertian) during Antarctica's short summer.

An Assessment of Precipitation Changes over Antarctica and the Southern Ocean since 1989 in Contemporary Global Reanalyses*

“…The operation of these new satellites has brought about a dramatic increase in the volume of satellite observations available for data assimilation (Dee et al 2009;Saha et al 2010)-to the point that some parallel can be drawn with the late 1970s ''shock'' in the global observing system (Bromwich and Fogt 2004). It is noteworthy that these transformations have been largely confined to areas outside Antarctica because the difficult detection of clouds and uncertainties in surface emissivity still largely preclude the assimilation of satellite radiances over ice sheets Guedj et al 2010). Over Antarctica, the reanalyses still rely primarily on a sparse network of terrestrial observations (Andersson 2007).…”

Influence of meter-scale wind-formed features on the variability of the microwave brightness temperature around Dome C in Antarctica

“…Typical applications for ice sheets aim to retrieve snow temperature (Shuman et al, 1995;Schneider and Steig, 2002;Schneider et al, 2004), snowmelt (e.g., Zwally, 1977;Abdalati and Steffen, 1995;Torinesi et al, 2003), snow accumulation (Vaughan et al, 1999;Arthern et al, 2006), grain size (Brucker et al, 2010;Picard et al, 2012), thermal properties (Koenig et al, 2007;Picard et al, 2009) or surface state (Shuman et al, 1993;Champollion et al, 2013). Passive microwave data are also widely used in assimilation schemes to constrain atmospheric analyses for which the surface emissivity is an issue, particularly over Antarctica (Guedj et al, 2010). These radiometers, however, have a coarse spatial resolution, typically 10-60 km depending on the frequency and the antenna size of the radiometer (Colton and Poe, 1999;Kerr et al, 2010).…”

Precipitation Changes in High Southern Latitudes from Global Reanalyses: A Cautionary Tale

“…First, the benefits of spaceborne remote-sensing data remain uneven in these regions. While abundant satellite observations have become available over the Southern Ocean, the situation over the Antarctic continent has changed little: the difficult detection of clouds and uncertainties in surface emissivity still largely preclude the use of satellite radiances over the ice sheet Bouchard et al 2010;Guedj et al 2010). Second, direct meteorological observations such as from radiosondes remain essential to calibrate satellite radiances and ensure the temporal consistency of the geophysical quantities derived from them (Dee and Uppala 2009).…”