Variability of sea ice emissivity estimated from airborne passive microwave measurements during FIRE SHEBA

Abstract

Abstract. Passive microwave radiometers with frequencies ranging from 37 GHz to 220 GHz were flown over the Surface Heat Budget of the Arctic (SHEBA) experimental site in May and July 1998. These measurements were motivated by the possibility of determining cloud liquid water path, ice water path, and precipitation over sea ice from these frequencies. The comprehensive cloud data set collected in this experiment offers a unique opportunity for improving and adapting passive microwave retrieval methods for application to arctic clouds. However, retrieval of cloud properties from a downward looking radiometer requires an estimate of the surface emissivity and its spectral, spatial, and temporal variation. In this study, brightness temperature measurements are used to calculate sea ice emissivity at each frequency using ancillary aircraft data to characterize the atmosphere and obtain surface temperature. Surface emissivities on clear sky days during the FIRE Arctic Cloud Experiment (FIRE ACE) aircraft campaign have been calculated and compared with previous estimates cited in the literature. Average emissivity at nadir for snow-covered sea ice in the experimental region during late May is estimated as 0.89 at 37 GHz, 0.74 at 89 GHz, 0.72 at 90 GHz, 0.73 at 150 GHz, and 0.84 at 220 GHz. In early July the average nadir emissivity for melting sea ice is 0.86 for 37 GHz and 0.84 for 90 GHz. Estimates of emissivity at 50 ø off nadir are compared with previous satellite and ground-based measurements of dual polarized emissivities at 37 GHz and 90 GHz. Significant variability exists in published emissivity values due to variations in dielectric and physical properties of snow and ice, but our results fall within previously observed ranges. Uncertainties in the emissivity calculations are estimated, and the accuracy required for use of surface emissivity estimates in cloud retrieval methods is discussed.

Remote sensing of sea ice: advances during the DAMOCLES project

“…This is compatible with the value of about 6 cm penetration depth for multi-year ice, as interpolated in Mathew et al (2008) from Haggerty and Curry (2001).…”

“…Knowledge about the surface temperature and atmospheric temperature and humidity profiles is provided from ECMWF ERA-40 reanalysis data available every 6 h in a grid of about 120 km resolution and with 60 vertical levels. Vertical temperature profile within the snow and sea ice pack are taken from the Heat Budget of the Arctic Ocean (SHEBA) observations in 1997(Ulbay et al, 2002 and penetration depth data from Haggerty and Curry (2001) are used to estimate the temperature of the emitting layer, using a linear regression. The discrepancy in time and space between satellite and in situ observations may introduce an error in the analysis, especially in view of a climatological change, but this was the best combination of data available at the time of the study.…”

Surface Emissivity of the Arctic Sea Ice at AMSR-E Frequencies

“…The error is small during late spring and summer when the snow or ice is relatively isothermal [7]. During winter, this is a potential source of error because of the temperature gradient in the sea ice and the resulting overestimation of emissivity [8].…”

An airborne imaging radiometer (AIR) for atmospheric and surface process studies

“…Global climate model parameterizations of radiative and turbulent heat fluxes between the sea-ice and atmosphere also require accurate measurement of surface temperature and the surface-air temperature difference. In addition, satellite retrievals of cloud and sea-ice surface properties in the Arctic rely on estimates of surface temperature 5,6 . While it is known that the consolidated ice pack exhibits significant spatial inhomogeneity in various physical properties, including surface temperature, there is little information documenting small-scale fluctuations in surface temperature over large regions.…”