Remote sensing of aerosols over snow using infrared AATSR observations

Abstract

Abstract. Infrared (IR) retrievals of aerosol optical thickness (AOT) are challenging because of the low reflectance of aerosol layer at longer wavelengths. In this paper we present a closer analysis of this problem, performed with radiative transfer (RT) simulations for coarse and accumulation mode of four main aerosol components. It shows the strong angular dependence of aerosol IR reflectance at low solar elevations resulting from the significant asymmetry of aerosol phase function at these wavelengths. This results in detectable values of aerosol IR reflectance at certain non-nadir observation angles providing the advantage of multiangle remote sensing instruments for a retrieval of AOT at longer wavelengths. Such retrievals can be of importance e.g. in case of a very strong effect of the surface on the top of atmosphere (TOA) reflectance in the visible spectral range. In the current work, a new method to retrieve AOT of the coarse and accumulation mode particles over snow has been developed using the measurements of Advanced Along Track Scanning Radiometer (AATSR) on board the ENVISAT satellite. The algorithm uses AATSR channel at 3.7 µm and utilizes its dual-viewing observation technique, implying the forward view with an observation zenith angle of around 55 degrees and the nadir view. It includes cloud/snow discrimination, extraction of the atmospheric reflectance out of measured brightness temperature (BT) at 3.7 µm, and interpolation of look-up tables (LUTs) for a given aerosol reflectance. The algorithm uses LUTs, separately simulated with RT forward calculations. The resulting AOT at 500 nm is estimated from the value at 3.7 µm using a fixed Angström parameter.Correspondence to: L. G. Istomina (lora@iup.physik.uni-bremen.de)The presented method has been validated against groundbased Aerosol Robotic Network (AERONET) data for 4 high Arctic stations and shows good agreement.A case study has been performed at W-Greenland on 5 July 2008. The day before was characterized by a noticeable dust event. The retrieved AOT maps of the region show a clear increase of AOT in the Kangerlussuaq area. The area of increased AOT was detected on 5 July on the ice sheet east of Kangelussuaq, opposite to the observed north easterly wind at ground level. This position can be explained by a small scale atmospheric circulation transporting the mobilized mineral dust upslope, after its intrusion into the upper branch of the circulation.The performed study of atmospheric reflectance at 3.7 µm also shows possibilities for the detection and retrievals of cloud properties over snow surfaces.

Improved cloud detection over sea ice and snow during Arctic summer using MERIS data

“…Most of the cloud screening approaches do not focus on the case of the snow surface; among those who do (Allen et al, 25 1990;Spangenberg et al, 2001;Trepte et al, 2001;Istomina et al, 2010;Istomina et al, 2011), even smaller fraction utilizes MERIS sensor for this task (Kokhanovsky et al, 2009, Krijger et al, 2011.…”

“…Depending on the retrieved parameter and sensor, the effect of a compromised cloud screening may be moderate (albedo and grain size, SGSP, Wiebe et al, 2013) to drastic (aerosol retrieval, Istomina et al, 2011; melt pond fraction retrieval, . The retrievals of MPF and albedo discussed in this work misinterpret the cloud contamination as melting sea ice surface which cannot be distinguished from the true melting surface and overlays the true values in the daily and weekly 15 means.…”

Radiometric calibration of a non-imaging airborne spectrometer to measure the Greenland Ice Sheet surface

“…While the CIMEL-retrieved aerosol loadings appeared to be indicative of the land ice target, the water vapor was not. Additionally, for satellite image data, it can be difficult to partition aerosol scattering from bright snow and ice surface scattering because atmospheric aerosols have relatively low reflectance by comparison (Istomina et al, 2011), and therefore, we did not attempt to use satellite aerosol retrievals.…”

Aerosol Optical Depth Over the Nepalese Cryosphere Derived From an Empirical Model

“…Atmospheric aerosol particles scatter, reflect, and absorb incoming solar radiation (as a direct effect) (Chylek and Wong, 1995;Solomon et al, 2007), and modify cloud properties (as an indirect effect) (Charlson et al, 1992;Kim et al, 2014). The uncertainty associated with these processes thusly is considered as one of the huge gaps in current climate prediction capabilities (Parry et al, 2007;Istomina et al, 2011;Alexandrov et al, 2016). Considering the significant role of aerosol in climate processes in the Himalaya (Ramanathan, 2001;Meehl et al, 2008;Nair et al, 2013), different studies have evaluated this region (Ramanathan and Ramana, 2005;Ramanathan et al, 2007;Srivastava et al, 2012) focusing on aerosol emissions, optical-physical properties, and its climatic implications (Tripathi et al, 2007;Srivastava et al, 2012;Lau, 2014;Soni, 2015;Paliwal et al, 2016;Zhang et al, 2017) as well as impacts for regional hydrology (Matt et al, 2018).…”