Abstract
Detection of clouds over polar areas from satellite radiometric measurements in the visible and IR atmospheric window region is rather difficult because of the high albedo of snow, possible ice covered surfaces, very low humidity, and the usual presence of atmospheric temperature inversion. Cold and highly reflective polar surfaces provide little thermal and visible contrast between clouds and the background surface. Moreover, due to the presence of temperature inversion, clouds are not always identifiable as being colder than the background. In addition, low humidity often causes polar clouds to be optically thin. Finally, polar clouds are usually composed of a mixture of ice and water, which leads to an unclear spectral signature. Single and bi-spectral threshold methods are sometimes inappropriate due to a large variability of surface emissivity and cloud conditions. The objective of this study is to demonstrate the crucial role played by surface emissivity in the detection of polar winter clouds and the potential improvement offered by infrared hyperspectral observations, such as from the Infrared Atmospheric Sounding Interferometer (IASI). In this paper a new approach for cloud detection is proposed and validated exploiting active measurements from satellite sensors, i.e., the CloudSat cloud profiling radar (CPR) and the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) on board the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO). For a homogenous IASI field of view (FOVs), the proposed cloud detection scheme tallies with the combined CPR and CALIOP product in classifying 98.11% of the FOVs as cloudy and also classifies 97.54% of the FOVs as clear. The Hansen Kuipers discriminant reaches 0.95.
Highlights
The advent of new satellite observations promises to transform cloud-climate studies [1], especially in the polar regions [2]
The Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) payload consists of three nadir-viewing instruments: the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP), the imaging infrared radiometer (IIR), and the wide field camera (WFC) [36]
This work reports on the results obtained investigating the effect of surface emissivity on polar night-time cloud detection algorithms
Summary
The advent of new satellite observations promises to transform cloud-climate studies [1], especially in the polar regions [2]. The radiation balance in the polar regions is yet to be understood due to the lack of accurate cloud climatology [7,8,9] This is mostly because of the difficulty in detecting clouds by passive satellite sensors over snow and ice surfaces, where it is difficult to distinguish between the reflectance and temperature of clouds and those of the underlying surface [2,10]. In particular cases the spectral emissivity of ice/snow surfaces may resemble the spectral signatures of clouds when observed by satellite and could potentially confuse detection techniques relying on thresholds This in particular affects the infrared (IR) window tests usually exploited in cloud detection schemes over the Arctic region [16,17,18].
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