Abstract
Abstract. A far infrared radiometer (FIRR) dedicated to measuring radiation emitted by clear and cloudy atmospheres was developed in the framework of the Thin Ice Clouds in Far InfraRed Experiment (TICFIRE) technology demonstration satellite project. The FIRR detector is an array of 80 × 60 uncooled microbolometers coated with gold black to enhance the absorptivity and responsivity. A filter wheel is used to select atmospheric radiation in nine spectral bands ranging from 8 to 50 µm. Calibrated radiances are obtained using two well-calibrated blackbodies. Images are acquired at a frame rate of 120 Hz, and temporally averaged to reduce electronic noise. A complete measurement sequence takes about 120 s. With a field of view of 6°, the FIRR is not intended to be an imager. Hence spatial average is computed over 193 illuminated pixels to increase the signal-to-noise ratio and consequently the detector resolution. This results in an improvement by a factor of 5 compared to individual pixel measurements. Another threefold increase in resolution is obtained using 193 non-illuminated pixels to remove correlated electronic noise, leading an overall resolution of approximately 0.015 W m−2 sr−1. Laboratory measurements performed on well-known targets suggest an absolute accuracy close to 0.02 W m−2 sr−1, which ensures atmospheric radiance is retrieved with an accuracy better than 1 %. Preliminary in situ experiments performed from the ground in winter and in summer on clear and cloudy atmospheres are compared to radiative transfer simulations. They point out the FIRR ability to detect clouds and changes in relative humidity of a few percent in various atmospheric conditions, paving the way for the development of new algorithms dedicated to ice cloud characterization and water vapor retrieval.
Highlights
During the Arctic polar night, the energy budget of the surface and atmosphere are mainly governed by long-wave radiative fluxes (Overland et al, 1997)
The sensitivity of the simulations to atmospheric inputs was estimated by adding a ±1 K vertically uniform offset to the temperature profile or by perturbing the specific humidity by ±10 %
The standard deviation of the measured radiances is less than 0.02 W m−2 sr−1 for all bands except for the 30–50 μm band, which is close to the resolution obtained in the laboratory (0.015 W m−2 sr−1)
Summary
During the Arctic polar night, the energy budget of the surface and atmosphere are mainly governed by long-wave radiative fluxes (Overland et al, 1997). It is well established that these radiative properties are highly sensitive to the amount of water vapor (Turner and Mlawer, 2010; Bianchini et al, 2011) and to the physical properties of clouds (Curry, 1983; Maestri et al, 2014). These two major components of the hydrological cycle and their radiative contributions remain poorly known in the Arctic winter. Decadal cloud cover trends are uncertain and the negative trend observed by Wang (2003)
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