A microbolometer-based far infrared radiometer to study thin ice clouds in the Arctic

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.

Thin ice clouds in the Arctic: cloud optical depth and particle size retrieved from ground-based thermal infrared radiometry

“…The spectral sensitivity of longwave (thermal) radiation to the microphysical properties of ice clouds has been investigated for satellite data (Chiriaco et al, 2004;Dubuisson et al, 2008), airborne data (Brogniez et al, 2004;Libois et al, 2016) and ground-based sensors (e.g. Comstock and Sassen, 2001;Yang et al, 2005, and articles cited above).…”

“…The Turner (2005) method was also employed at Eureka (Nunavut, Canada) to retrieve cloud optical depth and cloud microphysical parameters from AERI spectra acquired between (Cox et al, 2014. A proposed satellite-based instrument, with the goal of characterizing thin ice clouds in the Arctic using far and thermal infrared channels (Blanchet et al, 2011), was recently tested during an airborne campaign in the High Arctic (Libois et al, 2016).…”

“…The LI-RAD technique is based on spectral radiance/brightness temperature comparisons between measurements and radiative transfer calculations. It performs better in the presence of high thermal contrast and is thus well suited for cloud retrievals (Lubin, 1994). In more recent applications, cloud optical depth, effective radius and ice fraction were retrieved from AERI (Atmospheric Emitted Radiance Interferometer; Knuteson et al, 2004a, b) spectral downwelling radiance observations in Antarctic, during the surface heat budget of the Arctic Ocean (SHEBA) campaign and the ARM (Atmospheric Radiation Measurement) Alaska North Slope site Shupe et al, 2015;Mahesh et al, 2001, respectively).…”

“…As cloud altitudes (temperatures) can lead to large uncertainties in this latter technique, Platt (1973) proposed using lidar backscatter profiles along with IR radiometry to estimate cloud altitudes and accordingly improve the retrieval accuracy of cloud emissivity. This active/passive technique (called LIRAD for lidar/radiometer method by Platt) has evolved over the years with improvements such as the availability of high-resolution spectrometers (Smith et al, 1993;Lubin, 1994). The LI-RAD technique is based on spectral radiance/brightness temperature comparisons between measurements and radiative transfer calculations.…”

Airborne observations of far-infrared upwelling radiance in the Arctic

“…For this reason, most measurements taken with the Far-InfraRed Radiometer (FIRR; Libois et al, 2016) during the campaign consisted of vertical profiles of upwelling radiance from the surface up to about 6 km. The FIRR was developed as a technology demonstrator for the Thin Ice Clouds in Far-InfraRed Experiment (TICFIRE; Blanchet et al, 2011) satellite mission, the primary focus of which is on the water cycle in the Arctic, and ice clouds in particular.…”