The new potential of deep convective clouds as a calibration target for a geostationary UV/VIS hyperspectral spectrometer

“As one of geostationary earth orbit constellation for environmental monitoring over the next decade, the Geostationary Environment Monitoring Spectrometer (GEMS) has been designed to observe the Asia-Pacific region to provide information on atmospheric chemicals, aerosols, and cloud properties. In order to continuously monitor sensor performance after its launch in early 2020, we suggest in this paper deep convective clouds (DCCs) as a possible target for the vicarious calibration of the GEMS, the first ultraviolet and visible hyperspectral sensor onboard a geostationary satellite. The Tropospheric Monitoring Instrument (TROPOMI) and the Ozone Monitoring Instrument (OMI) are used as a proxy for GEMS, and a conventional DCC-detection approach applying a thermal threshold test is used for DCC detection based on collocations with the Advanced Himawari Imager (AHI) onboard the Himawari-8 geostationary satellite. DCCs are frequently detected over the GEMS observation area at an average of over 200 pixels within a single observation scene. Considering the spatial resolution of the GEMS (3.5 ×× 8 km2), which is similar to the TROPOMI and its temporal resolution (eight times a day), the availability of DCCs is expected to be sufficient for the vicarious calibration of the GEMS. Inspection of the DCC reflectivity spectra estimated from OMI and TROPOMI data also shows promising results. The estimated DCC spectra are in good agreement within a known uncertainty range with comparable spectral features even with different observation geometries and sensor characteristics. When DCC detection is improved further by applying both visible and infrared tests, the variability of DCC reflectivity from TROPOMI data is reduced from 10% to 5%. This is mainly due to the efficient screening out of cold, thin cirrus clouds in the visible test and of bright, warm clouds in the infrared test. Precise DCC detection is also expected to contribute to the accurate characterization of cloud reflectivity, which will be investigated further in future research.”

2.1. UV–VIS Hyperspectral Sensor

2.1.1. GEMS

The GEMS covers the Asia-Pacific region (5°S–45°N and 75°E–145°E), observing the Earth in an east-west direction with a fixed north–south field of view (FOV) of 7.73° [2]. For the retrieval of the concentrations of atmospheric gases (O3, NO2, SO2, and HCHO) and aerosol properties, the GEMS has been designed to provide a continuous spectrum from 300 to 500 nm, with a spectral resolution of better than 0.6 nm every 0.2 nm. As the first hyperspectral UV–VIS sensor onboard a GEO satellite, the GEMS is expected to provide critical information for the monitoring of the regional transport of atmospheric chemicals at hourly intervals during the daytime as part of the GEO constellation [35].
Prior to the launch of the satellite, on-ground sensor characterization and calibration have been conducted during the preparatory phase for the GEMS. While in orbit, the GEMS relies on the onboard calibration consisting of solar diffusers and LED to evaluate and maintain calibration quality. As part of the onboard calibration system, the LED serves as a stable light source to monitor the non-linearity of the electronic response and the aliveness of each pixel at the detector level. Solar measurements have also been designed to monitor and calibrate changes in the sensor response with two transmissive diffusers: a working and reference diffuser. The working diffuser has been designed to observe the sun on a daily basis which makes it to gradually degrade over the course of the mission. Thus, a reference diffuser identical to the working diffuser but observing the sun only once every six months has been included in the calibration system. However, because most components of the sensor are expected to degrade over time, it is important to isolate the degradation of each component of the sensor and accurately calibrate the changes. Because onboard calibration has been incorporated into the calibration system, an independent method for evaluating the overall performance of the calibration system would be useful for maintaining the data quality of the GEMS in the long-term as a back-up calibration strategy.

2.1.2. OMI and TROPOMI

The OMI and TROPOMI are hyperspectral sensors that encompass both the spectral range and the observation area of the GEMS. Operating in a sun-synchronous polar orbit, both sensors take radiance measurements in the ascending node of the satellites at around the local solar time (LST) of 13:30. The top-level specifications for the GEMS, OMI, and TROPOMI are summarized in Table 1. Launched in October 2017, the TROPOMI has stricter data quality requirements compared to other sensors. Because the spatial and spectral resolution of the GEMS is quite similar to the resolution of the TROPOMI, the GEMS and TROPOMI are strongly expected to be reciprocal candidates for inter-calibration once the GEMS goes into operation.
Table 1. Sensor specifications for the GEMS, OMI, and TROPOMI.
Orbit type Geosynchronous
(nadir at 128°E)
mean LST – 13:45)
(mean LST – 13:35)
Spectral range 300–500 nm UV-2 307–383 nm Band 3 320–405 nm
VIS 349–504 nm Band 4 405–500 nm
Spectral resolution < 0.60 nm UV-2 0.42 nm Band 3 0.55 nm
VIS 0.63 nm Band 4
Spectral sampling < 0.20 nm/pixel UV-2 0.14 nm/pixel Band 3 0.20 nm/pixel
VIS 0.21 nm/pixel Band 4
Spatial resolution 3.5 ×× 8 km2
(at Seoul)
13 ×× 24 km2
(along ×× across track)
5.5 ×× 3.5 km2
(along ×× across track)
* The spatial resolution of TROPOMI Band 3-4 has been updated from 7 to 5.5 km along track since 6 August 2019 [36]. UV-2 and VIS indicate the Level 1B products of OMI while Band 3 and Band 4 indicate the products of TROPOMI.

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