Abstract
Deep Convective Clouds (DCC) are bright, nearly isotropic earth targets extending up to the tropopause, making them favorable as pseudo invariant calibration sites (PICS). Located in all tropical domains, DCC can be observed by both geostationary (GEO) and low earth orbit (LEO) satellites, allowing for large ensemble statistical comparisons of the instrument sensors aboard the respective satellites to measure variation in sensor response. The stability of the GEO instrument can be monitored over time using monthly DCC PDF mode visible reflectances of GEO pixels over the corresponding domain. The temporal stability of DCC reflectances has been well documented. The inter-annual variability of the regional seasonal and diurnal cycle DCC reflectances are very small. The more difficult part of DCC calibration is tying the GEO DCC mode reflectance to the Aqua-MODIS 0.65 µm band calibration reference as recommended by the GSICS community. It has been shown that the DCC mode reflectance is dependent on the IR threshold temperature. If the GEO and MODIS imagers had the same IR temperature threshold, it can be assumed that the GEO and MODIS DCC monthly mode reflectance should be equal over the GEO domain during the MODIS overpass time provided they have the similar visible spectral responses. If this assumption is verified then any GEO in this GEO domain can be anchored to the MODIS calibration and therefor can be applied to historical GEOs. This assumption is verified by using GEO and MODIS ray-matched 10-km or 30-km pixel level averaged radiance pairs to transfer the MODIS calibration. In order to reduce the spectral band differences the ray-matched radiance pairs are limited over DCC. Another permutation of this method is to compare the GEO and MODIS monthly DCC mode reflectance using the same pixel-level ray-matched radiances. This will allow for a reliable transfer of a well-calibrated reference sensor calibration using DCC calibration.
Referencing the Deep Convective Cloud (DCC) Calibration to Aqua-MODIS over a GEO Domain
Deep Convective Clouds (DCC) are bright, nearly isotropic earth targets extending up to the tropopause, making them favorable as pseudo invariant calibration sites (PICS). Located in all tropical domains, DCC can be observed by both geostationary (GEO) and low earth orbit (LEO) satellites, allowing for large ensemble statistical comparisons of the instrument sensors aboard the respective satellites to measure variation in sensor response. The stability of the GEO instrument can be monitored over time using monthly DCC PDF mode visible reflectances of GEO pixels over the corresponding domain. The temporal stability of DCC reflectances has been well documented. The inter-annual variability of the regional seasonal and diurnal cycle DCC reflectances are very small. The more difficult part of DCC calibration is tying the GEO DCC mode reflectance to the Aqua-MODIS 0.65 µm band calibration reference as recommended by the GSICS community. It has been shown that the DCC mode reflectance is dependent on the IR threshold temperature. If the GEO and MODIS imagers had the same IR temperature threshold, it can be assumed that the GEO and MODIS DCC monthly mode reflectance should be equal over the GEO domain during the MODIS overpass time provided they have the similar visible spectral responses. If this assumption is verified then any GEO in this GEO domain can be anchored to the MODIS calibration and therefor can be applied to historical GEOs. This assumption is verified by using GEO and MODIS ray-matched 10-km or 30-km pixel level averaged radiance pairs to transfer the MODIS calibration. In order to reduce the spectral band differences the ray-matched radiance pairs are limited over DCC. Another permutation of this method is to compare the GEO and MODIS monthly DCC mode reflectance using the same pixel-level ray-matched radiances. This will allow for a reliable transfer of a well-calibrated reference sensor calibration using DCC calibration.