Tuesday, 13 January 2009
Solar channel calibration using deep convective clouds
Hall 5 (Phoenix Convention Center)
Over the tropical latitudes, there are abundant clouds overshooting the Tropical Tropopause Layer (TTL). These deep convective clouds (DCCs) were used for testing if solar channel calibration is possible, using collocated IR window channel measurements. The proposed method was tested first using MODIS measurements. DCCs were determined from MODIS 10.8 μm brightness temperature (TB) measurements by applying the criteria of TB ≤ 190 K, and then MODIS-derived cloud optical thickness (τ) and effective radius (re) of determined DCCs were examined to find typical values representing DCCs. It was found that most of τ of those selected DCCs are greater than 100 or appear to be larger than 200. In addition, re distributions show a sharp peak centered at around 20 μm. MODIS visible channel radiances were then simulated using a modified SBDART radiative transfer model with Baum scattering data (Baum et al., 2005a and 2005b) for homogeneous overcast ice clouds of τ = 200 and re = 20 μm, based on the assumption that reflected visible radiances are in near saturation when τ > 200. The comparison of simulated radiances with MODIS-observed radiances for one year of 2006 demonstrates that visible channel measurements can be calibrated within a ±5% uncertainty range on a daily basis. Furthermore, considering that DCCs are abundant over the tropical latitudes and that the algorithm only requires DCC determination, the method can be easily adopted for the calibration of visible sensors aboard both geostationary and low-orbiting satellites.