Satellite-based detection of deep-convective clouds: the sensitivity of infrared methods and implications for cloud climatology

Abstract

Reliable deep-convective cloud (DCC) climatology relies heavily on accurate detection. Infrared-based algorithms play a critical role, as they are the only ones that can be applied to the 6.7 μm water vapour (WV) absorption band and the 11 μm infrared (IR) window band. For over 40 years, the latter has been the only daytime/nighttime channel used in satellite cloud imaging. This study presents the first global validation of three commonly used DCC detection methods, which use brightness temperature (Tb) in the WV and IR bands. These methods are the infrared-window method (IRW; Tb11), the brightness temperature difference method (BTD; Tb6.7-Tb11), and the temperature difference method with the tropopause method (TROPO; Tb11-Ttropo). All methods were applied to 1 year (2007) of Moderate Resolution Imaging Spectroradiometer (MODIS) observations and validated against collocated CloudSat-CALIPSO lidar-radar cloud classifications. Results indicate that even with optimal parameter configurations, DCC detection accuracy remains moderate and is below 75 % (Cohen's κ < 0.4) for all methods. Global accuracy ranged from 56.6 % (for TROPO) to 72.8 % (for BTD) using an optimal threshold of -2 K. Regionally, the BTD method performs best, with an accuracy of 72.9 % over Europe and 67.9 % over Africa. Misclassifications are common with cloud types such as nimbostratus and altostratus (single-layer cloud regimes) and cirrus and altostratus (multilayer cloud regimes). Overall, the BTD method slightly outperforms the others, while TROPO is the least effective. Our study highlighted the high sensitivity of these methods to threshold selection. Even a ±1 K change in the threshold resulted in a 10 %-40 % variance in DCC frequency. This finding is of particular importance for the construction of homogenous DCC datasets, whether they are global mosaics or time series spanning multiple generations of satellite instruments.