Methods for detecting Deep Convective Clouds (DCCs) from METEOSAT Second Generation Imagers: A case study for 2005 summer season

As a result of progressive global warming, in some regions the frequency of Deep Convective Clouds (DCCs) is expected to increase. However, reliable information about DCCs presence still remains one of the greatest challenges in modern atmospheric sciences, so the misconclusions about existing trends can be avoid. The most accurate data seems to be radar and lidar profiles; unfortunately, the limited spatial resolution of those data, as well as the few years’ time of the operation of radar&lidar missions, makes them not sufficient in terms of providing long-term climatological research. On the other hand, traditional ground-based synoptic observations are mostly limited to land surfaces and are becoming to be provided increasingly rarely. The most promising datasets for present and future DCCs climatologies are those retrieved from geostationary satellite imagers, among which are the METEOSAT First (MFG) and Second Generation (MSG) instruments.

The most commonly used methods for detecting DCCs from satellites are the ones based on brightness temperature (BT), where the specified threshold indicate the presence of convective clouds on satellite images. The simplest of BT-based methods uses the radiances from only one spectral channel: 11 µm (BT11 method). More advances approaches additionally need radiances at 6 µm water vapor absorption channel (BT6-BT11 method) and 9.7 µm oxygen absorption channel (BT11-BT6-BT9.7 method). An utterly different method is to analyze cloud properties, such as cloud top pressure (CTP) and cloud optical thickness (COT), and to determine DCCs presence due to International Satellite Cloud Climatology Project (ISCCP) classification (COT-CTP method). Convective clouds detection can be also supported by meteorological reanalyses.

METEOSAT instruments operates since early 1980s. However, the first generation of METEOSAT imagers were retrieving radiances at only three spectral channels and thus MFG datasets would allow to use only two of the above-mentioned methods: BT11 and BT6-BT11. The rest of approaches could be applied only for MSG datasets (2000s+ time period). This study aims to investigate how many information about DCCs presence are being missed while using only the BT11 or BT6-BT11 methods (which are possible to employ for METEOSAT both First and Second Generation Imagers data) in comparison to the other methods (available for MSG, but not for MFG instruments).

We use data High Rate SEVIRI Level 1.5 Image Data - MSG - 0 degree data, as well as the CLAAS-2.1 (MSG, 0 degree) data containing cloud properties such as CTP and COT for summer season of 2005 (full disk). We compare the frequencies of DCCs determined in accordance to the following methods: BT11 (with ranging brightness temperature thresholds), BT11-BT6, BT11-BT6-BT9.7, COT-CTP, BT11+meteorological reanalyses. The study answers the question how the climatological statistics of DCCs vary depending on the method adopted for detecting these clouds.

This research was funded by the National Science Centre of Poland. Grant no. UMO-2020/39/B/ST10/00850.