The Role of NWP Filter for the Satellite Based Detection of Cumulonimbus Clouds

Abstract

This study is motivated by the great importance of Cbs for aviation safety. The study investigates the role of Numerical Weather Prediction (NWP) filtering for the remote sensing of Cumulonimbus Clouds (Cbs) by implementation of about 30 different experiments, covering Central Europe. These experiments compile different stability filter settings as well as the use of different channels for the InfraRed (IR) brightness temperatures (BT). As stability filters, parameters from Numerical Weather Prediction (NWP) are used. The application of the stability filters restricts the detection of Cbs to regions with a labile atmosphere. Various NWP filter settings are investigated in the experiments. The brightness temperature information results from the infrared (IR) Spinning Enhanced Visible and InfraRed Image (SEVIRI) instrument on-board of the Meteosat Second Generation satellite and enables the detection of very cold and high clouds close to the tropopause. Various satellite channels and BT thresholds are applied in the different experiments. The satellite only approaches (no NWP filtering) result in the detection of Cbs with a relative high probability of detection, but unfortunately combined with a large False Alarm Rate (FAR), leading to a Critical Success Index (CSI) below 60% for the investigated summer period in 2016. The false alarms result from other types of very cold and high clouds. It is shown that the false alarms can be significantly decreased by application of an appropriate NWP stability filter, leading to the increase of CSI to about 70% for 2016. CSI is increased from about 70 to about 75% by application of NWP filtering for the other investigated summer period in 2017. A brief review and reflection of the literature clarify that the function of the NWP filter can not be replaced by MSG IR spectroscopy. Thus, NWP filtering is strongly recommended to increase the quality of satellite based Cb detection. Further, it has been shown that the well established convective available potential energy (CAPE) and the convection index (KO) work well as a stability filter.

A Novel Approach for the Detection of Developing Thunderstorm Cells

“…The potential of the WV channel for Cb detection was first discussed in [1] and adapted to 2 water vapor channels by, e.g., [4]. Here, we present a novel approach for the detection of developing thunderstorms.…”

“…In accordance with the evidence given in [4], an NWP stability filter was used to reduce the false alarm ratio and to increase the CSI. For this purpose, the well-established Convective Available Potential Energy (CAPE) [17] and, as a supplementing filter, the Totals Total index (TT) were used, according to [4]. With the NWP filter switched on, developing thunderstorms are only allowed if CAPE exceeds a value of 60 or TT a value of 50.…”

“…The thresholds are very low because only stable and neutral atmospheres are aimed to be excluded. Please see Sections 3 and 4, as well as [4] for further discussions and details.…”

“…A detailed documentation of the model can be found on the ECMWF web-page; see [19]. Further details and discussions regarding NWP filtering are given in [4].…”

“…The approach is used currently for the operational detection of Cbs, e.g., [2,3]. More recently, it has been shown that mature Cbs can be detected by satellites with relatively high POD and low FAR, if the brightness temperature difference method according to [1] is applied to two water vapor channels (7.3 µm and 6.2 µm) in combination with Numerical Weather Prediction (NWP) filtering [4].…”