engleski

Satellite-based overshooting tops detection methods: comparison and validation

Overshooting convective cloud top (OT) is a dome-like protrusion above a cumulonimbus anvil, often penetrating into the lower stratosphere. It represents a very strong updraft. A single OT exists for less than 30 minutes and has a maximum diametar of ~15 km. According to investigations, storms with OTs often produce hazardous weather conditions such as heavy rainfall, damaging winds, large hail, cloud-to-ground lightning and tornadoes. The OTs also generate gravity waves which can produce significant turbulence. These events represent a considerable hazard to property and human lives, but also to the air traffic. OTs are most easily identified in the high-resolution visible (HRV) channel imagery as the lumpy textured (cauliflower-like) appearance, however only during day-time. Consequently, the techniques which use combinations with solar satellite channels perform well only during the day time. Therefore satellite based methods for the detection of convective clouds and the heights of their tops are usually based on the infra-red window (IRW) measurements, available during both day and night. In the infrared 10.8 μm satellite imagery, a small group of very cold pixels can indicate the appearance of the OT. Due to the restriction of using visible images only during daytime, four detection methods based on the IR window channel 10.8 µm and the absorption channels of water vapor (6.2 µm), ozone (9.7 µm) and carbon dioxide (13.4 µm) in form of brightness temperature differences were used. The theoretical background of all four methods is explained and the results of detection are compared with the High Resolution Visible (HRV) satellite images during day-time, in order to validate each method. Out of four tested methods best performance is found for the combination of brightness temperature differences 6.2-10.8 and 9.7-10.8 µm, which are connected to the overshootings in HRV images in 80% of the cases.

overshooting tops; satellite data; brightness temperature differences

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