Development of a Cut-off Low over South Africa.
22 October 2020
13 October 2010
by Jarno Schipper (ZAMG )
An Upper Tropospheric Cold Low is a conceptual model associated with bad weather and thus is important when making a forecast. The moisture at the surface and the dry air aloft, as well as the cold temperatures found in the upper layers of the cold core of the low, and its stark contrast to the warm surface conditions can cause thunderstorms to grow rapidly and extensively in the vertical therby producing extremely heavy rainfall, often with hail and thunderstorms. In Europe, during autumn, the development of an Upper Tropospheric Cold Low can cause local flash flooding, when the system is slow moving, locked in a so-called omega block of high pressure. The slow movement frequently results in some areas receiving very large rainfall.
The Meteosat-9 Airmass RGB product (see image below) shows an interesting case of a cut-off low over South Africa. The centre of the low is positioned right over the coastline, close to Cape Town. The blue lines represent isolines of Potential Vorticity (PV) on the 315 K isentropic surface (taken from the ECMWF model), which is the level usually used to identify PV anomalies (also called tropopause dynamic anomalies). In this case, the model PV field matches the satellite image very well. The development that led to the formation of this cut-off low is well depicted in this sequence of Airmass RGB images (hourly) (12 Oct 00:00 UTC–14 Oct 12:00 UTC).
Meteosat-9 Airmass RGB Product
Met-9, 13 October 2010, 12:00 UTC
RGB Composite WV6.2–WV7.3, IR9.7–IR10.8, WV6.2
with ECMWF Potential Vorticity on 315 K isentropic surface
Animation (hourly) (12 Oct 00:00 UTC–14 Oct 12:00 UTC)
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