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a record cold observed at the top of a cumulonimbus cloud!

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According to recent work, the record for the coldest cloudy peak on Earth dates back to December 2018. The study authors further report that the number of extremely cold stormy peaks has increased significantly over the past 20 years. The results are published in the journal Geophysical research letters this March 22.

In November 2019, we mentioned the observation of a minimum temperature of -109.4 ° C at the top of storm clouds accompanying the typhoon Kammuri in the Philippines. The temperature measured by the satellite radiometer in polar orbit NOAA-20 then presented itself as the coldest ever postponed for a cloudy summit.

However, a retrospective analysis published on March 22 uncovered the existence of even lower values. Indeed, on December 29, 2018, this same satellite was a minimum of -111.2 ° C at the top of a convective system located in the western tropical Pacific. A temperature about 30 ° C lower than the average for a typical peak. Incidentally, the instrument did not fly over cumulonimbus clouds at the time of peak activity, suggesting that the true minimum was even lower.

Brightness temperature measured by the NOAA-20 satellite on December 29, 2018. The storm cells are well characterized, one of them having extremely cold penetrating peaks (light blue spots). The temperatures are declined in Kelvin (Kelvin – 273.15 to obtain the temperature in ° C). Credits: Simon Richard Proud et al. 2021.

Record cold, signing of a very deep development of stormy convection

These exceptionally low temperatures testify to particularly intense vertical movements: the vigorous ascents propel the parcels of air so high thatthey locally penetrate the stratosphere. Also, the significant drop in pressure that results brings them to extremely cold conditions. A phenomenon which is accompanied by a cumuliform outgrowth overhanging the rest of the top cloud layer (the anvil). As such we speak of Mountain peak protuberant or penetrating.

During the event in December 2018, the latter was so deep that it brought the maximum altitude of the cumulonimbus to 20.5 kilometers. If we assume that in the stratosphere the air parcels are cooled by about 7 ° C per kilometer, we can estimate that the protruding summit protruded from the anvil by about 2 kilometers. A vigor that researchers link to unusually warm waters and a very favorable large-scale weather regime. In particular, at an important wet phase of theMadden-Julian Oscillation. ” This thunderstorm has reached an unprecedented temperature that pushes the limits of what current satellite sensors are capable of measuring »Relates Simon R. Proud, main author of the paper.

cumulonimbus
Example of protruding tops, here rather modest. Credits: flickr.

Another point raised by scientists is that these extremely cold storm summit events appear to be on the increase. ” Over the past 20 years, it seems that very cold thunderstorms are becoming a bit more common – with the same number of extremely low temperatures in the last 3 years as in the previous 13 years »Says the researcher. An important signal since very cold cumulonimbus clouds tend to give rise to more violent thunderstorms. However, it will be necessary to verify whether it is a real climatic trend or an effect linked to our better observation of the fine details of cloud tops.

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