Cross-Atlantic moisture transport

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Moisture transport from sub-tropical Atlantic brought precipitation to the UK and parts of western Europe on 9 and 10 March 2020.

Cross-Atlantic moisture transport
Date & Time
9 March 00:00 UTC–10 March 23:00 UTC
Meteosat-10, Metop-A & B, NOAA-18 & 19
Airmass RGB, 24-hour Microphysics RGB, Water Vapour, Total Precipitable Water, ECMWF Total Column Water

By Natasa Strelec Mahovic (EUMETSAT)

After the extremely rainy February, with several storms passing over and bringing extensive rain and snow, another rainy episode hit the UK on 9-10 March 2020. The Met Office issued yellow and amber warnings for some areas, expecting high precipitation amounts.

The loop of Meteosat-10 Airmass RGB images (Figure 2) shows a cloud band stretching all the way from subtropical Atlantic towards west Europe.

Figure 2: Meteosat-10 Airmass RGB animation, 9 March 00:00 UTC–10 March 23:00 UTC

Air with large amounts of Total Precipitable Water (TPW), usually referred to as an atmospheric river, was being transported towards Europe, as can be seen in the loop of the MIMIC-TPW2 product (Figure 3). This composite product is made from retrievals using AMSU-B and MSU from NOAA-18, NOAA-19, Metop-A and Metop-B. It also uses the retrieval from ATMS of Suomi-NPP.

Figure 3: MIMIC-TPW product, 9 March 00:00 UTC–10 March 23:00 UTC

The same atmospheric river feature is seen in ECMWF Total Column Water field shown in Figure 4, as a band of high values of total column water content.

Figure 4
Figure 4: ECMWF Total Column Water, 9 March 12:00 UTC

Due to high amounts of TPW, i.e. moisture, precipitation occurred over the majority of the UK during 9 March, as seen in radar images (Figure 5).

Figure 5: Meteosat-10 Airmass RGB product with 3-hourly radar accumulated precipitation overlaid, 9 March 00:00–21:00 UTC

Even after the clouds in the frontal cloud band started to dissolve, the transport of humidity in the upper troposphere could be seen as a blue stripe in Airmass RGB, due to lower brightness temperatures in WV 6.2 channel and larger WV 6.2-7.3 difference (Figure 6).

Image comparison
Airmass RGB image Water Vapour image
Figure 6: Comparison of Meteosat-10 Airmass & WV 6.2, 10 March 15:00 UTC. Full resolution Airmass RGB. Full resolution Water Vapour

Figure 7 shows a vertical cross section across the frontal cloud band. Marked by letter A on the cross-section line, there is a blue area in the Airmass RGB which corresponds to dry layer between 700 and 500 hPa, and moist level aloft, between 500 and 300 hPa. This results in larger WV 6.2-7.3 difference, as well as colder WV 6.2 temperatures, therefore, the blue-violet colour in Airmass RGB. In the same cross-section, lowering of the tropopause and a dry intrusion behind the frontal zone is found, seen as bright red in Airmass RGB — dry, stable, descending stratospheric air, also characterised by high values of isentropic potential vorticity (IPV).

Figure 7
Figure 7: Vertical cross-section of ECMWF fields of Equivalent potential temperature (black lines) and Relative humidity (orange and green lines), 10 March 15:00 UTC . Green lines stand for relative humidity larger than 80%. The location of the cross section is shown on the image to the right

High upper-tropospheric humidity, transported by this atmospheric river, reached as far as the Mediterranean and all the way to the North African coast, still detectable in WV 6.2, as well as Airmass RGB. Since that area was mostly cloud free, with only high clouds, many aeroplane contrails were visible in the 24-hour Microphysics RGB (as blue stripes, especially in the area around Corsica and Sardinia), due to a large content of humidity at flight levels (Figure 8).

Image comparison
Airmass RGB image  24-hour Microphysics image
Figure 8: Comparison of Meteosat-10 Airmass RGB and 24-hour Microphysics RGB, 10 March 18:00 UTC.
Full resolution Airmass RGB. Full resolution 24-hour Microphysics RGB.
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