Forecasting coloured rain in Bulgaria

By Jochen Kerkmann (EUMETSAT), Duncan Axisa (NCAR) and Gergana Kozinarova (National Institute of Meteorology and Hydrology, Bulgaria)

On 23 March a typical, spring season synoptic situation was prevailing over Europe, characterised by a cold meridional flow from Greenland through Western and Central Europe to the North African coast, and with an intensive transport of warm dry air towards the Balkans.

In this strong south-westerly airflow large amounts of dust particles were lifted from Northern Africa to high levels and transported from Libya across the Mediterranean Sea towards Greece, Turkey and eastern Bulgaria.

The dust can clearly be seen in the satellite image below by its intense pink colour. On the same image it can be seen that large amounts of dust got incorporated into a conveyor belt cloud system that stretched from Crete to the Black Sea.

Using this satellite information, it was possible to carry out a research flight around Istanbul/Turkey in the afternoon of 23 March in order to perform aerosol and cloud microphysics measurements (see poster, Duncan Axisa et al., 2008).

On 23 March 2008 the research aircraft measured dust aerosol and cloud over the Black Sea coast of Istanbul. Cloud extended from north-central Libya, across the Mediterranean and western Turkey, to the western Black Sea with the southern edge of the cloud mass ingesting Sahara desert dust. This cloud mass was associated with a weak, low-amplitude short wave trough aloft.

The 12:00 UTC Istanbul sounding had a 4°C surface inversion at 290m and a dry layer from 290m to 3.9km. A strong mid-level flow was present at 50 hPa with winds at 220° 84 knots, indicating that this transport of dust was associated with the subtropical jet. Several stations reported dust in the present weather conditions. Mikonos reported blowing dust at 13:00 UTC. Souda and Iraklion reported dust at 14:00 UTC. Several other stations in Greece, Turkey, Bulgaria and Russia reported dust.

The research aircraft took off at 12:50 UTC. The horizontal visibility was observed to be about 2km with light surface winds and surface pollution visible in the Istanbul area. A brown haze was present at low levels and close to the ground below the 290m inversion. Detailed aerosol profiles were made up to the cloud base.

When flying below cloud base the aircraft flew through virga. A few drops evaporated on the windshield and left a muddy residue. The aircraft proceeded to climb and conduct in-cloud penetrations. The cloud bases where at 12,000ft with virga. A dry layer was present below the cloud bases. Only slight amounts of supercooled water was present and this was only just above cloud base.

The cloud glaciated very quickly with lots of needle hydrometeors present. The poster shows the size distribution of the supercooled water (CDP) and the ice (CIP). The dust was most abundant just below cloud base as shown in the PCASP. CCN distributions where higher at lower altitudes due to the pollution. It is evident in this case that the dust was acting as very efficient ice nuclei.

Interestingly, the satellite information helped the Bulgarian Weather Service to correctly forecast 'coloured rain' on both 23 and 24 March. On Sunday 23 March there was a report from Varna that there had been red rain and from the news it can be deducted that other places in Bulgaria also received coloured rain or coloured snow (in the mountains). On 24 March, initially, the coloured rain continued, but the precipitations were stronger and the aerosols were gradually washed out.

Additional content

Met-9 Dust RGB 3-day composite (22–24 March)
Met-9 RGB NIR1.6, VIS0.8, VIS0.6 (23 March 12:00 UTC)
Met-9 RGB VIS0.8, IR3.9r, IR10.8 (23 March 12:00 UTC)
Met-9 RGB WV6.2-WV7.3, IR3.9-IR10.8, NIR1.6-VIS0.6 (23 March 12:00 UTC)
Met-9 RGB WV6.2-WV7.3, IR9.7-IR10.8, WV6.2 (23 March 12:00 UTC)
Aqua MODIS True color RGB image (24 March, source: NASA)