The United Nations decided to establish a common voice for all topics around the atmosphere, its interaction with the oceans, for weather and climate. The national weather offices all over the world realised that for accurate weather forecasts it would be necessary to share their data, and in the following years, more and more nations joined the organisation – 191 until today. Even nations at war or with critical relationships work together in this UN organisation (and have done so since its founding), a very nice indication that science can successfully ignore national thinking.

Fig1. Trough in the east, morning of 22 Mar 17

This year’s World Day of Meteorology has been titled “Understanding Clouds”, a very important and still open issue both for weather forecasts and climate models.

On the first days of our cruise we saw once again that understanding of clouds and their forecast is not easy.

In the last blog entry, I wrote that we expected more clouds on Wednesday, due to the trough that should swing east across our sailing area. In the early hours of Wednesday that looked quite right (see figure 1), but it seemed obvious that we were already on the rear of the trough.

Fig.2 Similar As Fig. 1, but looking to southeast

While in the east (figure 2) of our position the trough was still clearly visible, the situation to the south-west was less clear. The optical impression was that the clouds vanished in the west – figure 3 shows the view back to the south-west, with only few clouds on the right-hand side of the image.

The satellite image (figure 4) on the other hand that was taken at the very time when the photos were shot shows that there was a strip of reduced clouds just behind the vessel, but that we still were inside the cloudy area of the trough.

Fig.3 Same trough when looking to southwest

The cloud band located south-west of the vessel in the satellite image moved east with the trough, it took only two more hours to be passed by the last clouds of the trough. Behind, there were only some high and thin cirrus clouds left. The lower clouds vanished much faster than expected.

We got a wonderful afternoon and those who could be or must have been on deck needed to be careful about getting too much sun – with only 13°C you may not notice how strong the sun already is. And, as promised, the wind deceased during the day to, temporarily, only 1 Beaufort.

Fig. 4 Satellite image (NOAA-AVHRR RGB) At 10:50 UTC (cross at position of the RV Polarstern)

Towards late afternoon another effect could be observed. Water temperature had risen over the day to about 12°C, together with the dew point. With the air temperature coming down from 13°C when the sun came down, humidity increased clearly. It was noticeable first when finding wet places in areas inside shadow, and a look to the horizon clearly showed the humid air layer (figure 5 shows the view to northeast at about 16:45 local time).

Fig. 5 Humid layer against the horizon (northeast) during the afternoon


Posted by Oliver Sievers, DWD

Dr Oliver Sievers of the Deutscher Wetterdienst (DWD) is a forecaster specialising in sea-surface and wave models and ship-route forecasts

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