At the beginning of the reporting week we are approaching the equator and the south-east trade winds are slowly loosing strength. The clouds that we are after are becoming rare and it is getting unpleasantly hot. Fortunately, all instruments are functioning properly so that long working periods on the working deck are not required anymore. With 60 kgm-2 the microwave radiometer most likely measures the largest amount of water vapour above the ship along the entire expedition. The trade wind clouds contain about 0.03 kgm-2  liquid water. On Saturday we are crossing the inner tropical convergence zone and the first thundercloud towers are building up in front of us. The wind weakens in this tropical calm zone that was dreaded by seamen in former times. The sea surface appears strangely viscous, like painted in oil. Indeed, during Saturday night a strong thunderstorm is haunting us. We have to interrupt the atmospheric profiling. Sunday is absolutely cloud free which presents Alexei undisturbed aerosol optical thickness measurements.

On Monday we are reaching the north-east trade winds which immediately brings dust from the Sahel and later from the Sahara. This dust event is a stroke of luck for us as we can investigate now the effect of desert dust on air chemistry, radiation and clouds. We also see the dry Sahara air masses in our humidity profiles. With 6 storm forces the NE trade winds are blowing against us and the ride is getting somewhat rougher. The sea spray is reaching up to the observation deck and causes rain alerts in our microwave radiometer. The radiometer is going to be further belayed and we have to clean it from salt crusts on a regular basis.

The first data are analyzed. As an example the first diagram shows the diurnal cycle of the solar irradiance at the surface for April, 18, observed and parameterized by Alexei Sinitsyn. This day is characterized by Cumulus humilis clouds and longer cloud free passages. Measurements and parameterization fit well but we do also have many cases with large discrepancies. Here, John and Alexei are challenged to construct improved parameterizations e.g. for use in climate reconstructions or in climate models.