#16 - Climbing up for the clouds

Today we had very clean conditions and the lower atmosphere was predominantly influenced by marine aerosol. After a short shower we took off sharp at 13:51 UTC under cloud-free conditions. At the beginning we flew a short horizontal leg near sea level to Charly 1, followed by a vertical profile up to 8.000 ft to Charly 2, as usual. The CCN concentration was very low and almost constant with increasing altitude. It ranged from 50 to 100 #/cm³ at 0.3 % supersaturation. Afterwards we decided to go for low level cumulus clouds above the sea. But in reality the helicopter had to climb up to about 10.000 ft for these clouds, because they were quit convective and developed so fast, as can be seen in the first figure. At about 14:25 UTC, ACTOS had the first cloud contact and by the means of PICT a mean droplet size of about 35 µm was measured. We had the chance to fly on top and at the edge of this cumulus congestus cloud three times. A shift in droplet sizes to larger values was found. Possible explanations for this effect are (a) growing droplets in a still towering cloud or (b) PICT measured droplets at different cloud heights. During the whole flight no cirrus pertubated the radiation measurements, only a very thin fast disappearing stratocumulus was located at our flight altitude. Then we went for other cumulus clouds, some of them contained clearly smaller droplets of about 25 µm and others also larger ones of about 35 µm in diameter, see second figure. Being quite northward of Barbados over the sea and becoming really tired because of the lack of oxygen at this high altitude, we decided to turn back to Charly 2 and to descend down to 3.000 ft, which was approximately the height of the cloud base. A horizontal leg at 3.000 ft to Charly 1 and also one at 1.500 ft was performed on the way back to the airport under cloud-free conditions. Considering the former leg, a maximum CCN concentration of 250 #/cm³ was determined by the mini-CCNc operating in the scanning mode, see third figure. At the higher supersaturation values we can assume that all the potential CCN are already been activated to cloud droplets.

Now we are done, that was our last scheduled measurement flight of CARRIBA 2011 with the helicopter CFNHG carrying ACTOS and SMART-Helios. Many thanks to all contributors and the people who gave me the opportunity to be a part of this project!

Porpoise dives around cloud base

Today we performed (probably) our second last flight. After the low level leg at a height of 50 m (ACTOS height) above the sea, we started the profile up to 8000 ft (helicopter height). During this time almost all clouds in our measurement area disappeared. Only north of Ragged Point nice cumulus clouds were located above the sea with a cloud top at 6500 ft. So, we decided to chase these clouds. Afterwards, we flew back to our measurement area and investigated conditions around cloud base east of Ragged and Deebles Point. We started with a horizontal leg at a height of 3000 ft, followed by four porpoise flights between 1500 ft and 3000 ft and finished with a horizontal leg at 1500 ft (helicopter height). Before the last leg, the mini CCNC was switched to the scanning mode. We finished the flight with a low level leg above sea heading to the airport.

Flight #14

For the flight today we took off at 13:26 UTC. After profiling the atmosphere up to 8000 ft we directly started with cloud chasing over the ocean on the eastside of Barbados. Due to the large vertical extension of the first cumulus cloud we choose, we had to ascend again until we reached 8500 ft. For this first cloud PICT was measuring cloud droplets with a mean diameters of 40 µm. Outside the cloud at 0.1 % supersaturation the minature-Cloud Nucleation Counter measured an almost constant value of the total CCN number concentration with 40 cm^-3. After circling this cumulus cloud for several minutes we decided to investigate a slightly higher dissapearing shallow alto cumulus cloud layer. The cloud droplets of this cloud layer were smaller with a mean value of about 20 µm detected by PICT.
Afterwards we began to analyze some smaller low level cumuli. In the following we performed some porpoise flights between 1500 ft and 2500 ft. After almost two hours flight time and without breakdown of any measurement instruments we landed at 15:22 UTC. As you can see in the graph, the total number cocentration of the measured CCN did not changed significantly at different altitudes. At 0.3 % supersaturation it was roughly stable at 100 CCN cm^-3.

Very clean clouds

After the dust of the last few days we had some very clean clouds today. PICT measured mean droplet diameters of up to 40 μm (see exapmle on the right) and there were only few CCNs. First we started flying a profile in our measurement area, which was soon cancelled to catch the few clouds above the sea (north of Ragged Point) before they move over land. The cloud base was about 1600 ft high. Afterwards we went back to the measurement area and finished the profile from 8000 ft down to 1000 ft. Meanwhile a lot of nice cumuli were carried with the southeastern wind towards us, which we started chasing then. Moving towards Barbados they grew quite fast. During chasing we got too far north, so we went back to the measurement area, where we catched tinier cumuli. Unfortunately, there was cirrus above us almost all the time. Then cloud chasing was finished and we stayed at about 1600 ft below the clouds so the Mini-CCNC could perform the scan mode. Heading back to the airport we finished with a low level leg.

Growing and towering cumulus

During the weather briefing, we recognized cumulus clouds above the sea. So we decided to catch clouds over the ocean in contrast to the flights of yesterday. After the profile from 180 ft (ACTOS height) to 8000 ft (helicopter height) in our measurement area, only very thin cumulus clouds were below us (see upper picture). Additionally, a slightly increased concentration of aerosols and CCNs was detected in 8000 ft. Thus, we decided to stay there for a 5 minutes long leg. Afterwards, we tried to catch the thin cumulus clouds and recognized growing ones over the island. In the middle of the island, the cumulus clouds towered and their tops were partly separated and drifted eastwards resulting in a compact altostratus layer. That is shown in the lower picture, which also shows a cumulus congestus at the west coast of Barbados including some showers. We catched the thicker cumulus clouds at the east coast of Barbados, while the developed altostratus covered the sky above us. Thus, the radiation measurements were handicapped. On our way back to the airport we found some cumulus clouds, which were not influenced by this layer - but there was cirrus above.

To show you all what Birgit meant with "a full round-trip around the island" here is a screenshot of our GPS tracking of flight 10.

Biomass burning on Barbados

After our contact to probably polluted clouds in the morning we had the chance to sample clouds at the same place in the afternoon.
Again we started with a vertical profile from 200 ft (ACTOS) to about 8000 ft (helicopter) in our measurement area. Since there were no clouds off the east coast we decided to fly to the west coast where a couple of mature cumuli stayed almost the complete day - the ones Birgit already observed in the morning.
On our way there we performed a horizontal leg at 6000 ft and recognized increased aerosol number concentration and CCN concentration probably due to biomass burning (see upper picture with one bigger fire in the north of the island).
Finally, we were able to catch the clouds near the west cost (but over land) and performed several cloud passages at different altitudes with ACTOS inside the cloud and at the very top. It was really amazing to watch these big cumuli growing and disappearing right next to each other. It is a pity that my camera ran out of power just as we reached the first clouds... The lower pictures shows one of my last pictures for today.