Now that we had sampled the various types of continental convection, we needed to explore a contrasting regime – the convective clouds over the tropical Ocean. Here we expected to find the lowest aerosol particle concentrations, in combination with a greater input of moisture and latent heat. Again the morning sky was cloudless, and again the first few cu’s started up around 1030L.
Left: HALO's crowded home away from home - the whole airplane didn't even fit into the hangar!
Left: HALO's crowded home away from home - the whole airplane didn't even fit into the hangar!
We took off at1052L (1452Z) in an easterly direction (it’s always runway 10 in Manaus) and climbed to 39,000’, heading towards the Atlantic. We soon reached Santarem, where the Tapajos River joins the Amazon. Here we were able to observe an interesting phenomenon: the “river breeze” effect. It is analogous to the sea breeze, where greater heating over land causes local convection and a line of clouds along the coast. This draws in air from the cooler sea, the “sea breeze”. Over the cooler water, air subsides resulting in the absence of clouds over the water. The rivers in the Amazon are large enough to cause the same phenomenon, which could be easily seen along the wide Rio Tapajos.
After Santarem, we turned northeast towards Macapa. We passed a beautiful, large pyrocumulus way below us, and I was tempted to change the flight plan and descend to investigate it. But this would have taken so much time that we would have risked our chance to work over the Ocean. After a while, we descended to make our lower-level measurements, and for all intents must have looked to the air traffic controller at Macapa like we were making an approach to land there. She asked us where we planned to land, and when we answered “Manaus” she must have thought that we were a bunch of gringos who were REALLY lost! She told us “Standby”, probably to confer with the area controllers who must have told her that we were harmless. She then asked us to call her again when we were 37 miles away on the other side of her airport, surely looking forward to be rid of us again.
Soon we were over open water, which in this case looks like milk coffee from the giant plume of the Amazon River that stretches out over the Atlantic for hundreds of miles. Convection was not particularly abundant over the Atlantic, but we were lucky to find a long cloud street that we could use to make our cloud profiles in. It was nice to get lots of measurements by just zipping along this cloud street at different levels, back and forth.
Before we began our measurements over the sea, I had been a bit apprehensive about our prospects to get data in clean air. The air over the coastal region had still been quite polluted, and I had observed a number of brown pollution layers at various altitudes during descent. Down in the boundary layer, however, the particle concentration was about 300 per cc, typical of clean marine conditions. The clouds acted accordingly, producing ample rain at quite low altitudes.
But even some 100 nautical miles offshore, we still saw the brown pollution layers above 10,000’, which some of the taller clouds pushed through. As they were coming on easterly winds and made no sign of diminishing with distance from shore, they appear to be the result of long-distance transport of smoke from vegetation fires in Africa.
Our work over the Atlantic done, we headed back towards the Amazon at 14,000’ to measure the gradient in cloud properties from ocean to land. Over land, we happened upon a pyrocumulus over a fire northwest of Belem, which we took a few minutes to sample.
Eventually, we climbed to FL430 again for our way back. Close to Manaus we saw a large outflow from a huge Cb near Manaus, and since we had a little flight time left, we decided on the spot to make a measurement run through it.
On the approach to Manaus airport, we had a surprise: we were informed that because of “rubber removal from the runway” we had to enter a holding pattern. So we spent another 15 minutes circling at 4000’ before we could land at 2200Z. This was my last flight during ACRIDICON; tomorrow I’ll be flying back to Germany. Sitting on a cramped commercial flight, I’ll miss the excitement of being on HALO!