well by my time it is Day 6, but this counts as the Day 5 post… It is about the wind. First things first… thanks for the feedback and the great picture Bernhard. I was just wondering if the upside down guy in the cowboy hat in the picture was you?

Now to today’s topic: In Paris when we met a couple of weeks ago there was some discussion of where the planes would and could fly. There is an issue that the ATR 42 (https://www.eufar.net/aircrafts/41/details/) does not have large endurance, and so the plan is for it to fly for two 3-4 hrs shifts overlapping with HALO which can stay out for much longer. The shorter endurance of the ATR means that make more flight, but also more time is spent in ferrying from its airport to the area of operations. This, and the fact that we wanted the airflow it measured to also blow over our measurement station on Barbados (the BCO, or Barbados Cloud Observatory) influences our choice of area of flight operations. Now the ATR has a ferry speed of about 134 m/s so if we allow ourselves 30 min for the ferry then we can get about 240 km off shore. Given that it takes some time to get to crusing altitude, if we want to limit the ferries to 30 min, inclusive of take off and landing, that puts us more like 150km to 200 km off shore. This is also nice because we hope to have a big and beautiful scanning C-band radar which can also see that far. 

So now the question is in what direction. Well upwind of the observatory of course! … but that depends on which way the wind blows.  Here there seems to be some predictability. Looking at our surface wind measurements from now nine-years of data the winds are pretty consistently out of the east, or maybe just a tick north of east as shown in the wind histograms from three years (so the plot would not be too busy) for the EUREC4A period.

How long will it take the airmasses sampled by the ATR to reach the BCO. Well looking at the second attachment a mean wind speed of about 6 m/s is not a bad average. Winds at cloud base tend to be a bit stronger, so let’s say 8 m/s, which for 160 km is 20000s or 5.5 hours. One other thing that the plots suggest, and is consistent with my sense of the data, is that the winds are slightly stronger when they are more out of the North… stiffer trades with roll wave days.