First look at some data

Now that we have data, it's time to start looking at it!

I'll show a few pictures that were shown at the weather discussion we had today, a couple hours after the flight landed. The presentation was given by graduate students Lidia Huaman (Texas A&M) and Justin Whitaker (Colorado State) together with Prof. Larissa Back (Wisconsin), and I am cribbing material from them here. Their whole presentation will be available on the field catalog, and these pictures came from there as well in the first place. What's shown here is just a small sample of what was measured on this first OTREC flight, and I'll only begin to explain it. But we have to start somewhere.

Nothing beats satellite images to get a sense of the big picture quickly. Here is an infrared image from the GOES-16 satellite at 1600 universal time  - that's about 10 AM local time in Costa Rica, and a little more than halfway through the flight. Superimposed on the image is the flight pattern that the plane took today (on the left; also shown is the one that wasn't used today, but will be later (on the right). If you look closely you can see the actual position of the plane at the time of the image; it's at the left side of one of the rectangular zigzags near the middle of the pattern.

 

The colors show the strength of emission of infrared radiation from the atmosphere to space, and (perhaps counterintuitively) the more "warm" the color, the lower the emission, meaning the lower the temperature of whatever is emitting. In clear skies, the satellite sees emission from the sea surface and lower atmosphere, which are warm, and emit a lot. Where there are high, thick clouds - such as there tend to be above rain - the satellite sees emission from those clouds, which, being high, are cold, since the atmosphere gets much colder at higher altitudes. So the orange and red blobs are big deep convective systems; that is, organized thunderstorms of the kind we came here to study. These ones were connected to an "easterly wave", or large-scale tropical weather system moving east to west. We'll write more about easterly waves later.

On the plane itself is a downward-pointing cloud radar. Your typical weather radar, the ones you see on tv or your favorite weather app, is a precipitation radar, meaning that it sees raindrops but not clouds. This radar operates at a shorter wavelength, so that it can see the smaller droplets that make up non-precipitating clouds. It can see raindrops too, but if the raindrops are too many or too big, then it can only see the first ones it hits, and then those block the beam so that it can't see the ones further away. In this case since the radar points downward, that means if there is rain it only sees the top of it, and not anything below. This phenomenon is called "attenuation".

The image below is a "curtain" showing reflectivity - roughly, a measure of how much cloud is there - as a function of horizontal distance and altitude along 65 km of the flight track. The top of the plot is the altitude of the plane, about 42,000 feet, and the sea surface is a little bit above the bottom. (Ignore the fact that the radar seems to see below the sea surface - this is an artifact.)

 

So a quick interpretation of this figure is that the plane was flying near the top of a massive complex of stratiform cloud connected to stronger convective updrafts. The stratiform is the solid reddish thing that fills most of the picture from about 15-20 kft up. Nearer to the bottom are black areas, meaning the radar doesn't see anything; but we think most of that is not because there's nothing there to see, but because it's raining hard enough that the radar beam attenuates near the top of the rain and can't see what's below it. There's a lot more one could say about this picture, as well as the many others that were produced during this flight, but we'll leave it there for now.

Just one more for today: below is a plot showing measurements from a dropsonde tossed out of the plane during the same time period as in the images above. A dropsonde is a little package containing instruments that are similar to those on radiosondes (aka weather balloons) but they fall down instead of rising up. It measures temperature (degrees C, red), humidity (shown here as dew point, also degrees C, blue), and pressure (the vertical axis, logarithmic scale). Horizontal wind (knots, shown by the barbs at right) is measured by tracking the sonde's motion using GPS. 

I'll postpone explaining how to read this image. It uses a crazy plotting format called "skew-T log-p", that meteorologists take some time to learn in school. Just the bottom line for now: it shows an atmosphere that's warm and steamy at the bottom, and nearly saturated all the way up, just like one would expect having seen the satellite and radar images above showing deep clouds and rain up to the flight level. This is why we come to the tropics!