Planet Earth Online - The Big Dish

Chris -   Some clouds can be identified easily from the ground, others with more difficulty, but when weather forecasters and academic researchers need a bit more detail about what's up in the air, then they turn to the Chilbolton facility for Atmospheric and Radio Research in Hampshire.  These guys can use the world's largest fully steerable meteorological radar, known as the "Big Dish", as Planet Earth podcast presenter Sue Nelson found out with the facility's head, Charles Wrench...     

Charles -   This dish is 25 meters in diameter and it's fully steerable, which means that we can scan it in both elevation and azimuth to track storms or clouds as required by the researchers who come to use the facility.

Sue -   How easy is it to actually steer?  You say fully steerable, but I can't imagine how you would actually begin to steer something of this magnitude and height.

Charles -   It is a big structure; it was originally designed for radio astronomy.  We don't use it for that purpose right now, we're using it for meteorological research. We have our radar scientist who is just about to start it up for us, using computer control in the control room, and you'll be able to hear the system operating.

(Horn)

Charles -   That's the klaxon to warn anybody outside that the dish is about to start moving.  We're now doing an azimuth scan, so it's at a fixed angle of elevation and is going to rotate through 360 degrees.

Sue -   That's amazing because it's very smooth.  I mean, we heard a slight frictional screech of a start-up...

Charles -   That was the breaks letting go.  The breaks let go, but now, it is actually very quiet.  We've recently had the servo system upgraded.  Before that, it sounded a lot more impressive, a lot more noisy! But now it's very quiet, very smooth.

Sue -   This enormous dish has now turned through 90 degrees in a very short period of time.

Charles -   It does actually rotate at 1 degree per second, so a 360-degree scan will take you 6 minutes.

Sue -   Well as this huge dish looks out across the Hampshire countryside, let's just go in to what sort of technology you're using here, Charles?  Obviously there's an enormous antenna just there...

Charles -   Yes, the antenna is a very large structure.  There is a multi-parameter 3 Gigahertz radar on the dish, and with that radar we are able to characterise in great detail the structure of storms:  Precipitation, convective storms.  With that radar, we have a 0.25 degree beam, and that's a very narrow pencil beam which means we can move it around within the convective storms and look at the structure going on within those storms.

Sue -   So you're bouncing radio waves off of clouds, like the clouds we can see above us today?

Charles -   Yes, we're transmitting pulses of radio wave energy with a wavelength of about 10 centimetres. The pulses go out into the sky where they encounter targets, and a small amount of power gets reflected back.  We then analyse that power when it returns to us so we can establish particle size, type, phase (i.e. ice or liquid water) etc within storms and clouds.

Sue -   And you've also got a smaller dish only a meter or two across; how many other dishes do you have around the site?

Charles -   That's a 35 Gigahertz radar designed specifically for looking at cloud structures. We use higher frequencies for clouds because the particles within clouds are smaller, and therefore you need a shorter wavelength to detect them effectively.  In terms of dishes on the site, I think we probably have three of that type.

Sue -   What about behind it, because when I arrived there was an even smaller dish and what looked like a plastic owl?

Charles -   Yes, indeed that is a plastic owl!  We have a multi-frequency microwave radiometer which we use for measuring water vapour profiles in the atmosphere and the liquid water content of cloud.  You mentioned the plastic owl- that does a very good job of protecting the instrument from the attack of birds in the vicinity!