[Music]
STEPHEN PALM: CPL stands for Cloud Physics Lidar,
and it is a remote sensing tool
used on high altitude aircraft.
It measures the backscatter from the atmosphere,
which gives you a picture of the cloud structure
and aerosol structure in the atmosphere.
WILLIAM HART: The light gets attenuated as it goes through clouds.
And so, if you have a dense water cloud,
it becomes attenuated very quickly
and goes to zero, basically,
so you get no measurements.
But cirrus clouds can have many kilometers of tenuous density
and so you can get a nice profile many times
through the cirrus cloud.
[Music]
STEPHEN PALM: You get high vertical resolution of 30 meters
and horizontal resolution of about 200 meters.
So you get a very high resolution picture of the atmosphere.
The main uses of CPL
are for cloud properties and aerosol properties.
It can very accurately tell you the height of the clouds,
which then can be transferred into temperature,
which is very important to know,
because that affects the radiative balance
of the atmosphere.
So in ATTREX, what we are looking at is
the very high altitude clouds.
Clouds that are up close to the tropopause
because ATTREX wants to know about moisture transfer
between the troposphere
and the stratosphere.
And when you have cirrus clouds forming
at the at that boundary,
turbulence and gravity waves can transfer some moisture
from the troposphere to the stratosphere.
And knowing that those clouds are there
can only be done by CPL.
WILLIAM HART: The CPL is in the front of the plane
and what we can see while we are looking at the front
are the three units at the top, covered in gold foil.
These units are the electronics
for controlling the laser and the power supply
and the data system electronics
for recording the data that is detected.
The laser and the optics are covered, we can't see them,
they are in a pressurized box right in this area here.
So the optics directs the laser pulse out
from the bottom of the plane
STEPHEN PALM: It carries a laser, three wavelength laser.
We have 355 nanometers,
532 nanometers and 1064.
And the reason that you want to have multiple wavelengths
is because it can give you information on particle size,
mainly for aerosols, but also for clouds.
And then we have a fourth channel,
which is a depolarization channel,
and that is sensitive to the polarization state
of the return radiation from the laser pulse.
And when you have spherical particles
like water droplets,
you get very little depolarization.
But if you have ice crystals, you get a depolarized return.
So when you look at cirrus clouds, for instance,
you will have a higher depolarization
than you will from water clouds.
So that gives us a way to distinguish
between water and ice,
which is important in a number of applications
and is also important for radiative transfer.
WILLIAM HART: The backscattered signal is reported as a function of time
between the plane and the ground.
And, as that is being done, the reflective signal
comes back through the data system computer
which then stores the data on the disk
right up at the top.
[Music]
STEPHEN PALM: As we are taking data
we have a ground station here
where the data is actually sent down via KU,
communication band,
and we are able to display the profiles on our ground system.
And that enables us to
ascertain the performance of the system
and it also gives the mission scientist
and the flight planners information on
the cloud structure and where the clouds are.
For instance, I was saying before,
that we want to go to areas where we have cirrus clouds
close to the upper troposphere.
And you wouldn't know that
unless you had these real time data coming down
and you can look and say,
you know, we've got an area very interesting.
Let's, you know, stay there for a while
and look at that particular area.
[Music]
So that gives them the ability to instantaneously
react to the conditions.
Helps them in their flight planning.