Today's topic: The laser-ceilometer and its observations of the boundary layer.
The boundary layer is the lowest layer of the atmosphere and its name comes from the fact that it is the only atmospheric layer with a defined boundary (the ground). It is arguably one of the most important parts of the atmosphere since we spend the majority of our lives in it. When we ask a question like "How hot or cold will it be today?" or experience high wind gusts, we are having a conversation about boundary layer phenomena. Everything under the red dashed line in this COMET Program image are processes that take place in the boundary layer. For a more detailed explanation, I direct the reader to books such as "An introduction to Boundary Layer Meteorology" by Roland Stull, or perhaps just a quick peak at a Wiki (Wiki Link).
Back in graduate school I spent a great deal of time studying the boundary layer. It wasn't because I wanted to become a boundary layer expert, but rather I was tasked with writing a piece of code that simulated it. I remember distinctly learning from my Ph.D. adviser (Dr. Richard McNider) that there 'is no fuzzy thinking in coding' and thus I needed to really understand the physics that were represented in the equations I was about implement. In the process of learning everything I could on the topic, I inadvertently became fascinated with the boundary layer. I had always heard that this sort of thing could happen, but it wasn't until then I believed it. Boundary layer meteorology has since been my un-official area of expertise, but more importantly my official area of interest.
So lets move into the specifics of today's post....
The boundary layer is full of some of the highest concentrations of the gases that make up our atmosphere, and more importantly for this discussion, atmospheric particles called aerosols. Aerosols can be everything from dust and sand to sea salt (from sea spray) and carbon particles. They can come from both natural and anthropogenic (people) sources. These tiny particles are suspended in the atmosphere and during the day are often displaced to the top of the boundary layer. The commonly seen 'brown cloud' around or downwind of major cities is visual depiction of this.
The boundary layer is full of some of the highest concentrations of the gases that make up our atmosphere, and more importantly for this discussion, atmospheric particles called aerosols. Aerosols can be everything from dust and sand to sea salt (from sea spray) and carbon particles. They can come from both natural and anthropogenic (people) sources. These tiny particles are suspended in the atmosphere and during the day are often displaced to the top of the boundary layer. The commonly seen 'brown cloud' around or downwind of major cities is visual depiction of this.
It turns out that if you point a laser directly into the sky, these particles will scatter some of the laser's energy back to the ground. We have available to us today a variety of commercial lasers for this exact purpose. These lasers employ the concept of Mie scattering, a process by which electromagnetic radiation is scattered backwards. Mie scattering is definitely the more underrated scattering processes as most people only focus on Rayleigh scattering (blue sky anyone?). Over the past 6 months, I have had the opportunity to work with one of these instruments and it has been some of the interesting work I have been involved in to date.
The instrument I am speaking of is the laser-ceilometer.
Ceilometers are laser generating instruments that are pointed into the sky, primarily to detect the height of a cloud base. There is a ceilometer at every ASOS (Huh?) location in the U.S. and many additional located around the globe. Cloud base height detection is quite important for aviation as pilots need to know when they will no longer be able to see out of the window. Clouds return a fairly large amount of the laser's signal and are easy to see in any ceilometer return. Aerosols on the other hand, look more like noise in the signal. Over the last decade or so, researchers and scientists have been using the signal in the noise to observe the boundary layer.
Spoiler alert: A publication highlighting some of these uses is in the works!
The instrument I am speaking of is the laser-ceilometer.
Ceilometers are laser generating instruments that are pointed into the sky, primarily to detect the height of a cloud base. There is a ceilometer at every ASOS (Huh?) location in the U.S. and many additional located around the globe. Cloud base height detection is quite important for aviation as pilots need to know when they will no longer be able to see out of the window. Clouds return a fairly large amount of the laser's signal and are easy to see in any ceilometer return. Aerosols on the other hand, look more like noise in the signal. Over the last decade or so, researchers and scientists have been using the signal in the noise to observe the boundary layer.
Spoiler alert: A publication highlighting some of these uses is in the works!
In the meantime, I wanted to share some pretty neat examples highlighting the laser-ceilometers ability to observe the boundary layer. Here is an example of a 24 hour time series showing a ceilometer's return signal (time vs. height):
The colors on this plot represent backscatter density as seen by the ceilometer. In essence, the more energy that is scattered back to the instrument, the higher the backscatter density, and thus the warmer the color. Here, the majority of the signal is due to the presence of aerosols. When viewed over time, this signal depicts many of the classic structures you would expect to find in the boundary layer. For those that read the book I mentioned above or perhaps visited the Wiki, you will recognize some of the terms in the image above. All three stages of the boundary layer's diurnal patterns are evident here including the mixed layer, the residual layer, and the nocturnal layer. Pretty cool if I do say so myself.
Over the past couple of weeks, the Boulder, CO area has been getting a few snow events. I probably shouldn't use the word 'event' since the amount of snow we have been seeing has been less than impressive. Regardless, the laser-ceilometer isn't picky and snow has a knack for scattering the ceilometer signal. Here is a great example of snowfall as seen by a ceilometer and verified by a camera photo:
As the ceilometer beam intercepts the falling snow, a large amount of the signal is scattered back to the instrument. Since the snowflakes are so much larger than aerosols, the backscatter intensity is also larger. The signal shows up in red in the image on the left (I've drawn an arrow to the snow to help). In case you didn't believe me, I've also included a camera photo from less than 200 m away from the ceilometer. For the interested party, these ceilometer images are being taken from Vaisala's BLVIEW software and the camera image from Vaisala's Navigator software. The ceilometer being used here is the Vaisala CL31.
Following a snow 'event' a couple weeks ago, we got to see a pretty nice display of virga falling from the sky over lunch. I immediately ran to the ceilometer display and found this gem:
Virga is simply precipitation (snow in this case) falling from a cloud and evaporating or sublimating before reaching the surface. When seen with the naked eye, it resembles wisps or streaks falling from a cloud.
This particular ceilometer is sitting on the roof of the Boulder, CO Vaisala office and streams its data directly to my office. I am constantly on the lookout for interesting cases and can promise Ill share anything truly unique in the future. I personally feel that the use of a ceilometer in observing the boundary layer is a really unique concept and I hope you found this at least mildly interesting. If not, I apologize for putting you through reading this.
Cheers.
Scott M.
Ceilometer backscatter density
Over the past couple of weeks, the Boulder, CO area has been getting a few snow events. I probably shouldn't use the word 'event' since the amount of snow we have been seeing has been less than impressive. Regardless, the laser-ceilometer isn't picky and snow has a knack for scattering the ceilometer signal. Here is a great example of snowfall as seen by a ceilometer and verified by a camera photo:
Snowfall event as seen by ceilometer and camera near Louisville, CO
As the ceilometer beam intercepts the falling snow, a large amount of the signal is scattered back to the instrument. Since the snowflakes are so much larger than aerosols, the backscatter intensity is also larger. The signal shows up in red in the image on the left (I've drawn an arrow to the snow to help). In case you didn't believe me, I've also included a camera photo from less than 200 m away from the ceilometer. For the interested party, these ceilometer images are being taken from Vaisala's BLVIEW software and the camera image from Vaisala's Navigator software. The ceilometer being used here is the Vaisala CL31.
Following a snow 'event' a couple weeks ago, we got to see a pretty nice display of virga falling from the sky over lunch. I immediately ran to the ceilometer display and found this gem:
Backscatter density from ceilometer
Virga is simply precipitation (snow in this case) falling from a cloud and evaporating or sublimating before reaching the surface. When seen with the naked eye, it resembles wisps or streaks falling from a cloud.
This particular ceilometer is sitting on the roof of the Boulder, CO Vaisala office and streams its data directly to my office. I am constantly on the lookout for interesting cases and can promise Ill share anything truly unique in the future. I personally feel that the use of a ceilometer in observing the boundary layer is a really unique concept and I hope you found this at least mildly interesting. If not, I apologize for putting you through reading this.
Cheers.
Scott M.
