Tag: radar

NOAA Teacher at Sea
Jennifer Richards
Onboard NOAA Ship Ronald H. Brown
September 5 – October 6, 2001

Mission: Eastern Pacific Investigation of Climate Processes
Geographical Area: Eastern Pacific
Date: September 12, 2001

Latitude: 9º 56.5 N
Longitude: 95º 2.5 W
Temperature: 31.2º C
Seas: Sea wave height: 2-3 feet
Swell wave height: 4-5 feet
Visibility: 10 miles
Cloud cover: 5/8
Water Temp: 29.3ºC

Research Objective for the day: Begin taking measurements with the Lidar (ETL), the MMP (UW), weather balloons (CSU), and the SPMR (UCSB). Every group on the ship is in full swing, and will continue their operations for the next 18 days.

Science Log

Today I met with part of the group from NOAA’s Environmental Technology Laboratory in Boulder, Colorado. There are three sets of instruments being used by this team, and today I will introduce you to the researchers associated with two of those groups- the lidar group and the kaband group.

Ms. Janet Intrieri, an Atmospheric Scientist, and Dr. Raul Alvarez, a Physicist, have been working long hours each day on the Mini MOPA Lidar. This is the most labor-intensive piece of equipment on the ship, requiring constant watch and intervention to keep it running properly. It is also probably the fanciest piece of equipment on the ship, using CO2 lasers and an intricate network of lenses and mirrors to measure wind velocity and water vapor in the atmosphere. The really cool thing about the lidar is that it can measure these things at various altitudes simultaneously, up to 6-8 kilometers in range. Without the lidar, scientists could measure a specific point in the atmosphere using planes, satellites, or weather balloons, but the lidar allows Ms. Intrieri and Dr. Alvarez to see everything in a horizontal column of the sky at the same time.

How does lidar work? Lidar (which stands for Light Detection and Ranging, similar to the term Radar as used for radio waves) is a remote sensing technique that allows measurements of atmospheric conditions using laser light. The typical lidar system emits a short pulse of laser light that travels through the atmosphere. As this pulse of light goes through the atmosphere, it can interact or scatter off of various components in that atmosphere. These components can include dust, clouds, water vapor, pollutants, and even the air molecules themselves. When the light scatters off of these things, a small part of that scattered light is going back toward the receiver part of the lidar which is usually composed of a telescope (to collect as much of this light as possible) and a detector that converts the light signals into electronic signals that can be input to a computer.

How the signals that are collected are processed depends on what atmospheric properties are being measured. For information on the total amount of light scattering due to dust and clouds, we can simply look at the strength of the return signal as a function of time (which is proportional to the distance that the pulse has traveled). To gather information about the amount of water vapor in the atmosphere, one technique is to transmit two laser pulses that are at different wavelengths. One of the wavelengths is selected so that it is not affected by the water vapor, while the other is selected so that it is partially absorbed by water vapor. (Each different chemical that we might try to measure has a different absorption of light that will determine which wavelengths and types of laser must be used.) Now, as the laser pulses go through the atmosphere and as the scattered light returns to the receiver, one of the signals is attenuated (reduced) more than the other because it is being absorbed by the water vapor. The amount of water vapor that must have been in the atmosphere to cause a particular amount of signal reduction can then be calculated.

Another thing that can be measured with lidar is the wind velocity. To do this, we rely on the Doppler Effect. This effect states that as the light scatters off of the particles in the atmosphere, the frequency of the light may be shifted if the particles are moving. If they are moving towards the lidar, the frequency will be shifted up while the frequency will be shifted down for particles moving away. Since the frequency of light is extremely high and the Doppler frequency shift is very small, we need to bring the signal (light) frequency down to a manageable level. We can do this by a process called mixing. In essence, the light signal is shone onto a detector along with a small sample of laser light that is at the same frequency as the original pulse that was sent into the atmosphere. When these two beams interfere with each other, the result is a signal on the detector that is the difference in the two light frequencies. At this point, this difference signal tells us the speed of the wind, but not the direction of the wind. A shift of a few megahertz (MHz)(depending on the laser wavelength) could be due to a wind either towards or away from the lidar at a meter per second (m/s). To resolve this uncertainty, the transmitted laser pulse is shifted by a fixed amount of 10 megahertz. Now, when the atmospheric light signal and the laser sample are mixed, the shift in frequency will be offset by the 10 MHz signal. (As an example, let’s suppose that the Doppler shift due to the wind is 2 MHz. Then, the first example without a 10 MHz offset will give you simply a resultant 2 MHz signal for either a +1 m/s or -1 m/s wind, while the 10 MHz offset makes the resultant 12 MHz for a wind toward the lidar and 8 MHz for a wind away from the lidar.)

An additional piece of equipment being used by ETL is the Ka-band radar, operated by Ms. Michelle Ryan. Ms. Ryan uses Ka-band radar to study the clouds- water droplet size, condensation, and the changes between liquid, gas, and solid water. She also uses radiometers to study liquid water and vapor in a column from the ship to the sky. Her equipment complements the lidar by providing information about what’s going on above the cloud base (the lidar focuses on everything between the ocean surface and the clouds).

Thank you very much to Dr. Alvarez for translating enormously complex physics into what you just read about how the lidar works. If you read it through a couple times, it really makes sense! And they say laser physics is complex.

Travel Log

People always wonder what the food is like on the ship. Well, there is lots of it, and it’s better than what you would expect. In fact, I’ve heard some of the scientists challenging each other to see who can gain the most weight on the trip- just an excuse to try a little of everything on the buffet line, and dessert twice. There’s always a salad bar, a couple meat entrees, a couple meatless entrees, and several vegetables. One night we even had crab legs and steak! We eat during designated meal times in the mess hall, and since there are more people on the ship than there are seats in the mess, they try to get you to “eat it and beat it.” The most dangerous part of the mess is the freezer stocked with Haagen Daas ice cream, but I am challenging myself to avoid it until the last night on the ship. There are three stewards on the ship that do all the cooking and kitchen stuff. They’re really nice and friendly.

Question of the day: How much money did the U.S. spend last year on scientific research? What percent of the total budget does it represent? (Please cite your source when you send your answer)

Photo Descriptions:Today’s photos – Since today’s science log focused on the Lidar operated by NOAA Environmental Technology Laboratory (ETL), that’s what is highlighted in today’s pictures. You’ll see the ETL lab on the ship- a large container that travelled via tractor-trailor, plane, and barge to get onto the ship. There are two “vans” like this on the ship, which is where this group of ETL scientists spends most of their time. Inside the van, you’ll see Ms. Intieri at the computer controls, Dr. Alvarez tweaking the lenses in the Lidar, and in another picture, Dr. Alvarez pouring liquid nitrogen into the Lidar to keep the optics cool. Finally, you’ll see Ms. Ryan standing next to the kaband radar (looks like a large drum in the photo).

Until tomorrow,
Jennifer