sodar

The Wonderful World of SODAR

Could you hear me snorting at 0:06? I laughed when my professor said the sound isn’t there to be pleasant because one would really think otherwise! You can hear the cute little chirps from this SODAR-RASS machine 100 m away but much further than that and it fades into the sound of wind and creaking trees and rustling grass. But what is SODAR anyway, and why does it matter?

SODAR-RASS stands for Sonic Detection And Ranging with a Radio Acoustic Sounding System. It measures virtual temperature and wind from ground level up to ~200 m high, and can estimate turbulence. This particular SODAR is located in the Park Falls area of Northern Wisconsin, at the CHEESEHEAD field sight. However, it is part of a traveling laboratory owned by NCAR, called the Earth Observing Laboratory. For their more detailed explanation of SODAR click here.

At the end of the video, the narrator mentions “redundancy”. In research, especially intensive field campaigns, redundancy is a good thing! It ensures that measurements being made are accurate and precise, and if one instrument goes down another is there to cover the missing data. Redundancy can also tell us information about how different instruments relay the same information, or confirm interesting or unusual signals in data.

SODAR can be used for a variety of reasons and in a variety of situations, including weather monitoring and real-time display of convective or low-visibility events. For example, SODAR was used in a 2002 experiment to measure the wake effect of wind turbines.

clouds dawn dusk electricity
Photo by Flickr on Pexels.com

The wind farm in the study was located offshore, but “wake” here isn’t talking about big waves. The turbine wake effect is one of the largest barriers to harnessing wind power. Turbine hubs cause lower wind speeds in their surrounding area, resulting in lower efficiency and power output from nearby turbines. This issue can cost large-scale wind farms millions of dollars in revenue. That’s why wind farm design is super important.

According to the study, SODAR was located on a ship deck downwind from a turbine. Other meteorological measurements were taken from the mast. SODAR recorded data every minute, but then was averaged in 15-minute intervals. Positioning the ship correctly was the most time-consuming part of the study. Results showed that although wind turbines did not interfere with SODAR data collection, the wind turbines did indeed reduce wind speed downwind from the hub (say that five times fast!).

gray wind turbine
Photo by Trygve Finkelsen on Pexels.com

Some other studies have implemented SODAR to determine the best method for wind site assessment techniques in Pakistan, and investigate the stable surface-based turbulent layer of the atmosphere during polar winter in Antarctica. Below is a plot from the Antarctica study. It is called an echogram and displays the surface-based turbulent layer as measured by SODAR on August 27th, 2012. The y-axis represents height above ground and the x-axis relays time of day. The color shows the logarithm of the structure parameter C2T, also known as reflectivity, which has no units.

SODAR echogram
Petenko et al., Boundary-Layer Meteorology.

The next plot (Figure A) combines wind profiler and SODAR data to show horizontal wind speed, height, and direction. Using both instruments helped to increase spatial and temporal resolution. There was a change in high-altitude wind direction which can be seen in the changing direction of wind barbs from northwesterly at 0-12 h on July 30th to southwesterly around 18 h on July 31st. This coincided with a weaker wind field, or 3-D spatial pattern. Vertical wind velocity is displayed in Figures B & C for July 30th and 31st of 2017, respectively. Upward motion has positive vertical velocity values. Downward motion has negative values.

SODAR data plots
PW Chan, YY Chiu, & QS Li. Quality and applications of wind data from SODAR…Royal Met Soc.

Changing wind direction from day to nighttime, and different wind speeds at low versus high altitudes, are both common phenomena that can be seen at various locations. Here is a fun fact from North Carolina Climate Office regarding Sea and Land Breezes, which is when wind direction blows from sea to land at day, but from land to sea at night.

 

During the summer, the sea breezes are stronger … because of the large temperature differences between land and ocean water that time of year. The fronts caused by the sea breezes along the coast can provide a trigger to daily thunderstorm activity in coastal areas, particularly along the peninsula of Florida.”

The final study I will cover took place in San Francisco Bay Area during smog season of 1976. The purpose of the study was to determine characteristics of strong, low inversion layers to create better air quality simulation model inputs for San Francisco. Budget and air traffic limitations confined the amount of measurements that could be made with other instruments on towers, airplanes, or balloons. It appears data storage was a challenge in this study which used a total of 13 SODARs. Scientists maneuvered around this issue by taking photographs of facsimile records, storing them on microfilm, simplifying sodar mixing depth and stability information into hourly
digital parameters, then drawing contours of data onto maps for desired times. Coding in MATLAB almost sounds easier than all that…

San Francisco contour map
Contour maps of mixing height by PB Russell & EE Uthe, American Meteorological Society.

Results showed high spatial and temporal variability of Bay Area boundary layer structure, which was also seen in a previous study using airplane measurements. The authors recommended the use of SODAR for monitoring mixing depth over other more expensive or non-stationary methods. However, they suggested using periodic supplemental measurements to verify temperature, humidity, haze, and cloud
vertical profiles.

That’s the end of my spiel on the Wonderful World of SODAR, folks! I am so proud of myself for finally uploading this video and creating my own caption file- that was new and actually very fun. I hope you enjoyed, so if you did, let me know in the comments. All sources can be found by clicking the hyperlinks. Have a great night, Jess 🙂

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