If you think that wind directions are still estimated by holding your index finger into sky, you are not up to date. Star Wars technologies have entered the real scientific world of experimental atmospheric researchers. Traditional meteorological wind measurements (e.g. cup anemometers) are limited to near-ground levels. What if we are interested in winds at higher altitudes? Building up higher masts is challenging in terms of construction and cost efficiency. Wind measurements with newest laser technologies enable not only wind profiles up to several kilometres but also 2D pictures of the ambient flow field.
The measurement principle of so-called Light Detection and Ranging (LiDAR) instruments can be compared to Radars used at airports to detect aircrafts in the sky. Instead of radio waves, LiDAR instruments send light pulses into the atmosphere. The signal is scattered back by aerosols to the device. In order to be recognized again, the light signal needs amplification. This is why the history of LiDARs is linked to the invention of the Laser (Light Amplification by Stimulated Emission of Radiation) in 1960. The laser used in our LiDAR instrument cannot damage our eyes, because it emits waves slightly longer than visible light. This signal can be compared to waves connecting our mobile phones and transferring data.
Now we know that we send laser pulses into the sky and collect their backscattered version again. The question is: how could this give us information about the wind speed?
The magic lies in the so-called Doppler effect. This may surprise you, but you have “heard” the Doppler effect before. The horn of an ambulance car sounds differently before and after it passes. As the aerosols move with the wind this effect also appears to our backscattered signal. Therefore the signal from our moving aerosols is shifted into another frequency. This frequency shift is proportional to the speed of the particles, the wind speed. Knowing the traveling time between transmitted and received pulse enables us to measure wind speeds at different heights simultaneously.
Compared to traditional in-situ measurements, LiDARs provide more extensive temporal and spatial data coverage, which makes the profiling of the lower atmosphere possible. LiDARs are not limited to wind profiles. Using a scanning LiDAR one can take pictures of a certain wind field and observe its change over time. Suddenly, the invisible becomes visible. Watching the wind in color opens doors for scientists to analyze and characterize wind patterns on a new level. We are entering a new era of Star Wars wind measurements.
The University of Bergen is equipped with such a scanning LiDAR and we are highly motivated to really take a look at characteristic flow patterns as for example the sea breeze. So we packed our bags and started a field campaign at the airport of Stavanger at the Norwegian West Coast.
I think everyone who has been at the coast has experienced a nice breeze carrying with it the smell of the sea. In a more meteorological jargon this constantly blowing wind is called sea breeze. It can be explained quite nicely with the difference of heat capacities between land and sea. Land has a much higher heat capacity, meaning that it warms quicker, thereby warming near-ground air. Warm air rises over land and is replaced by air coming from the ocean. This results in a circulation cell, whose surface wind is called sea breeze. The circulation changes direction during nighttime, when the land becomes colder than the ocean. The wind at ground level is called land breeze.
As we can see in measurements from our super cool laser instrument the land breeze is not limited to night times. During winter days the sea can often be warmer than the land, leading to a more frequent land breeze. This wintertime land breeze is captured in a vertical cross-section of the flow field from West to East on March 12th, 2013. The LiDAR is located in the centre of this arc. The land is to the left and the sea is to the right. Red colours refer to motion towards the device while blue colours picture motion away from it. The white arrows indicate this flow direction. As in theory winds blow towards the sea at low levels and from the sea at high levels. A nice snap shot of a wintertime land breeze.