CLARA: RIVM Lidar

• Lidar: 1064 nm and 532 nm (range 0 - 4 km)

Here is an impressing example of recent measurements.




Global overview for November 26, 1996.

Lidar stands for Light Detection And Ranging. The technique is sometimes called ``laser radar'', because there are some similarities between the two techniques. The lidar measurement principle is as follows: a short pulse of laser light is emitted and the very faint reflections of the light on air molecules and particles are observed. The height resolution is abtained through measuring the lapse of time between the emission of the laser pulse and the reception of the echo (of course, the speed of light comes in between). Because the optical echoes that are received are so weak, very often signal averaging is applied to increase the signal to noise ratio in the measurements. For most purposes of lidar (for example boundary layer studies and trace gas concetration measurements) the averaging is allowed since the observed quantities vary relatively solwly with time. In other words: even if signals are averaged over time, a frozen atmosphere is observed. In clouds, however, the frozen atmosphere assumption does not apply. There is no such thing as a frozen cloud (although it may contain water in frozen form). In fact, the variations of the structure of a cloud are very large. In the RIVM cloud lidar system deployed in the CLARA campaigns the echos from single laser pulses were recorded, so that even the small scale variations of the cloud structure could be captured. The unique richness of detail in the measurements is illustrated in the following figures.

Figure 1 shows a fase color plot of a time series of lidar data on a timescale of 24 hours. On this timescale only large scale variations can be observed. Each pixel in the plot is 100 laser shots wide in time and 75 meters in altitude high (so in this figure averaging of data was applied). The drak areas in the figure are periods where no data was taken.

Figure 2 shows an exerpt from Figure 1. The dimensions of the pixels are again 100 laser shots wide in time and 75 meters in altitude high. On this timescale a lot more detail can be observed, even though averaging was applied.

Figure 3, finally, shows a period of only about three minutes where all single-shot lidar returns are plotted. Here each pixel is 0.6 seconds wide (the rate at which the data was stored) and 7.5 meters in altitude high (limited by the analog bandwith of the analog to digital converter). Comparing Figures 2 and 3 it is clear that many cloud features are ``smeared out'' by the averaging process. Using these high resolution data it is possible to study the effects of averaging in space and time of cloud data and to better understand the measurements of other instruments that had to use longer data intergration periods.

Other reseach goals are the retrieval of optical paramters at the measurement wavelengths (1064 nm and 532 nm) and measurement of cloud geometrical parameters (cloud base height, cloud top height) and to complement measurements from the other instruments participating in the CLARA campaigns.

For more information please contact Arnoud Apituley at the RIVM.