Here are some of my favorite measurements, made during my PhD in the CLARA-project. Clara was a national Dutch project which aimed at getting a better understanding of the interaction between clouds and radiation by improving the measurement of cloud properties.


Stratiform rain

 In Europe stratiform rain is mainly produced by the melting of ice particles into rain droplets. Normally this happens in a well-defined layer, just below the zero degree level. This melting layer is characterised by high radar reflections. In this picture the melting layer is at 2 km, with the cloud above and the rain below.
 In the upper part of the melting layer the reflections by the falling ice crystals increase as the refractive index of water is higher then that of ice. In the lower part of the melting layer the reflections decrease again as the particles become smaller. These smaller particles reflect less power, but also fall faster and thus decreases the number density and total reflection.


Altostratus cloud

 This picture shows a measurement of an altostratus cloud between 2 and 5 km. The reflections, up to -5 dBZ (the scale in the figure is about 8 dB to high), are much too high to be from a water cloud. So the radar reflections will probably mainly come from relatively big ice crystals. From this cloud also lidar measurements were made, showing that the cloud was optically thin.
 Below 2 km the measurement shows some vertical stripes. An FM-CW radar uses a Fourier transform to convert frequency into range information. The cross talk from this transform makes stripes from these point targets. The origin of these reflections is not known. For more on this phenomenon see the section on Angels below.



 Radar sometimes sees strong point reflections from still unknown origin. Some call them angels, ghosts, or UFO's, and they are speculated to come from the reflections of insects, birds, leaves, atmospheric plankton (anything organic in the air) or spontaneous turbulence.
 Normally they are seen as small dots (or stripes), see e.g. the reflections below the altostratus cloud. The picture below was made from a raw data file, which has a time resolution of 5 ms. The background is the radar reflection as function of height and time in seconds. The line is the average reflection, averaged over the heights shown. This high temporal resolution provides new insight on these angels. An interference pattern is seen in the power. The phase of this angel has a sinus pattern. The measured thickness of the object may be due to signal processing.
 More information is found in this article. I would like contact with people who have an idea on what can cause this phenomenon.


Cumulus cloud

 The background of the first picture is the radar reflection (Z); the contours are the reflections from the lidar (RIVM). The base of the cloud is detected by the lidar reflections. Rectangle A indicates a region in which the lidar does not detect a cloud but the radar still receives a lot of reflections; this is clear air scatter. Distinguishing between particle scatter and clear air scatter is a big problem for S-band radar. This problem can be solved by combining S-band and X-band radar. The values at the top of the cloud are approximately -5 dBZ, rectangle B. This is much too high to be from cloud droplets. This enhanced backscatter may be due to ice crystals, or changes in temperature and humidity. In Rectangle C the radar reflections are very low, but the lidar is attenuated totally, so in this region the particles are probably relatively small. Figure was made by A. Apituley from the RIVM.
 The second picture shows the dynamics of this cumulus cloud. The grey background is again the radar reflection; the contours give the velocity as measured by the radar. Negative velocity means the scatterers move upward, positive downward. There are two thermal plumes visible in this cloud, which move up with high speed, up to 5 m/s.


Lidar dark band in Cirrus

 The first five pictures are a measurement of Cirrus clouds. Cirrus clouds are optically thin, but the lidar reflections in the vertical direction are high because of specular reflections by horizontally aligned ice crystals. If a lidar is titled away from the zenith by a small angle, the lidar reflections can decrease enormously.
 The background of the first picture is the lidar reflection of this cloud. In the middle of the cloud, at 4300 m, the lidar receives almost no reflections. The radar does receive reflections from this layer. The explanation is probably that the crystals in this dark layer are not horizontally aligned. The blue line gives the wind direction measured by a radiosonde. The wind direction changes drastically in this layer. This can cause the crystals to be no longer horizontally aligned.
 The two small pictures give the lidar reflection and the width of the Doppler velocity spectrum. In this layer the lidar reflections are low; at the same height the width of the Doppler spectrum is much higher, indicating the presence of turbulence in this layer.



Lidar dark band in light rain

 The melting layer is characterised by high radar reflections for weather radars, the so-called bright band, for an short explanation see section on rain.
 This melting layer for the lidar is sometimes seen to give low reflections. The lidar measurement presented here has the deepest dark band we found up to now. In ten percent of the lidar backscatter profiles the dark band is more than 20 dB deep compared to the rain. More information on the lidar dark band is in these articles from 1998, and 2000.

Graph with radar reflection for 23rd April; click for bigger version. Graph with lidar reflection for 23rd April; click for bigger version.


 The first picture is the reflection of Darr, an S-band radar with wavelength 9 cm. In the lower 1.2 km there is some drizzle or mist. In the middle from 3 km to the ground one can see a downburst, giving a high and speckled reflection pattern. The second picture shows the same event with an X-band radar, with wavelength 3 cm. The last picture shows the difference in Radar reflection between the two radar's. Radar reflections from small Rayleigh particles (expressed in dBZ) should be the same for these radar's, irrespective of wavelength. This can be seen in the mist layer. The downburst however gives reflections which are about 18 dBZ lower. This is about what one would expect for clear air scatter. More information is found in this article.
 This set-up is not carefully calibrated yet. The beam width of both antenna systems is about the same and the range and time resolution of both radar's is the same.

Downburst by S-band radarDownburst by S-band radar

Downburst by S-band radar

For more information:
Victor Venema,
Last update: 04 June 2001