Lidar dark band


In the melting layer of precipitation a lidar often measures a dark band at about the same height as the radar bright band. There are many good ideas on why the lidar backscatter could decrease at the onset of melting at the top of the melting layer. I expect the most important to be the increased transparance of the particle as cracks and holes are filled with liquid water (just let a graupel particle or snow flacke melt on your hand).
More mysterious is that the lidar backscatter increases again at the end of the melting at the base of the melting layer. No credible explanation is known in literature. Also my suggestion that it may be related to a strong vertical peak in the backscatter due to the flatter shape of fastly falling droplets is likely wrong: In the mean time we have also seen the lidar dark band in lidar measurements that were tilted away from the vertical, and one would not expect this backscatter peak to be very broad as the deformations are very small.
My articles contain, as far as I know, the only quantitative analysis of the depth of the lidar dark band. In one case the drop in lidar backscatter at the top of the melting layer is about a factor 100 and the increase of the backscatter at the base of the melting layer is a factor 10. This factor 10 makes may theories on the cause of the increase of backscatter not supportable, e.g. those of Sassen and Chen (The lidar dark band: An oddity of the radar bright band analogy. Geophys Res. Lett., 22, 24, pp. 3505-3508, Dec 15, 1995).

The most complete description of the phemenon can be found in Chapter 8 of my thesis. Below you will find some of the earlier conference contributions as well.


 Cloud measurements with radar
PhD thesis, 13 November 2000.
Victor Venema
Chapter 8. Measurements of the melting layer using lidar and radar: the lidar dark band pdf (1.3 Mb)

 Radar and Lidar Observations of the Melting Process in the Bright Band
IGARSS 2000, Hawaii, USA.
Victor Venema, Herman Russchenberg, Arnoud Apituley, Andre van Lammeren, Susanne Crewell, and Leo Ligthart

 The effect of the orientation of ice crystals in the melting layer and ice clouds on measurements using radar and lidar
conference on light scattering by nonspherical particles: Theory, measurements and applications, 1998, New York, USA.
Victor Venema, Herman Russchenberg, Leo Ligthart, Andre van Lammeren and Arnoud Apituley

 The melting layer: The radar bright band is dark for lidar
URSI commission F international triennial open symphosium on Wave Propagation and Remote Sensing, pp. 159-162, 1998, Aveiro, Portugal.
V.K.C. Venema, H.W.J. Russchenberg, A. van Lammeren, A. Apituley and L.P. Ligthart
Last update: 20 October 2003