University of Wales, Aberystwyth 532 nm polarization lidar data

Introduction

The University of Wales, Aberystwyth 532 nm polarization lidar was operated at Chilbolton Observatory during the CWAVE 03 campaign. Since it is designed to measure lidar depolarization ratios, it was operated on a case study basis when ice clouds were present. Depolarization of the lidar beam is observed when it is scattered from asymmetric particles, such as ice crystals and some aerosol particles. The system is vertically pointing.

 

Parameters

Parameter Variable Name Value

Wavelength wavelength 532 nm

Pulse energy lidar_pulse_energy 0.03 J

Pulse repetition rate lidar_pulse_frequency 20 Hz

Transmitted beam diameter lidar_beam_diameter 0.03 m

Receiver mirror diameter lidar_mirror_diameter 0.1 m

Transmitted beam divergence lidar_beam_divergence 0.0057

Receiver field of view lidar_field_of_view 0.069

Beam elevation lidar_beam_elevation 90

Latitude latitude 51.4445N

Longitude longitude 358.5630E

Height above mean sea level lidar_height_amsl 84 m

 

Theory and calibration methods

The system transmits a plane polarized laser beam. The received signal is split into two components: that with the same direction of polarization as the transmitted beam (co-polar) and that with its polarization direction at 90 to the transmitted beam (cross-polar). The lidar depolarization ratio is defined as the ratio of the cross-polar to the co-polar lidar return. These quantities are measured by the detection system. The ratio must then be corrected for differences in sensitivity between the two measurement channels. This calibration factor is determined by measuring the depolarization ratio for scattering from clear air, which has a depolarization ratio of 0.02.

 

Data processing

The data were acquired at 5 second intervals, with 30 m height resolution. The flat background due to solar radiation in each measurement channel was calculated from the average signal from 13.3 to 13.6 km, the maximum height from which signals were recorded, and subtracted from the measurements. The data were then averaged over 60 second periods. If required, we can supply data averaged over shorter periods. No height averaging was applied. Depolarization ratios were calculated from the 60 s average data. It was necessary to remove some data, where the measured signal in either channel was large enough to cause saturation of the photon counting detection system. The cut-off level was determined to be a count rate of 30 MHz from calculations of the depolarization ratio from the large molecular returns at low altitudes. These points are indicated by the fill value of 99.0 in the data files and are shown as white in the quicklook plots. By comparison with cloud data from the co-located UV Raman lidar and IR ceilometer it was determined that depolarization lidar was overestimating cloud heights by around 500 m at all heights, so the data were corrected to account for this. As with any lidar system, the beam is strongly attenuated by clouds, so where clouds are present, the depolarization measurements are noisy above them.

 

Data format

The data are provided as daily files in NetCDF format, together with quicklooks using Portable Network Graphics (PNG) format. In additional to the variables listed in the Parameters, global variables (giving general information about the data) and data variables are defined.

 

Global variables

 

Conventions, title, history, institution, source, comments, references, British_National_Grid_ Reference.

 

Data variables

 

Parameter

Unit

Variable Name

Description

Time

s

time

Time at start of measurement, measured from midnight at the start of the current day

Acquisition time

s

acquisition_time

Integrated data acquisition time in each measurement

Height

m

height

Height of measurement

Depolarization ratio

m-1sr-1

lidar_depolarization_

ratio

Ratio of cross-polar to co-polar lidar return

 

 

Conditions of use

Use of these data is restricted to participants of the CWAVE 03 campaign and for academic uses only. If data are used in any publication or report then an acknowledgement must be given to the Radio Communications Unit at the CCLRC Rutherford Appleton Laboratory and the University of Wales, Aberystwyth for providing the data.

 

Who to contact

If you have any problems obtaining the data, please contact the British Atmospheric Data Centre. If you have problems, queries or comments regarding the data themselves that are not covered adequately by this document, please contact Judith Agnew, j.l.agnew@rl.ac.uk.

 

 

Judith Agnew

Lidar Project Manager

Radio Communications Research Unit

CCLRC Rutherford Appleton Laboratory

November 2003