Dr David Hooper

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Manufacturer & Model

Vaisala LD40

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Vaisala LD40 Laser Ceilometer

Vaisala LD40 Laser Ceilometer

Note that this instrument is operated in support of NCAS’s long term measurement programme. It is not bookable.

A Vaisala LD40 laser ceilometer has been in operation at the NCAS Capel Dewi Atmospheric Observatory (CDAO) since August 2005. It transmits short pulses of 855 nm wavelength (i.e. infra-red) laser light, which are scattered back to it from atmospheric aerosols (i.e. small particles such as dust and sand) and hydrometeors (e.g. cloud droplets and ice crystals). The profiles of backscattered power are used to derive the cloud base altitudes for up to the three lowest cloud layers and an estimate of the rate of precipitation. Clouds are only rarely detected above mid-tropospheric altitudes.

See the Data section below for more details about the available data products.

Instrument Details
  • Latitude: 52.424492°N
  • Longitude: -4.004323°E
  • British National Grid Reference: SN6369482616
  • Altitude above mean sea level (base of instrument): 52 m
Instrument Status

Although the instrument is still in operation, there is currently a gap in the provision of cloud base altitude data through the Centre for Environmental Data Analysis. The backscatter profile data have never been made publicly available. Both data types will soon be made available in netCDF files that conform to the NCAS data standard. Please contact the instrument scientist if you require access to the data more urgently. Plots of the latest 24 hours’ worth of data can be viewed through the CDAO’s supplementary website.

Cloud Base Dataset Details

Note that a Lufft CHM15k ceilometer was installed at the CDAO in January 2020. It is better able to detect high-level clouds than the Vaisala LD40. The data will start to be made publicly available in the near future.

The LD40 operates according to the LIght Detection And Ranging (LIDAR) principle, which is similar to that of radar. It transmits short pulses of 855 nm wavelength laser light, which are scattered back to it from atmospheric targets – primarily aerosols and hydrometeors. Aerosol concentrations tend to be relatively high within the boundary layer, i.e. approximately the lowest kilometre of the atmosphere, but are much lower within the overlying free troposphere. The distance of the targets from the instrument is determined by the time delay between the transmission and reception of a pulse.

The LD40 transmits 75 ns duration pulses at a repetition frequency of 6494 Hz. The temperature of the laser diode is maintained within a narrow range in order to ensure high stability of the emitted wavelength. This allows a very narrow band optical filter to be used at the input to the receiver. Nevertheless, the background noise level increases during daylight hours. The receiver signal is sampled at time intervals of 50 ns, corresponding to altitude intervals of 7.5 m. Owing to the low power of the eye-safe transmitter, the receiver signal at each range gate must be integrated over 15 s in order to increase the detectability of the desired returns. An automatic chain in gain is noticeable at a range of 3.8 km. The resulting backscatter power profiles are analysed by the manufacturer’s (undocumented) algorithms in order to determine cloud base altitudes and other data products

Although the LD40 produces 4 sets of measurements every minute, the data capture software (provided by the Met Office) only records one of these.

Refer to the Access to the data section of the Capel Dewi Atmospheric Observatory page for general details about:

  • Licensing and acknowledgements
  • Access to data files/plots
  • Availability of data files/plots
  • Centre for Environmental Data Analysis (CEDA) file naming convention and archive structure

The cloud base data are currently only available in NASA Ames (ASCII) format files using a File Format Index of 1001. Refer to Gaines and Hipskind (1998) for a full description of the file format. Here I will just describe the bare minimum that you need to know in order to read in the data. I’m using the convention that the first line/value number is 1 rather than 0.

  • Line 1: two values. The first gives the number of (metadata-containing) header lines, nr_header_lines. The second number is the File Format Index, whose value is always 1001.
  • Line 7: six values. These give the date (UTC) for which data are available followed by the date on which the file was created, both in YYYY MM DD format.
  • Line 12: ten values specifying (in order) the missing datum values for the primary variables, i.e. for all of the variables except the first one, which records the the sample time. Note that missing datum values are used for data line values 2 – 7 to indicate that a cloud layer was not detected. A missing datum value for data line value 11 (Instrument status flag) is used to indicate that no observation is available for a particular time.
  • Lines (nr_header_lines + 1) to line (nr_header_lines+ 1441) contain 11 values
    • Value 1: time of the measurement given as the number of seconds past 00:00:00 UTC for the day in question
    • Value 2: Cloud base altitude of first cloud layer (m)
    • Value 3: Cloud base altitude of second cloud layer (m)
    • Value 4: Cloud base altitude of third cloud layer (m)
    • Value 5: Laser penetration depth into first cloud layer (m)
    • Value 6: Laser penetration depth into second cloud layer (m)
    • Value 7: Laser penetration depth into third cloud layer (m)
    • Value 8: Maximum possible altitude of cloud detection (m)
    • Value 9: Estimated visibility in air looking vertically (m)
    • Value 10: Estimated precipitation rate index (0 – 3)
    • Value 11: Instrument status flag (00000000 – 99999999)

The Instrument status flag contains 8 digits, which encode information using the following values:

  • Status Digit 1 (most significant digit): Power supply
    • Value 0: Power supply OK
    • Value 1: +/-5 V or +/- 12 V supply failure
    • Value 2: Not used
    • Value 3: Avalanche photodiode voltage regulator failure
    • Value 4: Test laser voltage regulator failure
    • Value 5: Original laser voltage regulator failure
    • Value 6: No response from AD/DA converter
  • Status Digit 2: Window backscatter and solar shutter
    • Value 0: Window backscatter and solar radiation measurements within allowed range.
    • Value 1: Window scatter > 50%. Window requires cleaning.
    • Value 2: Solar radiation too strong. Shutter is closed.
  • Status Digit 3: Receiver
    • Value 0: Receiver OK.
    • Value 1: Offset too low.
    • Value 2: Offset too high.
    • Value 3: Not used.
    • Value 4: Receiver sensitivity too low. Error. No more measurements possible.
    • Value 5: Receiver sensitivity too low and offset too low.
    • Value 6: Receiver sensitivity too low and offset too high.
  • Status Digit 4: Original laser and test laser
    • Value 0: Transmitter module OK.
    • Value 1: Laser power low. Warning only. Measurements continue.
    • Value 2: Laser power too low. Error. No more measurements possible.
    • Value 3: Not used.
    • Value 4: Not used.
    • Value 5: Test laser failure. Error. No more measurements possible.
    • Value 6: Original laser switched off.
  • Status Digit 5: Flash-ADC
    • Value 0: Flash-ADC OK.
    • Value 1: One flash-ADC error during regulation of the bias voltage of the avalanche photodiode.
    • Value 2: Several flash-ADC errors during regulation of the bias voltage of the avalanche photodiode.
    • Value 3: Flash-ADC failure.
    • Value 4: Flash-ADC too slow during normal measurements. No reply.
  • Status Digit 6: Temperature regulation
    • Value 0: Temperature regulation OK.
    • Value 1: Temperature controller defect.
  • Status Digit 7: Reset and data reception
    • Value 0: Data transmission OK.
    • Value 1: Sensor will perform a reset.
    • Value 2: Not used.
    • Value 3: Not used.
    • Value 4: Not used.
    • Value 5: Not used.
    • Value 6: Not used.
    • Value 7: Received and calculated checksum differ.
    • Value 8: Received command message is too long.
  • Status Digit 8 (least significant digit): Setup information
    • Value 0: Current setup is the setup stored in the EEPROM.
    • Value 1: EEPROM failure. Current setup if the default setup.
    • Value 2: Setup has been changed by the user and is different from the EEPROM and default setups.
    • Value 3: Not used.
    • Value 4: In case of failures, the message contains dashes instead of data.
    • Value 5: EEPROM is defective. Current setup is the default setup and the message contains dashes instead of data.
    • Value 6: Setup has been changed by the user and is different to the EEPROM and default setups. The message contains dashes instead of data.