Measures concentration of liquid water in clouds using a heated wire resistance bridge.

JW probe on port side of aircraft nose

Uncertainty in measurement

The Johson Williams probe is able to measure the liquid water content with a time resolution of 4Hz with a typical response time of 1s and a detection limit of 0.001 g/m3.

Significant contributions arise from the sensing head calibration, droplet collection efficiency, and baseline drift. The typical overall uncertainty under normal operation is estimated at ±10%, but additional points should be noted, see below.

Measurements on which this variable depends

The derived liquid water variable in the netCDF from the Johnson Williams instrument is called "LWC_JW_U". The calculations depends upon the calculation of True Airspeed (TAS_RVSM), Deiced True Temperature (TAT_DI_R), and Static Pressure (PS_RVSM).
Baseline correction of the data (by the user) may be assisted using Nevzorov, other cloud probe data.

Explanation of data flags

  • 1: Fewer than 8 good datapoints in either PS_RVSM, TAT_DI_R, TAS_RVSM for averaging (but at least one of each)
  • 1: Instrument measurement saturated (see below)
  • 2: Data out of range of ±10 g/m3
  • 3: Value cannot be calculated (data also generally -9999)

Calibration details

The JW sensing heads use manufacturers calibration data from resistance measurements of the elements in each head. The probes' validity is established in comparison with other existing cloud droplet measurement techniques. Experience has shown that this is a reliable, robust device showing little detectable measurement drift over time.

The probe response is, if possible, zeroed shortly after takeoff on each flight, whilst exposed to clear-air conditions.

Logged counts are characterised using a purpose-built resistance device, in place of the JW sensing head, designed to replicate the cooling effect of 1.0 g/m3 of liquid cloud droplets when operated at a true airspeed of 77.2 m/s. This is combined with a zero reading to produce a linear relationship equating measured liquid water with DRS counts, including the effect of the instrument's amplifier and control electronics.

Probe elements are hard-wearing and a visual inspection will in most cases determine the physical condition of the probe. Routine observation and comparison of JW readings against other techniques (Total Water Content, Nevzorov) during each flight, by the Flight Manager, will detect any gross errors.

Detailed calibration records are held at FAAM for every probe in use.

Limitations of measurements

  1. Saturation

    Saturation occurs when the available heat from the resistance wire is unable to evaporate all of the water vapour droplets impinging on the wire. The liquid water content (LWCsat) at which saturation occurs is approximately related to true air speed (TAS) as follows:

    LWCsat = 300/TAS (g/m3)

    As outlined above, data fulfilling this condition are flagged with a 1.

  2. Aircraft Attitude

    Measurements made while the aircraft is turning are unreliable owing to turbulent flow within the probe. The extent of this is difficult to quantify.

  3. Collection Efficiency

    Droplet collection efficiency has been shown to decrease for droplets larger than 30µm, as larger drops tend to shatter on impact and are only partially collected. Under these conditions the probe will tend to underestimate the true LWC. Again, the magnitude of this effect is difficult to quantify. [Ref. 3]

  4. Icing

    Icing conditions place a further load on the Johnson Williams heating current and will generally cause underestimation of the true LWC content. [Ref. 4]

Method outlining further processing requirement for this dataset

The instrument compensation arrangement introduces a drift in the instrument baseline response under flight conditions. This results from changes in heat loss as air density and air speed change [Ref. 2]. In order to correctly utilise the output from this parameter a baseline correction must be carried out by the user as follows:

Out of cloud the instrument should read zero. Data from either the Nevzorov, Cloud Physics probes or Mission Scientist Log Sheets may be employed to signify periods in/out of cloud. The Johnson Williams reading should be corrected by applying an offset to the data. Where the zero level changes within a cloud, standard practice is to assume a linear baseline drift from the point at which the instrument entered the cloud to the point at which the instrument is once again in clear air.

As many of these corrections should be applied as necessary to give the Johnson Williams liquid water data a level baseline. Further guidance may be obtained from FAAM.

Further References

  • [1] Spyers-Duran, P A, 1968: Comparative measurements of cloud liquid water using heated wire and cloud replicating devices. J. Appl. Meteor, 7, 674-678.
  • [2] Baumgardner, D, 1983: An analysis and comparison of five water droplet measuring instruments. J. Appl. Meteor, 22, 891-910.
  • [3] Knollenberg, RG, 1972. Comparative liquid water content measurements of conventional instruments with an optical array spectrometer. J Appl Meteor, 11, 501-508.
  • [4] Strapp, J W and Schemenauer, 1982: Calibrations of Johnson-Williams liquid water content meters in a high-speed icing tunnel. J. Appl. Meteor, 21, 98-108.

Further Details

Contact Duncan MacLeod at FAAM.