Two instruments on the T-28 provide information about cloud water. The Droplet Measurement Technologies (DMT) liquid water sensor is a heated coil for which heat loss can be correlated with cloud liquid water concentration. The Particle Measuring Systems, Inc. (PMS) Forward Scattering Spectrometer Probe (FSSP) is an optical particle-counting instrument capable of giving estimates of cloud droplet population characteristics.

Although the T-28 FSSP instrument is one of the first models of its kind, it can produce, with proper calibration, very good data on cloud droplet concentrations and size distributions. A description of the instrument and its data is presented in Cooper (1988), and in Baumgardner and Spowart (1990).

Bench test was done on June 3 in Greeley. Glass microspheres of three different sizes were passed through the instrument, using a vacuum cleaner to draw air beads through the sample volume.  Based on these bench tests, we computed channel size assignments for equivalent water droplets applicable for the Greeley project data. Comparison of integrated liquid water concentration (LWC) from the FSSP with LWC from the DMT instrument suggests that the FSSP calibration based on July 27 bead tests should be used for flights that were done during July, 2003. A table with calibration results is shown in Table 1.

Table 1: FSSP calibration for Greeley 2003 project.

In several occasions the DMT sensor was broken by the impact with hail while the airplane was flying through the storms, and no data were recorded after that event. The FSSP worked most of the time, therefore the cloud liquid water concentration is also computed by integrating the FSSP droplet size spectra. When the DMT was working,  generally good agreement was found between the DMT liquid water concentration and the liquid water concentration computed from FSSP recordings. Figure 6 shows an example for a flight when both instruments worked most of the time. It appears that the FSSP may have iced up from just before 23:06 until just after 23:18. Figure 7 shows a plot of DMT vs. FSSP derived liquid water concentration for the same flight, which demonstrate a good correlation between the data from the two instruments.

Figure 6: DMT and FSSP liquid water concentrations for the flight 813 on 23 July, 2003.

Figure 7: DMT vs. FSSP liquid water concentration for the flight 813 on 23 July, 2003.

References

Baumgardner, D., and M. Spowart, 1990: Evaluation of the forward scattering spectrometer probe. Part III: Time response and laser inhomogeneity limitations. J. Atmos. Ocean. Tech., 7, 666-672.

Cooper, W.A., 1988: Effects of coincidence on measurements with a foreward scattering spectrometer probe. J. Atmos. Ocean. Tech., 5, 823-832.