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 tests were done in Goodland on 27 May and after the project was over, on 20 and 26 September. 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 STEPS 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 27 May tests should be used for flights that were done through June 9, 2000. After this date, there was a small shift in FSSP response and a FSSP calibration based on the post-season microsphere tests produces better agreement with the DMT LWC. A table with calibration dates and results is shown in Table 1.

Table 1: FSSP calibration for STEPS 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. Figure 7 shows a plot of DMT vs. FSSP derived liquid water concentration for the same flight, which demonstrate a reasonable correlation between the data, and a tendency for FSSP water concentration to be slightly lower than DMT water concentration.

Figure 6: DMT and FSSP liquid water concentrations for the flight on 11 June, 2000. During the period beginning just before 2248 and ending just after 220000, the FSSP indicated water concentrations from about 1/5 to 1 g m^-3, while the DMT probe indicated negligible water. These data were obtained during a long pass through a trailing stratiform region behind a bow-echo storm. There actually was little liquid water. The indication by the FSSP is an artifact caused by non-spherical ice particles passing through the probe sample volume. There also was an upward shift in the baseline response of the DMT probe just prior to 2206, caused by impact of graupel or hail with the sensing element.

Figure 7: DMT vs. FSSP liquid water concentration for the flight on 11 June, 2000.

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.