Cloud Water
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).
FSSP ground test was done on May 24 in Norman. 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 Norman 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 May 24 bead tests should be used for flights that were done during May-June, 2003. A table with calibration results is shown in Table 1.
Table 1: FSSP calibration for Norman 2003 project.
|
Bead Test Predictions |
Valid For Flights |
|
|
Channel |
Min (m m) |
Max (m m) |
|
1 |
1.4 |
5.50 |
|
2 |
5.50 |
9.0 |
|
3 |
9.0 |
12.0 |
|
4 |
12.0 |
15.0 |
|
5 |
15.0 |
18.30 |
|
6 |
18.30 |
22.0 |
|
7 |
22.0 |
26.7 |
|
8 |
26.7 |
30.3 |
|
9 |
30.3 |
34.50 |
|
10 |
34.50 |
37.50 |
|
11 |
37.50 |
40.40 |
|
12 |
40.40 |
45.90 |
|
13 |
45.90 |
48.80 |
|
14 |
48.80 |
53.60 |
|
15 |
53.60 |
57.30 |
| 16 | 57.30 | 58.60 |
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 803 flight, which demonstrate a correlation between the data, but a tendency for DMT water concentration to be lower than FSSP water concentration for values above ~0.6 g m-3. Further comparisons between these instruments, including data from multiple flights, is warranted before using the data in a quantitative study.
Figure 6: DMT and FSSP liquid water concentrations for the flight 803 on June 1, 2003.
Figure 7: DMT vs. FSSP liquid water concentration for the flight 803 on June 1, 2003.
References
Baumgardner, D., and M. Spowart, 1990: Evaluation of the forward scattering spectrometer probe. Part III: Time response an d 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.