Units of Measurement

One often overlooked problem concerns the units to use when reporting measurements. Modelers tend to use mass units giving the concentrations as a function of a unit mass of air. This has the advantage of being independent of pressure and temperature. Chemists often use such units as well. However, today, most cloud microphysical measurements are reported in volume units. This would

-■-50%

-o-95%

-n-75%

-*-99%

♦90%

-a-99.5%

. a

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60 120 180 240 300 Averaging interval (s)

60 120 180 240 300 Averaging interval (s)

E 600

E 600

60 120 180 240 300 Averaging interval (s)

Figure 4.11 Scale effects of cloud LWC and droplet concentration as described by Cober et al. (2001c). 30- and 300-second averaging intervals represent 3 km and 30 km scales, respectively. The plot shows various percentile values of the distribution.

not be a problem if the corresponding pressure and temperatures were also reported. However, this is not done routinely. When analyzing measurements from many different days, the altitude and temperature variations are often ignored and the volume unit concentrations are used for averaging.

Isaac et al. (2004) examined this problem and showed the typical type of "errors" that might result. The data were obtained during four different field projects in eastern Canada, central Canada, and the Arctic. Table 4.5 shows the data analyzed in their original volume units at the measurement level and in volume units that were referenced to a standard temperature and pressure (STP) level of 0°C and 1013 hPa. The STP data presentation is similar to a mass concentration because a simple multiplication factor would convert the numbers to mass units. Presenting the data in this manner, however, allows a direct comparison of the volume and "mass" measurements. It is clearly shown that there can be a factor two between the different methods of presenting the data. The rate of change of LWC and TWC with temperature is less when the data are converted into mass units because measurements at colder temperatures were usually taken at lower pressure altitudes.

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