Vertical and Geographical Variations

Aerosol and cloud microphysical properties vary in vertical distribution, and their properties depend on the airmass where they were formed. Figure 4.10 shows the vertical variation of aerosol particles as a function of back trajectory

Summer 1988

Summer 1988

Spring 1990

Particle concentration (cm-3, STP)

Figure 4.10 Aerosol particle concentration as a function of height for measurements made during 1988 and 1990 field campaigns in southern Ontario (Isaac et al. 1998). A PMS active scattering aerosol spectrometer probe instrument was used in 1988 with a detection size range of 0.17-3 pm. For 1990, a PMS passive cavity aerosol spectrometer probe was used with a size range of 0.12-3 pm.

Particle concentration (cm-3, STP)

Figure 4.10 Aerosol particle concentration as a function of height for measurements made during 1988 and 1990 field campaigns in southern Ontario (Isaac et al. 1998). A PMS active scattering aerosol spectrometer probe instrument was used in 1988 with a detection size range of 0.17-3 pm. For 1990, a PMS passive cavity aerosol spectrometer probe was used with a size range of 0.12-3 pm.

for a study performed in southern Ontario during the summer of 1988 and the spring of 1990 (Isaac et al. 1998). There is only a small variation with height when airmass back trajectories were from the north, where there were few source regions, whereas there were steep gradients with height when back trajectories were from the south, where the air traveled over heavily populated and industrialized areas. Trajectories from the west showed concentrations between those from the north and south. The use of surface measurements of IN, CCN, or their precursor chemical constituents, without providing some mechanism for dispersing the aerosol particles in the vertical, would provide poor inputs for climate models.

Cloud microphysical properties also depend on geographical location. The distinction between maritime and continental clouds has been well known and documented (see Squires 1958). Isaac et al. (2001) show clear differences between the frequency distributions of droplet concentrations in stratiform clouds in a maritime (CFDE I) and continental (CFDE III and AIRS) environment (see Table 4.4); maritime clouds show lower droplet concentrations and larger drops for the same temperature level.

However, it is unclear whether the clouds in eastern Canada are similar to those in other places of the world. Such a study needs to be done. Korolev et al. (2001) did a statistical analysis of cloud properties by cloud type in the former USSR using a large dataset and showed distinct differences in cloud properties as a function of cloud type. Although the instrumentation and resulting analysis techniques were different, the data are similar to the stratocumulus and stratus measurements of Table 4.4, at least for cloud LWC. Korolev et

Table 4.4 Cloud microphysical summaries, using 30 s averages, for the maritime (CFDE I) and continental cases (CFDE III and AIRS) in terms of static temperature (Ta), droplet number concentration (Nd), total water content (TWC) and median volume diameter (MedVD). Ice crystal concentration is represented as I. For example, 25% of the CFDE I droplet concentrations (Nd) were less than 16 cm 3.

Table 4.4 Cloud microphysical summaries, using 30 s averages, for the maritime (CFDE I) and continental cases (CFDE III and AIRS) in terms of static temperature (Ta), droplet number concentration (Nd), total water content (TWC) and median volume diameter (MedVD). Ice crystal concentration is represented as I. For example, 25% of the CFDE I droplet concentrations (Nd) were less than 16 cm 3.

1%

25%

50%

75%

99%

Maritime

Points

= 1154* Ta < 0°C I < 1 l 1

1 TWC > 0.005

g m 3

Ta ( °C)

-20.6

-5.8

-4.1

-2.0

0.0

Nd (cm3)

1

16

52

108

406

TWC (g m 3)

0.01

0.07

0.13

0.20

0.47

MedVD (pm)

10

18

24

34

527

Continental

Points

= 4759* Ta < 0°C I < l L

1 TWC > 0.005

g m 3

Ta (°C)

-24.7

-9.1

-6.2

-3.2

-0.2

Nd (cm3)

2

55

121

233

643

TWC (g m 3)

0.01

0.05

0.11

0.21

0.49

MedVD (pm)

10

13

17

22

643

*Liquid and mixed phase, in-icing conditions

*Liquid and mixed phase, in-icing conditions al. (2000), Gultepe et al. (2001), Field et al. (2005) and Gayet et al. (2006) all showed similar ice particle (> 100 p.m) concentrations around the world, in different cloud types. More coordinated work along this line needs to be performed using similar instrumentation and analysis techniques, so that the measurements can be compared directly.

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