FIGURE 3.26 Vertical measurements of actinic fluxes below, in, and above a cloud. The dotted line shows calculated clear-sky values (from Vila-Guerau de Arellano et al., 1994).
base, the measured flux is 0.56 W m~2 nm-1, compared to a calculated value for a cloudless sky of 0.93 W m~2 nm-1. At the top of the cloud, the flux increased significantly to 2.1 W m~2 nm-1. Inside the cloud itself, the flux increased linearly.
In short, while the flux under a cloud is generally less than the clear-sky value, inside and above the cloud it can be significantly larger, leading to enhanced rates of photolysis of photochemically active species. For example, the photolysis of 03 to form electronically excited O('D), followed by the reaction of the latter with water vapor, is a major source of OH in the troposphere. As a result, actinometric measurements are often made by measuring the rate of production of OC'D) directly, known as /(O'D), or, alternatively, using a light detector calibrated for this photolysis process. Junkermann (1994), for example, has used a hang glider (Fig. 3.27) equipped with a photoelectric detector to fly spiral flight paths from the top of a mountain to the valley floor in Germany while measuring vertical profiles of the light intensity coming into a 2-77 radiometer as described earlier (Fig. 3.20). Two such hemispherical detectors are used, one of which is downward facing and one of which is upward facing; the sum of the two gives the spherically integrated actinic flux, which is the parameter of interest for measuring total photolysis rates in the atmosphere. To relate this measurement of light intensity to /(01D), a combination of optical filters and appropriate detectors is used (Junkermann et al., 1989).
Figure 3.28 gives a typical measurement of the upward component of /(O'D) as well as the total value during one flight made at a solar zenith angle of 62° (Junkerman et al., 1994). The values of /(O'D) are clearly reduced below the cloud, increase linearly inside the cloud, and are more than double the below-cloud values above the cloud top. As expected, the OH concentrations above the cloud are higher as well. For example, Mauldin et al. (1997) report OH concentrations of (8-15) x 106 OH cm"3 above clouds compared to (3-5) x 106 OH cm-3 in cloud-free regions. Similarly, Volz-Thomas et al. (f 996) measured values of /(N02) that were about twice as high above clouds compared to cloud-free days.
The intensity of all wavelengths is not affected equally by clouds. For example, Bordewijk et al. (1995) reported that the relationship between total solar radiation and UV in the 285- to 345-nm region measured at ground level is nonlinear, with relatively higher amounts of UV reaching the surface. Indeed, they suggest that even when the total solar radiation is decreased by 20% due to clouds, the UV intensity can be unchanged. Seckmeyer et al. (1996) also reported a wavelength dependence for radiation reaching the earth's surface through clouds. It has been shown that this dependence is not due to the properties of water in the clouds but rather to longer effective path lengths due to scattering (e.g., Kylling et al., f 997; Mayer et al., 1998). The increased path then gives a wavelength dependence through Rayleigh scattering and the enhanced light absorption by 03 and particles. The backscattered light from clouds (which can be measured by satellites) not only has been reported to be wavelength dependent but also differs for high-level clouds compared to low- or mid-level clouds (e.g., see Wen and Frederick (1995) and Chapter 14.C).
One final interesting aspect of clouds and actinic fluxes is that inside the cloud droplets themselves, an
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