Abstract

Midlevel clouds occur in the region of the troposphere, lying between high cirrus ice clouds and low boundary layer stratus, stratocumulus and cumulus clouds. Midlevel clouds are associated with moisture advected to the middle part of the troposphere between about 2km and 6km, or temperatures between about 0°C and -20°C. However, there is no sharp gradation between cirrus and altus clouds, rather, a blending of one into the other. Again, deep frontal clouds, which include midlevel cloud, can extend from cirrus level to the boundary layer. Both the depths and the compositions of midlevel clouds are thus very variable. The clouds can often exist as supercooled water drops, which sometimes glaciate rapidly; as the saturation vapour pressure over ice is lower than over water, and the deficit is maximum at about -15°C in the mid-troposphere, any tendency for ice crystals to form can cause rapid growth of the crystals at the expense of the water drops. Optical remote sensing of these clouds has been confined in the past to a few observations (e.g. Platt and Gambling, 1971, Platt and Bartusek, 1974, Platt, 1977, Sassen, 1984, Uchino et al., 1988). However, recently, a few more observations were made in the first FIRE experiment (e.g. Heymsfield et al., 1991) although observations tended to concentrate on cirrus.

© 1993 Optical Society of America