Background aerosol and cloud definitions

In order to understand the optical properties of cloud in the height region of 0.7-3 km, we need to distinguish the differences between the clouds and aerosols. We defined the background aerosols (BGA) based on their scattering properties which follows two criteria: (i) the backscattering coefficient should be smaller than five times that scattered by air molecules and (ii) the DR of aerosols should be smaller than the value of 0.1 at the aerosol layer (Fig. 2). BGA normally consists of pollutants, sea salt,

Depolarization Ratio (5 a)

Depolarization Ratio (5 a)

Backscattering Ratio

Fig. 2. The height distribution of backscattering ratio (—), depolarization ratio (—), and relative humidity (Q) on 7 March 2002.

Backscattering Ratio

Fig. 2. The height distribution of backscattering ratio (—), depolarization ratio (—), and relative humidity (Q) on 7 March 2002.

and other unknown particles. Dust is a kind of aerosol that only occurs occasionally in the spring time. So, they should be treated as separate cases in addition to BGA.

For clouds definition we have to consider two points: First, the backscattering coefficient should be five times greater than that scattered by air molecules. Such a higher backscattering coefficient usually occurs when the scattering is by larger particles (such as water drop), and it is independent of the measured wavelengths of the laser. Second, the local RH should be higher than 70% at the cloud height. Usually, the clouds observed in the regions (0.7-3km) have temperature higher than 0°C with DR close to 0, because they consist of water drops (refer to Fig. 5), except for some special events such as the occurrence of dust storm from Northern China (Fig. 9).

0 0

Post a comment