Cumulus Cloud Model - Rain Formation in Warm Clouds: General Systems Theory - page 40

Fig. 2.2 In-cloud updraft speed and cloud particle terminal velocities for the two input cloud-base
CCN size spectra with mean volume radius (mvr) equal to (i) 2.2 μm and (ii) 2.5 μm. CCN cloud
condensation nuclei

Cloud-base vertical velocity equal to 1 cm/s has been used for the model com-
putations in the following. The in-cloud updraft speed and cloud particle terminal
velocities are given in Fig. 2.2 for the two input cloud-base CCN size spectra with
mean volume radius (mvr) equal to (i) 2.2 μm and (ii) 2.5 μm. The in-cloud updraft
speed W is the same for both CCN spectra since Wwfz

= * (Eq. 1.6) and depends
only on the persistent cloud-base primary perturbation speed w * originating from
MFC by deliquescence on hygroscopic nuclei at surface levels in humid environ-
ment (see Sect. 1.3). Cloud LWC increases with height (Fig. 2.3 ) associated with
the increase in cloud particle mean volume radius (Fig. 2.4 ) and terminal velocities
(Fig. 2.2 ). The cloud particles originating from the larger size CCN (mvr = 2.5 μm)
are associated with larger cloud LWCs, larger mean volume radii, and, therefore,
larger terminal fall speeds at all levels.
The turbulent vertical velocity perturbation w * at cloud-base level (1 km) is equal
to 0.01 m/s or 1 cm/s. The corresponding cloud-base temperature perturbation θ * is
then computed from the equation:

g

w =

θ θ

* *

v

θ

v

θ = w .

* *

g

Substituting w * = 1 cm/s, θ v = 273 + 30 = 303 K and g = 980.6 cm/s 2 , the temperature