Fig.1 : Schematic representation of the eddies in the Atmospheric Boundary Layer
It was shown that the buoyant
production of energy by the Microscale-Fractional-Condensation (MFC)
in turbulent eddies is responsible for the sustenance and growth of the
large eddies (vortex rolls). The MFC takes place in turbulent eddies
even in the unsaturated environment. Under favourable synoptic conditions
the turbulent eddies get further amplified due to enhanced MFC and
lead to the growth of the large eddy in the vertical resulting in cloud
formation above the
LCL. Inside the cloud the turbulent eddies get
amplified faster due to higher degree of condensation and generate cloud-top-gravity
(buoyancy) oscillations which are responsible for vertical mixing in clouds
(Mary Selvam et al., 1985). The theory relating to the dynamics
of the ABL and a warm cloud model is presented below.
The circulation speed of the large eddy is related to that of the turbulent eddy according to the following expression (Townsend, 1956).
where W and w*are respectively the r.m.s.(root mean square) circulation speeds of the large and turbulent eddies with radii R and r. For a large eddy with R = 10r the increase in W is 25% of w*or W @0.25w*.
The wind profile in the ABL is governed by the physical processes relating to the growth of the large eddy. It was shown (Mary Selvam et al., 1985) that the vertical wind (W) profile can be expressed as
where w*is the increase in the vertical velocity per second of the turbulent eddy resulting from the MFC process, at the normalised height z (R/r), k the Von Karman constant which has been shown to be equal to 0.4 and represents the fractional volume dilution rate of the large eddy by the turbulent scale eddies for z = 10 (Mary Selvam et al., 1984 a, b).
On the basis of the conceptual cloud model discussed earlier, theory relating to the prediction of different cloud parameters is briefly discussed in the following.
In Eq.(3), f will be representative of the q / qa. The model predicted profile of q / qa is in close agreement with the observed profiles (Fig.2).
The logarithmic wind profile (Eq.2) can be expressed as
q = q* fz
Thus W andq follow the fz distribution
The in-cloud temperature lapse rate can be expressed as follows (Mary Selvam et al., 1984 a).
where t is the dry adiabatic lapse rate.
Analogous to Eq.(5) the expression for qt can be given as
Fig. 3 : Computed fz profile
Fig. 4 : Computed cloud growth time
The N* at level z can be expressed as
The computed cloud drop size spectra at different levels in the cloud are shown in Fig.5.
Fig. 5 : Computed cloud drop spectra at different heights in the cloud
The computed rain-drop size spectra at different levels in the cloud are shown in Fig.6.
Fig. 6 : Computed rain-drop spectra at different heights in the cloud
Mary Selvam, A., A. S. R. Murty and Bh. V. Ramana Murty, 1984 a : A new physical hypothesis for vertical mixing in clouds. Proc. 9th International Cloud Physics Conference, Tallinn, Estonian SSR, USSR, 21-28 August 1984, 383-386.
Mary Selvam, A., A. S. R. Murty and Bh. V. Ramana Murty, 1984 b : Role of frictional turbulence in the evolution of cloud systems, Proc. 9th International Cloud Physics Conference, Tallinn, Estonian, SSR, USSR, 21-28 August 1984, 387-390.
Mary Selvam, A., A. S. R. Murty, Poonam Sikka and Bh. V. Ramana Murty, 1985 : Some physical and dynamical aspects of warm monsoon clouds and their modification. Arch. Met. Geoph. Biokl., Ser. A (In press)
Murty, A. S. R., A. M. Selvam and Bh. V. Ramana Murty, 1975 : Summary of observations indicating dynamic effect of salt seeding in warm cumulus clouds. J. Appl. Meteor., 14, 629-637.
Murty, A. S. R., R. N. Chatterjee, A. M. Selvam, B. K. Mukherjee, L. T. Khemani and Bh. V. Ramana Murty, 1985 a : Results of the randomized warm cloud modifications experiment conducted using aircraft in Maharashtra State, India during the nine summer monsoon seasons (1973-74, 1976, 1979-84). Proc. 4th WMO Scientific Conference on Weather Modification, Honolulu, Hawaii, 12-14 August 1985.
Murty, A. S. R., A. M. Selvam, S. S. Parasnis and Bh. V. Ramana Murty, 1985 b : Warm cloud dynamical responses to salt seeding. Proc. 4th WMO Scientific Conference on Weather Modification, Honolulu, Hawaii, 13-14 August 1985.
Parasnis, S. S., A. M. Selvam, A. S. R. Murty and Bh. V. Ramana Murty, 1980 : Dynamical characteristics of warm monsoon clouds and their responses to salt seeding. Proc. 3rd WMO Scientific Conference on Weather Modification, France, 21-25 July, 1980, 127-132.
Townsend, A. A., 1956 : The structure of turbulent shear flow, Cambridge University Press, 113-130.
Woodcock, A. H. and A. T. Spencer, 1967 : Latent
heat released experimentally by adding sodium chloride particles to the
J. Appl. Meteor., 6, 95-101.