A. Mary Selvam and A.S. Ramachandra Murty
Indian Institute of Tropical Meteorology, Pune 411008, India.
Proc. of the 4th WMO Scientific Conf. on Weather Modification 12-14 August 1985, Honolulu, Hawaii, 503-506.
Extensive aircraft cloud physical observations made
in more than 2000 tropical cumulus clouds indicated new observational
evidence viz., (1) horizontal structure of the air flow inside the cloud
has consistent variations with successive positive and negative values
of vertical velocity representative of ascending and descending currents,
(2) regions of ascending currents are associated with higher LWC
and negative cloud drop charges and the descending currents are associated
with lower LWC and positive cloud drop charge, (3) width of the
ascending and descending currents is about 100 m, (4) ratio of the
measured LWC (q) to that of the adiabatic value (qa)
= 0.60 at cloud-base levels, (5) increase in the cloud electrical
activity with LWC, (6) cloud-drop size spectra is unimodal near
the cloud-base and multimodal at higher levels, (7) in-cloud temperatures
are colder than the environment, (8) environmental lapse rates are equal
to the saturated adiabatic value while inside the cloud they are lower,
(9) positive increments in the LWC are associated with the increments
in temperature inside the cloud and the immediate environment and (10)
variances of in-cloud temperature and humidity are larger in the regions
where the values of the LWC are higher. The observed dynamical and
physical characteristics of clouds cannot be explained by the presently
available cloud models. A simple cloud model based on certain physical
processes taking place in the atmospheric boundary layer (ABL) has
been developed. A brief summary of the cloud model is presented below.
The ABL consists of convective scale large
and turbulent eddies (Fig.1).
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,
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 @ 0.25 w*.
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
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.
2.1 Vertical Profile of q / qa
The fraction of the air mass of surface origin
which reaches the height z after dilution due to the vertical
mixing caused by the turbulent eddy fluctuations can be expressed as (Mary
Selvam et al., 1984 a).
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).
2.2 In-Cloud Vertical Velocity and Excess Temperature Profiles
The logarithmic wind profile (Eq.2) can be expressed as
The in-cloud temperature lapse rate can be expressed
as follows (Mary Selvam et al., 1984 a).
is the dry adiabatic lapse rate.
2.3 Total Cloud Liquid Water Content (qt) Profile
Analogous to Eq.(5) the expression for qtcan be given as
the production rate of the LWC at the cloud-base level and is equal
/L. The qt
profile follows the fz distribution. The computed fz
profile is shown in Fig.3.
Fig. 3 : Computed fz profile
2.4 Cloud Growth Time
The cloud growth time, t, can be expressed
Fig. 4 : Computed cloud growth time
2.5 Cloud Drop Size Spectra
It was shown that the cloud drop number concentration decreases with height according to the f distribution due to eddy mixing (Mary Selvam et al., 1985). The total water content increases with height according to the fz distribution. The cloud drop spectrum at any level will consist of the drops transported from the lower levels and the large size drops formed on less number of nuclei at the level.
It was shown that the cloud drop spectrum at any
level z consists of drops of radii r1,
with respective concentrations of n1 , n2,
...., nz . rz and nz
can be expressed as follows.
Where c = N*fz
at level z can be expressed as
Fig. 5 : Computed cloud drop spectra at different heights in the cloud
2.6 In-Cloud Raindrop Spectra
In the model it was assumed that the bulk conversion
of cloud water to rain water takes place mainly by collection and the process
is efficient due to rapid increase in the cloud water flux with height.
Analogous to the cloud drop size spectrum the raindrop size spectrum at
any level z above the cloud-base will consist of z
categories of raindrops of radii R1, R2,
...Rz with respective concentrations n1,
... nz respectively.
and Rz can be expressed as follows.
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
3. WARM CLOUD RESPONSES TO SALT SEEDING
In the previous section it was shown that the buoyant
production of energy by MFC in turbulent eddies is mainly responsible
for the formation and growth of the cloud. When warm clouds are seeded
with salt particles the turbulent buoyant production of energy increases
due to enhanced condensation and results in enhancement of vertical mass
exchange. This would enhance the convergence in the sub-cloud layer and
result in the invigoration of the updraft in the cloud. If sufficient moisture
is available in the sub-cloud air layer the enhanced convergence would
lead to increased condensation and cloud growth. The salt seeding can thus
alter the dynamics of warm clouds.
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.
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