theoretical estimation of the lifting condensation level (lcl)

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convective clouds. In this context, according to the concept of Lifting Condensation Level (LCL), from the microphysical point of view, the LCL height is optimal ...

THEORETICAL ESTIMATION OF THE LIFTING CONDENSATION LEVEL (LCL) FOR A RISING AIR PARCEL IN ORDER TO ENHANCE THE CLOUDS SEEDING PROCEDURES IN PERIODS OF LOW RAIN INCIDENCE Muñoz R, Erith A. (1)(2) , Mundaray, Rafael (2) , Estrella P, Carlos M (1) . (1) Ecuadorian Space Institute (Quito-Ecuador) (2) Meteorological and Hydrological National Institute (Caracas-Venezuela). ABSTRACT An alternative to solve problems related to periods of drought due to rainfall deficit is to increase rain by artificial methodologies that could be achieved by stimulation of cloud formation from the ground or atmosphere directly. In the case of cloud seeding from the atmosphere, a fairly used methodology consist in the bombing of external chemical agents that facilitate the process of nucleation and condensation of water vapor in cloud droplets, however this methodology requires a high knowledge about the thermodynamic state of the vertical column of the atmosphere, as well as considering the microphysical processes involved in the formation of convective clouds. In this context, according to the concept of Lifting Condensation Level (LCL), from the microphysical point of view, the LCL height is optimal for the artificial formation of convective clouds, and for this reason in this work a methodology for the estimation of the LCL height is presented.

1. INTRODUCTION A relevant aspect derived from the climate change is associated with drought periods

Barnes (1968) reported a estimating uncertainty of ±0,5C for LCL calculations using

due to variations in rainfall seasons. Particularly, drought seasons over Caribbean and

equation (3). So this expression is a good approach to calculate TLCL as a function of T

northern

and Td. The temperature of the ascending air parcel decrease linearly according to the

South

America are strongly interconected with wetter seasons over

the

southern North America (Nobre and Srukla,1996), and this interconection generates undesirables effects

Pseudo-Adiabatic Gradient Γs, until it reaches the LCL height (zLCL):

that adversely affect social and economical systems, and even

more serious sustainability of life over this regions.

(4)

(5)

(6)

(7)

Bruintjes (1999) presents a complete review in which several techniques for cloud seeding are debated. In this context, some researches have shown the potential to increase precipitation by seeding silver iodide (AgI) in strong convective clouds (Hsie etal., 1980),however there are some factors related with the AgI seeding process in order to reach optimum results. The foundations to hygroscopical seeding experiment using AgI, are based on cloud microphysical principles, which establish the possibility of

(8)

stimulate the nucleation and condensation process in atmospheric convective process by the seeding of external chemical agents to increase precipitation rates. From this point, a way to improve the condensation process in convective clouds development could be seeding AgI in the LCL height, considering that in this point, the nucleation and

In equation (6), Γ is the adiabatic gradient for dry air, To is the initial temperature for the air parcel, L is latent heat, ws is the saturation mixing ratio, R’ is the individual gas constant, cp is the specific heat for constant pressure.

condensation activities have great influence in the formation of convective structure, and 3. DISCUSSION AND RESULTS

also with its potential to precipitate. 2. LCL ESTIMATION Some relevant works have been published at the present to estimating the LCL height (Schrieber et al., 1996, Barnes, 1968, R., 1968), however, the contribution of this paper is to provide an equation for LCL estimation using as input the dew point temperature and the surface temperature, both measured generally in ground meteorological stations, furthermore, it also have been considered the variations associated with the adiabatic gradient due to the saturation mixing ratio dynamics. From the dynamical point of view, to estimate the ZLCL height for which an ascending

Table1: Aplications of the equations (3), (5) and (8) using as input To and Td values taken from virtual sounders of the MM5 model

air parcel reaches the LCL, it is needed to know either the temperature TLCL surface or

4. REFERENCES

the pressure PLCL that experiment the parcel at the LCL point. Barnes(1968) described a relation in which the temperature difference between the LCL point temperature (TLCL) and the dew point temperature(Td), divided by the differences between the temperature of the air parcel on surface (T) and the dew point temperature is constant, and also analyzed the functional dependence of k with Td, reporting:

(1)

(2)

In equation (2), Td is measured in Celsius. Combining the equations (1) and (2), and isolating TLCL:

(3)

Figure1: LCL estimation on summer condition over Caribbean region.