The design and protocol of a summertime rainfall enhancement program for West Texas



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Texas Tech University


Original analyses of data collected during three rainfall enhancement experiments are combined with other Individual studies to assess the state of knowledge about rainfall enhancement In West Texas. The small, multiple-cell convective system is chosen for study because treating it offers the potential for significantly increasing growing-season rainfall in the region.

Geostationary Operational Environmental Satellite Images are used to develop a climatology of the frequency, location and timing of the small, multiple-cell convective system formations within 150 km of Big Spring, TX. Enough treatment opportunities exist during each month In the Maythrough- August period to warrant experimentation. No area of preferential development appears to exist within the study region.

The small, multiple-cell convective system is initiated in a moist, conditionally-unstable atmosphere by weak forcing mechanisms. Precipitation appears to be first produced by either the cold or the warm rain process depending upon the strength of the vertical motions within the clouds. The cold rain process dominates later stages of the cloud system's lifetime. However, microphyslcal characteristics of the clouds are highly variable and lead to the conclusion that data In addition to those which already exist are needed before the precipitation process in the clouds is sufficiently understood to be altered in a verifiable manner.

The relationship between rainfall rate (R) and radar reflectivity factor (Z) In the small, multiple-cell convective system is examined considering the effects of evaporation and horizontal transport of rainfall. The equation Z-260R''^2 is derived and is found to produce the best radar estimates of overall mean rainfall volume. However, Z=155R^-^® best estimates rainfall volume for the small, multiple-cell convective system.

A field experiment is designed for sampling of the small, multiplecell convective system with Doppler radars and instrumented aircraft, This experiment Is guided by a conceptual model developed from the results of the analyses discussed above. The cloud sampling experiment is the first stage in a multi-phase program expected to produce statistically significant findings about the effectiveness of seeding summertime convective clouds. The annual cost of the three-year cloud sampling experiment is estimated to be $3,283,900, but could be reduced to $1,695,200 through the elimination of certain equipment and personnel.