Gary L. Achtemeier

The Relationship Between the Surface Wind Field and Convective Precipitation over the St. Louis Area

Abstract Rainfall, wind and temperature data at the surface for a mesoscale area surrounding St. Louis, Missouri for seven summer days in 1975 were used to determine qualitative and quantitative relationships between divergence, and the location, timing and intensity of rainfall. The study used 30 prominent raincells that formed over a 1600 km2 wind network under different synoptic and subsynoptic weather conditions. The results indicate that the physical relationships between convergence and convective rainfall over the Middle West are quite complex. Widespread convective rainfall seldom occurred in the absence of some larger scale forcing. Possible forcing mechanisms occurred at various levels within the troposphere and over several different space scales. On the cell scale, convergence preceded some raincells but not others. Potential mechanisms that could explain the dichotomy were investigated. The results from several statistical studies include: 1) Pre-rain average network convergence was weakly re...

Planned Weather Modification and the Severe Weather Threat in the Central High Plains

Abstract Operational and experimental convective cloud-seeding projects are often planned without regard to the number of seeding-opportunity days that can be lost because of the need to suspend operations during the threat of severe weather. June daily rainfall, severe storm and tornado watches, and observed tornadoes within a hypothetical (proposed) operational area over southwest Kansas were compared within the context of five procedures for severe weather related operations suspensions. These procedures varied in the restrictions placed on operations. The results show that anywhere from 45–87% of the June rain can fall when operations have been suspended. The length of a scientific seeding experiment could be increased anywhere from 45–426%. Finally, 46% of the tornadoes occurred when there were no concurrent tornado watches. This failure rate is so large that severe weather watches may not be useful for operations suspension procedures.

On the Temporal Decay of the Relationship between Environmental Covariates and Convective Rainfall for the Kansas High Plains

Abstract Covariates derived from 1200 GMT tropospheric soundings taken for 13 June at Dodge City, Kansas, were compared with 3 h rainfall within the surrounding area. Correlation magnitudes decreased beginning with the first period for which precipitation lagged the soundings. There were larger decreases thereafter. It is suggested that the correlation magnitudes decrease because the measured environment is not representative of the environment within distant precipitation-producing migratory disturbances. This implies that atmospheric measurements should be taken simultaneously with the initiation of rainfall if maximum correlative power is to be attained. Spatial analyses are presented as an alternative to frequent observations so that migratory precipitation-producing disturbances can be detected prior to the on-site rainfall. Covariates obtained from spatial fields gave higher correlations with 3 h rainfall than did the covariates obtained from single soundings.

Design of a Hall Suppression Project for Illinois

Abstract A description is given of a broad program to Design and Experiment to Suppress Hail (DESH) in Illinois. This program draws on results acquired during 17 years of extensive hail research in Illinois. There are two principal tasks to DESH: the determination of the desirability and the feasibility of hail suppression experimentation in Illinois and the Midwest. Socio-economic studies have led to an affirmative conclusion on the desirability issues. The feasibility decision appears affirmative and rests on certain key results. Airborne cloud base seeding in the humid midwestern environment is possible but will be more difficult and expensive than in less humid areas. Radar will be needed for short-term forecasting, aircraft operations, identification of potential hailstorms, and in the evaluation of seeding effectiveness. Weather forecasting by objective techniques will be valuable in both operations and evaluation, and adequate objective techniques have been largely developed. The overall shape of t...