Richard G. Semonin

METROMEX: an Investigation of Inadvertent Weather Modification

Abstract METROMEX, a field project designed and now in progress at St. Louis, involves 4 research groups planning and working cooperatively to study inadvertent weather modification by urban-industrial effects, and, in particular, man-made changes of precipitation. Urban areas affect most forms of weather and some, such as winds, temperature, and visibility, are obvious and their changes are easily measured. inadvertent precipitation changes are harder to measure, and except for the well-documented La Porte anomaly, urban-related rain changes have had only limited study. Examination of historical data at St. Louis has revealed summer increases in the immediate downwind area of. 1) rainfall (10–17%); 2) moderate rain days (11–23%); 3) heavy rainstorms (80%) 4) thunderstorms (21%); and 5) hailstorms (30%). METROMEX field measurements in the summer of 1971 involved 220 raingages and hailpads, 3 radar sets, 70 rainwater collectors, 14 pibal stations, 4 meteorological aircraft, unique atmospheric tracers, and ...

Resolving Power of a Zone Plate

The resolution (Rayleigh criterion for resolution) of two point sources by a zone plate is analyzed with respect to the number of zones. It is determined that for N (number of zones) less than 200 the limit of resolution is greater than that of a lens with the same aperture diameter, D, if only the zones which contribute light with a positive phase to the point of observation are used, and less than that of the lens if only the zones which contribute light with a negative phase to the point of observation are used. But, in the former case the maximum value of the side lobe (secondary maximum, first ring) is less than that of an equivalent-diameter lens whereas for the latter case the maximum value is greater. For N greater than 200 the resolution and side-lobe values approach those of a lens in both cases.

A Hypothesis for Urban Rainfall Anomalies

Abstract METROMEX was the first major field program aimed at studying the reality and causes of urban rainfall anomalies suggested in several climatological studies. The results from the 1971–74 METROMEX data portray statistically significant increases in summer rainfall, heavy (>2.5 cm) rainstorms, thunderstorms and hail in and just east (downstorm) of St. Louis. Examination of the rainfall yield of individual showers (cells), the spatial distribution of echo (rain) developments, and areal distribution of afternoon rain clearly point to the urban-industrial complex as the site for the favored initiation of the rain process under certain conditions. The greater frequency of rain initiations over the urban and industrial areas appear to be tied to three urban-related factors including thermodynamic effects leading to more clouds and greater incloud instability, mechanical and thermodynamic effects that produce confluence zones where clouds initiate, and enhancement of the coalescence process due to giant n...

Chicago Area Program: A Major New Atmospheric Effort

A series of mesoscale meteorological research projects have developed since 1975 in the area over and around the south end of Lake Michigan. These regionally focused projects, under the label of the Chicago Area Program (CAP), are being performed by scientists from 12 research groups or universities using funds from a variety of state and federal agencies. Efforts to date have led to the installation and operation of a major rain gage network, other weather networks and sondes, several weather radars, meteorological aircraft, and a ship. This sizeable program is addressing five major study areas including lake meteorology, water resources and hydrometeorology, inadvertent weather modification, air pollution and its impacts, and severe weather. Multigroup field experiments and the exchange of data are coordinated at the scientist level.

The Production of Equal and Unequal Size Droplet Pairs

Experimental examination of interactions between small water droplets, such as wake interactions, collision‐coalescence phenomena, and the dependence of these interactions on the electrical charge and field, has awaited the development of a dependable source of multisize droplets. A simple droplet generator has been developed to produce controlled pairs of equal and unequal size droplets. The generator and its applicability to droplet interaction studies are described.

Particle-Size Measurement Using Forward-Scatter Holography*

This study is an experimental and theoretical investigation of the application of forward-scatter Fresnel holography to water droplets. The theoretical holograms were constructed on a digital computer by determining the irradiance of the interference pattern formed by the light scattered from a droplet and a reference source, at discrete points on a hologram. The forward-scattered light is evaluated by assuming that the droplet is an opaque disk and utilizing the Rayleigh–Sommerfeld theory of diffraction. The reconstructed wave front is evaluated by performing the Fresnel transform on the hologram matrix. The corresponding experimental holograms were made with a helium–neon continuous-wave laser and using glass beads from 80 to 250 μm in diameter as models of the water droplets. The half-radiance width of the reconstructed wave-front radiance distribution is shown theoretically to increase linearly as the distance of the object from the hologram is decreased. Also, the half-radiance width increases logarithmically with hologram area. The theoretical and experimental radiance distributions in the plane of reconstruction were compared. Proper alignment of the hologram during reconstruction is necessary for the reconstruction of the original radiance distribution. Size can be measured by noting edge enhancement (radiance peak at the edge of the droplet) in the reconstructed image. A relationship between the diameter of the peak and the actual droplet diameter is determined for sphere diameters from 20 to 250 μm and object distances from 10 to 20 cm. Experimental accuracy to within 4% is achieved with this technique.

The variability of pH in convective storms

The rainwater pH was measured in a total of 22 storms which occurred in 1972 and 1974 in the METROMEX (METROpolitan Meteorological EXperiment) rainwater sampling network. The network consists of 81 collectors in an area of 1800 kM2 over and east of St. Louis, Missouri.The data set is composed of dry fallout samples as well as precipitation samples which have been analyzed independently. An analysis of the frequency distribution of these data shows the rainfall samples form a bimodal distribution of pH with relative frequency maxima at pH 7.0 and 4.0 while the dry samples are unimodal with the maximum frequency at pH 7.0.The areal distribution of precipitation weighted mean pH indicates a variability unrelated to total precipitation. However, the derived deposition of H+ is more similar to the areal rainfall pattern than to the pH. The dH is defined which incorporates both the rainfall and pH which is useful for climatological trend studies of acid rainfall.Examples of two individual storm events illustrate an inexplicable area-wide variation from nearly an all acid rain (pH ⩽ 4.5) to an all alkaline rain (pH ⩾ 5.5). These case studies indicate some of the meteorological as well as chemical problems which must be considered when attempting to characterize convective storm rainfall pH.

Interactions between Equal-Sized Droplets Due to the Wake Effect

Abstract In the development of raindrops from cloud droplets in warm rain, the collision-coalescence process is considered to be the main growth mechanism for droplets of unequal size greater than 20 μm in diameter. However, due to the wake effect, the possibility of equal-sized droplets colliding does exist for some maximum vertical separation of the droplets. An empirical study has been performed which led to the determination of the maximum vertical separation required, as a function of droplet size, for equal-sized droplets to be influenced by the wake effect.

Atmospheric Issues Identified in State Water Resource Planning

Abstract Illinois is completing a comprehensive statewide water plan. The plan selects three atmospheric issues, among the 11 identified as key issues facing the states water resources. The issues selected include climate change and prediction, inadvertent weather and climate modification, and planned weather modification. Each atmospheric issue presents major resource or policy problems, with capabilities needed to enhance the quality and/or quantity of the states waters. The identification of these atmospheric issues reveals awareness at the policy level of their importance. Policy and programmatic needs found to be common to each issue include 1) collection of more data and continued research (with an increasing state role; 2) coordinated policy development around atmospheric expertise from several agencies and universities; and 3) an expanded public information program. A Climate Detection and Assistance Board is to be established in Illinois to provide the planning, coordination, and assistance nee...

Film resolution and holographic recordings

The limitation imposed by the film resolution (N) on the size of a hologram, the Rayleigh resolution criterion, and the holograms ability to resolve the interference patterns are discussed. It is shown, for configurations with a plane wave or point source and specified λ (wavelength), that all the interference patterns recorded on the film will be resolved if Nλ = 2.