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JOANNE SIMPSON and VICTOR WIGGERT

Abstract

A one-dimensional numerical cumulus model was tested against data from a randomized seeding experiment made in South Florida in 1968. Fourteen GO clouds were studied. Nine were seeded by pyrotechnics with 1 kg of silver iodide each, while five were studied identically as controls.

Various seeding subroutines and assumptions regarding the ice phase are compared. The experimental aircraft data are used to guide the modeling assumptions and to select the most realistic ones. Seedability and seeding effect correlate to 0.96 for seeded clouds in the three best models. A high correlation is found between seedability and radar-measured rainfall increase from seeding. Also, a high correlation is found between model predictions of the difference in precipitation fallout between seeded and control clouds and the measured rainfall differences, although the model predictions are much smaller in magnitude. A calculation is undertaken showing that coalescence within the cloud body on descent of the raindrops easily accounts for the discrepancy.

The model predictions for each GO cloud are discussed in comparison with actual measurements on the cloud.

The 1968 experiment was found to subdivide into two periods, one fair and one disturbed, with quite different effects of seeding. The two periods and corresponding cloud behavior are compared. It is concluded that the disturbed period was less favorable for seeding because of higher unseeded cloud growth and strong wind shear. Implications of this result for future modeling efforts are discussed.

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JOANNE SIMPSON and VICTOR WIGGERT

Abstract

This paper presents a model of the growth of cumulus clouds. The water content and maximum height of rising towers are calculated using a buoyancy equation with consideration of effects of entrainment and water load. The latter is subject to effects of modeled microphysical effects. Precipitation growth is parameterized in terms of an autoconversion equation and a collection equation. A precipitation fallout scheme is devised that depends on water content, drop spectrum, and the vertical rise rate of the tower.

Then “freezing subroutines” are devised to model the effects of silver-iodide seeding. A hierarchy of seeding routines, using different ice collection efficiencies and terminal velocities, is partially tested against the data of the Stormfury 1965 tropical cumulus-seeding experiment.

Some preliminary numerical experiments on warm clouds are performed, assuming changes in drop spectra from hygroscopic seeding.

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Victor Wiggert and Stellan Ostlund

Weather radar power can be electronically assessed and digitally quantified within many small “range bins.” The tape recorded output from a radar digitizer linked to the Miami WSR-57 is being processed post hoc by a sequence of computer programs written at the Experimental Meteorology Laboratory. One program assesses radar-derived rainfall rates and total rain volumes over preselected areas and for preselected time periods; another isolates and tracks radar echoes and, while so doing, calculates the rainfall from each echo as it grows, moves, splits, merges, or dies. Sample results are displayed and future applications discussed.

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Victor Wiggert, Robert I. Sax, and Ronald L. Holle

Abstract

The potential for enhancing rain output over central Illinois through modification of summertime convective clouds was investigated by use of model predictions of cumulus growth, as well as direct, internal measurements of cloud physical characteristics. No clouds were seeded in Illinois. Comparisons were made with results gathered during seeding experiments on single convective clouds in Florida and also during the Florida Area Cumulus Experiment (FACE).

Predictions of the top heights of unseeded and seeded Illinois clouds (and the predicted, post-seeding height increase, the “seedability”) were made using a one-dimensional cumulus model and 10 summers of regularly-gathered radiosonde data from Rantoul and Peoria. Seedability (which in Florida is correlated with enhanced rain volume) was small (<2 km) or zero in Illinois much more frequently than in Florida, and was largest, in the mean, in July and smallest in June; it varied markedly from morning to evening, from month to month within a summer, and from summer to summer.

Measurements made during two July days in 1977, in cumuli over central Illinois, described the natural evolution of the ice-water budget and the life history of the updraft. In-cloud microphysical characteristics, near the −10°C level (∼6 km) in Illinois, in convective clouds that were developing in moist air in advance of a weak cold front, were equivalent to those characteristics encountered at the same penetration level of Florida cumuli of similar size and depth. Clouds that were penetrated in the dry air behind the cold front had microphysical structures that differed greatly from those in clouds developing in tropical maritime air mass conditions.

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Victor Wiggert, Gloria J. Lockett, and Stellan S. Ostlund

Abstract

Data from rainshowers observed by the WSR-57 radar at the NOAA National Hurricane Center in Miami were tape-recorded during three recent summers as part of the Florida Area Cumulus Experiment (FACE). Past studies of convective clouds in the 13 × 1033 km2 FACE target area have indicated that enhancement of rainfall, following silver iodide seeding, was favored on those days that had light winds in the lower troposphere, along with some movement of rainshower echoes.

In this study, we investigated the effect of wind speeds above and below 3 m s−;1 and, alternatively, the effect of echo motion (or stationarity) on the behavior of radar-observed rainshower echoes that were over water or over land, that had undergone merger with other echoes or had never merged, as functions of time of day and of echo growth cycle. We ignored any alterations in echo behavior that may have been the result of cloud seeding activities that were conducted over the target area on half of the afternoons.

PPI radar scans were tape-recorded every 5 min from 0900 to 2100 LT. Individual rainshower echoes were isolated by computer and objectively “tracked” in a square domain of 11 × 104 km2 that was centered on Miami. Echo characteristics such as area, population, volumetric rain rate and area-average rain intensity were extracted from each of the recorded PPI scans during 16 days of the FACE program. Variations in the echo characteristics were studied from data that were composited into 8-day sets. The compositing was based on echo motion and, alternatively, mean speed of the lower tropospheric wind.

The 8-day averages of daytime echo areas and rain volumes were generally larger when echoes had resulted from the merger of preexisting echoes, or were over land, or stationary, or embedded in a lower tropospheric wind field with speed <3 m s−1. Unmerged echoes were 5–14 times more numerous and, per echo, had W 12–429% of the rain volume of merged echoes in the same set of days. There were ∼15% more echoes over land than over water. Over land and over the total domain, total rain volume was 30–110& greater on days with stationary, rather than with moving, echoes and 70–200% greater on days with weak, rather than strong. mean winds at low levels.

Differences among the 16 days of the hour-by-hour averages of four parameters were tested for significance at the <59% level (one-tailed Wilcoxon-Mann-Whitney test). The parameters were average echo area, average volumetric rain rate, average population and echo-average rain intensity. For nonmerged echoes, each parameter, except rain intensity, was shown to be significantly different from that of echoes that resulted from the merger of other echoes. This significance was found especially after noon and regardless of whether wind speed was weak or strong, or whether echoes were moving or stationary. After 1400 LT, echoes over land had significantly larger volumetric rain rates, areas and rain intensities than did those over water, but only when the echoes were stationary or embedded in light winds in the lower troposphere. When days with strong wind were compared to those with weak wind, unmerged echoes were found to have been affected more strongly by wind speed than were merged echoes, and echoes over land were more affected than were echoes over water.

Volumetric rain rate was significantly larger on days with stationary, rather than moving, echoes, for a greater number of hours, when echoes wore unmerged rather than when merged. Thus. even before merger, the stationary rainshowers produced greater rain volumes for more hours than did moving echoes. This is a factor that underlies the lack in improvement of rain volumes by artificial means, which has been observed on days with stationary echoes.

Growth tendencies of rain intensity for the four 8-day composite sets were similar in that echoes at any given percentage of their maximum size, and while growing in area, had higher intensities than did dying rainshowers having the same relative size. Highest rain intensities preceded maxima in echo area. Growth tendencies revealed that greater average rain intensities, over the ocean as well as over land, were found on the same kinds of days (i.e., with light winds but some echo motion) that provided the most favorable conditions for seeding cumuli in the FACE target area.

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William L. Woodley, Anthony R. Olsen, Alan Herndon, and Victor Wiggert

Abstract

Gage and radar methods of convective rain measurement are compared in the context of the continuing multiple cloud seeding experiment of the Experimental Meteorology Laboratory. An optimal system, combining the best features of both, is recommended.The nature of the Florida convective rainfall to be measured is documented using measurements from a dense raingage mesonet (about 3 km2 per gage over 570 km2) that was operated for a total of 93 days in 1971 and 1973, and the gaging requirements for detection and measurement of 24 h rainfalls in the mesonet are determined using the full complement of gages as the standard. For the measurement of areal convective rainfall greater than 0.25 mm within a factor of 2 on 90, 70 and 50% of the days, gage densities of 31, 91 and 208 km2 per gage, respectively, are required.Radar performance in estimating convective rainfall over south Florida is determined using two collocated, calibrated 10 cm radars (UM/10-cm of the University of Miami and WSR-57 of the National Hurricane Center). In all cases, the radar estimates of rainfall are compared with the rainfall as determined by raingages (densities 3 to 8 km2 per gage) in cluster arrays. The relative performances of the two radars are compared.In 1973, WSR-57 radar-derived rainfalls were computed by hand as in 1972 and by computer using taped radar observations. On a daily basis, 80% of the radar estimates were within a factor of 2 of the cluster standard. The combined accuracy of the WSR-57 radar in 1972 and 1973 in estimating convective rainfall approximated that which one would obtain with a gage density of 65 km2 per gage over an area the size of the mesonet.The daily representation of rainfall by the radar improves if one adjusts it using gages. In the mean, adjustment produced a statistically significant 15% improvement (<1% level with two-tailed “t” test) in radar accuracy. The adjusted radar measurements then had an approximate gage density equivalence of 25 km2 per gage.The gaging requirements for the estmation of area mean rainfall for an area the size of the EML target (1.3 × 104 km2) is inferred using the digitized radar observations. To meet a specification that the area-mean rainfall be measured to within a factor of 2 of the true value 99% of the time requires 143 km2 per gage, compared to a requirement of at least 13 km2 per gage for the mesonet.An optimum method of rain measurement is suggested. For the measurement of the rainfall from individual showers anywhere, the gage-adjusted radar is far superior to gages alone. For measurement in a fixed area the size of the mesonet, gages are superior to the radar. To measure rainfall over the EML target either gages alone, or a radar adjusted by gages, can accomplish the task. About 90 evenly spaced gages in the EML target should provide area rain measurements within a factor of 2 of the true value 99% of the time. The radar estimates adjusted by gages should be as accurate as those provided by the network of 90 gages. The final choice as to the measurement system will probably be determined by other considerations such as budget, personnel and terrain over which the measurements are to be made.

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