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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