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Doron Nof and Leslie M. Simon

Abstract

The mutual interaction of two isolated lenslike eddies is examined with the aid of a laboratory experiment on a rotating table. The isolated anticyclonic eddies are formed by withdrawing two cylinders containing a mixture of Freon (with a density of 1.53 gm cm−3), and silicone oil (with a density of 0.853 gm cm−3). The cylinders are embedded in water, and the collapse of the mixture forces two identical lenslike eddies (with an anticyclonic circulation) on the bottom of the tank. Initially, the lenslike eddies are completely separated from each other so that one vortex does not “know” about the presence of the other.

Due to small bottom friction, the vortices spin down slowly so that after some time their edges meet and they touch each other, forming a “figure 8” structure. After this happens there is a rapid (i.e., within 20 revolutions) interleaving of the two eddies. Arms are extended from one vortex to the other and the vortices become one unit consisting of two main lenses. As the interaction continues, the two lenses become less distinct and, ultimately, a single lenslike vortex is formed.

A total of about 20 experiments were performed and all showed that merging takes place after the eddies touch each other. Experiments with vortices whose densities are not identical were also performed and these also resulted in vortices that merged. The experiments suggest that the potential vorticity of the eddies is altered during their interaction and that no external source of energy is needed for the merging.

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John R. Mecikalski, Kristopher M. Bedka, Simon J. Paech, and Leslie A. Litten

Abstract

The goal of this project is to validate and extend a study by Mecikalski and Bedka that capitalized on information the Geostationary Operational Environmental Satellite (GOES) instruments provide for nowcasting (i.e., 0–1-h forecasting) convective initiation through the real-time monitoring of cloud-top properties for moving cumuli. Convective initiation (CI) is defined as the first occurrence of a ≥35-dBZ radar echo from a cumuliform cloud. Mecikalski and Bedka’s study concluded that eight infrared GOES-based “interest fields” of growing cumulus clouds should be monitored over 15–30-min intervals toward predicting CI: the transition of cloud-top brightness temperature to below 0°C, cloud-top cooling rates, and instantaneous and time trends of channel differences 6.5–10.7 and 13.3–10.7 μm. The study results are as follows: 1) measures of accuracy and uncertainty of Mecikalski and Bedka’s algorithm via commonly used skill scoring procedures, and 2) a report on the relative importance of each interest field to nowcasting CI using GOES. It is found that for nonpropagating convective events, the skill scores are dependent on which CI interest fields are considered per pixel and are optimized when three–four fields are met for a given 1-km GOES pixel in terms of probability of detection, and threat and Heidke skill scores. The lowest false-alarm rates are found when one field is used: that associated with cloud-top glaciation 30 min prior to CI. Subsequent recommendations for future research toward improving Mecikalski and Bedka’s study are suggested especially with regard to constraining CI nowcasts when inhibiting factors are present (e.g., capping inversions).

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