SOME OBJECTIVE QUANTITATIVE CRITERIA FOR SUMMER SHOWERS AT MIAMI, FLORIDA

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  • 1 Weather Bureau Airport Station, Miami, Fla.
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Abstract

A brief review of previous efforts in Florida and elsewhere to cope with the problem of summer air mass showers is followed by a new attempt to apply to the Miami problem empirical methods of determining the combined effect of several often contradictory shower parameters. Employing the hypothesis that criteria differ seasonally, geographically, and diurnally, this study classifies Miami summer soundings into four rainfall producing types for each of the two diurnal periods and presents averaged dry bulb, wet bulb, and dew point temperatures by 50-mb. intervals to 450 mb. for each type. In addition the heights of the 700-mb. surface and of the freezing level, together with corresponding changes by half days up to three days, are recorded by types. From these and related data, inductive reasoning suggests mechanisms for endemic shower types, and parameters are selected for determining precipitation during the 12-hour period following either sounding. Probability curves that represent also quantitative rainfall are drawn from four summer seasons' data. Contingency tables are given for the dependent objective data, two seasons' independent objective data, and one season of corresponding 12-hour forecasts. In terms of skill score the subjective forecasts are found to be inferior at 1500 gmt and slightly superior at 0300 gmt. Principal conclusions include the finding that Miami showers are more closely related to the absolute humidity through a broad layer centered near 800 mb. than to moisture in higher or in surface levels, and that heavier showers may be inhibited by excessive absolute humidity above 650 mb.

Abstract

A brief review of previous efforts in Florida and elsewhere to cope with the problem of summer air mass showers is followed by a new attempt to apply to the Miami problem empirical methods of determining the combined effect of several often contradictory shower parameters. Employing the hypothesis that criteria differ seasonally, geographically, and diurnally, this study classifies Miami summer soundings into four rainfall producing types for each of the two diurnal periods and presents averaged dry bulb, wet bulb, and dew point temperatures by 50-mb. intervals to 450 mb. for each type. In addition the heights of the 700-mb. surface and of the freezing level, together with corresponding changes by half days up to three days, are recorded by types. From these and related data, inductive reasoning suggests mechanisms for endemic shower types, and parameters are selected for determining precipitation during the 12-hour period following either sounding. Probability curves that represent also quantitative rainfall are drawn from four summer seasons' data. Contingency tables are given for the dependent objective data, two seasons' independent objective data, and one season of corresponding 12-hour forecasts. In terms of skill score the subjective forecasts are found to be inferior at 1500 gmt and slightly superior at 0300 gmt. Principal conclusions include the finding that Miami showers are more closely related to the absolute humidity through a broad layer centered near 800 mb. than to moisture in higher or in surface levels, and that heavier showers may be inhibited by excessive absolute humidity above 650 mb.

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