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Glenn W. Brier
,
Ralph Shapiro
, and
Norman J. Macdonald

Abstract

A number of different analyses are used to examine 63 years of daily United States rainfall data to determine whether a suggested periodicity of 18 cycles per year exists. Although a slight tendency was found for such a period to persist in phase during the 63 years, the amplitude is extremely small. On the basis of the several tests performed we conclude that there is no reality to a periodicity at or near 18 cycles per year.

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Joanne Simpson
,
Glenn W. Brier
, and
R. H. Simpson

Abstract

A randomized seeding experiment was carried out on 23 tropical oceanic cumulus clouds on 9 days in the summer of 1965 as part of the joint Navy-ESSA Project Stormfury. Following instructions in sealed envelopes, an aircraft seeded 14 of the clouds with 8-16 pyrotechnic silver iodide generators called Alecto units. Each unit releases about 1.2 kg of silver-iodide smoke. The nine remaining clouds were studied in an identical manner as controls, using the same stack of four instrumented aircraft to penetrate the cloud before and after the seeding run. Cloud growth was documented by aircraft, radar and photogrammetry. The seeded clouds grew vertically an average of 1.6 km more following the seeding run than did the control clouds; the difference is significant at the 0.01 level.

A numerical model of cumulus dynamics was specified in advance of the field program. This model integrates the equation for the vertical acceleration of an entraining cumulus tower, predicting top heights of unseeded and seeded clouds as a function of ambient sounding and horizontal tower dimension. Seedability is defined as the predicted difference between the seeded and unseeded top of the same cloud. Effect of seeding is defined as the difference between the observed top and the predicted unseeded top of the same cloud. Both parameters are computed and graphed for all 23 clouds. Seeded and unseeded clouds separate into distinct populations. This statistical analysis demonstrates that 1) seeding has a clear effect on cumulus growth under specifiable conditions and 2) the model has considerable skill in predicting the amount of growth and in specifying the conditions.

Sources of subjectivity and bias are shown to be small and not to affect the results. The sensitivity of the model predictions to variations in input data is investigated with two examples, one each of large and of negligible cloud growth following seeding. Some possible effects of natural glaciation are examined with the model and future phases of the program are described.

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Glenn W. Brier
,
Ralph Shapiro
, and
Norman J. Macdonald

Abstract

A statistical analysis is applied to 63 years of daily United States precipitation data to determine whether a tendency exists for rainfall anomalies to occur near specific calendar dates. The results clearly indicate no tendency for rainfall anomalies near specific calendar dates.

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WILLIAM L. KISER
,
THOMAS H. CARPENTER
, and
GLENN W. BRIER

Abstract

Twelve years of hourly surface observations from Wake Island are analyzed to isolate the atmospheric tides and to study possible relations between the tides and weather over a very small land mass. In addition to determining the solar diurnal and the solar semidiurnal variation of temperature and pressure, the study demonstrates that the lunar semidiurnal pressure oscillation can be satisfactorily determined while the lunar semidiurnal temperature variation cannot be isolated with 12 years of hourly observations. The frequency of occurrence of precipitation shows only a slight suggestion of a lunar tidal period; however, the lunar semidiurnal precipitation component has the same relation to the lunar semidiurnal pressure wave as the solar semidiurnal precipitation wave has to the solar semidiurnal pressure wave. There is some evidence for a solar semidiurnal variation of cloudiness and a relationship between cloudiness and the semidiurnal pressure wave rather than between cloudiness and the predominantly diurnal temperature wave. A pronounced observational bias in sky cover associated with the moon's position makes it impossible to conduct any valid study of the relationship between sky cover and the lunar tidal period.

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Paul W. Mielke Jr.
,
Glenn W. Brier
,
Lewis O. Grant
,
Gerald J. Mulvey
, and
Paul N. Rosenzweig

Abstract

A reanalysis of the Climax I and II experiments is described. The concern prompting this reanalysis is a suggestion arising from Colorado State University analyses of extended area effects. Those analyses suggested a regionwide pattern of precipitation that, by chance, may have favored the randomly selected seeding days for some of the important meteorological partitions used in earlier analyses. In order to address this concern, this reanalysis employs excellent covariate relationships developed before the initiation of Climax II and which account for over half of the target variability for most meteorological partitions of major interest (e.g., warm 500 mb temperatures and southwest 700 mb wind directions). The statistical evidence of seeding-induced increases for this reanalysis is generally much stronger than in the previous analysis, which did not utilize covariates. For example, the joint one-sided Wilcoxon test statistic P-value for testing the null hypothesis that seeding did not induce a precipitation increase during warm 500 mb temperatures of the Climax I and II experiments is now 0.0013, compared to 0.0550 in the previous analysis. However, the reanalysis also indicates that previous estimates of increases attributed to seeding based strictly on ratios of seeded to non-seeded precipitation amount means are apparently too large. For example, the estimated precipitation increase of the combined Climax I and II data for the warm 500 mb temperature partition is reduced from 41 to 25% when the full set of data is employed in this reanalysis.

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Panel Discussion on Forecast Verification

(Held by the District of Columbia Branch on December 12, 1951)

Mr. Roger A. Allen
,
Mr. Glenn W. Brier
,
Mr. Irving I. Gringorten
,
Captain J. C. S. McKillip
,
Mr. Conrad P. Mook
,
Dr. George P. Wadsworth
, and
Mr. Walter G. Leight
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