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W. M. Frank and W. M. Gray

Abstract

Over 8500 rawinsonde soundings within 8° radius of tropical cyclones in the northwest Pacific and West Indies regions are statistically analyzed to determine occurrence frequencies of 15 m s−1 (30 kt) winds at the sea surface. At any radius the likelihood of encountering winds >15 m s−1 is found to increase with increasing central wind speed and with increasing radius of the highest closed isobar. There are significant right-side wind maxima and left-side minima with respect to the direction of storm motion. This reflects the natural storm asymmetry. Much variability is found in the radius of 15 m s−1 winds between cyclones of similar inner core intensity.

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William M. Gray and Christopher W. Landsea

This paper describes a predictive relationship between West African rainfall and U.S. hurricane-spawned destruction, which is based on information for the 42-yr period 1949–90. It is shown that above-average rainfall during the previous year along the Gulf of Guinea, in combination with above-average rainfall in the western Sahel during June and July, is linked to hurricane-spawned destruction along the U.S. East Coast occurring after 1 August, which is 10–20 times greater than in years when pre-1 August precipitation for these West African regions is below average. Similar hurricane-spawned damage along the U.S. Gulf Coast shows only a negligible relationship with African rainfall. Hurricane-caused deaths for both U.S. coastal regions also show a similar association with West African rainfall.

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William M. Gray and Robert W. Jacobson Jr.

Abstract

This paper presents observational evidence in support of the existence of a large diurnal cycle (one daily maximum and one daily minimum) of oceanic, tropical, deep cumulus convection. The more intense the deep convection and the more associated it is with organized weather systems, the more evident is a diurnal cycle with a maximum in the morning. At many places heavy rainfall is 2–3 times greater in the morning than in the late afternoon-evening. Many land stations also show morning maxima of heavy rainfall. The GATE observations show a similar diurnal range in heavy rainfall, but the time of maximum occurrence is in the afternoon. This occurrence is 6–7 h later than in most other oceanic regions and is probably a result of downwind influences from Africa and the fact that the GATE heavy rainfall was often associated with squall lines.

Diurnal variations in low-level, layered and total cloudiness show a much smaller range. The variability of deep convection and heavy rainfall is not readily observable from those satellite pictures which cannot well resolve individual convective cells nor is it easily obtained from surface observations of the percent of sky coverage which are heavily weighted to the presence of low-level and layered clouds. A comparison of previous observational studies is made. It is hypothesized that the diurnal cycle in deep convection with a morning maximum is associated with organized weather disturbances. This diurnal cycle likely results from day versus night variations in tropospheric radiational cooling between the weather system and its surrounding cloud-free region.

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Christopher W. Landsea and William M. Gray

Abstract

Seasonal variability of Atlantic basin tropical cyclones is examined with respect to the monsoon rainfall over West Africa. Variations of intense hurricanes are of the most interest, as they are responsible for over three-quarters of United States tropical cyclone spawned destruction, though they account for only one-fifth of all landfalling cyclones. Intense hurricanes have also shown a strong downward trend during the last few decades. It is these storms that show the largest concurrent association with Africa's western Sahelian June-September rainfall for the years 1949–90.

Though the Sahel is currently experiencing a multidecadal drought, the relationship between Atlantic tropical cyclones and western Sahelian rainfall is not dependent on the similar downward trends in both datasets. A detrended analysis confirms that a strong association still exists, though reduced somewhat in variance explained. Additionally, independent data from the years 1899 to 1948 substantiate the existence of the tropical cyclone-western Sabelian rainfall association.

The fact that the Sahel periodically experiences multidecadal wet and dry regimes suggests that the current Sahelian drought, which began in the late 1960s, could be a temporary condition that may end in the new future. When this occurs, the Atlantic hurricane basin—especially the Caribbean islands and the United States East Coast—will likely see a large increase in intense hurricane activity associated with abundant Sahelian rainfall similar to the period of the late 1940s through the 1960s.

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MARVIN W. BURLEY, EARL M. RITCHIE, and CHARLES R. GRAY

Abstract

Daily wind data from United States stations for six pressure surfaces (200, 150, 100, 80, 50, 30 mb.) for the period April 1951 through January 1956 are used to study upper winds along the 80th and 120th meridians. At 5-degree intervals along the two meridians, mid-season average wind speeds and prevailing directions have been computed and are compared.

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W. Ho, H. H. Wang, W. F. Hall, W. Norris, W. N. Hardy, K. W. Gray, and G. M. Hidy

Abstract

No abstract available.

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Christopher W. Landsea, William M. Gray, Paul W. Mielke Jr., and Kenneth J. Berry

Abstract

Western Sahelian rainfall during the primary rainy season of June through September is shown to he significantly associated with concurrent intense U.S. landfalling hurricanes during the last 92 years. The meet intense hurricanes (i.e., Saffir–Simpson Scale Category 3, 4, or 5) have an especially strong relationship with Sahelian rainfall, whereas weaker hurricanes show little or no association. The hurricane-Sahelian rainfall association is most evident along the U.S. East Coast but is negligible in the U.S. Gulf Coast region.

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William M. Gray, Christopher W. Landsea, Paul W. Mielke Jr., and Kenneth J. Berry

Abstract

A surprisingly strong long-range predictive signal exists for Atlantic-basin seasonal tropical cyclone activity. This predictive skill is related to two measures of West African rainfall in the prior year and to the phase of the stratospheric quasi-biennial oscillation of zonal winds at 30 mb and 50 mb, extrapolated ten months into the future. These predictors, both of which are available by 1 December, can be utilized to make skillful forecasts of Atlantic tropical cyclone activity in the following June-November season. Using jackknife methods to provide independent testing of datasets, it is found that these parameters can be used to forecast nearly half of the season-to-season variability for seven indices of Atlantic seasonal tropical cyclone activity as early as late November of the previous year.

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Paul W. Mielke Jr., Kenneth J. Berry, Christopher W. Landsea, and William M. Gray

Abstract

The results of a simulation study of multiple regression prediction models for meteorological forecasting are reported. The effects of sample size, amount, and severity of nonrepresentative data in the population, inclusion of noninformative predictors, and least (sum of) absolute deviations (LAD) and least (sum of) squared deviations (LSD) regression models are examined on five populations constructed from meteorological data. Artificial skill is shown to be a product of small sample size, LSD regression, and nonrepresentative data. Validation of sample results is examined, and LAD regression is found to be superior to LSD regression when sample size is small and nonrepresentative data are present.

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Paul W. Mielke Jr., Kenneth J. Berry, Christopher W. Landsea, and William M. Gray

Abstract

An estimator of shrinkage based on information contained in a single sample is presented and the results of a simulation study are reported. The effects of sample size, amount, and severity of nonrepresentative data in the population, inclusion of noninformative predictors, and least (sum of) absolute deviations and least (sum of) squared deviations regression models are examined on the estimator. A single-sample estimator of shrinkage based on drop-one cross-validation is shown to be highly accurate under a wide variety of research conditions.

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