A global observational study of atmospheric conditions associated with tropical disturbance and storm development is presented. This study primarily uses upper air observations which have become available over the tropical oceans in the last decade. Climatological values of vertical stability, low level wind, tropospheric vertical wind shear and other parameters relative to the location and seasons of tropical disturbance and storm development are discussed. Individual storm data are also presented in summary form for over 300 development cases (with over 1,500 individual observation times) for four tropical storm genesis areas. Results show that most tropical disturbances and storms form in regions equatorward of 20° lat. on the poleward side of doldrum Equatorial Troughs where the tropospheric vertical shear of horizontal wind (i.e., baroclinicity) is a minimum or zero. Storm development occurring on the poleward side of 20° lat. in the Northwest Atlantic and North-west Pacific takes place unde...

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Global view of the origin of tropical disturbances and storms

The years 1995 to 2000 experienced the highest level of North Atlantic hurricane activity in the reliable record. Compared with the generally low activity of the previous 24 years (1971 to 1994), the past 6 years have seen a doubling of overall activity for the whole basin, a 2.5-fold increase in major hurricanes (>/=50 meters per second), and a fivefold increase in hurricanes affecting the Caribbean. The greater activity results from simultaneous increases in North Atlantic sea-surface temperatures and decreases in vertical wind shear. Because these changes exhibit a multidecadal time scale, the present high level of hurricane activity is likely to persist for an additional approximately 10 to 40 years. The shift in climate calls for a reevaluation of preparedness and mitigation strategies.

https://science.sciencemag.org/content/sci/293/5529/474.full.pdf

The Recent Increase in Atlantic Hurricane Activity: Causes and Implications

This study investigates the origin and structure of easterly waves that form in the lower troposphere of North Africa and have a periodicity of 3–5 days,. From June to early October these waves propagate across the Atlantic and occasionally reach the eastern Pacific. Although only a few of these disturbances actually intensify after reaching the Atlantic, they account for approximately half of the tropical cyclones that form in the Atlantic. Spectral analysis of five years of upper air data shows that African waves produce a spectral peak of the meridional wind at periods of 3–5 days with a maximum amplitude of 1–2 m sec−1 near 700 mb. These waves normally originate between Khartoum (32E) and Ft. Lamy (I5E) and affect a greater depth of the atmosphere as they propagate westward. Wind statistics at stations flanking the mountains in Ethiopia indicate that airflow over these mountains is not the cause of the easterly waves. This study shows that the African waves are directly related to the mid-tro...

/pdf/the-origin-and-structure-of-easterly-waves-in-the-lower-26g6o6c8gz.pdf

The Origin and Structure of Easterly Waves in the Lower Troposphere of North Africa

The sensible heat budget of a large-scale area containing an idealized tropical cloud cluster is analyzed. The cluster is assumed to have spatial dimensions and precipitation rates typical of observed cloud clusters. In its early stages of development the idealized cluster consists of isolated deep precipitating convective cells, or \"hot towers.\" A simple model using the assumed precipitation rates as input is employed to compute the condensation and evaporation rates and sensible heat fluxes associated with the precipitating hot towers. The condensation dominates the contribution of the hot towers to the large-scale heat budget, and the net effect of the towers is warming distributed through the full depth of the troposphere. In its mature stage of development, the idealized cluster contains not only convective towers but a widespread cloud shield interconnecting the towers. The cloud shield is dynamically and thermodynamically active, and processes associated with it also contribute significantly to the large-scale sensible budget. Stratiform precipitation falls from the cloud shield, and in the stratiform precipitatioi region, condensation occurs in mesoscale updraft aloft, evaporation occurs in a mesoscale downdraft at low levels and melting occurs in a middle-level layer. The condensation, evaporation and sensible heat transports associated with the mesoscale updraft and downdraft are determined from simple models using the cluster's assumed stratiform precipitation rate as input. The evaporation and melting in the stratiform precipitation region are also estimated from vertical profiles of radar reflectivity in real cloud clusters. The total effects of the stratiform precipitation processes on the largescale heat budget are warming of the middle to upper troposphere, where condensation in the mesoscale updraft is the dominant effect, and cooling in the lower troposphere, where melting and mesoscale downdraft evaporation dominate. The widespread cloud shield present in the mature and later stages of a cloud cluster's life cycle is also an important absorber and emitter of radiation. Radiative transfer models applied to tropical cloud shields show substantial heating effects in the middle to upper troposphere. These effects are nearly as important as the heating by convective towers and the heating and cooling associated with the stratiform precipitation processes. As the idealized cloud cluster progresses from early to mature stages of development, its net effect on the large-scale heat budget changes. As the cloud shield develops, the mesoscale updraft condensation and radiation reinforce the heating by convective towers aloft, while the mesoscale downdraft evaporation and melting counteract the convective-tower heating at low levels. Thus, the net heating by the cluster increases in the upper troposphere and decreases in the lower troposphere as the system develops. Large-scale upward motion, which is required to balance the large-scale heat budget against the effects of the cluster, is thus expected to increase aloft and decrease at low levels. Vertical motions deduced from large-scale wind observations in the tropics confirm this expectation. Thus, it is concluded that the mesoscale stratiform and radiative processes associated with the cloud shields of developing cloud clusters are sufficiently strong to alter the large-scale vertical motion field in the tropics.

https://www.jstage.jst.go.jp/article/jmsj1965/60/1/60_1_396/_pdf

Cloud Clusters and Large-Scale Vertical Motions in the Tropics

Physical mechanisms responsible for the contemporaneous association, shown in earlier studies, of North Atlantic basin major hurricane (MH) activity with western Sahelian monsoon rainfall and an equatorial eastern Pacific sea surface temperature index of El Nino are examined, using correlations with 200- and 700-mb level wind data for the period 1968–92. The use of partial correlations isolates some of the relationships associated with the various parameters. The results support previous suggestions that the upper- and lower-level winds over the region in the basin between ∼10° and 20°N where most MHs begin developing are critical determinants of the MH activity in each hurricane season. In particular, interannual fluctuations in the winds that produce changes in the magnitude of vertical shear are one of the most important factors, with reduced shear being associated with increased activity and stronger shear with decreased activity. The results show that most of these critical wind fluctuations...

Physical Mechanisms for the Association of El Niño and West African Rainfall with Atlantic Major Hurricane Activity

A census of Atlantic tropical systems of 1969 presents information on the history of each tropical wave or disturbance, including the dates these systems passed three key stations: Dakar, Barbados, and San Andres Island.

Atlantic tropical systems of 1969

Atlantic tropical disturbances, 1967

“Easterly waves” in the tropical Atlantic have been found to be associated with a characteristic cloud pattern which has the appearance of an “Inverted V.” Cloud bands are aligned approximately parallel to the lower tropospheric winds and change orientation along the wave axis.

The “inverted v” cloud pattern—an easterly wave?

Atlantic tropical disturbances of 1968

The 1974 hurricane season produced 96 “tropical systems,” of which 25 acquired the closed circulation of a depression. The origin of nearly half of these, 52, was over the continent of Africa. African seedlings initiated five of the seven named Atlantic storms, and disturbances of Atlantic origin spawned 12 of the 17 East Pacific named storms.

Neil L. Frank

Papers

Atlantic tropical systems of 1969

Atlantic tropical disturbances, 1967

The “inverted v” cloud pattern—an easterly wave?

Atlantic tropical disturbances of 1968

Atlantic Tropical Systems of 1974