Abstract

This is the first of two papers on Atlantic seasonal hurricane frequency. In this paper, seasonal hurricane frequency as related to E1 Niño events during 1900–82 and to the equatorial Quasi-Biennial Oscillation (QBO) of stratospheric zonal wind from 1950 to 1982 is discussed. It is shown that a substantial negative correlation is typically present between the seasonal number of hurricanes, hurricane days and tropical storms, and moderate or strong (15 cases) El Niñ off the South American west coast. A similar negative anomaly in hurricane activity occurs when equatorial winds at 30 mb are from an easterly direction and/or are becoming more easterly with time during the hurricane season. This association of Atlantic hurricane activity with El Niño can also be made with the Southern Oscillation Index. By contrast, seasonal hurricane frequency is slightly above normal in non-El Niño years and substantially above normal when equatorial stratospheric winds blow from a westerly direction and/or are becoming more westerly with time during the storm season.

El Niño events are shown to be related to an anomalous increase in upper tropospheric westerly winds over the Caribbean basin and the equatorial Atlantic. Such anomalous westerly winds inhibit tropical cyclone activity by increasing tropospheric vertical wind shear and giving rise to a regional upper-level environment which is less anticyclonic and consequently less conductive to cyclone development and maintenance. The seasonal frequency of hurricane activity in storm basis elsewhere is much less affected by El Niño events and the QBO.

Seasonal hurricane frequency in the Atlantic and the stratospheric QBO is hypothesized to be associated with the trade-wind nature of Atlantic cyclone formation. Tropical cyclone formation in the other storm basins is primarily associated with monsoon trough conditions which are absent in the Atlantic. Quasi-Biennial Oscillation-induced influences do not positively enhance monsoon trough region vorticity fields as they apparently do with cyclone formations within the trade winds.

Part II discusses the utilization of the information in this paper for the development of a forecast scheme for seasonal hurricane activity variations.

Abstract

This is the second of two papers on Atlantic seasonal hurricane activity. It is an extension of Part I, which discussed the association of El Niño and the phases of the stratospheric Quasi-Biennial Oscillation (QBO) of equatorial zonal wind with Atlantic seasonal hurricane variability. It is shown how the addition of regional sea-level pressure data from Caribbean basin meteorological stations can be combined with the more global El Niño and QBO information to form a forecast scheme for Atlantic seasonal hurricane activity. Seasonal forecasts might be issued on 1 June of each year and updated prior to the commencement of the most active part of the hurricane season on 1 August.

Although this forecast scheme, of necessity, has been developed on dependent data, the expected forecast skill degradation when applied to independent data sets has been estimated. It appears not to be large enough to significantly negate the rather substantial degree of potential forecast skill that is evident in the developmental data set.

Abstract

This paper (part I) is a discussion of the magnitude and implication of the vertical circulation patterns of the summertime tropical atmosphere as derived from synoptic scale considerations. Part I is compared to the vertical circulation patterns derived from cumulus scale considerations as discussed by López in part II, a companion paper. From the synoptic scale considerations, we show that a very significant subsynoptic or local vertical motion is occurring within the cloud regions of the Tropies. This mass-cancelling local up- and-down circulation is not resolved by the mean or synoptic scale flow patterns. The magnitude of this local or up- and-down vertical circulation can be estimated from cloud-cluster scale (approx. 4°) mass, vapor, energy, and rainfall-evaporation budgets. Results are closely comparable to those obtained by López from an independent small-scale approach through modelling of individual cumulus elements. This local vertical circulation is shown to be fundamental for the mass, vapor, and energy balances of the tropical atmosphere. Other discussions of the characteristics of the cumulus convective atmosphere are included.

Abstract

Abstract

This paper uses extensive aircraft, composited rawinsonde data, and an idealized hurricane structure model to analyze the physical processes that maintain the transverse circulation of the steady-state hurricane. It is shown that convective available potential energy (CAPE) or processes other than frictional forcing plays an important role in maintaining the hurricane's inner-core (radius < 60 km) in-up-and-out radial circulation. But this is not true at outer radii (60–250 km or 250–700 km) where surface friction forcing is dominant and larger than the resulting upward vertical motion.

Overall, there is less vertical motion within the hurricane's 0–250-km area than that specified by frictional forcing and, overall, CAPE or buoyancy plays a negative role in enhancing vertical motion. But this is not true of the inner-core eye-wall cloud region where nonfrictionally driven eye-wall vertical motion has an important buoyant contribution and a strong ocean-to-air energy flux is present. Frictionally forced vertical motion resulting from low-level relative vorticity is typically not balanced locally. Quasi balance between frictional forcing and vertical motion is observed only for the larger-scale vortex (approximately 0°–3° radius) as a whole.

PROLOGUE

Joanne Simpson tells the story that in the mid-1940s, when she was a young (and precocious!) graduate student at the University of Chicago, she told Carl Rossby that she wanted to study clouds and that he responded by saying that that was a good subject for a girl. We now more fully appreciate the role of clouds as the fundamental component of the hydrologic cycle. Most of us would agree that understanding the physics behind cumulus convection is a fundamental challenge for all, girl or boy. Joanne's choice of cloud studies as a career endeavor was a wiser choice than most meteorologists of that day (and many of this day) realized. Attention in the 1940s and 1950s had been focused more on the requirements of wind for the transfer of energy from the tropical to the polar regions. There is no doubt that horizontal transport of energy is a fundamental ingredient of the general circulation. But vertical energy transport to balance the troposphere's continuous radiational cooling of ∼1°C per day is more important. Globally averaged, the required vertical transport of energy from the surface up into the troposphere is about four times larger than the required horizontal transport. It is this vertical energy transport that is so messy and so difficult to understand, and so hard to treat in a realistic and quantitative fashion. Many modelers and theoreticians have chosen to neglect the many hydrologic cycle complications (by assuming that the troposphere's radiational cooling is balanced by condensation warming) and to concentrate only on the horizontal energy imbalances. This has been the approach of the dishpan or annulus experiments. But this is not satisfactory for a full understanding of how the troposphere really functions. We have to face up to the need for the development of a realistic quantitative treatment of the globe's hydrologic cycle. The cumulus convection schemes in current GCMs are still inadequate. It is this continuing need to better understand the full range of cloud processes that has made Joanne's decision in the mid-1940s to concentrate on clouds such a wise one. She has since made many contributions to the understanding of the role of clouds. The paper she wrote with Herbert Riehl in 1958 (Riehl and Malkus) had much influence on the thinking of the important role of cumulus convection. Her recent work with the Tropical Rainfall Measuring Mission (TRMM) experiment is an example of her continuing drive to better understand clouds and the hydrologic cycle.

I first met and worked with Joanne in the late 1950s when I was a graduate student of Herbert Riehl's at the University of Chicago. I participated in the study she was directing on the variations of tropical Pacific cloudiness from aircraft time-lapse photography. This was before the satellite and the computer. We had more time to think and to speculate in those days. I have been most grateful to both Joanne and Bob Simpson for their interest and encouragement of my research efforts since that time.

It is a pleasure to make a contribution to this symposium honoring Joanne. The paper to follow has many similarities to the early and original paper of Joanne in 1958 titled “The Structure and Maintenance of the Mature Hurricane Eye.”

Abstract

In cumulus convective atmospheres the most significant length scale of vertical motion is of cumulus draft size—of horizontal lengths two to four orders of magnitude less than the synoptic scale and of magnitude one to three orders greater. This draft motion on scales between the gust and synoptic motion has been least investigated. The purpose of this paper is to propose a new method of calculating draft velocities from aircraft and present results of calculations with this method in hurricanes.

An aircraft Doppler radio navigation instrument which is capable of measuring horizontal wind variations to a space resolution of a fraction of a nautical mile can be employed with other instruments to measure an aircraft's pitch angle changes. From this determination, along with other standard aircraft measurements such as radar and pressure altitude, power setting, etc., it is possible to make determinations of average vertical air motion to space resolutions of 0.4 to 0.7 nautical miles.

Calculations of vertical motion are performed along a number of radial leg flight tracks flown by the National Hurricane Research Project B-50 aircraft during the 1958 season with the above method. Typical draft velocity was 5–15 knots and draft widths 1–3 nautical miles. Maximum derived gust velocities within the drafts were typically in the range of 5–15 knots. Comparison of results with those of the Thunderstorm Project is made. Vertical accelerometer data are also presented.

Abstract

In cumulus convective atmospheres where important energy and momentum interactions are occurring on the cumulus cloud scale (width 1 to 3 nautical miles) dynamical processes may be significantly different than middle latitude baroclinically driven circulations. This is the case especially in the intense cumulus-convective atmosphere of the hurricane. The purpose of this study is to present recent observational information of the cloud-scale and meso-scale wind fluctuations in the hurricane and to discuss their possible significance with regard to understanding the dynamics of the cumulus convective atmosphere.

Detailed investigation is made of the wind observations collected during the 1958 season by the National Hurricane Research Project (NHRP) B-50 aircraft from 28 radial penetrations in hurricanes at levels between 830 and 560 mb. Horizontal wind velocities are measured with the aid of an AN/APN-82 radio navigation instrument utilizing Doppler frequency shift. These measurements, together with the author's (Gray, 1965) previous calculation of vertical air velocity along these same radial legs, give the complete three-dimensional cylindrical wind representation to a space resolution of approximately one-half nautical mile. From the characteristic width of the component fluctuations, space smoothing along the radial legs is performed. With certain approximations this allows determination of the three component space-smoothed (mean) and eddy winds. Computations of the turbulent Reynolds stress from these cloud-scale wind fluctuations are made. Observational evidence of the correlation of cloud-scale horizontal and vertical wind components is presented. Other aspects of the hurricane circulation are discussed.

Abstract

No abstract available.

It is proposed that attention be given to the possibility of tropical cyclone intensity determination through upper-tropospheric jet aircraft reconnaissance. The cyclone's upper-level temperature anomaly and its gradient can be related to surface pressure and wind. This is particularly relevant to foreign countries affected by these cyclones that do not have a dedicated low altitude aircraft reconnaissance capability, but have available jet aircraft. Only the ordinary aircraft instrumentation for measuring temperature and pressure-altitude would be required. Jet flights are faster, longer ranged, and less turbulent (if echoes are avoided) than propeller flights. Many more aircraft are available for such missions.

Abstract

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 under significantly different environmental conditions, which are described. These latter developments make up but a small percentage of the global total.

Observations are also presented which indicate that over the tropical oceans where disturbances and storms form, there is a distinct Ekman or frictional veering of the wind in the subcloud layer (surface to 600 m.) of approximately 10°. This produces or enhances synoptic-scale low level convergence and cumulus convection in regions of large positive relative vorticity which exist in the cyclonic wind shear areas surrounding doldrum Equatorial Troughs.

Tropical disturbance and later storm development is viewed as primarily a result of large-scale Ekman or frictionally forced surface convergence (with resulting cumulus production and tropospheric heating), and a consequent inhibition of tropospheric ventilation by initially existing small vertical wind shear, and later inhibition of ventilation by cumulus up- and downdrafts acting to prevent increase of vertical shear as baroclinicity increases. The above processes produce the necessary condensation heating and allow for its concentration and containment in selective areas. Development is thus explained from a simple warming, hydrostatic adjustment point of view with the energy source analogous to Charney and Eliassen's proposed “conditional instability of the second kind.”