On the Transverse Circulation of the Hurricane

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  • 1 Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado
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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

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.”

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