Some Aspects of Hurricane Daisy, 1958¹

1 Contribution No. 1156 from the Woods Hole Oceanographic Institution.

Abstract

Observations from research flight missions into hurricane Daisy (1958) are utilized to compute the budgets of heat, moisture, kinetic energy, and momentum. Calculations are performed for a period when the cyclone was just becoming a hurricane, and for a second period, two days later, when it had attained maturity.

The level of non-divergence separating lower inflow from upper outflow was quite high, near 500 mb. Mass flow increased considerably between the two days, but rainfall remained quite moderate for a severe hurricane. The radiation heat sink was small compared to the oceanic source so that the system was “open” within the confines of the study. If all ascent took place in the hard radar echoes, the mean vertical speed there was about 7 meters per sec. Without such localized ascent, heat balance could not be achieved. Heat budget computations permitted assessing the fraction of mass flow reaching upper levels in undilute “hot” cumulonimbus towers and estimation of the number of such towers required, to compare with previous photographic studies. Hot towers proved to be the dominant mechanism of raising warm air to upper levels, particularly in the inner core. However, because of the high level of non-divergence the constraint known as ventilation was active. It is shown not to have been a severe deterrent upon Daisy, nor was it the reason she did not become an extreme storm.

Kinetic energy balance is obtained readily on the first day. Import of kinetic energy through the outermost radius was small, so that most of the energy dissipated in the interior was also produced there by the pressure forces. Ground and internal friction were about equal. The kinetic energy source shifted outward as the storm grew, so that on the day of maturity large inward transport took place. The internal source also increased but wind speeds failed to rise correspondingly. Hence ground friction was inadequate to balance production plus import, and large internal dissipation had to be invoked. This dissipation must be assigned largely to vertical eddies. If ascribed to horizontal eddies, the coefficient of lateral exchange becomes so large that momentum budget requirements cannot be fulfilled by a large amount.

It is concluded from the study that the important mechanisms of the hurricane core can be well represented in a two-dimensional framework, provided the essential effects of the buoyant cumulonimbus towers are properly introduced or parameterized therein. The two-dimensional flow of mass is shown in relation to the fields of heat, momentum, and vorticity. The law of conservation of potential vorticity appears to be well satisfied along the stream tubes of the mean motion.

Notes

1 Contribution No. 1156 from the Woods Hole Oceanographic Institution.