On hurricane energy

Abstract

Warm seawater is the energy source for hurricanes. Interfacial sea-to-air heat transfer without spray ranges from 100 W m⁻² in light wind to 1,000 W m⁻² in hurricane force wind. Spray can increase sea-to-air heat transfer by two orders of magnitude and result in heat transfers of up to 100,000 W m⁻². Drops of spray falling back in the sea can be 2–4 °C colder than the drops leaving the sea, thus transferring a large quantity of heat from sea to air. The heat of evaporation is taken from the sensible heat of the remainder of the drop; evaporating approximately 0.3 % of a drop is sufficient to reduce its temperature to the wet bulb temperature of the air. The heat required to evaporate hurricane precipitation is roughly equal to the heat removed from the sea indicating that sea cooling is due to heat removal from above and not to the mixing of cold water from below. The paper shows how case studies of ideal thermodynamic processes can help explain hurricane intensity.

Appendix: calculation procedure

Pressure P ₃ is the pressure for which the net work: the isentropic work of expansion, h ₃ − h ₄, minus the increase in the potential energy of the air (1 + r ₃) gz, in process 3–4 is zero. Hurricane eyewall air temperature and relative humidity are fairly well known from observation. The temperature and relative humidity of eyewall air were 24.5 °C and 97 % in hurricane Isabel and 26 °C and 95 % when hurricane Ophelia passed over National Moored Buoy 41049 in 2011. P ₃ is calculated by iteration, taking case 2 of Table 1 as an example: the net work when air at 98 kPa, 24.5 °C and 97 % RH is raised is +503 J/kg and the net work when air at 96 kPa, 24.5 °C and 97 % RH is raised is −416 J/kg. The pressure at which the net work is zero can then be found to be 96.91 kPa by linear interpolation. A second linear interpolation between 96.8 and 97.0 kPa gives a pressure of P ₃ of 96.90 kPa, the lowest pressure for which the net work in process 3–4 is positive. Once P ₃ is known, temperature T ₄ and work W ₁₂ can be calculated. This example is based on P ₄ and z ₄ of 12 kPa and 15,500 m, respectively. The graph of Fig. 2 was produced using a similar interpolation approach to find the relative humidity required to produce a given pressure with air of a given temperature. The equations used to calculate entropy and enthalpy equations are available from Michaud (2012c) where the Hewlett Packard HP48SX calculator program used to calculate both parts of Table 1 is also available. Thermodynamic properties per unit mass of dry air are commonly used because in many processes such as pseudo-adiabatic expansion, the mass of dry air is constant while the mass of the total substance changes. The (1 + r ₃) factor in the total energy equation would not be required if thermodynamic properties per unit mass of total substance were used.