Latent Heat Induced Energy Transformations during Cyclogenesis

Abstract

Using real-data numerical simulation experiments, latent heat induced energy transformations during the development of the wave cyclone of 20 May 1977 are investigated. During a 24 h period over 5 cm of precipitation fell despite baroclinically inactive synoptic conditions. The numerical experiments which were conducted included two 24 h fine-mesh forecasts, one with and the other without latent heating. The following conclusions resulted from kinetic energy budget calculations performed on isobaric surfaces at 100 mb increments from 900 mb to 100 mb. 1) Heating enhanced the generation of kinetic energy at all levels, slightly weakened its dissipation (to sub-grid scales) in the lower troposphere and increased this dissipation in the upper troposphere. 2) Because of the rapid increase of kinetic energy with height the latent heat's contribution to the kinetic energy balance was, in a relative sense, most significant in the lower troposphere. It is shown that while the maximum latent heating rates occurred in the middle to upper troposphere, the most significant response to the warming appeared in the lower troposphere. The enhancement of ageostrophic generation of kinetic energy and the reduction of sub-grid scale dissipation provides an important source of energy for the maintenance of the lower tropospheric circulation. From potential energy calculations it was found that although the heating rates within the simulation domain were quite large, the condensation processes were not efficient in increasing the total potential energy of the model atmosphere. The contribution of heating to generation of total potential energy was 60 ? 1055 J m?2 while the actual increase of total potential energy from the dry simulation to the wet simulation was 5 ? 105 J m?2. The bulk of the discrepancy between the generation and the net gain was due to the changes in the boundary flux in the simulation's upper troposphere as a result of beating. The growth of this midlatitude cyclone did not depend on the short-term generation of potential energy by condensation processes to provide a source of energy. Rather, latent heat acted as a catalyst to enhance the conversion of potential to kinetic energy within the cyclone. The induced upper-level kinetic energy then was very effective at increasing the export of potential energy from the cyclone to its large-scale environment.