An Interpretation of extratropical cyclogenesis using adjoint methods
Adjoint methods are used to examine the development of idealized and real extratropical cyclones. This research represents the first use of adjoint sensitivity that includes moist physical processes to study complete cyclone life cycles. Adjoint sensitivity is a computationally efficient technique for determining, in a comprehensive sense, the sensitivity of a forecast aspect (J) to small perturbations of model variables at earlier times in a numerical forecast, including initial conditions. In these simulations, J is selected to represent central pressure or vorticity of forecast cyclones. Specification of lower tropospheric (500-800 hPa) temperature and moisture near the incipient cyclone at the beginning of the storm track appears especially critical to cyclone prediction. Rapid cyclone intensification appears related to enhancement of dry baroclinic instability by latent heat release from nonconvective precipitation near the cyclone warn front. Cyclones can also be intensified by reduced surface stress and higher sea-surface temperature in the warm sector of the storm. The cyclone life cycle may be viewed in terms of an initially small-scale instability that propagates upward from a baroclinic zone in the lower troposphere, and leads to intensification of anomalies in both the upper and lower troposphere at the end of the storm track.
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H ADJOINT METHODOLOGY
IDEALIZED EXTRATROPICAL CYCLOGENESIS
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00UTC 20 January 760 hPa temperature 90 h cyclone adjoint model adjoint sensitivity advection anticyclogenetic average vorticity baroclinic instability basic center of 90 Chapt contour interval convective and nonconvective convective precipitation cyclogenesis cyclone center cyclone central pressure cyclone development cyclone intensification cyclone life cycle cyclone position dashed dJ/dq dJ/dT dJ/du dJ/dv dot product East-west vertical cross-sections Errico extratropical cyclones Forecast aspect Grace Hopper grid points Heavy solid line hPa and surface idealized cyclone indicates location initial perturbation J95 cyclone jet streak latent heat flux lower troposphere magnitude maximum sensitivity meridional wind mixing ratio model level moist processes Negative values NOGAPS nonconvective precipitation nonlinear forecast nonlinear model physical processes potential vorticity pressure at center rapid deepening region sea-surface temperature sensible heat sensible heat flux shown in Fig singular vectors solid contour surface pressure tangent linear model tendencies transfer coefficient tropopause upper troposphere values are shaded warm sector zonal and meridional