ADJOINT SENSITIVITY OF AN IDEALIZED EXTRATROPICAL CYCLONE WITH MOIST PHYSICAL PROCESSES

An adjoint model (MAMS1) that includes parametrizations for convective (subgrid-scale), and non-convective (grid-scale) precipitation, and s urface latent-heat flux is used to investigate an idealized extratropi cal cyclogenesis. The adjoint sensitivity information demonstrates the effects that perturbations of model variables and parameters at vario us times during the cyclone life cycle have on forecast cyclone intens ity. For a nonlinear trajectory that includes precipitation processes and surface latent-heat flux, the accuracy of the tangent-linear and a djoint model is much higher when moist physical processes are included . Inclusion of moist processes in the adjoint model increases sensitiv ity magnitude compared with sensitivity obtained with a dry adjoint mo del, but does not alter the primary spatial pattern of sensitivity. Th e larger cyclone deepening rates that occur with the inclusion of mois ture are related to latent-heat release from condensation of water vap our in areas of the middle and lower troposphere (the warm-front regio n) that are strongly sensitive to temperature perturbations in both dr y and moist cyclone simulations. The effects of diabatic heating on cy clone development are interpreted as a reinforcement of dry baroclinic instability and not a separate development mechanism (which would hav e a unique non-baroclinic sensitivity signature). The sensitivity patt erns explain why favourably positioned latent-heat release is an ingre dient that can lead to explosive baroclinic development. Cyclone inten sity is very sensitive to the vertical distribution of temperature per turbations, so this feature of diabatic heating is critical to the cyc lone forecast. An increase in the transfer coefficient C-E, for the su rface latent-heat flux can intensify the cyclone by adding moisture to the lower troposphere in the cyclone warm sector before the release o f latent heat by precipitation processes. Perturbations of C-E have mo re effect on cyclone intensity than perturbations of the transfer coef ficients involved in surface sensible-heat flux and surface stress dur ing most of the cyclone life cycle.