EVALUATION OF PHYSICAL PROCESSES IN AN IDEALIZED EXTRATROPICAL CYCLONE USING ADJOINT SENSITIVITY

An adjoint model is used to examine the sensitivity of an idealized dr y extratropical cyclogenesis simulation to perturbations of predictive variables and parameters during the cyclone life cycle. The adjoint s ensitivity indicates how small perturbations of model variables or par ameters anywhere in the model domain can influence cyclone central pre ssure. Largest sensitivity for both temperature and wind perturbations is located between 600 and 900 hPa in the baroclinic zone above the d eveloping cyclone. Perturbations of a given size have more influence o n cyclone intensity when located in high-sensitivity regions (the midd le and lower troposphere in this simulation). The effects of physical processes can be interpreted with adjoint sensitivity by considering p erturbations that are proportional to temperature and wind tendencies in the basic state (nonlinear forecast). In the early phase of the cyc lone life cycle, temperature advection near the steering level in the lower troposphere (about 800 hPa) is strongly cyclogenetic and resembl es a Charney mode of baroclinic instability. During the phase of most rapid deepening, temperature advection in the lower troposphere remain s important, while interpretation of sensitivity to wind perturbations suggests that increased vorticity in the middle and upper troposphere above the surface low-pressure centre may also be significant for cyc lone intensification. Adjoint techniques can provide insight into spat ial and temporal sensitivity not easily obtained from other methods. H igher sea surface temperature (SST) has a cyclogenetic effect mainly i n a localized region corresponding to the cyclone warm sector. Outside the areas of high sensitivity, small perturbations of SST have very l ittle effect on central pressure of the forecast cyclone. When strong upward sensible-heat flux, F-s, exists, it can have a cyclogenetic (pr econditioning) influence early in the cyclone life cycle, although dow nward F-s in the cyclone warm sector is anticyclogenetic during the ph ase of most rapid deepening. The sensitivity indicates that F-s can be cyclogenetic in one location and anticyclogenetic at the same time in another location, so that F-s effects on cyclone intensity are partia lly self-cancelling. Surface momentum stress is anticyclogenetic, with sensitivity highly localized in the cyclone warm sector.