Predictability of weather and climate

Abstract:

With the introduction of diabatic processes in the forward and adjoint tangent models of the European Centre for Medium-Range Weather Forecasts's model, it is possible to determine singular vectors (SVs) for situations where diabatic physics may be important in producing perturbation growth. In this paper, the linear physical parametrizations are used to compute SVs for the tropical region, or subsets thereof, with an optimization time of 48 h. Perturbation growth is measured in terms of the so-called total energy norm, augmented with a term for specific humidity. Difficulties that may arise in computing tropical SVs, such as associated with spurious upper-tropospheric perturbation growth, are described. Also, the impact on the SV structure by including a specific-humidity term in the defining norm is discussed. Using a term for specific humidity based on background-error statistics in the norm at initial time yields SVs with a more realistic specific-humidity vertical profile. SVs are determined in various configurations for two tropical cyclones. Results show that targeting in the vicinity of the cyclone is required to obtain SVs associated with the cyclone dynamics. The dominant targeted SVs for tropical cyclones show resemblance to fast-growing structures found for idealized vortices.