The predictability of convective storms over the ocean
Lean, Peter William
PublisherUniversity of Reading
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The predictability of individual convective storms over the mid-latitude ocean is investigated by quantifying the divergence of pairs of perturbed forecasts in a cloud-resolving model. The Met Office non-hydrostatic Unified Model is used in an idealised configuration to simulate moist convection initiating under homogeneous destabilisation. All convection is represented explicitly since a convective parameterisation scheme is not used. The growth of potential temperature perturbations at a single height is quantified as a function of time and spatial scale. The perturbations are found to grow in two distinct stages. Firstly, changes in the regime diagnosed by the boundary layer parameterisation scheme lead to rapid but limited perturbation growth before growth by convective instability becomes dominant. Both error growth mechanisms are found to contribute independently to the total error growth in the forecast. The range of predictability in this perfect model framework is quantified for different spatial scales and initial condition error. The upper limit (provided by O.OO2K perturbations) is shown to be around 200 minutes at scales of 10km. Initial condition perturbations of similar magnitude to those of typical analysis errors (i.e. of order I K) were found to saturate almost immediately at all scales. The short time taken for the forecasts to become uncorrelated in all cases indicates that individual showers will always be unpredictable beyond approximately four hours. The asymmetry in the evolution of initially equal and opposite perturbations highlights the nonlinear nature of the growth, which could prove problematic for convective scale data assimilation and the design of 'optimal' perturbations for convective scale ensemble forecasting.