The predictability of convective storms over the ocean
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.