Predictability of weather and climate

Abstract:

The dominant singular vectors of the tangent propagator of the European Centre for Medium-Range Weather Forecast (ECMWF) numerical weather-prediction model are an essential component of the ECMWF Ensemble Prediction System (EPS). These singular vectors describe the principal finite-time linear instabilities of the northern extratropical atmospheric circulation. The impact of increasing the horizontal resolution of the tangent model from T21 to T42 on three different types of initial perturbation, which make use of these singular vectors, is considered. The increase of resolution allows the possibility of describing more accurately instabilities with an upscale cascade of energy from subsynoptic to synoptic scales. Two of the perturbations are referred to as the pseudo-inverse and sensitivity vectors. These are both diagnostic, and involve estimating from the short-range forecast error the component of initial error in the unstable subspace. The third type of perturbation is used to construct the set of initial states for the ensemble prediction scheme. Linear and nonlinear integrations are described using these different types of perturbation. All the results point to the conclusion that the higher-resolution calculations lead to more accurate results. Specifically, it is shown that significant amounts of energy in the pseudo-inverse and sensitivity vectors reside in scales smaller than T21, and that the skill of the EPS is improved when generating the initial perturbations using T42 singular vectors. This is found to be especially true in a number of cases when the control forecast was particularly poor. As a consequence, the predictability of synoptic-scale disturbances in the atmosphere on time-scales of a few days is likely to be determined by errors in the initial state on subsynoptic scales. In addition to these conclusions, it is proved that the analysis of the relationship between the EPS and the pseudo-inverse perturbations can be used to define the initial amplitude of the EPS perturbations.

Abstract:

The combined use of the European Centre for Medium-Range Weather Forecasts high-resolution, at T213 spectral triangular truncation and with 31 vertical levels (T213L31, operational model and ensemble prediction system (EPS), during cases of intense Mediterranean storms, is studied. In particular, it is discussed how EPS products can be used to provide a measure of confidence in the high-resolution precipitation forecast. Three case studies (two extreme events plus one false alarm case) are analyzed. For the first two cases, the EPS probability values for precipitation occurrence supported the medium-range T213L31 prediction, which proved to be successful. By contrast, for the third case, the high-resolution forecast suggested heavy rainfall over northern Italy but was not supported by the EPS. The T213L31 prediction for this case was poor. EPS forecasts of extreme weather events are necessarily compromised by the moderate resolution of the T63L19 model (a version of the operational high-resolution T213L31 model, at T63 spectral triangular truncation and with 19 levels) used to generate the ensembles. In future studies, ensembles will be made using at least T106L31 resolution combined with an increase in ensemble size.