Downstream influence of mesoscale convective systems. Part 2: Influence on ensemble forecast skill and spread. (11th October 2019)

• Record Type: Journal Article
• Authors: Clarke, Samantha J.; Gray, Suzanne L.; Roberts, Nigel M.
• Is Part Of: Quarterly journal of the Royal Meteorological Society. Volume 145:Number 724(2019)
• Page Start: 2953
• Page End: 2972
• Publication Date: 2019-10-11
• Subjects: convection -- convection‐permitting -- forecast error -- MCS -- Met Office Unified Model -- MOGREPS‐G -- potential vorticity; Meteorology -- Periodicals; 551.5
• DOI: 10.1002/qj.3613
• Languages: English
• ISSNs: 0035-9009

Abstract

Abstract : Ensemble forecasts are run operationally to determine the forecast uncertainty arising from initial condition, model physics and boundary condition uncertainty. However, global configuration ensembles, which use a convection parametrization scheme, may miss uncertainty because of the misrepresentation of intense convection by such schemes. Here, the impacts of the misrepresentation of mesoscale convective systems (MCSs) on downstream ensemble forecast skill and evolution are determined for a case study. MCS perturbations (calculated‐15 from the difference between outputs from convection‐parametrizing and convection‐permitting Met Office model configurations) are added to six members of a global configuration ensemble created by downscaling forecasts from the global version of the Met Office Global and Regional Ensemble Prediction System. For the first 36 hours, differences grow on the convective scale related to the MCSs, leading to systematic deepening of a developing UK cyclone, although there is damping of the perturbations found in root mean square difference calculations between the forecasts with and without the perturbations (particularly in mean sea level pressure). Subsequently, differences grow rapidly to the synoptic scale, and by five days impact the entire Northern Hemisphere. The MCS perturbations can have systematic effects on ensemble forecasts (e.g., a systematic displacement of a downstream cyclone is found), but, for this case, there is no discernible change in forecast skill as measured by the root mean square error of the ensemble means and the effects of the MCS perturbations are smaller than those generated by the initial condition perturbations. The spread of the combined ensemble (the two ensembles with and without the MCS perturbations) is larger than that of the individual ensembles. Thus, perturbing convection‐parametrizing models to include potential vorticity anomalies associated with MCSs represented in convection‐permitting forecasts, or idealized representations of them, produces alternative realizations from those generated by initial condition perturbations and has the potential to be useful operationally. Abstract : Perturbing convection‐parametrizing simulations to include potential vorticity anomalies from convection‐permitting simulations of a mesoscale convective system (MCS) produces systematic impacts on the downstream forecast in an ensemble, leading to alternate realizations of the forecast. The root mean square difference analysis shown here demonstrates that the effects of localized MCS perturbations are smaller than those of global operationally generated initial condition perturbations. Despite this, the combined ensemble spread is enhanced when including MCS perturbations, so they could be useful operationally.