Heavy precipitation events in the Mediterranean: sensitivity to cloud physics parameterisation uncertainties

Abstract

Abstract. In autumn, southeastern France is often affected by heavy precipitation events which may result in damaging flash-floods. The 20 October and 1 November 2008 are two archetypes of the meteorological situations under which these events occur: an upper-level trough directing a warm and moist flow from the Mediterranean towards the Cévennes ridge or a quasi stationary meso-scale convective complex developing over the Rhone valley. These two types of events exhibit a contrasting level of predictability; the former being usually better forecast than the latter. Control experiments performed with the Meso-NH model run with a 2.5 km resolution confirm these predictability issues. The deterministic forecast of the November case (Cévennes ridge) is found to be much more skilful than the one for the October case (Rhone valley). These two contrasting situations are used to investigate the sensitivity of the model for cloud physics parameterisation uncertainties. Three 9-member ensembles are constructed. In the first one, the rain distribution intercept parameter is varied within its range of allowed values. In the second one, random perturbations are applied to the rain evaporation rate, whereas in the third one, random perturbations are simultaneously applied to the cloud autoconversion, rain accretion, and rain evaporation rates. Results are assessed by comparing the time and space distribution of the observed and forecasted precipitation. For the Rhone valley case, it is shown that not one of the ensembles is able to drastically improve the skill of the forecast. Taylor diagrams indicate that the microphysical perturbations are more efficient in modulating the rainfall intensities than in altering their localization. Among the three ensembles, the multi-process perturbation ensemble is found to yield the largest spread for most parameters. In contrast, the results of the Cévennes case exhibit almost no sensitivity to the microphysical perturbations. These results clearly show that the usefulness of an ensemble prediction system based upon microphysical perturbations is case dependent. Additional experiments indicate a greater potential for the multiprocess ensemble when the model resolution is increased to 500 m.

An ensemble study of HyMeX IOP6 and IOP7a: sensitivity to physical and initial and boundary condition uncertainties

“…For the other ten members, the time tendencies of the warm rain processes of the ICE3 microphysical scheme were perturbed by a random factor ranging between 0.5 and 1.5. This random factor was generated in the same manner as in Hally et al (2013) and Fresnay et al (2012). Each random factor multiplied simultaneously the sources and sinks of a given microphysical process to ensure mass conversation was met.…”

“…Different methodologies have been employed, ranging from the use of different physical parameterisation schemes to stochastic perturbations applied upon the time tendencies of physical processes. More recently, Gebhardt et al (2011), Clark et al (2011, Fresnay et al (2012), Leoncini et al (2013) and Hally et al (2013) constructed convection-permitting short-range ensembles. The existence of such a breath of ensemble methodologies demonstrates that the most suitable approach remains open to debate, as no one methodology is found to be superior to the others.…”

“…The present work uses the methodology described in Hally et al (2013) and inspired by the previous works of Buizza et al (1999) and Fresnay et al (2012). These studies described ensemble simulations using stochastic perturbations upon the physical processes.…”

“…These studies described ensemble simulations using stochastic perturbations upon the physical processes. Hally et al (2013) and Fresnay et al (2012) concentrated on errors associated with the boundary layer turbulence and warm microphysical processes. They investigated the possible use of ensembles containing perturbations upon these processes in the forecasting of HPEs in the Mediterranean region.…”

Influence of targeted observations on short-term forecasts of high-impact weather events in the Mediterranean

“…This configuration produces a warm and wet maritime low-level flow over the Mediterranean coastal regions. Heavy precipitation affected southern France (the Cevennes region), with values over 300 mm 24 h −1 (Vié et al, 2011;Fresnay et al, 2012) and north-eastern Spain (Catalonia), with values larger than 100 mm 24 h −1 (Bech et al, 2011). As a result of the quasi-stationary upper-level system, sensitive regions are near the same in all six cases of the event, and they locate around the cut-off, with maxima over the Canary Islands and the Iberian Peninsula (Fig.…”