Influence of the sea state on Mediterranean heavy precipitation: a case‐study from HyMeX SOP1

Abstract

Sea state can influence the turbulent air–sea exchanges, especially the momentum flux, by modifying the sea‐surface roughness. The high‐resolution non‐hydrostatic convection‐permitting model MESO‐NH is used here to investigate the impact of a more realistic representation of the waves on heavy precipitation during the Intense Observation Period (IOP) 16a of the first HyMeX Special Observation Period (SOP1). Several quasi‐stationary mesoscale convective systems developed over the western Mediterranean region, two of them over the sea, and resulted in heavy precipitation on the French and Italian coasts on 26 October 2012. Three different bulk parametrizations are tested in this study: a reference case (NOWAV) without any wave effect, a parametrization taking into account theoretical wave effects (WAV) and a last one with realistic wave characteristics from the MFWAM analyses (WAM). Using a realistic wave representation in WAM significantly increases the roughness length and the friction velocity with respect to NOWAV and WAV. The three MESO‐NH sensitivity experiments of the IOP16a show that this surface‐roughness increase in WAM generates higher momentum fluxes and directly impacts the low‐level dynamics of the atmosphere, with a slowdown of the 10 m wind, when and where the wind speed exceeds 10 m s−1 and the sea state differs from the idealized one. The turbulent heat fluxes are not significantly influenced by the waves, these fluxes being controlled by the moisture content rather than by the wind speed in the simulations. Although the convective activity is globally well reproduced by all the simulations, the difference in the low‐level dynamics of the atmosphere influences the localization of the simulated heavy precipitation. Objective evaluation of the daily rainfall amount and of the 10 m wind speed against the observations confirms the positive impact of the realistic wave representation on this simulation of heavy precipitation.

Influence of sea surface roughness length parameterization on Mistral and Tramontane simulations

“…(2). This result is in agreement with the findings of Thévenot et al (2015), who showed that an increase in wave height (and a resulting increase in z 0 ) leads to lower wind speeds.…”

High‐resolution air–sea coupling impact on two heavy precipitation events in the Western Mediterranean

“…A further step towards operational real‐time forecasts would be to explore strategies for combining ocean data assimilation and atmosphere data assimilation for the AROME‐NEMO system. Another perspective is to take into account the sea state with the introduction of a wave model into the coupled system, as waves strongly impact the sea surface turbulent fluxes and thus can significantly modify the weather forecast (Renault et al , 2012; Ricchi et al , 2016; Thévenot et al , 2016; Bouin et al , 2017; Voldoire et al , 2017).…”

“…Two case‐studies have been chosen from the HyMeX SOP1 period because they include the two kinds of intense weather events of interest: first a moderate mistral episode followed by an HPE during IOP13 (Rainaud et al , 2016), and secondly, an HPE followed by a severe mistral event during IOP16a/b (Ducrocq et al , 2014; Duffourg et al , 2016). Moreover, for these two IOPs, it has been suggested that air–sea exchanges played a significant role (Rainaud et al , 2016; Thévenot et al , 2016). A brief description of the events is given in the following.…”

Offshore deep convection initiation and maintenance during the HyMeX IOP 16a heavy precipitation event

“…Other studies have shown that the simulation of the convective system is very sensitive to model parametrizations. For example, Thévenot et al () also shows for this IOP how the turbulent air–sea exchanges can modify the low‐level dynamics of the atmosphere and the precipitation location. In Rainaud (), a change in the sea surface temperature or the coupling of the atmosphere with an oceanic model is found to have a large impact on the simulated precipitation amount overland.…”