Lagrangian dynamics of the mistral during the HyMeX SOP2

Abstract

The mistral refers to a severe wind blowing over the Gulf of Lions after being channeled in the Rhone valley. It influences the western Mediterranean climate as it brings cold and dry continental air over the warm western Mediterranean, generating intense air‐sea heat exchanges and sea surface cooling, inducing the formation of the western Mediterranean deep water that moves into the Atlantic Ocean. The mistral is frequently observed to extend as far as a few hundred kilometers from the coast, and its fine‐scale dynamics over the sea is still only partially understood as finely resolved observations in time and space are lacking. The boundary layer pressurized balloons (BLPB) developed by the Centre National d'Etudes Spatiales and deployed during HyMeX SOP2 allowed the Lagrangian documentation of the mistral events that occurred between beginning of February to mid‐March 2013. Analyzed in synergy with the AROME‐WMED weather forecast model, all the terms of the Lagrangian formulation of the momentum conservation equation could be quantified showing three different regions: (1) an injection zone where the mistral flow is directed toward the center of the Genoa cyclone due to a strong zonal pressure gradient, enhanced friction, and entrainment in the mountain wake; (2) an ejection zone where the flow is deflected outward of the cyclone due to either the nonnegligible inertia pseudoforce or an inertial oscillation caused by a sudden friction decrease; and (3) a region of geostrophic deceleration due to the weakening of the pressure gradient.

Mistral and tramontane simulations with changing resolution of orography

“…Including SSO, therefore, could reduce this westward shift. It also resembles the wind speed during mistral events observed by Drobinski et al (2017) using balloons. Figure 4 shows the change in wind direction.…”

“…When surface momentum transfer and, therefore, surface friction, is introduced the maximum wind speed decreases again by about 10 m/s (Figure 2c) and both the wind direction and the pressure gradient rotate counterclockwise (Figure 2g,k). Experiment oro2 SSO also shows a better agreement with wind patterns during mistral and tramontane events from satellite data and ERA-Interim Mean wind speed (m/s, upper row) and mean wind direction ( , middle row) at lowest model level and mean sea level pressure (hPa, lower row) of day 11-14 for simulations oro2 none (a, e, i), oro2 cori (b, f, j), oro2 flux (c, g, k), and oro2 SSO (d, h, l) (Obermann et al, 2016a) as well as balloon observations (Drobinski et al, 2017) than the other setups. Experiment oro2 SSO also shows a better agreement with wind patterns during mistral and tramontane events from satellite data and ERA-Interim Mean wind speed (m/s, upper row) and mean wind direction ( , middle row) at lowest model level and mean sea level pressure (hPa, lower row) of day 11-14 for simulations oro2 none (a, e, i), oro2 cori (b, f, j), oro2 flux (c, g, k), and oro2 SSO (d, h, l) (Obermann et al, 2016a) as well as balloon observations (Drobinski et al, 2017) than the other setups.…”

“…This study surveys the effect of orographic grid spacing (2.6-45 km) on mistral and tramontane by performing idealized simulations with a 1.3 km horizontal numerical grid. Drobinski et al (2017) surveyed the structure of mistral using a Lagrangian analysis. If orographic features on smaller scales are parameterized, the effects of the smoothing are weaker, but a systematic decrease in channeling and wind speed still persists.…”

Offshore winds obtained from a network of wind‐profiler radars during HyMeX

“…Figure shows the results of the zonal and meridional components of the acceleration ( ΔU / Δt and ΔV / Δt ) for the R11 network. Drobinski et al () compared the acceleration of the BLPBs during HYMEX with the three components of the wind equation (pressure gradient, Coriolis and shear forces) obtained by the AROME–WMED analyses. They found that the offshore Mistral first accelerates during its first 170 km from the coast (stage 1) to the open sea, due to the strong zonal pressure gradient linked to the low in the Gulf of Genoa.…”

“…The deviation of the Mistral plume to the east is a well-known feature, linked with the sheltered effect of the South Alps and the low pressure occurring in the Gulf of Genoa. According to Drobinski et al (2016), the deeper the low in the Gulf of Genoa, the stronger the deviation of the Mistral plume towards the east. The issue here is that outside the Gulf of Lion the network overestimates the cyclonic gyre, since most of the radar trajectories are located to the east of the BLPB ones.…”