Temperature And Wind Velocity Oscillations Along a Gentle Slope During Sea-Breeze Events

The flow structure on a gentle slope at Vallon dOl in the northern suburbs of Marseille in southern France has been documented by means of surface wind and temperature measurements collected from 7 June to 14 July 2001 during the ESCOMPTE experiment. The analysis of the time series reveals temperature and wind speed oscillations during several nights (about 60--90 min oscillation period) and several days (about 120–180 min oscillation period) during the whole observing period. Oscillating katabatic winds have been reported in the literature from theoretical, experimental and numerical studies. In the present study, the dynamics of the observed oscillating katabatic winds are in good agreement with the theory.In contrast to katabatic winds, no daytime observations of oscillating anabatic upslope flows have ever been published to our knowledge, probably because of temperature inversion break-up that inhibits upslope winds. The present paper shows that cold air advection by a sea breeze generates a mesoscale horizontal temperature gradient, and hence baroclinicity in the atmosphere, which then allows low-frequency oscillations, similar to a katabatic flow. An expression for the oscillation period is derived that accounts for the contribution of the sea-breeze induced mesoscale horizontal temperature gradient. The theoretical prediction of the oscillation period is compared to the measurements, and good agreement is found. The statistical analysis of the wind flow at Vallon dOl shows a dominant north-easterly to easterly flow pattern for nighttime oscillations and a dominant south-westerly flow pattern for daytime oscillations. These results are consistent with published numerical simulation results that show that the air drains off the mountain along the maximum slope direction, which in the studied case is oriented south-west to north-east

ROLE OF THE HOGGAR MASSIF ON THE WEST AFRICAN MONSOON ONSET

It has been observed that the West African monsoon onset is concomitant with the enhancement of the Saharan heat low. We show here through a combined diagnostic and modeling study a possible interaction between northern Africa orography and the deepening of the Saharan heat low at the time of the monsoon onset. The amplification of an anticyclonic circulation above and north of the Hoggar massif leads to an increase and a southeasterly-northeasterly rotation of the wind ahead of the Hoggar which contribute to an increased leeward-trough effect enhancing the Saharan heat low. The Atlas does not play any role during the monsoon onset but contributes to the mean climatological location of the Saharan heat low

Secondary instability of stratified Ekman layer roll vortices

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Aerosol indirect effects on the temperature-precipitation scaling

Aerosols may impact precipitation in a complex way involving their direct and indirect effects. In a previous numerical study, the overall microphysical effect of aerosols was found to weaken precipitation through reduced precipitable water and convective instability. The present study aims to quantify the relative importance of these two processes in the reduction of summer precipitation using temperature-precipitation scaling. Based on a numerical sensitivity experiment conducted in central Europe aiming to isolate indirect effects, the results show that, all others effects being equal, the scaling of hourly convective precipitation with temperature follows the Clausius-Clapeyron (CC) relationship, whereas the decrease in convective precipitation does not scale with the CC law since it is mostly attributable to increased stability with increased aerosol concentration rather than to decreased precipitable water content. This effect is larger at low surface temperatures at which clouds are statistically more frequent and optically thicker. At these temperatures, the increase in stability is mostly linked to the stronger reduction in temperature in the lower troposphere compared to the upper troposphere, which results in lower lapse rates

SENSIBILITE D'UN MODELE A AIRE LIMITEE A SA PARAMETRISATION PHYSIQUE : APPLICATION EN AFRIQUE AUSTRALE

Cette étude examine pour la première fois en Afrique australe les incertitudes d'un modèle à aire limitée (Advanced Weather Research Forecast (WRF-ARW V3011)) liées à sa paramétrisation physique. Les incertitudes sont analysées au pas de temps saisonnier en déterminant les principaux points communs et différences de 27 expériences numériques, avec un focus sur le champ pluviométrique. Ces 27 expériences documentent le trimestre Décembre-Janvier-Février 1993-94, coeur de la saison des pluies de l'Afrique du Sud à régime pluvial tropical, et correspondent à toutes les combinaisons possibles entre 3 schémas de couche limite, 3 schémas de convection et 3 schémas de microphysique. Quelle que soit la paramétrisation testée, la distribution spatiale des pluies est similaire et relativement proche des estimations de pluies. WRF sous-estime la pluviométrie des deux zones de convergence de la région (ZCIT et ZCSI) et la surestime en Afrique subtropicale, surtout sur les reliefs. Les principales différences inter-membres concernent l'amplitude des cumuls saisonniers et les processus pluviogènes qui dépendent principalement des schémas de convection. Grell simule des quantités comparables aux observations in situ alors que Kain-Fritsch et Betts-Miller-Janjic les surestiment nettement, ce qui peut résulter d'une sous-estimation (surestimation) de l'humidité spécifique en moyenne et basse couche observée avec Grell (Kain-Fritsch et Betts-Miller-Janjic). Grell et Kain-Fritsch simulent essentiellement des pluies convectives, ce qui semble cohérent avec l'influence de la circulation tropicale sur cette région. Comparée aux réanalyses ERA40 utilisées pour le forçage latéral, la convergence d'humidité associée à la majorité des expériences est renforcée sur le subcontinent, de même que la vitesse verticale de l'air en moyenne atmosphère. C'est la raison pour laquelle WRF corrige généralement les biais secs d'ERA40. Les différences inter-membres des champs thermo-dynamiques sont fonction des schémas de convection à ce pas de temps, mais aussi d'alliances non systématiques entre les trois types de schémas testés

WAVE BREAKING OVER LOCAL TOPOGRAPHY DURING THE MAP IOP 15 MISTRAL EVENT: OBSERVATIONS AND HIGH-RESOLUTION NUMERICAL SIMULATIONS

This study investigates the fundamental processes involved in the severe Mistral’s windstorm occurring during the MAP IOP 15 (from 06 to 09 November 1999). It is based on numerical high-resolution simulations performed with the RAMS non-hydrostatic model at 3 km resolution. The simulation is found able to capture the flow complexity both upstream of the Alps and in coastal regions affected by the Mistral. The simulations accurately reproduce dynamical and thermodynamical fields observed by the observational network consisting of two UHF wind profilers set up near Marseille and Toulon and two radiosoundings at Lyon and Nîmes. The model indicates that the Mistral is mainly governed by flow splitting and downslope accelerations occurring at different scales. Wave breaking are triggered above local topography resulting in hydraulic jumps that in turn induce two mountain wakes trailing from the Alps and the Massif Central. The flow is accelerated by channelling effects between these two wakes. The simulations therefore evidence that channelling effects by the Rhône valley do not explain the Mistral’s dynamics, which are fundamentally three-dimensional, multi-scaled and formed by a multitude of orographic processes

Stochastic Downscaling Method: Application to Wind Refinement

International audienceIn this article, we propose a new stochastic downscaling method: provided a numerical prediction of wind at large scale, we aim to improve the approximation at small scales thanks to a local stochastic model. We first recall the framework of a Lagrangian stochastic model borrowed from S.B. Pope. Then, we adapt it to our meteorological framework, both from the theoretical and numerical viewpoints. Finally, we present some promising numerical results corresponding to the simulation of wind over the Mediterranean Sea

La transition vers la turbulence de la couche d'Ekman stratifiée

L'écoulement d'Ekman se forme au voisinage d'une paroi dans un fluide tournant.Dans la couche d'Ekman stratifiée, la transition vers la turbulence est contrôlée par les nombres de Reynolds, de Richardson et de Prandtl. Pour des Ri inférieurs au Ri critique, des tourbillons se forment par saturation de l'instabilité linéaire primaire. Nous montrons que lorsque Pr>1 des tourbillons d'amplitude finie existent lorsque Ri atteint sa valeur critique et au-delà. Nous étudions l'instabilité secondaire tridimensionnelle de ces tourbillons

Wind power predictions from nowcasts to 4-hour forecasts: a learning approach with variable selection

We study the prediction of short term wind speed and wind power (every 10 minutes up to 4 hours ahead). Accurate forecasts for those quantities are crucial to mitigate the negative effects of wind farms' intermittent production on energy systems and markets. For those time scales, outputs of numerical weather prediction models are usually overlooked even though they should provide valuable information on higher scales dynamics. In this work, we combine those outputs with local observations using machine learning. So as to make the results usable for practitioners, we focus on simple and well known methods which can handle a high volume of data. We study first variable selection through two simple techniques, a linear one and a nonlinear one. Then we exploit those results to forecast wind speed and wind power still with an emphasis on linear models versus nonlinear ones. For the wind power prediction, we also compare the indirect approach (wind speed predictions passed through a power curve) and the indirect one (directly predict wind power)

On the late northward propagation of the West African monsoon in summer 2006 in the region of Niger/Mali

International audienceThis paper investigates the fine-scale dynamical processes at the origin of the late northward migration of the monsoon flow in summer 2006 in the region of Niger and Mali (onset on 3 July 2006 compared to the climatological onset date, 24 June). Compared to a 28-year climatology, 2006 NCEP-2 reanalyses show evidence of an anomalous pattern during 10 days between 25 June and 3 July 2006, characterized by the African Easterly Jet (AEJ) blowing from the northeast along a narrow northeast/southwest band located over the Hoggar and Air mountains associated with an unusually strong northeasterly harmattan in the lee of the mountains. Using data collected during the African Monsoon Multidisciplinary Analysis (AMMA) experiment and mesoscale numerical simulations, this study shows evidence of interaction between the AEJ and the orography supported by the reduced gravity shallow water theory which explains the enhancement of the harmattan downstream of the Hoggar and Air mountains in summer 2006. The enhanced harmattan contributes to move southward the intertropical discontinuity (ITD) defined as the interface between the cool moist southwesterly monsoon flow and the warm dry harmattan. Finally, an interaction between the ITD and African Easterly waves contributes to propagate the ITD southward retreat about 1500 km to the west of the Hoggar and Air mountains