conditional sampling and scale analysis of the marine atmospheric mixed layer - SOFIA Experiment

International audienceDuring the SOFIA experiment, performed in the Azores region in June 1992, airborne missions were conducted in the atmospheric boundary layer with two aircraft instrumented for turbulence measurements. We show how the conditional sampling technique, applied to the velocity, temperature and moisture fluctuations, is able to describe the various parcels which constitute the turbulent field. Each parcel, so identified, is characterized by its fractional area and by its contribution to the transfers of sensible heat and latent heat. On the other hand, a scale analysis is conducted by filtering the turbulent signals in five non-overlapping frequency bands, defined according to the characteristic turbulent scales. The contribution of each band to the turbulent energy and to the transfers is thus presented. The importance of the lowest frequencies, which are generally removed from the signals by high-pass filtering before computing turbulent fluxes, is shown. In the final section, the conditional sampling technique is applied to the signals filtered in the various bands. Despite a slight deformation of the eddies due to the filtering technique, the contribution of each parcel can be estimated at the various scales analysed

Planetary-wave modulation of PMSE

International audienceVariations in PMSE (Polar Mesosphere Summer Echo) occurrence with 4-6 day periods were observed by the ESRAD radar during the summer of 1997. These are compared with temperature fluctuations in 5-day planetary waves extracted from the UKMO assimilated global data analyses. At the beginning and end of the PMSE season, the PMSE variations are closely anti correlated with temperature variations associated with 5-day planetary waves at the 1 mb level. The planetary wave amplitudes expected at the mesopause are 1-2 K. This is found, by comparison with the seasonal decay of PMSE as the temperature rises at the end of the summer, to be sufficient to explain the observed 5-10% amplitude fluctuations in PMSE daily occurrence

Rain kinetic energy measurement with a UHF wind profiler: application to soil erosion survey of a volcanic tropical island

International audienceCommunication about Rain kinetic energy measurement with a UHF wind profiler: application to soil erosion survey of a volcanic tropical islan

Dynamic circulations and Windward Flow over Reunion Island

International audienceNumerical Weather Prediction models still have difficulties to predict local-scale phenomena, such as thermal breezes circulation. They are local driven wind systems that form over coastal zones (sea/land breeze) or mountainous terrain (slope/valley breeze), produced by the buoyancy effects associated with the diurnal cycle of heating and cooling of the lower atmospheric layers (Zardi et Whiteman, 2013). These circulations can drive abrupt changes that generate localized wind gusts, extreme precipitation, air pollution episodes in the lower layers, or sea state perturbations. The characteristics of the volcanic and tropical island of Reunion Island (Indian Ocean, 21°07’S, 55°32’E) offer an exceptional natural field of investigation for these process studies. The meteorological circulations on Reunion Island have been extensively studied by Lesouëf et al. (2010), Durand et al. (2014), Tulet et al. (2017), Foucart et al. (2018), and Réchou et al. (2019). These works show that the island is affected by a regime of southeast trade winds, which is intense in winter (June-August) and moderate to weak in summer (December to February). This weather regime is the cause of intense winds on the southwest and northeast edges of the island and a branch of northwesterly leeward circulation forcing in the northwest of the island (Maïdo area). In this region, thermal circulations are added to this regional circulation. This return loop occurs almost daily in this part of the island in the boundary layer. The oceanic air masses are advected on the slopes of the Maïdo area by the sea and valley breezes. This convection on the mountain slopes causes an almost daily formation of clouds, which are generally weakly developed vertically and generally with low water content. An intensive measurement campaign BIOMAÏDO (Bio-physicochemistry of tropical clouds at Maïdo) took place from 11 March to April 7, 2019, at Réunion Island, in order to study the chemical and biological composition of the air mass, the formation processes of secondary organic matter in heterogeneous environments, the dynamics and the evolution of the boundary layer, and the macro and micro-physical properties of clouds. In this study, we detail and analyze the dynamics circulations using the observations of the campaign and compare them to a high-resolution (100m horizontal resolution) numerical simulation with the Meso-NH model. Such a model turned during the selected days in which a dynamical connection between the sites was found (Rocco et al., 2022).The preliminary results have shown that a vertical resolution smaller than a few meters (~1m) is needed to capture the katabatic flows and the structure of the valley boundary layer, these circulations have an abrupt variation (~1 hour) and the anabatic flow takes nearly 1 h to arrive to the top of the mountain. The temporal and spatial structure of this breezes regimes is analyzed with the use of the wet bulb potential temperature (Davies-Jones., 2007), and the turbulence kinetic energy budgets determined by the numerical model; this study aims to quantify which processes have the most important role during the diurnal breeze evolution

One-Year analysis of rain and rain erosivity in a tropical volcanic island from UHF wind profiler measurements

International audienceCommunication about One-Year analysis of rain and rain erosivity in a tropical volcanic island from UHF wind profiler measurement

Short vertical-wavelength inertia-gravity waves generated by a jet-front system at Arctic latitudes - VHF radar, radiosondes and numerical modelling

International audienceInertia-gravity waves with very short vertical wavelength (λz ≤ 1000 m) are a very common feature of the lowermost stratosphere as observed by the 52MHz radar ESRAD (Esrange MST radar) in northern Scandinavia (67.88◦ N, 21.10◦ E). The waves are seen most clearly in radar-derived profiles of buoyancy frequency (N). Here, we present a case study of typical waves from 21 February to 22 February 2007. Good agreement between N^2 derived from radiosondes and by radar shows the validity of the radar de- termination of N^2 . Large-amplitude wave signatures in N^2 are clearly observed by the radar and the radiosondes in the lowermost stratosphere, from 9 km to 14-16 km height. Vertical profiles of horizontal wind components and poten- tial temperature from the radiosondes show the same waves. Mesoscale simulations with the Weather Research and Forecasting (WRF) model are carried out to complement the analysis of the waves. Good agreement between the radar and ra- diosonde measurements and the model (except for the wave amplitude) shows that the model gives realistic results and that the waves are closely associated to the upper-level front in an upper-troposphere jet-front system. Hodographs of the wind fluctuations from the radiosondes and model data show that the waves propagate upward in the lower stratosphere confirming that the origin of the waves is in the troposphere. The observations and modelling all indicate vertical wave- lengths of 700 ± 200 m. The radiosonde hodograms indicate horizontal wavelengths between 40 and 110 km and intrinsic periods between 6 and 9 h. The wave amplitudes indicated by the model are however an order of magnitude less than in the observations. Finally, we show that the profiles of N2 measured by the radar can be used to estimate wave amplitudes, horizontal wavelengths, intrinsic periods and momentum fluxes which are consistent with the estimates from the radiosondes

Investigations of the possible relationship between PMSE and tides using a VHF MST radar

International audienceThe possible relationship between Polar Mesospheric Summer Echoes (PMSE), tidal winds and vertical wind shear associated with the tidal winds is examined. PMSE were observed by the Esrange VHF MST Radar (ESRAD) 5 2 MHz radar at 67ø56'N, 21ø04'E during May-August 1997. Three data series in the beginning, middle and the end of the PMSE season are considered. The seasonal variation of PMSE shows a steep increase in occurrence at the end of May and fade-out in mid-August. Daily variation of echo power in the middle of the season has a minimum around 20-22 UT. Wind measurements reveal terdiurnal, semidiurnal and diurnal tides. Between 82-87 km we found a strong wind shear which tends to increase through the season, but does not show significant correlation with SNR (signal-to-noise ratio). It seems likely that PMSE appearance is conditioned by small scale dynamics and tidal effects on aerosols, and not by large scale wind shear

ESRAD observations of lee waves and associated atmospheric structures

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1997 Turbulence structure of the Marine Atmospheric Boundary Layer during the SEMAPHORE experiment

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1997: MST radar observations of lee waves in the Kiruna area: A complement to optical measurements of polar stratospheric clouds

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