Hassan Bencherif

Examination of the 2002 major warming in the southern hemisphere using ground-based and Odin/SMR assimilated data: stratospheric ozone distributions and tropic/mid-latitude exchange

Following an exceptionally active winter, the 2002 Southern Hemisphere (SH) major warming occurred in late September. It was preceded by three minor warming events that occurred in late August and early September, and yielded vortex split and break-down over Antarctica. Ozone (O3 and nitrous oxide (N2O) profiles obtained during that period of time (15 August – 4 October) by the Sub-Millimetre Radiometer (SMR) aboard the Odin satellite are assimilated into MOCAGE (Modelisation Isentrope du transport Mesoechelle de lOzone Stratospherique par Advection), a global three-dimensional chemistry transport model of Meteo-France. The assimilated algorithm is a three-dimensional-FGAT built by the European Centre for Research and Advance Training in Scientific Computation (CERFACS) using the PALM (Projet dAssimilation par Logiciel Multi-methode) software. The assimilated O3 and N2O profiles and isentropic distributions are compared to ground-based measurements (LIDAR and balloon-sonde) and to maps of advected poten...

Fine‐scale study of a thick stratospheric ozone lamina at the edge of the southern subtropical barrier

A large-scale transport event resulting in a thick ozone lamina originating from midlatitudes is observed in the tropical stratosphere over Reunion island (55°E, 21°S). This isentropic transport was detected from stratospheric balloon-borne ozone measurements that showed the occurrence of the lamina and was investigated using different tools based on Ertels potential vorticity (Epv) analyses. An original software (DYBAL) using surface coordinate and the equivalent length of Epv contours as diagnostic tools in conjunction with high-resolution outputs from an Epv advection model MIMOSA allows us to specify the origin of the lamina. The results indicate that a broad layer of stratospheric air was isentropically advected from midlatitudes across the southern edge of tropical reservoir and reached Reunion island on 12 July 2000. In addition, Eliassen-Palms flux vectors, calculated from ECMWF analysis, show that planetary wave activity was quite large during that time period, with wave-breaking occurring around 30 km, and could have driven that exchange. In contrast with analyses of filamentation events based on model and satellite data, the present study focuses on a fine-scale vertical survey from in situ measurements. The filament reported in this paper is characterized by a large vertical extension and is located around the maximum of ozone concentration in the tropical stratosphere (600 K). The analysis of such events, poorly documented in the tropics, could complement satellite studies and contribute to a better determination of the transport between the tropics and the midlatitudes.

Evidence of tidal perturbations in the middle atmosphere over Southern Tropics as deduced from LIDAR data analyses

Abstract Systematic measurements of the middle-atmosphere temperature by a RAYLEIGH LIDAR located at La Reunion Island (20.8°S–55.5°E) has led to a preliminary study of the tidal effects in the height range of 30–70 km. Two analysis methods able to estimate the mean nighttime evolution of the temperature have been compared. Method 1 consists in averaging the temperature deviations from the nightly mean over several successive nights of measurement for given local solar times (LST); method 2 consists in averaging the raw data over the period of observation for given LST and in deriving afterwards the mean nighttime evolution of the temperature profiles. Some consistent LST-related structures have been observed with both methods, though better results have been obtained with method 1. One possible explanation for the differences between the two methods is the use of a series of data ranging from 4 to 8 h / night , depending on the meteorological conditions. In contrast to method 2, method 1 allows to correct the mean temperature for a given night, when the measurement window is different from night to night. Method 1 has been applied to two time series recorded in October and November 1995. The results clearly show the presence of tidal components with a downward phase propagation, specifically a warmer early night and a colder midnight in the stratosphere and the lower mesosphere. This behaviour is consistent with other LIDAR measurements made at similar latitudes in the Northern Hemisphere. In addition, a close comparison with the Global-Scale Wave Model (GSWM) tidal model predictions has also pointed out some similarities. Yet, large discrepancies in magnitude are also observed: as already reported in previous studies, the amplitudes predicted by GSWM are more than two times smaller compared to the corresponding values observed with the LIDAR.

Seasonal oscillations of middle atmosphere temperature observed by Rayleigh lidars and their comparisons with TIMED/SABER observations

The long-term temperature data sets obtained by Rayleigh lidars at six different locations from low to high latitudes within the Network for the Detection of Atmospheric Composition Change (NDACC) were used to derive the annual oscillations (AO) and semiannual oscillations (SAO) of middle atmosphere temperature: Reunion Island (21.8°S); Mauna Loa Observatory, Hawaii (19.5°N); Table Mountain Facility, California (34.4°N); Observatoire de Haute Provence, France (43.9°N); Hohenpeissenberg, Germany (47.8°N); Sondre Stromfjord, Greenland (67.0°N). The results were compared with those derived from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument onboard the Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics (TIMED) satellite

LIDAR observations of lower stratospheric aerosols over South Africa linked to large scale transport across the southern subtropical barrier

Abstract The study of the variability of stratospheric aerosols and the transfer between the different atmospheric regions improves our understanding of dynamical processes involved in isentropic exchanges that take place episodically in the lower stratosphere through the subtropical barrier. One useful approach consists in combining in situ ground-based and global measurements with numerical analyses. The present paper reports on a case study of a horizontal transfer evidenced first by Rayleigh–Mie LIDAR observations over Durban (29.9°S, 31.0°E, South Africa). Additional data from MeteoSat and SAGE -2 experiments, and from ECMWF meteorological analysis have been used in this study. Contour advection maps of potential vorticity from the MIMOSA model derived from ECMWF fields, were also used. By the end of April, 1999, LIDAR observations showed that aerosol extinction, in the lower stratosphere, has increased significantly and abnormally in comparison with other LIDAR and SAGE -2 observations recorded for the period from April 20 to June 14, 1999. The dynamical context of this case study seems to exclude the possibility of a local influence of the subtropical jet stream or tropical convection, which could inject air masses enriched with tropospheric aerosols into the stratosphere. On the contrary, a high-resolution model based on PV advection calculations and ECMWF meteorological analyses shows that air masses are isentropically advected from the equatorial zone close to Brazil. They cross the southern barrier of the tropical reservoir due to laminae stretching and reach the southern subcontinent of Africa 5–6 days later.

Tropopause Characteristics and Variability from 11 yr of SHADOZ Observations in the Southern Tropics and Subtropics

In this paper, tropopause characteristics observed from tropical to subtropical Southern Hemisphere stations using Southern Hemisphere Additional Ozonesonde (SHADOZ) data are presented for the 11-yr period of 1998‐2008. Three different definitions of tropopause—cold-point tropopause (CPT), lapse-rate tropopause (LRT), and ozone tropopause (OT)—are determined, and their variability for nine different SHADOZ sites is studied for the purpose of evaluating their usefulness as indicators of possible tropopause trends. For each station, the OT is uniquely defined by the ozone gradient and is found to be more variable than either LRT or CPT. The OT roughly coincides with the upper boundary of the region of most active convective mixing over the western Pacific Ocean and with the lower boundary of the transition region from the troposphere to the lower stratosphere that is generally referred to as the tropical tropopause layer. The monthly and year-to-year variations in the tropopause are examined, and the annual cycle in OT, the dominant signal, is described. The distance of separation of the OT from the CPT or LRT is smaller for the tropics (stations at 08‐158S) than for the subtropics (158‐258S). The decadal trend in tropopause heights is measured

Tropopause characteristics over a southern subtropical site, Reunion Island (21°S, 55°E): Using radiosonde‐ozonesonde data

[1]xa0In this paper, we present tropopause characteristics using in situ radiosonde-ozonesonde observations carried out over the past 13 years (September 1992 to February 2005) from a southern subtropical site, Reunion Island (21°S, 55°E). Three kinds of tropopause definitions, namely, cold-point tropopause (CPT), lapse rate tropopause (LRT), and ozone tropopause (OT), are characterized. The ozone tropopause for Reunion is appropriately defined as the height at which the vertical gradient of the ozone mixing ratio exceeds 55 ppbv/km and the ozone mixing ratio is over 75 ppbv. The overall height distribution of the three kinds of tropopause ranges in between 12.5 and 18.5 km. Of the three definitions, CPT follows higher heights of distribution, followed by LRT and then OT. The sharpness of tropopause height detection for LRT and OT has been examined, and it has been found that the detection frequency of a definite OT is larger than that for LRT. The results also showed no indefinite OT for a subtropical station in comparison to the results obtained for Northern Hemisphere midlatitude and high-latitude stations. After adopting a few modifications in the sharpness of ozone tropopause detection, the result remains the same, with a single indefinite case. The indefinite ozone tropopause case was identified when Reunion Island was located nearer to the divergence zone of wind as well as in the anticyclonic region of jet stream entrance. The LRT sharpness results are found to be in good agreement with the other published definition.

Fine‐scale study of a thick stratospheric ozone lamina at the edge of the southern subtropical barrier: 2. Numerical simulations with coupled dynamics models

The modeling of an event such as an ozone lamina requires reproducing both the global and the small scales. In this study we report on a specific model capable of resolving such scale issues: the COMMID model, which has been developed by coupling a mechanistic model, MSDOL, with a high-resolution advection model, MIMOSA. MSDOL, which is forced toward National Centers for Environmental Prediction (NCEP) reanalyses below 100 hPa, provides a consistent picture of the stratospheric large-scale circulation from which MIMOSA simulates the fine-scale filaments generated by breaking planetary waves in the stratosphere. To evaluate the performances of the model, we present results for a particular event of tropical-air intrusion at midlatitudes across the southern subtropical barrier observed in July 2000 and described in part 1 (Portafaix et al., 2003). The model is used to examine the contribution of each wave to the structure and the development of that event. The methodology consists in filtering the NCEP tropospheric forcing by zonal wave number and by phase speed. Our results show that mixing is significantly reduced precisely at the locations where the phase speeds of the filtered waves are close to the speed of the mean zonal wind, thus confirming the findings of previous studies. However, what is important here is that they validate the use of an approach based on the coupling of two models. The next step will consist in using the COMMID model in a more general way for further investigations of the impact of the tropospheric circulation on the isentropic transport in the stratosphere for climate sensitivity purposes.

Non-solid non-rigid optics for high power laser systems

Non-solid and non-rigid optics employ gas and liquid transmission and reflection, as well as flexible membranes to influence laser beams, laser driven particle beams and harmonic generators. Some examples are acoustic gratings, phase conjugate mirrors, Raman cells, gas-jets, gas and flame lenses, gas capillaries, plasma cones, mercury mirrors and rotating and aerodynamic windows. Industrial scale laser propulsion, laser fusion, laser accelerators, lithography and laser isotope separation will necessitate handling average beam powers very different from present day single shot demonstrations. Standard solid state optics may prove incapable of handling such conditions.

Laser propulsion experiments in South Africa

Two sets of experiments indicate a renewal of interest in South Africa in the topic of laser propulsion. Both sets were conducted under the auspices of the new National Laser Center. In the first set, a 1 kW, CO2 laser (1 kHz, 1 J, 100 ns) was used to propel small (ca 1 gram) targets through a vertical tube-launcher and the momentum-coupling coefficient for a variety of conditions was estimated. The somewhat disappointing results were accounted for in terms of the poor beam quality from a single oscillator and premature break-down of the exhaust vapor in the tube. These experiments were conducted with one module of the now dismantled MLIS uranium isotope separation system. The second set of experiments being conducted in Durban with a small but more energetic marking laser (CO2 20 Hz., 4 J, 100 ns). The chief purpose of this, was to better understand the discrepancies between the recent vertical propulsion experiment at Pelindaba and earlier propulsion attempts with the original MLIS chain. Preliminary pendulum experiments were carried out. Burning targets exhibited enhanced coupling for single pulses.