Béatrice Morel

Diurnal changes in middle atmospheric H2O and O3: Observations in the Alpine region and climate models

In this paper we investigate daily variations in middle atmospheric water vapor and ozone based on data from two ground-based microwave radiometers located in the Alpine region of Europe. Temperature data are obtained from a lidar located near the two stations and from the SABER experiment on the TIMED satellite. This unique set of observations is complemented by three different three-dimensional (3-D) chemistry-climate models (Monitoring of Stratospheric Depletion of the Ozone Layer (MSDOL), Laboratoire de Meteorologie Dynamique Reactive Processes Ruling the Ozone Budget in the Stratosphere (LMDz-REPROBUS), and Solar Climate Ozone Links (SOCOL)) and the 2-D atmospheric global-scale wave model (GSWM). The first part of the paper is focused on the first Climate and Weather of the Sun-Earth System (CAWSES) tidal campaign that consisted of a period of intensive measurements during September 2005. Variations in stratospheric water vapor are found to be in the order of 1% depending on altitude. Meridional advection of tidal nature is likely to be the dominant driving factor throughout the whole stratosphere, while vertical advection becomes more important in the mesosphere. Observed ozone variations in the upper stratosphere and lower mesosphere show amplitudes of several percent in accordance with photochemical models. Variations in lower stratospheric ozone are not solely governed by photochemistry but also by dynamics, with the temperature dependence of the photochemistry becoming more important. The second part presents an investigation of the seasonal dependence of daily variations. Models tend to underestimate the H2O diurnal amplitudes, especially during summer in the upper stratosphere. Good agreement between models and observations is found for ozone in the upper stratosphere, which reflects the fact that the O3 daily variations are driven by the photochemistry that is well modeled.

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.

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.

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.

Regionalizing Rainfall at Very High Resolution over La Réunion Island Using a Regional Climate Model

AbstractRegional climate models (RCMs) should be evaluated with respect to their ability to downscale large-scale climate information to the local scales, which are sometimes strongly modulated by surface conditions. This is the case for La Reunion (southwest Indian Ocean) because of its island context and its complex topography. Large-scale atmospheric configurations such as tropical cyclones (TCs) may have an amplifying effect on local rainfall patterns that only a very high-resolution RCM, forced by the large scales and resolving finescale processes, may simulate properly.This paper documents the capability of the Weather Research and Forecasting Model (WRF) RCM to regionalize rainfall variability at very high resolution (680 m) over La Reunion island for daily to seasonal time scales and year-to-year differences. Two contrasted wet seasons (November–April) are selected: 2000–01 (abnormally dry) and 2004–05 (abnormally wet). WRF rainfall is compared to a dense network of rain gauge records interpolated...

A Method for Mapping Monthly Solar Irradiation Over Complex Areas of Topography: Réunion Island’s Case Study

The aim of this study is to build a high-resolution mapping model for Reunion, a mountainous island with highly complex terrain. The dataset used here, which consists of solar irradiation, is not available from the regular weather station network over the island. This network is relatively dense and includes quality-monitoring stations, thus providing enough information to tackle the problem of climate data interpolation over the complex terrain. A model for mapping the monthly means of such variables is presented. It combines Partial Least Squares (PLS) regression with kriging interpolation of residuals. For all the variables, the same set of nine predictors, including altitude, geographical and topographical features, was selected for PLS regression. The regression model gives statistically good estimates of monthly solar irradiation. Accuracy improves significantly using solar radiation mapping built with regression+kriging than for mapping built with regression only.

Evaluation of balloon and satellite water vapour measurements in the Southern tropical UTLS during the HIBISCUS campaign

Balloon water vapour in situ and remote measurements in the tropical upper troposphere and lower stratosphere (UTLS) obtained during the HIBISCUS campaign around 20 S in Brazil in February–March 2004 using a tunable diode laser ( μSDLA), a surface acoustic wave (SAW) and a Vis-NIR solar occultation spectrometer (SAOZ) on a long duration balloon, have been used for evaluating the performances of satellite borne remote water vapour instruments available at the same latitude and measurement period. In the stratosphere, HALOE displays the best precision (2.5%), followed by SAGE II (7%), MIPAS (10%), SAOZ (20–25%) and SCIAMACHY (35%), all of which show approximately constant H2O mixing ratios between 20–25 km. Compared to HALOE of ±10% accuracy between 0.1–100 hPa, SAGE II and SAOZ show insignificant biases, MIPAS is wetter by 10% and SCIAMACHY dryer by 20%. The currently available GOMOS profiles of 25% precision show a positive vertical gradient in error for identified reasons. Compared to these, the water vapour of the Reprobus Chemistry Transport Model, forced at pressures higher than 95 hPa by the ECMWF analyses, is dryer by about 1 ppmv (20%). In the lower stratosphere between 16–20 km, most notable features are the steep degradation of MIPAS precision below 18 km, and the appearance of biases between instruments far larger than their quoted total uncertainty. HALOE and Correspondence to: N. Montoux (nadege.montoux@latmos.ipsl.fr) SAGE II (after spectral adjustment for reducing the bias with HALOE at northern mid-latitudes) both show decreases of water vapour with a minimum at the tropopause not seen by other instruments or the model, possibly attributable to an increasing error in the HALOE altitude registration. Between 16–18 km where the water vapour concentration shows little horizontal variability, and where the μSDLA balloon meaurements are not perturbed by outgassing, the average mixing ratios reported by the remote sensing instruments are substantially lower than the 4–5 ppmv observed by the μSDLA. Differences between μSDLA and HALOE and SAGE II (of the order of−2 ppmv), SCIAMACHY, MIPAS and GOMOS (−1 ppmv) and SAOZ ( −0.5 ppmv), exceed the 10% uncertainty of μSDLA, implying larger systematic errors than estimated for the various instruments. In the upper troposphere, where the water vapour concentration is highly variable, AIRS v5 appears to be the most consistent within its 25% uncertainty with balloon in-situ measurements as well as ECMWF. Most of the remote measurements show less reliability in the upper troposphere, losing sensitivity possibly because of absorption line saturation in their spectral ranges (HALOE, SAGE II and SCIAMACHY), instrument noise exceeding 100% (MIPAS) or imperfect refraction correction (GOMOS). An exception is the SAOZ-balloon, employing smaller H 2O absorption bands in the troposphere. Published by Copernicus Publications on behalf of the European Geosciences Union. 5300 N. Montoux et al.: Balloon and satellite H2O measurements in the tropical UTLS

Regionalizing rainfall at very high resolution over La Réunion island: a case study for Tropical Cyclone Ando.

Ensemble simulations of Tropical Cyclone (TC) Ando (31 December 2000–9 January 2001) are performed over the southwest Indian Ocean using the nonhydrostatic WRF Model. Nested domains centered over the island of La Reunion allow for the simulation of local rainfall amounts associated with TC Ando at very high resolution (680-m grid spacing). The model is forced by and nudged toward ERA-Interim during the first (1–6) day(s) of the TC’s life cycle. The nudging ends at various dates to constrain either the whole life cycle or only parts of it. As expected, results show weakened member dispersion, as the relaxation lasts longer, with more members producing similar cyclone tracks and intensities. The model shows reasonable skill to simulate local rainfall amounts and distribution, as soon as the simulated TC approaches La Reunion with a realistic distance and azimuth. Strong lower-level wind associated with the TC is forced to ascend over the slopes of the island. The model is able to successfully simulate the extreme daily precipitation amounts (>1200 mm) and their distribution over the highest parts of La Reunion. Nevertheless, smaller-scale features of the rainfall field are less realistic in the simulations. The wind speed and direction upstream of the island are the main drivers of such local uncertainties and errors, and they appear as an important issue to assess the local impacts of the TC over such a complex terrain.