Miloud Bessafi

Modulation of South Indian Ocean Tropical Cyclones by the Madden–Julian Oscillation and Convectively Coupled Equatorial Waves

Abstract The subseasonal modulation of tropical cyclone (TC) genesis by large-scale atmospheric wave modes is studied using data from the south Indian Ocean region. The modes considered are the Madden–Julian oscillation (MJO), and the convectively coupled equatorial Rossby (ER), Kelvin, and mixed Rossby–gravity (MRG) waves. Analysis of all TCs west of 100°E reveals a large and statistically significant modulation by the MJO and ER waves, a small yet significant modulation by Kelvin waves, and a statistically insignificant modulation by MRG waves. Attribution of the observed TC modulation was made through examination of the wave-induced perturbations to the dynamical fields of low-level vorticity, vertical shear, and deep convection. Possible thermodynamic influences on TC genesis were neglected. Different combinations of the three dynamical fields were necessary for successful attribution for each of the large-scale wave modes. For example, for the MJO, the modulation was best attributable to its perturba...

How do Tropical Temperate Troughs Form and Develop over Southern Africa

AbstractThis paper aims at separating the respective influences of tropical and midlatitude variability on the development and life cycle of tropical temperate troughs (TTTs) over southern Africa in austral summer (November–February). Cluster analysis is applied to 1971–2000 40-yr ECMWF Re-Analysis (ERA-40) daily outgoing longwave radiation (OLR) anomalies to identify TTTs and monitor tropical convection. The same analysis applied to the zonal wind stretching deformation at 200 hPa (ZDEF) characterizes midlatitude transient perturbations. Results based on the comparison between these two classifications first confirm that midlatitude baroclinic waves are a necessary condition for TTT development, but they are not sufficient. Roughly 40% of those occurring in austral summer are associated with a TTT. They tend to be stronger than the baroclinic waves not associated with TTT development. In the tropics, additional conditions needed to form a TTT consist of an excess of latent energy over the Mozambique Chan...

Diurnal and seasonal variation of carbonaceous aerosols at a remote MBL site of La Réunion island

In this study, we characterise the variability of black carbon (BC) obtained at Sainte-Rose, a tropical marine site of La Reunion island (21.5°S, 55°E) from one and a half-year (1998–1999) semi-continuous ground-based measurements. BC diurnal variation shows low concentrations during night-time (mean ∼10–20 ng/m3, following the season) and moderate levels at daytime (mean ∼35 ng/m3), while BC peaks are observed at the beginning and end of the day. Morning (mean ∼50 ng/m3 at 0700 h) and evening (mean ∼70 ng/m3 at 1800 h) BC peaks are thought to be due to local pollution induced by anthropogenic activities, most probably car exhausts from nearby roads. Night-time BC exhibits a seasonal variation too, with maximum levels observed during autumn and winter (∼20 ng/m3) and lowest values measured during spring and summer (∼10 ng/m3). BC seasonality suggests a local-to-regional anthropogenic origin of carbonaceous aerosols following the season. These suggestions are confirmed by wind sector and BC concentrations analysis at Sainte-Rose, which show that daytime BC aerosols originate mainly from inland while night-time BC concentrations originate from northeasterly (regional inhabited areas) to southeasterly (marine) regions, following the season. Wind sector analysis also points out important seasonal shift in origin of night-time air samples. These assumptions are confirmed by trajectory analysis indicating a spatial shift in the origin of air samples following the season. Finally, it is shown that during austral spring and summer, low carbonaceous aerosol concentrations encountered at this site are weakly subjected to scavenging processes. Results point out the potential of ground-based measurements to assess the origin and causes of carbonaceous aerosol variability at remote locations under favourable conditions (minimum local anthropogenic inputs and meteorology).

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...

Intra-seasonal rainfall variability over Madagascar

Using daily rain-gauge records for Madagascar and nearby islands, this paper investigates rainfall intraseasonal variability at local and regional scales during the austral summer season (November–February), as well as the respective influences of recurrent convective regimes over the South-West Indian Ocean (SWIO) and the Madden-Julian Oscillation (MJO). Our results show a general consistency between local-scale rainfall variability in Madagascar and regional-scale features of climate variability. The influence of Tropical-Temperate Troughs in their mature phase and/or their easternmost locations is first underlined. The development of such systems over Southern Africa and the Mozambique Chanel can be considered as precursors for Malagasy wet spells, especially over the southern part of the island. Regional and local effects of the MJO are weaker on average, and only concern the northwest of the island and the north of the Mozambique Chanel. MJO and convective regimes are finally shown to explain distinct fractions of regional rainfall variability.

Clustering of Solar Irradiance

The development of grid-connected photovoltaic power systems leads to new challenges. The short or medium term prediction of the solar irradiance is definitively a solution to reduce the storage capacities and, as a result, authorizes to increase the penetration of the photovoltaic units on the power grid. We present the first results of an interdisciplinary research project which involves researchers in energy, meteorology, and data mining, addressing this real-world problem. In Reunion Island from December 2008 to March 2012, solar radiation measurements have been collected, every minute, using calibrated instruments. Prior to prediction modelling, two clustering strategies have been applied for the analysis of the data base of 951 days. The first approach combines the following proven data-mining methods. principal component analysis (PCA) was used as a pre-process for reduction and denoising and the Ward Hierarchical and K-means methods to find a partition with a good number of classes. The second approach uses a clustering method that operates on a set of dissimilarity matrices. Each cluster is represented by an element or a subset of the set of objects to be classified. The five meaningfully clusters found by the two clustering approaches are compared. The interest and disadvantages of the two approaches for classifying curves are discussed.

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.

The seasonality of ozone profile at Reunion Island: the role of biomass burning and of transport

Vertical ozone profiles obtained from ozone soundings at Reunion island (21/spl deg/S-55/spl deg/E) present a seasonal variability. High values of ozone concentration are observed in the troposphere above the inversion level that caps the marine boundary layer during September to December. Satellite observations using NOAA (National Oceanic and Atmospheric Administration) data indicate that this period corresponds to active biomass burning and the beginning of deep convection clouds in Southeastern Africa and Madagascar. Furthermore, analysis of backtrajectories obtained from Reunion Island clearly evidence the role of long distance transports from biomass burning sources. Strong variation of ozone profiles are associated, through this transport, with fires, smoke plumes and deep convective structures present in the studied regions.

Analysis of Solar Irradiation Time Series Complexity and Predictability by Combining Kolmogorov Measures and Hamming Distance for La Reunion (France)

Analysis of daily solar irradiation variability and predictability in space and time is important for energy resources planning, development, and management. The natural variability of solar irradiation is being complicated by atmospheric conditions (in particular cloudiness) and orography, which introduce additional complexity into the phenomenological records. To address this question for daily solar irradiation data recorded during the years 2013, 2014 and 2015 at 11 stations measuring solar irradiance on La Reunion French tropical Indian Ocean Island, we use a set of novel quantitative tools: Kolmogorov complexity (KC) with its derivative associated measures and Hamming distance (HAM) and their combination to assess complexity and corresponding predictability. We find that all half-day (from sunrise to sunset) solar irradiation series exhibit high complexity. However, all of them can be classified into three groups strongly influenced by trade winds that circulate in a “flow around” regime: the windward side (trade winds slow down), the leeward side (diurnal thermally-induced circulations dominate) and the coast parallel to trade winds (winds are accelerated due to Venturi effect). We introduce Kolmogorov time (KT) that quantifies the time span beyond which randomness significantly influences predictability.

Mapping the incoming solar irradiance over Mauritius using time-series records of Satellite Application Facility on Climate Monitoring

In its long-term energy strategy, the tropical island of Mauritius does not plan to fully exploit its abundant solar energy resources. Several barriers have been identified for curtailing the development of solar energy systems in the island. One of the major difficulties is that Mauritius is not equipped with a high quality radiometric network that would allow an accurate mapping of its solar resources. Moreover, the presence of microclimates all over the island implies that it is essential for the assessment to be performed at a small spatial resolution. This problem has been addressed in many countries by using satellite-based datasets as a feasible alternative to acquire site-specific solar irradiation time-series records. In this work, the monthly and annual average global solar irradiance maps for Mauritius have been devised using mean monthly data obtained from the EUMETSAT Satellite Application Facility on Climate Monitoring. The climatic data records not only span over a long period ranging from 1983 to 2005 but also have a small spatial resolution 0.03° × 0.03°. The maps reveal that average monthly solar irradiance of 300 W/m2 are obtained in the northern part of the country during peak summer. A south-north positive solar irradiance gradient is also observed along the island. In the absence of precise ground measurements, these maps provide a first insight into the solar potential of Mauritius and can support formulation of energy policy that will foster solar energy development.