Typical disturbances of the daytime equatorial F region observed with a high-resolution HF radar

HF radar measurements were performed near the magnetic equator in Africa (Korhogo 9°24'63''N–5°37'38''W) during the International Equatorial Electrojet Year (1993–1994). The HF radar is a high-resolution zenithal radar. It gives ionograms, Doppler spectra and echo parameters at several frequencies simultaneously. This paper presents a comparative study of the daytime ionospheric structures observed during 3 days selected as representative of different magnetic conditions, given by magnetometer measurements. Broad Doppler spectra, large echo width, and amplitude fluctuations revealed small-scale instability processes up to the F-region peak. The height variations measured at different altitudes showed gravity waves and larger-scale disturbances related to solar daytime influence and equatorial electric fields. The possibility of retrieving the ionospheric electric fields from these Doppler or height variation measurements in the presence of the other possible equatorial ionospheric disturbances is discussed

Analysis of the temporal structure of the daily rainfall observed at upper Oueme valley catchment in northern of Benin (west Africa) during 1999-2012.

West African rainfall is characterized by a strong variability. In order to quantify the hydrological impacts of such variability, analysis of rainfall patterns is highly essential. This study aims to characterize the Sudanese rainfall, using a raingauge data set collected on the upper Oueme River catchment (Benin) between 1999 and 2012. In this study, we choose a homogeneous network of fourteen rain gauges divided into four zones in the catchment: zone A (Affon, Bori, Sonoumon, Tobre), zone B (Copargo, Djougou, Donga, Tebou), zone C (Dogue, Pelebina, Sarmanga), zone D (Beterou_C, Wewe, Koko). The analysis of the frequency distributions of daily rainfall revealed that 25% of rainy days give rainy heights greater than 15mm while 50% of rainy days occur with rainy heights less than 5mm. The frequency distribution of daily rainfall has been well modeled by Gamma and Weibull law. The seasonal cycle of daily rainfall in the study region is mono-modal. The analysis of the seasonal cycle indicated that its bandwidth is wider in the South than in the North of the region such as: bandwidth (zone A) = bandwidth (zone B) < bandwidth (zone C) < bandwidth (zone D). The results of this study will allow us to fully realize the monofractal analysis of the precipitations in this region.Keywords: Sudanese climate; precipitation; West Africa, seasonal cycle, Exponential, Gamma, Weibull model.RESUMELes précipitations en Afrique de l'Ouest sont caractérisées par une forte variabilité spatio-temporelle. Afin de quantifier les impacts hydrologiques de cette variabilité, l'analyse de la structure temporelle des précipitations est très essentielle. Cette étude vise à caractériser les précipitations soudanaises, en utilisant un ensemble de données recueillies sur le pluviomètre bassin versant supérieur du fleuve Ouémé (Bénin) entre 1999 et 2012. Dans cette étude, nous choisissons un réseau homogène de quatorze pluviomètres répartis en quatre zones dans le bassin versant : zone A (Affon, Bori, Sonoumon, Tobre), zone B (Copargo, Djougou, Donga, Tebou), la zone C (Dogue, Pelebina, Sarmanga), la zone D (Beterou_C, Wewe, Koko). L’analyse des distributions fréquentielles des pluies journalières a révélé que 25% des jours pluvieux donnent lieu à des cumuls supérieurs à 15mm. Cette distribution des cumuls journaliers a été bien modélisée par la loi gamma et la loi de weibull. Le cycle saisonnier des pluies journalières de la région est monomodal. Son analyse par zone a indiqué que sa bande passante est plus large au Sud qu’au Nord de la région : la bande passante (zone A) = bande passante (zone B) <bande passante (zone C) <bande passante (zone D). Les résultats de cette étude nous permettront de réaliser pleinement l'analyse de monofractal des précipitations dans cette région.Mots clés : Climat soudanien, précipitations, Afrique de l'Ouest, cycle saisonnier, exponentiel, modèle Gamma, Weibull

First wind profiler radar observations in Africa.

(24-29 avril 2005)

Lightning characteristics observed by a VLF/LF lightning detection network (LINET) in Brazil, Australia, Africa and Germany

International audienceThis paper describes lightning characteristics as obtained in four sets of lightning measurements during recent field campaigns in different parts of the world from mid-latitudes to the tropics by the novel VLF/LF (very low frequency/low frequency) lightning detection network (LINET). The paper gives a general overview on the approach, and a synopsis of the statistical results for the observation periods as a whole and for one special day in each region. The focus is on the characteristics of lightning which can specifically be observed by this system like intra-cloud and cloud-to-ground stroke statistics, vertical distributions of intra-cloud strokes or peak current distributions. Some conclusions regarding lightning produced NOx are also presented as this was one of the aims of the tropical field campaigns TROCCINOX (Tropical Convection, Cirrus and Nitrogen Oxides Experiment) and TroCCiBras (Tropical Convection and Cirrus Experiment Brazil) in Brazil during January/February 2005, SCOUT-O3 (Stratospheric-Climate Links with Emphasis on the Upper Troposphere and Lower Stratosphere) and TWP-ICE (Tropical Warm Pool-International Cloud Experiment) during November/December 2005 and January/February 2006, respectively, in the Darwin area in N-Australia, and of AMMA (African Monsoon Multidisciplinary Analyses) in W-Africa during June-November 2006

Equatorial electrojet as part of the global circuit: a case-study from the IEEY

International audienceGeomagnetic storm-time variations often occur coherently at high latitude and the day-side dip equator where they affect the normal eastward Sq field. This paper presents an analysis of ground magnetic field and ionospheric electrodynamic data related to the geomagnetic storm which occured on 27 May 1993 during the International Equatorial Electrojet Year (IEEY) experiment. This storm-signature analysis on the auroral, mid-latitude and equatorial ground field and ionospheric electrodynamic data leads to the identification of a sensitive response of the equatorial electrojet (EEJ) to large-scale auroral return current: this response consists in a change of the eastward electric field during the pre-sunrise hours (0400-0600 UT) coherently to the high-, mid-, and equatorial-latitude H decrease and the disappearance of the EEJ irregularities between the time-interval 0800-0950 UT. Subsequent to the change in h'F during pre-sunrise hours, the observed foF2 increase revealed an enhancement of the equatorial ionization anomaly (EIA) caused by the high-latitude penetrating electric field. The strengthening of these irregularities attested by the Doppler frequency increase tracks the H component at the equator which undergoes a rapid increase around 0800 UT. The ?H variations observed at the equator are the sum of the following components: SR, DP, DR, DCF and DT.Keywords. Equatorial electrojet · Magnetosphere-ionosphere interactions · Electric fields and currents · Auroral ionosphere · Ionospheric disturbance