Search results for "Atmospheric Circulation"

2019

Abstract. The cumulative occurrence of heavy precipitation and flood events during recent years in various Central European locations emphasises the urgent need to improve extreme rainfall observations and forecasts. Precipitation gauges based on a weighing system allow the recording of intense short-term precipitation events with a very high temporal resolution (down to 1 min). In this study, observational data that were collected during the period 2000 to 2016 for 126 stations of two corresponding measuring networks in the Central German state of Hesse were investigated for the first time to answer the following questions: (1) Are the recorded high-resolution precipitation data plausible …

Interannual variability of rainfall in the eastern horn of Africa and indicators of atmospheric circulation

1993

Relationships between rainfall variations in the Eastern Horn of Africa, the Southern Oscillation, and the Indian Ocean temperature and pressure surface fields are studied for the period 1932–83. Rainfall data consist of stations and regional indices calculated for three selected areas experiencing quite different rainfall patterns. The results indicate significant negative correlations between northern autumn rains in Somalia and the Southern Oscillation during the same season. These rains are also negatively correlated with pressure in the Western Indian Ocean, and positively correlated in the Eastern Indian Ocean. The reverse pattern is shown with sea-surface temperature. This agrees ver…

Climate Adjustments over Africa Accompanying the Indian Monsoon Onset

2010

Abstract Rainfall and circulation changes accompanying the Indian monsoon onset are examined, focusing on the African continent and neighboring areas. The Indian Meteorological Department official monsoon onset dates over Kerala (MOK; on average on 1 June) are used. Composites are formed at a pentad (5 days) time scale to compare pre- and postonset conditions. Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) data for 1979–2007 indicate that a substantial rainfall decrease over several parts of Africa is associated with MOK. Significant rainfall anomalies, after removal of the mean seasonal cycle, are found in eastern Africa and the nearby western Indian Ocean. Indian …

Global atmospheric response to specific linear combinations of the main SST modes. Part I: numerical experiments and preliminary results

1996

Abstract. This article investigates through numerical experiments the controversial question of the impact of El Niño-Southern Oscillation (ENSO) phenomena on climate according to large-scale and regional-scale interhemispheric thermal contrast. Eight experiments (two considering only inversed Atlantic thermal anomalies and six combining ENSO warm phase with large-scale interhemispheric contrast and Atlantic anomaly patterns) were performed with the Météo-France atmospheric general circulation model. The definition of boundary conditions from observed composites and principal components is presented and preliminary results concerning the month of August, especially over West Africa and the …

Global equatorial variability of 850 and 200 hPa zonal winds from rawinsondes between 1963 and 1989

1995

The longitude-height-time variability of 3-month averaged zonal wind anomalies at 850 and 200 hPa over the equatorial area (5{degrees}N-5{degrees}S) is analyzed using a three-dimensional dataset constructed from rawinsonde data (1963-1989). The first mode, closely related to the Southern Oscillation Index, suggests a strong vertical coupling associated with a horizontal out-of-phase pattern between the central/western Pacific and the remainder of the equatorial belt. The vertical coupling appears to be phase-locked to the annual cycle with strongest intensities found over South America and near the maritime continent early in the calendar year and over the Pacific basin and Africa during th…

Sahel droughts and Enso dynamics

1996

Correlations between summer Sahel rainfall and Southern Oscillation Index has increased during the last thirty years. At high frequency time scale (periods lower than 8 years), an intertropical Atlantic zonal divergent circulation anomaly is forced by the difference of sea surface temperature (SST) anomalies between the eastern equatorial parts of Pacific and Atlantic. This zonal connection worked well during most of the E1 Nino/Southern Oscillation (ENSO) events occurring after 1970; positive/negative SST anomalies in the eastern Pacific/Atlantic led to rainfall deficits over the whole West Africa. At low frequency time scale (periods greater than 8 years), positive SST anomalies in the In…

Types de configurations pluviométriques sur le bassin méditerranéen, forçages atmosphériques et évènements intenses (1979-2013).

2015

Sixrecurrent and redundant daily rainfall patterns have been extracted over the Mediterranean area from 1979 to 2013: five areassociated to regionals rainfall excess (South-West Mediterranean, Mid-North Mediterranean, East Mediterranean, WestMediterranean and north-west Mediterranean) and one to generalized deficit. Each rainfall pattern is associated with atypical barometric spatial configuration leading to local wind bringing moisture to the coast leading to specific local rainfallsurplus. Rainfall patterns also explain a large part of the spatial characteristics and the probability of occurrence of intenseevents. For example, more than 20% of days of “West-Mediterranean” rainfall pattern…

Épisodes secs en hiver dans le bassin méditerranéen : variabilité et forçages atmosphériques (1957-2013).

2016

The mean number of dry days in winter increased in the Mediterranean Basin during the 1957-2013 period,mainly because of dry spells becoming longer. The shortest events (Very Short Dry Spells) were less frequentwhereas longer events (Short Dry Spells, Medium Dry Spells and Long Dry Spells) were more frequent on theMediterranean Basin. 76 very long events (Very Long Dry Spells – VLDS) occurred during the 1957-2013period. A hierarchical clustering analysis of these VLDS events allows to detect 5 mainly Mediterraneanconfigurations: North-West Mediterranean, West Mediterranean, North-East Mediterranean, South-EastMediterranean and localized events.

Pluviométrie et circulation atmosphérique simulées par le modèle régional WRF en Afrique australe : sensibilité à la physique et variabilité interne

2011

This study evaluates the capability of the regional climate model WRF to simulate Southern African rainfall and associated atmospheric circulation, particularly over South Africa, a country covered by a dense network of in situ observations. Three sets of simulations are performed at a 35km horizontal resolution during the core of the austral rainy season (December to February: DJF) with 6-hourly forcings provided by ERA40 reanalysis. The first set quantifies sensitivity of the simulated climate to the parameterizations of atmospheric convection, planetary boundary layer and microphysics. The retained case study (DJF 1993-1994: DJF94) is representative of the South African rainfall climatol…

545. Un período fundamental para la climatología española: el “descubrimiento” de la circulación atmosférica en altitud, 1950-1980

2016

El período que transcurre entre los años 1950 y 1980 resulta decisivo para la consolidación de la climatología española. Los trabajos de una serie de geógrafos españoles (López Gómez, García Fernández, Gil Olcina, Burriel de Orueta, Albentosa Sánchez y Capel Molina) se convertirán en referencias básicas al presentar en nuestro país los avances sobre la circulación atmosférica en las capas altas de la troposfera que se desarrollan después de la 2a Guerra Mundial en Estados Unidos y otros países europeos. Este trabajo analiza el contexto científico en el que tiene lugar el “descubrimiento” de la circulación atmosférica en altitud por parte de la geografía española y su repercusión para la mej…