Search results for "Atmospheric convection"
showing 7 items of 7 documents
Definition and predictability of an OLR based West African monsoon onset
2008
The monsoon onset is documented in terms of latitudinal shift of deep convection areas within the ITCZ using an interpolated version of the National Oceanic and Atmospheric Administration (NOAA) Outgoing Longwave Radiation (OLR) at a 5-day time-step over West Africa for the period 1979–2004. Signals in moist convection derived from OLR values lower than 180 W/m2 allow better determination of onset dates (ODs) than the use of other thresholds or of the raw values of OLR. Such ODs are defined without any time filtering or spatial averaging along the meridional plane. They are also significantly correlated with ODs based on other datasets such as the CMAP and Global Precipitation Climatology P…
Influence of the Madden-Julian Oscillation on East African rainfall. Part I: Intraseasonal variability and regional dependency
2006
The influence of the Madden–Julian Oscillation (MJO) on rainfall amounts over Equatorial East Africa (Kenya and northern Tanzania) is analysed for the period 1979–95 at the intraseasonal (pentad) time-scale. The two rainy seasons (March to May and October to December) are considered. Intraseasonal wet events in East Africa are embedded in large-scale zonal circulation anomaly patterns along the equator, showing distinct eastward propagation. It is further found that these ‘wet’ events display a clear phasing with respect to the MJO cycle. This phasing is expressed as out-of-phase variations between the Highland and the coastal areas. Such a pattern is suggested to reflect different rain-cau…
Tropical–extratropical interactions related to upper-level troughs at low latitudes
2007
Abstract Momentum and kinetic energy fluxes associated with low-latitude transient disturbances at upper-levels play an important role in the general circulation of the atmosphere. They are related to eastward and equatorward propagating, positively tilted wave trains from the extratropics. Theoretical, modelling and observational studies show that this particular kind of tropical–extratropical interaction is most common in regions of mean upper-level westerlies at low latitudes, i.e. over the central and eastern Pacific and Atlantic Oceans during boreal winter and spring. The penetration of an upper-level trough into the Tropics is often associated with enhanced convection and the formatio…
Adaptive discontinuous evolution Galerkin method for dry atmospheric flow
2014
We present a new adaptive genuinely multidimensional method within the framework of the discontinuous Galerkin method. The discontinuous evolution Galerkin (DEG) method couples a discontinuous Galerkin formulation with approximate evolution operators. The latter are constructed using the bicharacteristics of multidimensional hyperbolic systems, such that all of the infinitely many directions of wave propagation are considered explicitly. In order to take into account multiscale phenomena that typically appear in atmospheric flows nonlinear fluxes are split into a linear part governing the acoustic and gravitational waves and a nonlinear part that models advection. Time integration is realiz…
Multi-model simulations of a convective situation in low-mountain terrain in central Europe
2008
The goal of the present study is to investigate the variability of simulated convective precipitation by three convection-resolving models using different set-ups and initial and boundary conditions. The COSMO, MM5 and WRF models have been used to simulate the atmospheric situation on 12 July 2006, when local convection occurred in central Europe under weak synoptic forcing. The focus of this investigation is on the convective precipitation in the northern Black Forest in South-West Germany. The precipitation fields from the nine model simulations differ considerably. Six simulations capture the convective character of the event. However, they differ considerably in the location and timing …
Variability of the African convection centre as viewed by outgoing longwave radiation records and relationships with sea-surface temperature patterns
1995
International audience; The main African convection centre (ACC) is delimited by the 5"-square areas with outgoing longwave radiation less than 235 W m-' in the domain 17.5°N-17.5"S and 18"W-46"E. Four parameters (latitude and longitude of the centre of gravity, mean intensity, and extension) were analysed during the period June 1974 to December 1991. The relationships between the variability of the ACC and the main sea-surface temperature (SST) patterns were then studied. Correlation and composite analyses demonstrate that the first principal component of SST, representing mainly the variability of the eastern and the central tropical Pacific, is associated strongly with the extension and …
Influence of the Madden-Julian Oscillation on Southern African summer rainfall
2007
Rain-causing mechanisms over Southern Africa (south of 15˚S) involve both tropical and temperate dynamics. Most studies focused on the synoptical timescale, while the intraseasonal (20-120 days) variability has more been neglected to date. This study aims at determining whether the dominant mode of intraseasonal variability in the Tropics, namely the Madden-Julian Oscillation (MJO), has a significant impact on Southern African rainfall and associated atmospheric dynamics. The examination of outgoing longwave radiation (OLR) over Southern Africa shows indeed significant intraseasonal fluctuations at the 30-60 day timescale, i.e. in the pe- riods that are typically reminiscent of the MJO. In ord…