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African Easterly Waves (african + easterly_wave)
Selected AbstractsMulti-scale analysis of the 25,27 July 2006 convective period over Niamey: Comparison between Doppler radar observations and simulationsTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue S1 2010Christelle Barthe Abstract The present study investigates the multi-scale processes associated with a sequence of convective events that occurred over Niamey during the period 25,26 July 2006. This period corresponds to the active stage of the first intense monsoon surge over Sahel for 2006. During this two-day period, two successive sequences of mesoscale convective systems (MCSs) were located ahead of and in phase with the trough of an African Easterly Wave (AEW). They were followed by suppressed or isolated convection behind the trough and in the vicinity of the ridge. The large AMMA-SOP dataset, in particular the UHF radar and the MIT Doppler radar in Niamey, are used in combination with a low-resolution (5 km) cloud-resolving model to understand the convection organization and its interaction with the environment. Several initial and boundary conditions have been tested, but only the simulation starting with the ECMWF AMMA reanalysis succeeds in reproducing the observed features; this emphasizes the importance of the initial state. From the simulated MCSs, the along-line component of the apparent source of momentum due to the convection is found to be up to 1 m s,1h,1. It seems that MCSs globally reduce the monsoon flow and generate southerlies at mid levels which can reinforce the rotation of the wind at the passage of the trough. During the afternoon of 26 July, the local convection over Niamey resulted from some favourable factors (humidity, CAPE, CIN) that triggered convection, while inhibiting factors (mid-level dry layer, weaker low-level wind shear pointing to the north, anticyclonic curvature of the streamlines at 700 hPa) prevented it organizing itself and propagating. In particular, the low-level wind shear seems of critical importance. Copyright © 2010 Royal Meteorological Society [source] Simulation of the intraseasonal and the interannual variability of rainfall over West Africa with RegCM3 during the monsoon periodINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 12 2010M. B. Sylla Abstract Intraseasonal and interannual variability of rainfall is simulated using the International Centre for Theoretical Physics (ICTP) Regional Climate Model (RegCM3) over West Africa. The intraseasonal variability of rainfall showing three distinct phases and the monsoon jump is well reproduced in the simulation. In addition, the regional model shows that while the monsoon rainbelt moves to the Sahel, the African Easterly Jet (AEJ) undergoes a northward migration and a weakening from June to August, when the core is at its northernmost location. This coexists with the appearance and the strengthening of the Tropical Easterly Jet (TEJ), the development and increased activity of the African Easterly Waves (AEWs), and the intensification and northward shift of the ascent between the AEJ and the TEJ core levels and axis. Similarly, the simulated interannual variability of rainfall over West Africa, the Guinea region, and the Sahel, as well as the variability of atmospheric features during contrasting wet and dry years, is also well captured. In fact, in the simulation during dry years the AEWs activity is decreased while the AEJ is strengthened and migrates southward, the TEJ becomes weaker, and the ascent between the levels of the AEJ and the TEJ decreases. The simulated rainfall variability and the behavior of the related features during the rainy season and during contrasting wet and dry years are in line with previous studies that used observations and reanalysis. We conclude that this model performance is of sufficient quality for application to the study of climate processes and mechanisms over West Africa. Copyright © 2009 Royal Meteorological Society [source] The impact of vertical resolution on regional model simulation of the west African summer monsoonINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 10 2008Leonard M. Druyan Abstract The RM3 regional climate model is used to simulate the west African summer monsoon for six June,September seasons using NCEP reanalysis data for lateral boundary forcing. The study compares the performance of the previously published 16-level version with a newly tested 28-level version, both running on a horizontal grid with 0.5° spacing, in order to determine what improvements in simulations are achieved by increased vertical resolution. Comparisons between the performances include diagnostics of seasonal mean precipitation rates and circulation, vertical profiles of cumulus heating rates, frequencies of shallow and deep convection and diagnostics related to transient African easterly waves (AEWs). The characteristics of a composite AEW simulated at both vertical resolutions are presented. Results show that the most significant impact of increasing the vertical resolution is stronger circulation, stronger vertical wind shear and higher amplitude AEWs. The simulations with higher vertical resolution also achieve higher peaks of cumulus latent heating rates. Spatial,temporal correlations between simulated daily 700 mb meridional winds versus corresponding NCEP reanalysis data and simulated daily precipitation versus estimates from the Tropical Rainfall Measurement Mission (TRMM) archive were equally high at both vertical resolutions. Copyright © 2007 Royal Meteorological Society [source] Understanding African easterly waves: a moist singular vector approachATMOSPHERIC SCIENCE LETTERS, Issue 3 2009Rosalind J. Cornforth Abstract Moist singular vectors (MSV) have been applied successfully to predicting mid-latitude storms growing in association with latent heat of condensation. Tropical cyclone sensitivity has also been assessed. Extending this approach to more general tropical weather systems here, MSVs are evaluated for understanding and predicting African easterly waves, given the importance of moist processes in their development. First results, without initial moisture perturbations, suggest MSVs may be used advantageously. Perturbations bear similar structural and energy profiles to previous idealised non-linear studies and observations. Strong sensitivities prevail in the metrics and trajectories chosen, and benefits of initial moisture perturbations should be appraised. Copyright © 2009 Royal Meteorological Society [source] | ||||||||||