Sensitivity of African easterly waves to boundary layer conditions

A linearized version of the quasi-geostrophic model (QGM) with an explicit Ekman layer and observed static stability parameter and profile of the African easterly jet (AEJ), is used to study the instability properties of the environment of the West African wave disturbances. It is found that the growth rate, the propagation velocity and the structure of the African easterly waves (AEW) can be well simulated. Two different lower boundary conditions are applied. One assumes a lack of vertical gradient of perturbation stream function and the other assumes zero wind perturbation at the surface. The first case gives more realistic results since in the absence of horizontal diffusion, growth rate, phase speed and period have values of 0.5 day<sup>−1</sup>, 10.83 m s<sup>−1</sup> and 3.1 day, respectively. The zero wind perturbation at the surface case leads to values of these parameters that are 50 percent lower. The analysis of the sensitivity to diffusion shows that the magnitude of the growth rate decreases with this parameter. Modelled total relative vorticity has its low level maximum around 900 hPa under no-slip, and 700 hPa under free slip condition

RETRACTED ARTICLE: Hydrologic evaluation on the ERA-Interim output using observed river discharge data

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Investigating the diurnal cycle of precipitation over Central Africa

AbstractThe present study investigated the reliability of downscaling tool RegCM4.4 to simulate 2002–2006 June–September diurnal cycle precipitation characteristics. Besides their diurnal cycles, the spatial and temporal patterns in precipitation intensity, amount and frequency over Central Africa (CA) are investigated. Diurnal variance, phase and amplitude based on 3‐hourly model simulations are obtained by diurnal harmonics from each 24‐h period. Two statistical measures are used to evaluate model performance: the root mean square error and the index of agreement. The result shows that the RegCM outputs are well simulated compared with reference data in revealing the temporal and spatial patterns of precipitation amount and frequency over the continental area with some systematic wet biases over Cameroon highlands area. Diurnal variability of precipitation frequency and amount are properly well reproduced by the model with an afternoon peak around 1800 LST over entire domain except Atlantic Ocean sub‐region. The model does not properly describe the observed diurnal variation of precipitation intensity over the study area. One of the prominent results is that the pattern of precipitation frequency is quite similar to that of precipitation amount. This strong relationship between these two precipitation characteristics over the entire region of interest suggests that the diurnal precipitation variability is generally determined by how often it rains

Solitary Rossby waves in the lower tropical troposphere

Weakly nonlinear approximation is used to study the theoretical comportment of large-scale disturbances around the inter-tropical mid-tropospheric jet. We show here that the Korteweg de Vries (KdV) theory is appropriated to describe the structure of the streamlines around the African easterly jet (AEJ) region. The introduction of the additional velocity C1 permits to search the stage where the configuration of the wave will correspond in this zone to those of Rossby solitary waves. It was also shown that the configurations of disturbances can be influenced by this parameter so that we can look if the disturbances are in the control or not of their dispersive effects.Keywords: Soliton; Rossby waves; KdV equation; African easterly wave

Variability of static stability over West Africa during Northern summer 1979-2005

The occurrence and duration of precipitations in West Africa are a function of several local atmospheric processes and conditions. The tropospheric static stability is one of these processes and conditions that determine whether the growing season atmosphere over West Africa is conducive to convection, and thus to rainfall. This study has investigated the spatial variability of static stability in West Africa using 27 years of NCEP/NCAR reanalysis and satellite data. Six stability measures were examined. The relationship of the stability measure field to the convection, represented by Outgoing Longwave Radiation (OLR) field, was investigated by means of a Canonical Correlation Analysis (CCA)

Seasonal variations in the diurnal patterns of convection in Cameroon - Nigeria and their neighboring areas

Precipitations in tropical Africa are highly convective. Therefore, knowledge of the diurnal cycle of convective processes enhances understanding and prediction of rainfall patterns. In this study the satellite database is subsetted to look carefully for the regional variability of the diurnal cycle, and geographical patterns of convection in Cameroon-Nigeria and neighbouring areas. Strong spatial variations exist in the seasonal amplitude of diurnal cycle, highlighting that complex orography, land-sea contrast and coastline curvature play an important role in modulating the spatial and diurnal patterns of convection. Results reveal features that can be used to improve convection simulation in models

Evaluation of Vapor Pressure Estimation Methods for Use in Simulating the Dynamic of Atmospheric Organic Aerosols

The modified Mackay (mM), the Grain-Watson (GW), Myrdal and Yalkovsky (MY), Lee and Kesler (LK), and Ambrose-Walton (AW) methods for estimating vapor pressures ( vap ) are tested against experimental data for a set of volatile organic compounds (VOC). vap required to determine gas-particle partitioning of such organic compounds is used as a parameter for simulating the dynamic of atmospheric aerosols. Here, we use the structure-property relationships of VOC to estimate vap . The accuracy of each of the aforementioned methods is also assessed for each class of compounds (hydrocarbons, monofunctionalized, difunctionalized, and tri-and more functionalized volatile organic species). It is found that the best method for each VOC depends on its functionality

Evaluation of Vapor Pressure Estimation Methods for Use in Simulating the Dynamic of Atmospheric Organic Aerosols

The modified Mackay (mM), the Grain-Watson (GW), Myrdal and Yalkovsky (MY), Lee and Kesler (LK), and Ambrose-Walton (AW) methods for estimating vapor pressures () are tested against experimental data for a set of volatile organic compounds (VOC). required to determine gas-particle partitioning of such organic compounds is used as a parameter for simulating the dynamic of atmospheric aerosols. Here, we use the structure-property relationships of VOC to estimate . The accuracy of each of the aforementioned methods is also assessed for each class of compounds (hydrocarbons, monofunctionalized, difunctionalized, and tri- and more functionalized volatile organic species). It is found that the best method for each VOC depends on its functionality