No abstract

No abstract

Because the rain rate R is easier to observe than the drop size distribution (DSD), it is useful to express the DSD as a function of R only, i.e. N(D,R). Disdrometer data from four sites in West Africa are used to compare observed DSDs with fitting to a lognormal function in which the three parameters X i , with i = 1, 2, 3, are expressed as functions of R only. Observed DSDs are found well represented by such N(D,R) function. Writing the N(D,R) moments for a calculation of useful physical quantities, such as water content M, rain rate, or radar reflectivity factor Z, raises constraints of self consistency of the various equations. Modifying the analytical form of DSD in order to take into account the constraint of self-consistency is not found to increase significantly the quality of the DSD fitting with respect to the direct use of X i (R) in the distribution, ignoring the lack of self-consistency. Then Z-R relations obtained by direct regression of observed data are compared with the relations derived analytically from N(D,R) and the expressions X i (R) observed in each site. The two groups of relations compare very well.

The study is focused on the water diffusion phenomenon through the Raffia vinifera fibre from the stem. The knowledge on the behavior of those fibres in presence of liquid during the realization of biocomposite, is necessary. The parameters like percentage of water gain at the point of saturation, modelling of the kinetic of water absorption, and the effective diffusion coefficient were the main objectives. Along a stem of raffia, twelve zones of sampling were defined. From Fick's 2nd law of diffusion, a new model was proposed and evaluated compared to four other models at a constant temperature of 23°C. From the proposed model, the effective diffusion coefficient was deduced. The percentage of water gain was in the range of 303–662%. The proposed model fitted better to the experimental data. The estimated diffusion coefficient was evaluated during the initial phase and at the final phase. In any cross section located along the stem of Raffia vinifera, it was found that the effective diffusion coefficient increases from the periphery to the centre during the initial and final phases.

No abstract

This paper investigates the relationship between the Normalized Difference Vegetation Index (NDVI) and extracted rainfall in the Global Precipitation Climatology Project (GPCP) in Central Africa between latitudes 15• S and 20• N and longitudes 0 • E and 31• E. Monthly NDVI and GPCP datasets for the period 1982-2000 have been used.The Index of Segmentation of Fourier Components (ISFC) has been applied on the NDVI dataset to segment Central Africa into four bioclimatic ecoregions (BCERs). In order to compare the differential response of vegetation growth to rainfall, an analysis of the inter-annual, intra-annual and seasonal variability for each BCER has been carried out, and the correlations between NDVI and rainfall have been assessed. The plot of the annual cycles of both variables revealed a coherent onset, peak and decay, with a time lag of 1 month for almost all the zones, except the zones, semi-desert and steppe, where a season of short and intense rainfall was observed. The correlation coefficients computed between the two variables are relatively high, especially in brush-grass savannah, where they reach up to 0.90 at a time lag of 1 month. The phenological transition points and phases show that the rangeAtmosphere 2013, 4 412 between the +1 and −1 time lags corresponds to the duration of the maturity of vegetation.Overall, there is a strong similarity between temporal patterns of NDVI and rainfall, showing that the NDVI can be considered a sensitive indicator of the interannual variability of rainfall.

The area-time integral (ATI) method has previously been successfully used to estimate the area-averaged rain-rate distribution and the rainfall volume over an area from radar or from satellite infrared (IR) data. In most cases, the method was implemented over regions or test areas with an assumed homogeneous climatic character, that is, without a strong spatial variation of the rain regime throughout the test area. In the present paper, the behavior of the ATI method is discussed for a test area displaying two strong gradients of the cumulative annual rainfall: one meridional, at the transition between regions having, respectively, a desertic and an equatorial climate and the other zonal, at the transition between land and sea. The studied area is divided into four subtest areas (north, south, land, and sea) over which the ATI computation is applied separately. The linear coefficient relating the radar-observed area-averaged rain rate and the fractional area where the rain is higher than a threshold calculated over the four subtest areas is found to be almost constant, in agreement with the ergodic character of the rain-rate distribution observed in this region. Similarly, the linear coefficient relating the rain volume over the subtest areas to the IR satellite-derived ATI, a parameter analogous to the Geostationary Operational Environmental Satellite (GOES) Precipitation Index (GPI), is found to be very steady, with a mean value of 3.02 mm h Ϫ1 and a coefficient of variation of only 8%. These coefficients, as well as the underlying dynamic and microphysical processes, do not seem significantly influenced by the climatic character, even at a short space scale, in the studied area. The ratio of radar rain areas to cloud areas is, notably, almost constant. For a brightness temperature of 235 K, the ratio of the cloud area to rain area is around 1.68.FIG. 9. Plot of the linear coefficient G( TB ) of (7) vs TB for the 1996-99 dataset for the four subareas in Fig. 4. The calculation has been made for R ϭ 5 mm h Ϫ1 .

High-resolution (5-km, 30-min) Meteosat 7 infrared images are used to document seasonal variations of the diurnal cycle of convective activity over Cameroon and Nigeria during the period 1998-2002. Cloud fraction, considered to diagnose the properties of cold clouds, is derived by using a threshold of brightness temperatures less than 235 K, thus providing a detailed view of regional differences in the diurnal patterns. The features of the diurnal cycle of convection were investigated for six sampled regions. Over land, the diurnal cycle of convection is mainly controlled by the radiative heating and associated low-level destabilization. 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. There is consistent noon diurnal peak without obvious seasonal variations of the phase over the ocean.