Diurnal Variability of the Radiative Impact of Atmospheric Aerosols in Ouagadougou, Burkina Faso: A Seasonal Approach

The objective of this work is to study the diurnal evolution of the radiative impact of atmospheric aerosols in an urban city located in the West African Sahel and the correlations with the main influencing factors of local climate dynamics. The simulation was performed using a treatment chain including the GAME code. In the methodology, the atmosphere is modeled by 33 plane parallel layers and the effects of absorption, multiple scattering by particles and gas are taken account. An hour-by-hour calculation of radiative forcing at the top of the atmosphere, in the atmospheric layer and at the earth’s surface was performed. The data used as input are the monthly averages of optical properties, radiosonde measurements, daily synoptic measurements and surface albedo. The results show a parabolic diurnal course of a negative radiative impact at the top of the atmosphere with an extremum at 12 o'clock. Maximum cooling is observed shortly after sunrise and shortly after sunset. The largest annual deviations are noted between the months of March and December with respective maximum cooling values of -34 W/m2 and -15.60 W/m2. On the earth’s surface, a cooling impact is observed with two diurnal peaks at sunrise and sunset, the greatest difference between the diurnal maximums is noted between March (-104.45 W/m2) and August (-54 W/m2). In the atmospheric layer, there is almost constant diurnal warming between 9 a.m. and 4 p.m. The maximum difference between the diurnal extremes is also noted between March (about 85 W/m2) and August (35 W/m2). Likewise, the study of the diurnal warming of the first atmospheric layer showed the extreme values in March (5.6°C) and August (2.4°C), these maximum values being always observed at around 12 o’clock. An analysis of similar works carried out in urban cities in various locations of the world has shown a relatively good accordance with the values obtained. This study highlights the radiative impact of Saharan desert dust, the effect of the local climate and the succession between dry season (November to May) and the rainy one (July to October), as well as the zenith solar angle and human activity.

Long-Term Behaviour of Temperature in the Lower Atmosphere of Niamey a West African Tropical Station

This paper highlights the global warming of the lower atmosphere of West African tropical area using in-situ data. The study is based on the analysis of 500-m interval vertical profiles of radiosonde temperature above Niamey (13.47°N;2.16°E) a sub-Saharan meteorological station. The annual cycle of surface temperatures shows seasonally two peaks located in April/May and in October and two minimum in December/January and August respectively. In the mid-troposphere (between 5 km and 10 km height), time series of monthly mean temperatures from January 2001 to December 2018, shows an annual variability with a slight downward trend of -0.036°C per decade. In the lower stratosphere (25 - 30 km altitude) a cooling of -0.64°C/decade is observed. Temperatures time series also highlight the presence of two breaking points associated with the El Niño-Southern Oscillation (ENSO) events. When performing a separation regarding Southern Oscillation Index (SOI) time series, model parameters of the linear regression indicate a tropospheric warming during the neutral and La Niña phases and a stratospheric cooling. The analysis of the lower stratosphere zonal wind highlights different behaviours of the quasi-biennial oscillation (QBO) during the neutral and La Niña phases.

The Radiative Forcing of Aerosols in a West Africa Sahelian Urban City: Case Study of Ouagadougou

This paper is an assessment of radiative forcing caused by atmospheric aerosols in an urban city in West Africa. It is carried out in Ouagadougou in Burkina Faso and is an illustration of the radiative impact in most of the large Sahelian urban cities which are under the same climatic influences and whose populations present similarities in their socio-economic aspects. Using the GAME code, the radiative forcing was calculated at the top of the atmosphere, in the atmospheric layer and at the earth’s surface. The results showed overall a cooling effect at the top of the atmosphere due to the backscattering in space of the incident radiation, a heating in the atmospheric layer due to the absorption effect and a surface cooling justified by the attenuation of radiation crossing the atmosphere. Using monthly average values of optical properties, vertical temperature and humidity profiles, daily temperatures and surface albedo, the simulation yielded forcing values ranging from -6.77 W/m2 to -2.56 W/m2 at the top of the atmosphere, from 15.8 W/m2 to 34.7 W/m2 in the atmospheric layer and from -41.00 W/m2 to -21.68 W/m2 at the earth’s surface. In addition, the warming was simulated in the first atmospheric layer (in contact with the surface), and the results show values ranging from 0.8°C to 1.8°C. The study of the annual variability of the results showed a strong correlation between the radiative forcing and the seasonal succession characteristic of the climate in West Africa with the extreme values in the month of March (characteristic of the dry and hot season) and in the month of August (characteristic of the rainy season).