Diurnal and seasonal variations of the incoming solar radiation flux at a tropical station, Ile-Ife, Nigeria

Abstract

The diurnal and seasonal variations of the incoming solar radiation have been studied by analysing two years data measured between January, 2016 to December, 2017 at a Tropical station, Ile-Ife (7.53°N; 4.54°E), Nigeria. The maximum incoming solar radiation flux which occurs between 13:00 – 14:00 LT and varies in the course of the year from 639.5 ± 171.6 Wm -2 (with large fluctuations) in the wet months (March – October) to 700.7 ± 105.2 Wm -2 in the dry months (November – February). The large differences in the values, diurnal and seasonal variation of the measured incoming solar radiation between the dry and wet seasons are attributed to the attenuation of the flux by aerosol particles in the dry season and increased cloudiness and humidity in the wet season. The monthly maximum values of 760.3 Wm -2 and 732.8 Wm -2 indicated a double peak from March to May and October to November respectively while a minimum of about 492.7 Wm -2 was recorded from July to August. Similarly, the daytime average had a double peak of 412.5 Wm -2 and 361.3 Wm -2 in March/April/May and October/November respectively, equally a minimum value of about 249.8 Wm -2 was recorded in July/August. The maximum value of the air temperature (which occurs around 15:00 LT) was observed to lag behind the maximum value of the incoming solar radiation (which occurs around 13:00 LT) by 2 hours at the study site. The statistical analysis of the monthly daytime averages of the incoming solar radiation showed that the intensity of the flux received at Ile-Ife (a tropical location) is high (about 67% of the incoming solar radiation are between the interval 325 and 400 Wm -2 ) throughout the year.

Evaluation and Calibration of Downward Longwave Radiation Models under Cloudless Sky at Ile-Ife, Nigeria

“…The best results were obtained from Idso (1981) and Niemela, Raisanen and Savijarvi (2001) pressure and air temperature by all the models (Iziomon, Mayer and Matzarakis, 2003). Also, the under/overestimation of the downward longwave radiation flux by the models when compared to the measured values at the study site can be attributed to the differences between the geographical location, cloud formation, precipitation, atmospheric turbulence and pollution levels of Ile-Ife and the various sites where all the models' coefficients were developed (Sozzi et al, 1999;Alaa, Ali and Yaseen, 2017).…”

“…This intersection migrates and attains its most northerly position between 22.25°N in August (Okogbue, 1997 (Okogbue, 1997;Falaiye, Babatunde and Willoughby, 2014;Soneye, 2018). In this area, the sunrise time and sunset time are around 07:00 LT and 19:00 LT respectively (Ayoola et al, 2014;Soneye et al, 2019) while the yearly average air temperature and relative humidity are about 26.7 °C and 80.1 % respectively. Mean yearly rainfall ranges between 1000 and 1500 mm with weak surface wind flow less than 1.5 ms -1 (Hayward and Oguntoyinbo, 1987;Ayoola et al, 2014;Soneye et al, 2019).…”

“…In this area, the sunrise time and sunset time are around 07:00 LT and 19:00 LT respectively (Ayoola et al, 2014;Soneye et al, 2019) while the yearly average air temperature and relative humidity are about 26.7 °C and 80.1 % respectively. Mean yearly rainfall ranges between 1000 and 1500 mm with weak surface wind flow less than 1.5 ms -1 (Hayward and Oguntoyinbo, 1987;Ayoola et al, 2014;Soneye et al, 2019). At Ile-Ife, the intensity of global solar radiation received at the surface is high all year round with maximum values of 1100 Wm -2 (at 13:00 LT) for the month of March, and 800 Wm -2 (at 13:00 LT) for the month of August (Balogun, Jegede and Olaleye, 2003).…”

“…March/April to October and November to February, respectively (Griffiths, 1974;Ayoola et al, 2014;Soneye et al, 2019). In this area, variations and changes in the pattern of the season are as a result of the meridional (North -South) movement of the Inter-Tropical Convergence Zone (ITCZ) across West Africa.…”

“…The calibration constants of the net radiometer and Vaisala probe provided by the manufacturer were used. All the sensors were recalibrated before and after measurements and no significant drift was observed in the calibration constants of the instruments after the measurement period (Soneye, 2018;Soneye et al, 2019). In this study, the cleaning of all the sensors was performed on a regular basis by cleaning their lenses and domes gently with a moistened cotton swab dipped in distilled water and alcohol to avoid the scratching of the thin optical coating on the window.…”