This paper used the International Centre for Theoretical Physics (ICTP) Regional Climate Model, Version 3 (RegCM3) and rain gauge data selected from the Ghana Meteorological Agency (GMet) from 1990 to 2008 to investigate the extent and nature of variability in the annual rainfall and pattern of the raining seasons in Ghana. In the study, six meteorological stations selected from three rainfall distribution zones according to the divisions of the GMet were used to study the pattern of rainfall and its departure from the normal trend. The study also assessed the performance of the RegCM3 simulation with reference to the observed gauge data. Results confirmed the unimodal nature of the rainfall annual cycle over the northern belt and bi-modal rainfall nature over the middle and southern belts of Ghana. Negative departures of rainfall implying consistent downward trend were observed at all the stations. Our analysis showed that RegCM3 captured the average rainfall over Ghana but demonstrated an underestimation as compared to the observed gauge data. The model also had difficulty stimulating the departures accurately in direction and in magnitude in all the stations except for Accra where RegCM3 simulated the right direction of the departures.
Ghana shares boundary with the Gulf of Guinea to the south and extends between latitude 4˚N and 11˚N and longitude 4˚W and 2˚E. It is a region that is strongly influenced by the West African Monsoon. It undergoes various variations in climate that are marked by severe droughts and floods. Ghana’s rainfall seasons are mostly influenced by the movement of the tropical rain belt (also known as the Inter-Tropical Convergence Zone, ITCZ), which swings back and forth between the northern and southern tropics in each year. The prevailing wind direction in regions south of the ITCZ is southwesterly, blowing moist air from the Atlantic Ocean onto the continent and the prevailing winds to the north of the ITCZ is known as the Harmattan. The Harmattan originates from the northeast, bringing in hot and dusty air from the Sahara Desert between December and March. As the ITCZ moves north and south over the year, the zones between the northern and the southernmost positions of the ITCZ experience a shift between the two opposing prevailing wind directions. This pattern is known as the West African Monsoon. The northern part of Ghana experiences a single wet season occurring between May and November of the year when the ITCZ is in its northern position and the prevailing wind is south-westerly. The northern part of the region receives 150 - 250 mm of rainfall per month in the peak months of the wet season (July to September). The southern parts of Ghana have two wet seasons: the major season from March to July, and a minor season from September to November [
Studies have shown that the annual rainfall in Ghana is highly variable on inter‐annual and inter‐decadal timescales [
The inhabitants of Ghana depend heavily on rainfall for many activities [
The majority of research work undertaken on rainfall in Ghana focused on selected stations or zones. For example, [
In this paper we group the annual and seasonal variability of rainfall in three rainfall distribution zones in Ghana according to the GMet (
Daily rainfall data from 1990 to 2008 for six synoptic weather stations obtained from the GMet were considered in the study. The data for the chosen period are consistent for all the synoptic weather stations.
the map of Ghana with the distribution of these stations. Navrongo and Tamale stations represent uni-modal rainfall distribution region of Ghana, Kumasi and Wenchi stations represent bi-modal rainfall distributions with two (2) peaks similar in magnitude, and Accra and Axim represent bi-modal rainfall distributions with a higher first peak. For each station, the monthly rainfall values were computed. These values were then used to compute the monthly mean rainfall for the period (1990-2008) of study.
The model data considered in this work is the International Center for Theoretical Physics Regional Climate Model version 3 (RegCM3) from the Coordinated Regional Climate Downscaling Experiment (CORDEX).
RegCM3 is a 3-dimensional, sigma-coordinate, hydrostatic, compressible, primitive equation regional climate model. It was originally developed in the late 1980s, and it was the first limited area model applied to climate studies [
The deviations of the annual rainfall (Equation (1)) from the annual mean rainfall for each station were computed and subsequently, the annual rainfall departures (Equation (2)) for each station over the period of study.
where:
Xij = annual rainfall departure.
xij = the annual total for station i and year j.
͞xi = the annual mean rainfall in station i averaged over the entire 19-year period.
σi = the standard deviation of the annual totals.
The stations were grouped based on their rainfall distribution patterns in Ghana as shown in
Bar and line graphs were plotted to show various forms of information, including trend of average monthly rainfall and rainfall departure. Rainfall departure is a way of measuring the deviation of each of the yearly rainfall values from the mean over the 19-year period.
The RegCM3 has been used for simulation of different meteorological parameters like rainfall, temperature, pressure, humidity, wind field, radiation, soil moisture, surface runoff, cloud fraction etc. In this analysis, the model simulated rainfall is compared with the observed data for 19 years from 1990 to 2008. The average monthly rainfall and the rainfall departure were also calculated for the model data and plotted along with the rain gauge data for good comparison.
This section examines the rainfall variability for a 19-year (1990-2008) period observed rain gauge data and its comparison with the RegCM3 model data.
The results from the rain gauge data confirm that stations with uni-modal rainfall distribution have rainfall periods of April to October with September and October recording the highest amount of rainfall. Stations in the middle belt of Ghana with bi-modal rainfall distribution of almost equal peaks also exhibit similarities in the nature of their rainfall patterns with lower amount of rainfall in August separating the two peaks. Stations in southern Ghana exhibit the bi-modal rainfall distribution and have its first peak higher than the second peak.
Figures 2-7 show the average monthly rainfall pattern from 1990 to 2008 for six selected stations. While the
highest average rainfall at Navrongo was in month of August (
Generally, rainfall increases gradually from March for both Navrongo and Tamale at 6 mm and 26 mm respectively and reaches the maximum in August at 275 mm for Navrongo (
From the RegCM3 data, Navrongo shows a similar rainfall pattern to that of the rain gauge data but grossly underestimates the amount of rainfall in all the months except for March and September where RegCM3 overestimates the amounts of rainfall by 2 mm and 10 mm respectively. At Tamale, RegCM3 rainfall peaks earlier in August at about 139 mm and equally underestimates it amount in all the months except for February and March where RegCM3 overestimates the rainfall amount by about 1mm and 10 mm respectively.
The minor rainfall season starts in September and peaks in October recording 222 mm of rainfall. This city experiences a little dry season between the major and minor rainfall seasons. Similar pattern of rainfall is observed in the RegCM3 data in both Accra and Axim but with a high underestimation of rainfall amount at both stations except for Accra which recorded an overestimation of rainfall amount in September and October where RegCM3 overestimates by close to 3 mm and 8 mm respectively. The case presented in Axim, using the RegCM3, deviates from the normal pattern of rainfall in southern Ghana. Generally, Axim recorded a low rainfall amount over the period of study (1990-2008) as compared to the rain gauge data. This might be due to inconsistencies in the model data over this period, since the model data shows a good correlation with the rain gauge data (
Rainfall in Figures 4-7 is very similar in nature in the period June-October, whereas rainfalls in
Analysis of the regions with the two peaks (the middle belt and southern belt), the first around May/June/July and a second in September/October, saw a relative minimum rainfall in August (Figures 4-7). This rainfall minimum in August by [
From the rainfall departure plots (Figures 8-13), it was realized that rainfall generally recorded negative departures from the mean over the 19-year period in all the stations. The negative departure in the 1990’s may be due to the influence of ENSO on West Africa [
A consistent downward trend implying occasional droughts is observed throughout the years. The study of [
In
[
Station | Correlation coefficient |
---|---|
Navrongo | −0.0176 |
Tamale | 0.0621 |
Kumasi | 0.1264 |
Wenchi | 0.1826 |
Accra | 0.5720 |
Axim | 0.0667 |
From the analysis in the departure in rainfall amount, it was realized that there is a decrease in the accuracy of the model data as compared to the rain gauge data. This is due to the low correlation between the model and rain gauge data. Rainfall from most synoptic stations showed little or no correlation except for Accra which showed a moderate positive correlation of 0.57 between the model and rain gauge data. It is therefore observed in
indicates that there was almost no major change in annual rainfall depths over the period of study (1990-2008).
The study investigated the extent and nature of variability in the annual rainfall and pattern of the rainy seasons in Ghana. An assessment of the suitability of the RegCM3 data commonly used for rainfall variability and trends studies over Africa was carried out over Ghana and compared with rain gauge data from meteorological stations in Ghana. In the analysis, data from six synoptic stations were used as reference for the period 1990-2008.
The results confirmed the uni-modal nature of rainfall over the northern and bi-modal rainfall nature over the middle and southern belts of Ghana. Negative departures of rainfall implying consistent downward trend were observed in most of the stations especially in the 1990s. This has the implication on droughts and even longer dry season [
This research was completed at the Department of Physics, University of Cape Coast, Ghana under the PEER science project under the kind sponsorship of the USAID.