The socio-economic sector of West African countries is rain-fed agriculture driven. Information regarding the onset, cessation and duration of the rainy season is thus, very essential. In this paper, a comparison of the onset, cessation and duration of the rainy season has been carried out using simulated rainfall data from the fourth generation Regional Climate Model (RegCM4) and rain gauge measurements from Ghana Meteorological Agency (GMet), covering a period of 1998 to 2012. Similar onset and cessation dates were seen in both the simulated and guage rainfall measurements for the various agro-ecological zones, resulting in similar duration of the rainy season. The average duration of the rainy season were less than 200 days for the savannah and coastal zones whereas the duration of the rainy season were beyond 200 days for the forest and transition zones. The bias of these comparisons was less than 30 days and the root mean square error (RMSE) values were less than 15 days for all stations, except Saltpond. The Pearson’s correlation (r) typically ranged between 0.4 and 0.8. However, negative correlations were observed for Tamale in the savannah zone, and the entire coastal zone. These findings are indications that RegCM4 has the potential to clearly simulate the movement of the rain belt, and thus, could fairly determine the onset, cessation and duration of the rainy season. The findings have significant contributions to effective water resource management and food security in Ghana, as the thriving of these sectors depend on the dynamics of the rainfall seasons.
Rainfall is a major factor that determines crop choice and yield, as well as, success of other socio-economic activities in West Africa. The variability of rainfall onset, cessation and duration affect planting (sowing) dates, crop growth, yield and food production [
In an earlier paper, Amekudzi et al. (2015) [
Although numerous onset and cessation studies have been carried out, very few utilized an integrated network of gauge and regional climate models (RCMs). RCMs, in rainfall analysis over West Africa, have the potential to enhance prediction of onset, cessation and duration of the rainy season. For example, RegCM4 allows for the capturing of detailed climate variations over shorter spatial scales [
The goal of this paper is to inter-compare the onset, cessation and duration of the rainy season for Ghana using RegCM4 simulation and gauge data. This will provide the opportunity to test the potential of RegCM4 in simulating the rainfall events over West Africa. The paper is structured as follows: datasets and the study site are described in Section 2 with methodology given in Section 3. The results are discussed in Section 4 and finally, the conclusion of this study is presented in Section 5.
The study area, Ghana, is divided into four agro-ecological zones (savannah, transition, forest, coast) based on GMet’s classification [
Daily rainfall data from twenty (20) synoptic stations over Ghana (see
Furthermore, RegCM4 was used to simulate rainfall for the 20 respective stations. RegCM4 is a regional climate model of the Earth Science Department of the Abdus-Salam International Centre for Theoretical Physics (ICTP) [
The fuzzy-logic approach was used in determining the onset, cessation and duration of the rainy season. The onset date for the rains was defined as the first day of a 5-day period with a total of at least 20 mm of rainfall recorded. Also, the starting day and at least two other days in this 5-day period must be wet (at least 1 mm of rainfall recorded), with no dry spell of seven or more consecutive days occurring in the subsequent 30 days [
where OD is the onset date and D is the total number of days in first month with effective rain (MER: the accumulated rainfall totals equal or exceeds 20 mm). F (mm) is the accumulated rainfall total of earlier months and R is the accumulated rainfall within the MER.
Rainfall cessation dates were also calculated using Equation (2), given by
where CD is the cessation date and is defined from the equation as any day from 1st October after which there are more than 7 consecutive days of rainfall with amounts below 50% of the soil water requirement and b denotes the number of days in which there is maximum build-up of pre-season moisture. The duration of the rainy season is the difference between the cessation and onset, which is expressed as (CD − OD + 1).
Three statistical measures, described in [
where x is the gauge data, y is the RegCM4 model data, μx and μy are the arithmetic mean of x and y respectively and Sx and Sy are the standard deviation of x and y respectively.
The onset of rains was determined from both the gauge and the RegCM4 model rainfall data over the 20 synoptic stations. For the stations in the savannah zone, the mean rainfall onset dates were determined to be the third dekad of April and the first dekad of May for the gauge and model respectively (see
Station | GMet Mean OD | RegCM4 Mean OD |
---|---|---|
Abetifi | 91 ± 10 | 93 ± 10 |
Accra | 123 ± 07 | 112 ± 05 |
Ada | 116 ± 07 | 110 ± 05 |
Akatsi | 109 ± 07 | 76 ± 07 |
Akim Oda | 74 ± 07 | 115 ± 05 |
Axim | 87 ± 10 | 85 ± 05 |
Bole | 100 ± 05 | 110 ± 05 |
Ho | 70 ± 07 | 76 ± 07 |
Kete Krachie | 121 ± 07 | 105 ± 05 |
Koforidua | 94 ± 05 | 93 ± 05 |
Kumasi | 80 ± 07 | 83 ± 05 |
Navrongo | 132 ± 05 | 120 ± 05 |
Saltpond | 112 ± 07 | 61 ± 05 |
Sunyani | 83 ± 05 | 115 ± 05 |
Takoradi | 104 ± 10 | 86 ± 05 |
Tamale | 127 ± 07 | 128 ± 05 |
Tema | 120 ± 07 | 112 ± 05 |
Wa | 111 ± 05 | 130 ± 05 |
Wenchi | 90 ± 07 | 88 ± 05 |
Yendi | 92 ± 05 | 123 ± 05 |
On the whole, earliest onsets were observed at Akim-Oda and Ho in the forest zone whereas the model identifies the earliest onsets at Saltpond in the coast and Akuse and Ho in the forest zone (see
The mean rainfall cessation dates, for the savannah zone, were determined to be the second dekad of October for both the gauge and RegCM4 model. The model rightly captured the cessation of rainfall in the savannah zone. In the forest zone, the mean cessation was found to be in the second dekad of November for the gauge and the first dekad of November for the RegCM4 model. The cessation dates are similar and again this is an indication of better performance of the model in estimating the cessation of rains in the forest zone. Furthermore, the mean rainfall cessation dates were also identified to be within the third dekad of November for gauge and the second dekad of November for the model. Finally, the first dekad of November and the third dekad of October were observed as the mean cessation dates for the gauge and model respectively, in the coastal zone. The model again underperformed in estimating the cessation date of the rainy season for the coastal zone. Rainfall cessation, over the entire country, was observed after September. The latest cessation, as observed from the gauge, was at Wenchi and Kete Krachi in the transition zone. The RegCM4 model also identified Wenchi as the station with the latest cessation (see
As represented in
Station | GMet Mean OD | Duration (days) | GMet Mean CD |
---|---|---|---|
Abetifi | 91 ± 10 | 226 | 317 ± 07 |
Accra | 123 ± 07 | 187 | 313 ± 05 |
Ada | 116 ± 07 | 194 | 310 ± 05 |
Akatsi | 109 ± 07 | 184 | 293 ± 07 |
Akim Oda | 74 ± 07 | 240 | 314 ± 10 |
Axim | 87 ± 10 | 239 | 326 ± 10 |
Bole | 100 ± 05 | 215 | 315 ± 05 |
Ho | 70 ± 07 | 250 | 320 ± 10 |
Kete Krachie | 121 ± 07 | 213 | 334 ± 05 |
Koforidua | 94 ± 05 | 220 | 314 ± 10 |
Kumasi | 80 ± 05 | 244 | 324 ± 10 |
Navrongo | 132 ± 05 | 183 | 315 ± 05 |
Saltpond | 112 ± 07 | 202 | 314 ± 05 |
Sunyani | 83 ± 05 | 236 | 319 ± 10 |
Takoradi | 104 ± 10 | 222 | 327 ± 05 |
Tamale | 127 ± 07 | 185 | 312 ± 05 |
Tema | 120 ± 07 | 187 | 308 ± 05 |
Wa | 111 ± 05 | 194 | 306 ± 05 |
Wenchi | 90 ± 07 | 242 | 332 ± 07 |
Yendi | 92 ± 05 | 203 | 315 ± 05 |
Station | RegCM4 Mean OD | Duration (days) | RegCM4 Mean CD |
---|---|---|---|
Abetifi | 93 ± 10 | 192 | 284 ± 05 |
Accra | 112 ± 05 | 196 | 308 ± 05 |
Ada | 110 ± 05 | 194 | 304 ± 05 |
Akatsi | 76 ± 07 | 203 | 279 ± 05 |
Akim Oda | 115 ± 05 | 188 | 303 ± 05 |
Axim | 85 ± 05 | 246 | 330 ± 05 |
Bole | 110 ± 05 | 179 | 289 ± 05 |
Ho | 76 ± 07 | 223 | 299 ± 07 |
Kete Krachie | 105 ± 10 | 201 | 306 ± 05 |
Koforidua | 93 ± 10 | 192 | 285 ± 05 |
Kumasi | 114 ± 07 | 236 | 319 ± 05 |
Navrongo | 120 ± 05 | 181 | 301 ± 05 |
Saltpond | 61 ± 05 | 264 | 325 ± 05 |
Sunyani | 115 ± 15 | 200 | 315 ± 05 |
Takoradi | 86 ± 05 | 232 | 318 ± 05 |
Tamale | 128 ± 07 | 160 | 288 ± 05 |
Tema | 112 ± 05 | 186 | 298 ± 05 |
Wa | 130 ± 07 | 171 | 300 ± 05 |
Wenchi | 88 ± 15 | 249 | 306 ± 05 |
Yendi | 123 ± 07 | 165 | 288 ± 05 |
mean rainfall within these zones are normally observed to be between 740 mm and 890 mm, which supports very little rain-fed agriculture. Rainfall patterns for the forest and transition zones are bi-modal and rainfall values are typically higher than that of the coastal and savannah zone. However, the peak rainfall of the transition is mainly in September and that of the forest is in June. These zones tend to have a longer growing season and as such, form the productive zones of the country in terms of cash crop and food production [
The upper panels of
Zone | GMet Mean OD | Duration (days) | GMet Mean CD |
---|---|---|---|
Savannah | 117 ± 05 | 196 | 313 ± 05 |
Transition | 98 ± 07 | 230 | 328 ± 07 |
Forest | 86 ± 07 | 234 | 320 ± 10 |
Coastal | 117 ± 07 | 191 | 307 ± 05 |
Zone | GMet Mean OD | Duration (days) | GMet Mean CD |
---|---|---|---|
Savannah | 122 ± 05 | 171 | 293 ± 05 |
Transition | 103 ± 10 | 216 | 319 ± 05 |
Forest | 90 ± 07 | 216 | 306 ± 05 |
Coastal | 94 ± 05 | 209 | 302 ± 05 |
dates typically ranged between 0.4 and 0.8. Negative correlations were observed for Tamale and the entire coastal zone. These are likely attributable to the poor performance of the RegCM4 model in determining the rainfall onsets over the aforementioned locations. The correlation for the cessation was generally low over the country. However, Akuse, Wenchi and Navrongo showed higher correlations.
In this paper, we investigated the onset, cessation and duration of the rainy season using both simulated rainfall data from RegCM4 model and GMet gauge data, over Ghana from 1998 to 2012 using the fuzzy-logic approach. The mean onsets from both the gauge and model were found to be within the second dekad of March and first dekad of May countrywide. On the other hand, the mean cessation was identified from RegCM4 to be in the third dekad of October whereas the gauge found the third dekad of October to the second dekad of November as the mean cessation date. Furthermore, the average duration of the rainy season was found to be less than 200 days in the savannah and coastal zones whereas the transition and forest zones recorded more than 200 days (see
These findings support the forth and back migration of the ITD, which regulates the pressure system of West African Monsoon. In addition, topography and nature of land cover are major moderators of convective activities, and consequently, rainfall onsets and cessation. These factors were accounted for in RegCM4. Further study to assess the contributions of thermodynamic processes and other local effects is under consideration. These findings have very useful applications for water resource management, food security and other socio-economic activities in Ghana, considering their strong dependence on the dynamics of the rainfall season.
We thank the Abdus-Salam International Center for Theoretical Physics (ICTP) and Ghana Meteorological Agency (GMet) for provision of RegCM4 simulation and guage data respectively.
Caleb Mensah,Leonard K. Amekudzi,Nana Ama B. Klutse,Jeffrey N. A. Aryee,Kofi Asare, (2016) Comparison of Rainy Season Onset, Cessation and Duration for Ghana from RegCM4 and GMet Datasets. Atmospheric and Climate Sciences,06,300-309. doi: 10.4236/acs.2016.62025