Rainfall Extremes from Satellite-Retrieved Rainfall Estimates and Rain Gauge Measurements - Coursework Example

Supervisors:

Ross Maidment, Thorwald Stein, Carlo Cafaro

Abstract

The climate of a place is characterized by the amount of rainfall the area receives, temperature, humidity, and many other factors. Although most parts of the world have had a precise type of climate with predictable weather patterns, the trend is gradually changing to a point where one cannot quickly tell the annual amount of rainfall the area receives (Lupo, 2020). This is mainly because of climate change that is currently facing the world. Climate change has disrupted the standard weather patterns in fairly all regions. Due to climate change, unpredictable floods continue to hit most countries unexpectedly as the floods might come in a period that has been known to be a low-rainfall or a dry season (Mcaweeney, 2020). On the contrary, a very low amount of rainfall that leads to drought can be experienced in times that are known to be wet or high-rainfall seasons.

Keywords: Extreme rainfall, TAMSAT, rain gauge, West Africa, Ghana, Climate change, floods, drought

Chapter one: Introduction

Different parts of the world experience different types of climatic conditions. As such, the world is divided into several climatic regions. The regions are divided depending on the type of climate experienced (Karpourozs, 2020). The major types of climates are tropical climate, temperate climate, continental climate, equatorial climate, desert and semi-desert climate, and savannah climate. The environment of a place is determined by factors such as the amount of rainfall received annually, the temperature of the area, humidity, pressure, wind, and so on.

1.1 Background

Rainfall plays a big role in the developing countries of sub-Saharan because these countries mainly depend on rainfall for agricultural production. And since agriculture is the backbone for most West African countries and Africa at large, there have been many concerns about the trends of climate change that is being experienced globally (Nouaceur and Murarescu, 2020). These changes lead to extreme rainfall conditions where some parts receive too much rainfall that cause floods, while others experience very little that leads to drought and famine. Ghana, as a country in West Africa, is not an exception to these changes.

1.2 Motivation

Many sources show that West Africa is one of the world's regions that experiences massive rainfall annually. Unlike other areas experiencing a fluctuating amount of Rainfall, West Africa has been receiving continuously adequate rain. However, in the past few years, the country has been reported to experience extreme rainfall conditions whereby unpredicted floods have been experienced as well as drought. This has been a concern to the local weather experts in Ghana and the TAMSAT body that uses a satellite to monitor weather between the tropics (Mcaweeney, 2020). All these bodies come up with reports about the extreme rainfall conditions. The researcher of this dissertation was motivated to discover the reasons behind the extreme rainfall conditions in this region, how the community is affected, and compare the results from both TAMSAT and local meteorological stations to come up with a final report

1.3 Objectives

This research dissertation was carried out basing on some research objectives that were formulated by the researcher. Below are the research objectives which were used by the researcher of this paper to conduct the research.

• to determine the factors behind the environment experienced in the region
• to find out how the climate in Ghana has affected the distribution of human population in the area
• to determine how the extreme rainfall conditions, affect social activities in Ghana
• to identify any changes in the pattern of the climate of the region over the past fifty years
• To investigate the reason behind climate change that is affecting the world at large

1.4 Study Area

Ghana is a country that is in the West of the African continent. It is located along the Gulf of Guinea and the Atlantic Ocean. The country borders Togo in the East, Burkina Faso on the North, Ivory Coast on the West, and the Atlantic Ocean on the South. The country is found a few degrees on the North of the Equator, which means it is a warm country. Ghana is located between longitude 1015’ E and 3015 W and Latitude 4045’N and 110 N. the famous prime meridian passes through Ghana at the port of Tema (Prothero and Boateng, 2019).

1.5 Dissertation Outline

The rest of this paper contains chapter 2, which explains the type of climate experienced in Ghana with much interest in the amount of rainfall received and how the received rainfall affects human activities (Mcaweeney, 2020). Chapter 3 provides information from reviewed literature sources on data and statistics about Rainfall in Ghana. Chapter 4 of this paper describes the research methodology adopted, as chapter 5 provides a discussion on the collected data (Andrew, 2020). The researcher concludes the dissertation in chapter 6 to present their views and recommendations about the research subject.

Chapter two: Overview of Climatology in West Africa, Ghana’s rainfall, and extreme rainfall events

The western part of the African continent is the region that lies to the South of the Sahara while bordering the Atlantic Ocean on the South. According to the United Nations, sixteen countries make up West Africa. The countries are Ghana, Nigeria, Ivory Coast, Mali, Liberia, Burkina Faso, Benin, Gambia, Togo, Niger, Mauritania, Cape Verde, Guinea, Guinea-Bissau, Sierra Leon, and Senegal. This region lies between the tropics, along the equator to be precise. Thus, the area experiences a tropical type of climate.

2.1 West Africa climate

In general, West Africa experiences high humid temperatures with relatively high rainfall per year because it is a low-altitude area. However, different climates patterns are experienced across West Africa. The Northern part of the region, which borders the Sahara Desert experiences semi-arid conditions. This area receives little rainfall of less than 250mm per year (Mcaweeney, 2020). The western and southern part bordering the Atlantic Ocean experiences a tropical monsoon climate characterized by a relatively high rainfall of between 100 cm to 250 cm per annum. The region has two wet seasons: one from March to July and the other from September to November (Nouaceur and Murarescu, 2020). Savannah's type of climate characterizes the Eastern part of the region bordering Central Africa. The area receives moderate rainfall of between 75cm to 150 cm per annum.

2.2 Rainfall climate in Ghana

Ghana is one of the West African countries that experiences tropical climatic conditions. Tropical climate usually has two seasons: wet and dry seasons. The wet season comes to the northern part of Ghana experiences wet season between April and October (Manzanas et al., 2014), while the wet season for the southern part comes between March and November. Annually, Ghana receives rainfall of between 78 cm and 216 cm, which is approximately 31 to 85 inches. The pattern of rainfall in Ghana is brought about by the seasonal migration of the Inter-Tropical Convergence Zone (ITCZ) (GMet, 2020).

2.3 The overview of Extreme rainfall events

Rainfall received in Ghana is irregular. As such, the country ends up experiencing extreme floods and drought during specific periods, which can cause environmental disasters. For instance, a severe drought hit the Sahel region in the early 1970s. This led to the devastating famine in the region. On the other hand, the country was hit by severe floods in 1994, with a volume of 300mm of Rainfall (Tshiala, 2020). This led to the bursting of dams and riverbanks, resulting in loss of lives and property damage. Floods also affected Ghana in 2014 and 2015, where more than ten people died.

2.4 Ghana: Observed and projected extreme rainfall characteristics under climate change

2.4.1 Observed extreme rainfall events

Due to global warming, different parts of the world experienced unusual rainfall. One of the impacts of climate change is desertification. Although Ghana is located where rain is expected to be adequate throughout the years, over the past few decades, the amount received continued to reduce at an alarming rate (Tshiala, 2020). About 35% of the northern part of the country is now a semi-arid area, which was initially a rich agriculture area. The amount of rainfall received has been reduced in most regions over the years due to climate change. However, some parts have experienced a significant increase in the amount of rain received (Jena et al., 2016).

2.4.2 Projected Rainfall characteristics under climate change

In the past 20 years, most parts of Ghana continued to experience very high rainfall amounts, which resulted in floods. Most of the regions experiencing this are those with tropical and equatorial climates. It is expected that the rainfall will continue to be extreme as long as climate change is not handled. However, the northern part of the country will continue to experience low rainfall due to proximity to the Sahara Desert (Al-Hassan, 2009).

Chapter three: Data set-up

3.1 Station rain gauge observations

Several sources reveal the trend of the received rainfall amount recorded by rain gauges in different meteorological stations in Ghana. This data has been used by meteorologists in Ghana to predict the rainfall patterns in the country (Issahaku et al., 2016). However, reports on each region are also established to help in planning and public preparation.

There are approximately six meteorological stations distributed evenly across Ghana. GMet set these stations in the three GMet zones (Asare-Nuamah and Botchway, 2019). The stations are used to investigate changes in the extent, nature, and pattern of rainfall that are received annually in all regions. Research data is obtained from these stations.

3.2 TAMSAT estimates

Gauge stations are inadequate and sparsely distributed in most Sub-Saharan counties. The stations are also not well equipped with technical equipment such as precipitation radar. As a result, a continental body (TAMSAT) was established to help in studying weather patterns by the use of satellites. Although local meteorologists often come up with reports on the climatic conditions, TAMSAT (Tropical Applications of Meteorology using SATellite), an international body, also carries out reports on climatic regions across Africa (Arku, 2012). Data from this body has also been used in this dissertation.

Chapter four: Methodology

Due to limited resources and time frames, this research was conducted through a review of published literature sources. The GMet usually publish their annual reports every year on the amount and variability on rainfall pattern. Similarly, TAMSAT, as a body, also provides their yearly reports on rainfall patterns and variations across Africa. These reports were used by the researcher to make a comparison between the data provided by the two bodies. The researcher provides summary information from the two sources about rainfall.

4.1 Comparison of Rain gauge to TAMSAT

In this dissertation, several literature sources are reviewed to collect data about rainfall patterns in West Africa. Data from both TAMSAT and rain gauge stations were converted to the same spatial resolution to make a comparison. This means the gauge data was first to be interpolated. Accurate interpolation requires a dense set of gauge stations that could not be achieved in this research. It was not possible to access rainfall data records from all the gauge stations across Ghana. Despite this challenge, the research was undertaken using the little accessible reports from a few selected meteorological stations. These reports could still provide a general overview of comparison in question. Data from the gauge stations were resampled, using the inverse distance method, to the same resolution used by TAMSAT to record rainfall data to enable comparison. This method was the most suitable, given the fact that the stations are sparsely distributed.

4.1.1 Daily, Monthly, Annual, Seasonal

• • Daily rainfall data

An analysis was done on the number of days that are likely to receive rainfall every month. Data from both stations and TAMSAT are compared to get a conclusive overview of the same. Although the days vary from one location to another, a comprehensive report is provided.

• monthly rainfall data

The collected data about the rainfall received is grouped into months. The purpose of the grouping is to understand the rainfall distribution across Ghana, which will help the residents plan adequately when carrying out agricultural practices.

• annual rainfall analysis

This analysis gives the overall rainfall received in every year. It shows the trends in rain that is received as years pass. Due to climate change, it is expected that the rainfall received in every year will not be the same. Data for rainfall received in every year are compared from 1981 to 2010.

• seasonal rainfall data

The trends of rainfall received over a defined period of 10 years is analyzed from the data obtained, and a comprehensive report generated. The average rainfall received for the period of consecutive ten years is compared. The data is collected from the annual reports provided by TAMSAT and GMet.

4.2 Analyzing Extreme rainfall events, trends

The researcher examines TAMSAT and GMet to understand if there are extreme rainfall conditions experienced within the last 30 years. Extreme rainfall conditions can be either too much rainfall that results in floods or too little rain resulting in drought and famine (Hoscilo et al.,2014). By analyzing the collected data, the researcher can view abnormalities in the trend and amount of precipitation.

4.3 Validation statistics (RMSE, BIAS, R², POD, FAR)

Validation of the data obtained from both the GMet stations and TAMSAT must be validated to obtain a clear understanding of the exact rainfall changes across West Africa. Validation is done using the following methods.

RMSE

Root Mean Square Error (RMSE) is used in this paper to calculate the distance between data points and regression line data. The method defines how the residuals have been spread out. As such, it shows how the data is concentrated around the line of best interest by verifying the research data. The formula is:

• f represents forecasts
• O represents observed values (known results).
• The average is represented by the bar above the squared differences.

BIAS

There is likely to be a difference between the mean of the observations made and the mean of the forecasts. This difference is what is known as bias. Bias is calculated as a percentage of observation mean. There are two types of Bias calculation: absolute Bias and relative bias.

When the bias is constant to the used range of concentration, then absolute Bias is the best approach to be used. The method is also best when different concentrations will be used in the evaluation and in separate instances. To calculate the absolute bias, the following formula is used.

Bias = Xbb - Xref

Xbb= mean of observations Xref=mean of forecasts

On the other hand, Relative bias best fit in cases where the bias is a percentage of the analytic concentration and the bias estimate is believed to be effective in concentrations at different levels. Relative bias is calculated using the following formula.

Bias (%) = Xbb – Xref

XrefXbb= mean of observations

Xref=mean of forecasts

False Alarm Ratio (FAR)

FAR is calculated by dividing the number of false alarms by the total number of forecasted events. From the contingency 2X2 table, FAR = b/(a+b). There is a difference between False Alarm Ration and Probability of False Detection (POFD) which is sometimes referred to as False Alarm Rate). The Latter is based on observations and not forecasts.

b=number of false alarmsa=forecasted events

Probability of Detection (POD) or Hit Rate

This is calculated by dividing the number of hits by the total number of observed events. POD is calculated from the (2x2) contingency table using the formula.

POD = a/(a+c)

a=number of hitsc=observed events

R squared (R2)

This is the method of validating the goodness of fit of a regression residual. This method is referred to as the coefficient of determination. The goodness of fit is determined by listing squares whose intercept range is between 0 and 1.

Chapter five: Results and discussion

5.1 Daily, Decadal, Monthly, Annual, Seasonal analysis

5.1.1 Daily rainfall analysis

Daily analysis of the research data is analyzed to determine the number of days in every month that are likely to receive rainfall. The data was analyzed and presented in the graph below. The graph shows that July is the month where most days experience precipitation. On the opposite, January has the least days where precipitation is experienced with only one day on average. The graph is obtained from GMet.

5.1.2 Monthly rainfall analysis

The graph below shows some of the data obtained from stations during the research. It shows the distribution of precipitation in Ghana throughout the year. From the graph, it can be said that there are two distinct wet and dry seasons experienced in Ghana (Zhang and Cook, 2014). The first wet season begins in March and ends in July. The second one starts in September and ends in November.

The graphs below show the distribution of rainfall annually across the different regions in Ghana.

Data obtained from GMet

TAMSAT data showing mean monthly rainfall for different regions in Ghana

5.1.3 Annual rainfall

The figure below shows a comparison of annual rainfall received in Ghana from 1981 to 2010. The researcher obtained this data from both TAMSAT and GMet and analyzed them to conclude.

Mean annual rainfall in Ghana for the period 1981-2010 obtained from TAMSAT

It can be concluded from the above image that there has been a significant decrease in the amount of rainfall experienced in recent years. A change in the climate probably causes this.

5.1.4 Decadal comparison

The research used the set of graphs below to explain the obtained results about the trends of precipitation received in decades since 1980. According to the graphs, the rainfall received between 1980 and 2010 was almost uniform. However, the amount gradually slightly increased from 2001 to 2010 (Maidment et al, 2014).

Data obtained from TAMSAT

5.2 Extreme Rainfall events and trends

In the past years, Ghana received adequate rainfall that was fairly distributed throughout the years. The southern part of Ghana was known to have double maxima seasons, while the northern part had a single maxima period followed by a relatively dry season (Sanogo et al., 2015). However, climate change has affected the region in that it is now impossible to predict the wet season, and the dry season—seasons that were expected to receive medium rainfall end up receiving too much rainfall that result in floods. For instance, the month of December, which is expected to be a dry season, received too much rain in 2015, resulting in the loss of lives through extreme floods. Seasons that are expected to receive high rainfall end up receiving too little (Akumaga and Tarhule, 2018). Climate change has majorly led to desertification in Ghana, with 35% of the total land area becoming a semi-arid area.

The leading cause of climate change is global warming. Global warming is caused by human activities such as deforestation and the emission of greenhouse gases (Ian-Bin et al., 2012). These practices destroy the ozone layer, which later leads to a change in the global temperature. Change in temperature has a significant reflection on the amount of precipitation.

Chapter six: Conclusion and recommendations

Most of the regions in the world have experienced climate change in the past few years. The change has affected the amount and pattern of rainfall received. As such, unexpected floods and dry seasons have been experienced. These changes have a great impact on human activities, especially in sub-Saharan Africa, where rainfall is the critical driver of agriculture.

Although climate change has brought adverse effects on human lives, humans have played a huge role in creating the condition. Therefore, humans must address the causes of climate change to solve the problem. Precautions include preserving forests, avoid the use of leaded fuel and other chemicals. Otherwise, worse climatic conditions will be experienced in the future.

1.3 Objectives

This research dissertation was carried out basing on some research objectives that were formulated by the researcher. Below are the research objectives which were used by the researcher of this paper to conduct the research.

• to determine the factors behind the environment experienced in the region
• to find out how the climate in Ghana has affected the distribution of human population in the area
• to determine how the extreme rainfall conditions, affect social activities in Ghana
• to identify any changes in the pattern of the climate of the region over the past fifty years
• To investigate the reason behind climate change that is affecting the world at large

1.4 Study Area

Ghana is a country that is in the West of the African continent. It is located along the Gulf of Guinea and the Atlantic Ocean. The country borders Togo in the East, Burkina Faso on the North, Ivory Coast on the West, and the Atlantic Ocean on the South. The country is found a few degrees on the North of the Equator, which means it is a warm country. Ghana is located between longitude 1015’ E and 3015 W and Latitude 4045’N and 110 N. the famous prime meridian passes through Ghana at the port of Tema (Prothero and Boateng, 2019).

1.5 Dissertation Outline

The rest of this paper contains chapter 2, which explains the type of climate experienced in Ghana with much interest in the amount of rainfall received and how the received rainfall affects human activities (Mcaweeney, 2020). Chapter 3 provides information from reviewed literature sources on data and statistics about Rainfall in Ghana. Chapter 4 of this paper describes the research methodology adopted, as chapter 5 provides a discussion on the collected data (Andrew, 2020). The researcher concludes the dissertation in chapter 6 to present their views and recommendations about the research subject.

Chapter two: Overview of Climatology in West Africa, Ghana’s rainfall, and extreme rainfall events

The western part of the African continent is the region that lies to the South of the Sahara while bordering the Atlantic Ocean on the South. According to the United Nations, sixteen countries make up West Africa. The countries are Ghana, Nigeria, Ivory Coast, Mali, Liberia, Burkina Faso, Benin, Gambia, Togo, Niger, Mauritania, Cape Verde, Guinea, Guinea-Bissau, Sierra Leon, and Senegal. This region lies between the tropics, along the equator to be precise. Thus, the area experiences a tropical type of climate.

2.1 West Africa climate

In general, West Africa experiences high humid temperatures with relatively high rainfall per year because it is a low-altitude area. However, different climates patterns are experienced across West Africa. The Northern part of the region, which borders the Sahara Desert experiences semi-arid conditions. This area receives little rainfall of less than 250mm per year (Mcaweeney, 2020). The western and southern part bordering the Atlantic Ocean experiences a tropical monsoon climate characterized by a relatively high rainfall of between 100 cm to 250 cm per annum. The region has two wet seasons: one from March to July and the other from September to November (Nouaceur and Murarescu, 2020). Savannah's type of climate characterizes the Eastern part of the region bordering Central Africa. The area receives moderate rainfall of between 75cm to 150 cm per annum. Read More