hydrology and water engineering

Examining the future changes in the amount and rate of occurrence of short duration rainfall
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Abstract
The understanding of amount and rate of occurrence of short duration rainfall relationships is key in water engineering applications. The occurrence of extreme rainfall amounts at an increasing rate in the global scale has raised concerns recently. Most of the research conducted shows that much of the increases occur particularly in the short duration rainfall which majorly lasts for about an hour or so. This mainly increases the rate occurrence of flash floods. Thus, this study aims at examining the various evidence that suggests the occurrence of short duration rainfall amounts which is majorly caused by climate change due to human or anthropogenic factors. The study also provides an understanding of the relationship between the short duration rainfall amounts and the atmospheric temperatures. The study also provides a framework of observational and modeling methodology studies that explain the processes that contribute to short duration rainfall amount characteristics. The study provides future directions to facilitate prediction the possibility of changes in short duration rainfall amounts.
Keywords: short duration rainfall amount, future changes, observational, modeling, atmospheric temperatures.
Introduction
Background to the research study
The effects of increasing short duration rainfall amounts due to anthropogenic climatic change factors raise global concerns.

This is because of the natural hazards the short duration rainfall are likely to cause such as the high magnitude floods which are most dangerous worldwide. (Hallegatte et al. 2013). The costs incurred in mitigating the natural hazards caused by short duration rainfall globally are majorly high. For instance, in the year 2011, the cost incurred in resolving flood damages was approximated at seventy billion U.S dollars. ($70b) and the lives lost were around six thousand (6000) people. (Center of Research on Epidemiology Disasters, 2014). Therefore it is very important to understand the behavior changes as well as the impacts of short duration amounts for purposes of supporting and implementing the urban and rural policies of planning and the general designs of infrastructure for the protection against floods. This will help in saving human lives, property as well as minimizing costs that result from effects of short duration rainfall amounts in the future.
The evidence of human or anthropogenic influence on the extreme occurrence of high rates and amounts of short duration rainfall was provided by Min et al. (2011). He provided evidence that links the extremes weather conditions like increasing atmospheric temperatures caused by the emission of greenhouse gases from industrialization activities as anthropogenic climate change factors. Such conditions are likely to cause extreme rates of short duration rainfall amounts in most major industrialized countries. (Alexander, 2006). There is approximately sixty-five percent of the global regions and areas that experience positive trends in the increasing rates of short duration annual rainfall amounts. This is a relatively above average number which in turn raises global concerns since the percentage are still rising daily due to the intensification of the global industrialization. Currently, in the study by Westra et al. (2013), the rates of increase in the amount of daily short duration rainfall was approximated at 7.7% per degrees Celsius of global averages near-surface temperatures of the atmosphere. This averages majorly represents greater occurrences of changes particularly in the high latitudes in the Northern Hemispheres. The lower rates were majorly recorded in the mid-latitudes. (Groisman et al. 2005).
The daily time scale output from the general circulation models also suggest that extremes of short duration rainfall amounts are likely to increase with the increasing rates of global warming. (Meehl, 2007). Also, there is evidence that links the increase in atmospheric temperatures to extremes of short duration rainfall over time scales, of about an hour, which in turn also influences the general distribution of rainfall. (Haerter and Berg, 2009). The evidence indicates that daily extremes of rainfall intensifies and increases more rapidly than rainfall measured in the time scales. This, in turn, has societal effects of floods produced by short duration rainfall, majorly known as the “flash floods.” This types of floods are often more dangerous than the other onset floods due to challenge in providing timely warning and facilitating emergency responses. (Ahern et al. 2005).
There is great importance in understanding the daily rainfall patterns from impact centered as well as a scientific point of view. There are limited information and research that provides a framework on how short duration rainfall will change in future with climatic changes. The reliable, quality and controlled data on global short duration rainfall amounts are still limited. The modeling studies that majorly provide insights and evidence of the physical processes that cause the daily rainfall extremes are still scarce. (Knote et al. 2010). There also exists conflicting and alternative hypothesis about the avenues that results to extremes of short duration rainfall to intensify with atmospheric temperatures with their significant implication in the future predictions. The significance of the localized effects like those of orography has also been identified. This conditions together with rainfall responses, in turn, confound the likelihood to derive tangible conclusions at global scales. (Siler and Roe, 2014).
Research question
The research study will address the question: What are the future changes in the amount and rate of occurrence of short duration rainfall?
Research objective
The major objective of the research study will be to examine and provide a better understanding of the future changes in the amount and the rates of occurrence of short duration rainfall through linking anthropogenic climate change factors like industrialization, global warming and the variations in the atmospheric temperature.
Research methodology
The research study will encompass a deep literature review to examine whether the rates of short duration have increased or are in the process of increasing and intensifying due to anthropogenic climatic change factors. This will be achieved by reviewing the link between atmospheric temperature, global warming, and occurrence of the greenhouse gases to extremes of short duration rainfall amounts. It involves examining the observational studies, which explains how changes have occurred over time as well as the climatic modeling studies how the short duration rainfall have changed as a result of climate change. Also, the relation between extreme amounts of short duration rainfall and flooding effects will be explored.
Eventually, the research study will identify the knowledge gaps that might exist as well as providing a framework for future research direction to improve the capacity for predicting changes in the extremes in the amount of short duration rainfall patterns.
Literature review
The temperatures of the atmosphere greatly influence the rate of short duration rainfall. This is because warm air can hold more water vapor than the cooler air. This implies that the warm air has the potential of providing and releasing more moisture amounts during rainfall events. According to Trenberth (2011), regarding global warming and intensification of extremes of rainfall, it is crucial to understand the relationships that exist between extremes of short duration rainfall and atmospheric temperatures. Thus, the extremes of short duration rainfall amounts are expected to increase in warmer climatic conditions.
Westra et al. (2013) already stated that the available long term data indicates that the median rate of increase of annual short duration rainfall annually is approximated to be at 7.7% per degree Celsius of the average of near-surface temperatures of the atmosphere. Temperature variations are experienced greatly at the high latitudes as well as in the tropics. In a study done by Utsumi et al. (2011) and Mishra et al. (2012), shows that extreme rates of short duration rainfall increase linearly with increase in daily surface air temperatures at the high latitude areas. The same study shows a corresponding decrease in the rainfall amounts especially in the tropics.
233362561277500The atmospheric capacity to hold water vapor related to an equation called the Clausius-Clapeyron (CC) equation. (Trenberth, 2003). As shown below: where, es is the saturation vapor pressure.
Observational studies have indicated that there is a CC relationship between extremes of temperatures of the atmosphere and short duration rainfall, especially an hour. Thus, based on the observational studies, there is evidence that suggests an increase in temperature intensities to hourly duration rainfall at CC rates of about 7% per degrees Celsius.
With regard to examining climatic models, high amounts of short duration rainfall lasting for less than an hour are majorly associated with convective storms. (Hand et al. 2004). This provides a strong indication that short duration rainfall of greater rates and amounts are linked to convection. Using the global climatic models, (GCM), (Kunkel, 2013), found that there is a future increase in the rates and amounts of short duration rainfall, especially within an hour or less.
The potential increases of flooding as a result of climate change and global warming provides a basis for understanding future changes in the amount and rates of occurrence of short duration rainfall. According to Kundzewicz (2005), there are projections of extremes of short duration rainfall intensifying in the future, although there remain significant uncertainties regarding both the magnitude and direction of change to risks associated with flooding. The magnitude and intensity of floods caused by rainfall largely depend on the time of rainfall event, the direction of flow, the speed of travel of flood, the slope as well as the surface vegetation within the catchment area. (Merz and Bloschl, 2003). Also, the interaction between the rainfall and the catchment size is a key determinant for flooding events. Thus, smaller catchments are majorly sensitive to short duration rainfall of about an hour or so while large catchment areas are prone to long periods of rainfall lasting for either days or even longer.
Implications and the future directions of the research
According to Flato, (2013), amounts and the rates of short duration rainfall are predicted to intensify with increasing anthropogenic factors causing climate change. Most of the evidence for future intensification is based on the time scale observational studies as well as the climatic models. The review has discussed few the relationships between short duration rainfall occurrence rates and its relation to atmospheric temperature, as well as the contribution of observational and climatic modeling studies in enhancing the understanding of future changes of short duration rainfall.
However, there will be likely limitations in enhancing the understanding. For instance, the observational data studies of short duration rainfall are limited due to changes in global weather instruments over time. Also, the available standard instruments are scarce, present in locations globally. The studies of climatic modeling experience the difficulty of in resolving the processes the leads to the short duration rainfall extremes.
Thus, in the future research studies, there is a need for developing global sub-daily data for examining the systematic as well as the future changes likely to occur in short duration rainfall amounts. There is a need for improving the techniques used for purposes of detecting future changes. For instances, improving the gauging stations to enhance the provision of high-quality records. Also, for future prediction of changes, the existing global climatic models need to be improved. Also, there is a need for participatory and collective approach by bridging the existing gap between scientist of flooding hydrology and the atmospheric science.
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