Rainfall is the climatic parameter of greatest importance to the populations of the tropical continents. The arrival of monsoon rains drives a rapid transformation of the landscape, allowing crops to grow and river networks to refill. Yet predicting where and when rain will fall in the tropics is a notoriously difficult problem.
Great progress has been made in predicting how remote ocean conditions, such as El Niño, can affect tropical rainfall. However local factors such as vegetation also play a role. Thus when tropical forests are cut down for agriculture, we expect this to affect rainfall both locally, and across neighbouring countries. Indeed, climate scientists have to take into account future deforestation rates as well as greenhouse gas emissions when they assess how tropical climate will change in the 21st century.
Vegetation affects rainfall through the process of transpiration. When plants convert carbon dioxide and sunlight into carbohydrates via photosynthesis, they lose water through their leaves. With their deep roots, trees are able to extract this water from several metres below the soil surface. This allows them to continue photosynthesising for months without rainfall.
Crops and grasses on the other hand can run out of water during dry spells. When this happens, the land surface no longer provides water vapour to the atmosphere via transpiration. Instead the solar radiation absorbed by the plant canopy raises the air temperature. Replacing forests with crops and grasslands changes the rates of moistening and heating of the atmosphere, particularly when the shallow-rooted species start to run out of soil water. These changes in turn affect the development of winds, cloud and rain.
Above: Various cloud regimes above Tai National Park, Côte d'Ivoire. Photos: NASA Worldview
The details of how the atmosphere responds to vegetation is an area of significant scientific debate. There is evidence that clearing patches of forest may increase rainfall over the cleared area and reduce it over the remaining forest, depending on the particular weather patterns. On the other hand, new results have shown that as air masses cross the continent, they pick up additional moisture from forests, which then leads to more rain several hundred kilometres further downwind. Finally, by controlling the balance between heating and moistening of the atmosphere, the vegetation can affect the winds bringing moist air off the ocean, delaying or extending the rainy and dry seasons which characterise tropical climate.
Although these vegetation effects are known to be important individually for predicting weather and climate change in the tropics, there is uncertainty about how they work in combination. We have little confidence in current computer model projections of how climate will change at regional scales. This is partly due to large and long-standing uncertainties in the description of cumulonimbus storms (thunderstorms, which dominate the rainfall of many tropical regions) within the models. However through recent advances in computing power, we are now able to run these models for entire seasons with sufficient spatial detail to properly capture storms.
In this collaborative project between CEH and the University of Leeds we will use data from satellites and the latest weather and climate models to get to the heart of how vegetation affects rainfall. Focusing on West Africa, we will examine cloud and vegetation observations from the last 30 years to detect where deforestation has changed rainfall, and how the rapid greening of the savannah each year affects the monsoon rains. We will perform new computer simulations, incorporating the detailed development of thousands of individual storms, and examine what happens when we artificially deforest a region in the model. We will examine how the less-detailed computer simulations used for climate change projections actually capture the effects of vegetation.
Above left: The forested region of Tai National Park (centre of the image) in a cloud-free satellite image. Image source: NASA Worldview.
Above right: Cloud-free area above the northern part of Tai National Park and forested regions to the west. Image source: NASA Worldview.
The over-arching aim of the project is to quantify the impact of vegetation on rainfall in West Africa via energy and water cycle feedbacks.
To achieve this aim we have three specific objectives:
- To determine the net local and regional effects of deforestation on rainfall
- To identify circumstances where vegetation provides intraseasonal predictability (e.g. dry spells)
- To assess the credibility of different models’ depiction of rainfall responses to vegetation
Our results will help to improve the models used for weather and climate prediction, and provide robust conclusions on deforestation to guide land managers.