Forecasts and Projections

Weather forecasts are the most widely known class of future information and are usually provided by the NMS. They are used in EWS and other short term (a few days) outlooks. In the last two decades additional forecast systems have been implemented by the WMO to make predictions over  longer time scales. The most advanced systems currently available provides seasonal climate forecasts in the tropics.

SEASONAL FORECASTS

Seasonal forecasts take advantage of the fact that Sea Surface Temperatures (SSTs) influence moisture and atmospheric circulation in the tropics, but tend to evolve more slowly than the atmosphere by several months.As a result, SSTs can be used to predict the overall state of the atmosphere in a given region/climatic system in the coming months. A number of regions have been running seasonal forecasts over past decades and, in Africa, most notable seasonal forecasts are elaborated during the Climate Outlook Fora organized by ACMAD and ICPAC. During such fora the general tendencies of the season are predicted and presented in probabilistic format (Figure 3.2). These predictions can be helpful in planning water resource and on-farm management at national, subnational and individual levels.

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Figure 3.2 Probabilistic seasonal forecast for the June-August 2017 season in West Africa issued by ACMAD.

CLIMATE PROJECTIONS

A wide range of regional and international initiatives allow access to climate change projection data. Some of the portals giving access to projections are listed below:
However, to make best use of the available data and information, the following needs to be considered:

  • Scope of the study – how much climate information is really needed. E.g., general tendencies vs. precise high resolution data for further impact modeling
  • Accuracy of climate models over the region of interest – at local scale GCMs can present strong biases (e.g., precipitation can be strongly underestimated and using direct raw data may lead to biased conclusions with respect to future conditions); a thorough literature review is needed to assess such biases and eventually correct them
  • Uncertainty – is usually assessed through the use of a variety of models and assessment of minimum vs. maximum changes projected
  • Documentation about data source – sufficient explanations with respect to how the data were processed; this is particularly important when working with downscaled data.

DOWNSCALED INFORMATION

Although GCMs are valuable predictive tools, they cannot account for fine-scale heterogeneity of climate variability and change due to their coarse

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Figure 3.3 Many of the processes that control local climate, e.g. topography, vegetation, and hydrology, are not included in coarse-resolution GCMs. The development of statistical relationships between the local and large scales may include some of these processes implicitly. Source: Viner, 2012

resolution. Numerous landscape features such as mountains, water bodies, infrastructure, land-cover characteristics, and components of the climate system such as convective clouds and coastal breezes, have scales that are much finer than 100–500 kilometers. Such heterogeneities are important for decision makers who require information on potential impacts on crop production, hydrology, species distribution, etc. at scales of 10–50 kilometers.
Various methods have been developed to bridge the gap between what GCMs can deliver and what stakeholders require for decision-making. The derivation of fine-scale climate information is based on the assumption that the local climate is conditioned by interactions between large-scale atmospheric characteristics (circulation, temperature, moisture, etc.) and local features (water bodies, mountain ranges, land surface properties, etc.). It is possible to model these interactions and establish relationships between present-day local climate and atmospheric conditions through the downscaling process. It is important to understand that the downscaling process adds information to coarse GCM outputs so that information is more realistic at a finer scale, capturing sub-grid scale contrasts and inhomogeneities. Figure 3.3 presents a visual representation of the concept of downscaling.

Downscaling can be used to increase spatial or temporal resolution. There are two principal downscaling methods:

  • Dynamical: Similar to GCMs, this method involves incorporating additional data and physical processes in regional- or local-scale models, at a much higher resolution than is seen in GCMs. This method has numerous advantages but is computationally intensive and requires large volumes of data and a high level of expertise to implement and interpret. The resources required place this method beyond the capacities of most institutions in developing countries.
  • Statistical: This method involves establishing statistical relationships between large-scale GCM-modeled climate features and local climate characteristics. In contrast to the dynamical method, statistical methods are easy to implement and interpret. They require minimal computing resources but rely heavily on historical climate observations and the assumption that currently observed relationships will carry into the future. However, high quality historical records often are not available in developing countries.

The diversity of existing downscaling methods reflects the diversity of goals of and resources for each downscaling exercise. Thus, there is no single best downscaling approach, and downscaling methods will depend on the desired spatial and temporal resolution as well as the impacts being assessed. In most cases, a sequence of different methods is needed to obtain results at the desired resolution.
When working with downscaled information the following needs to be kept in mind:

  • Information on downscaling and the limitations of the results need to be understood and taken into account. Any results at resolutions higher than the native resolution of the GCMs has undergone downscaling and may mislead the user that the results are automatically more accurate because of their finer scale.
  • Downscaling processes usually adds uncertainty to projections. Additional uncertainty should be factored into the interpretation of the results or indeed into the decision of whether downscaling is appropriate.
  • Downscaling methods should be validated first on historical data to assess their potential biases and uncertainties.

In summary, downscaling is not a trivial process and requires some expertise. It may require significant investment in time, tools and capacity. The user should not believe that the results are automatically more true or valid because they are presented at higher resolution. The need for downscaling and the tradeoff between higher-resolution and increased uncertainty need to be carefully assessed.

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