Archive ScienceWatch

Sakari Uppala talks with and answers a few questions about this month's Emerging Research Front Paper in the field of Geosciences. The author has also sent along images of their work.
Uppala Title: The ERA-40 re-analysis
Authors: Uppala, SM, et. al
Journal: QUART J ROY METEOROL SOC, 131 (612): 2961-3012 Part B, OCT 2005
Addresses: European Ctr Medium Range Weather Forecasts, Shinfield Pk, Reading RG2 9AX, Berks, England.
European Ctr Medium Range Weather Forecasts, Reading RG2 9AX, Berks, England.
(addresses have been truncated.)

Why do you think your paper is highly cited?

Atmospheric research in general tries to understand, model, and predict the global hydro-dynamical processes. Climate research tries to understand why and by how much the climate is changing. Both fields need to know the so-called true state of the atmosphere at any one time or as statistics over a long period. Observations of the "true state" have historically been made by in-situ measurements and, during later periods, increasingly by remote sensing instruments.

It has been a common practice to use observations and their statistics as the true state without making a synthesis of the information from different observation types. Since the observation coverage is irregular and varies in time, large "unobserved" areas will cause uncertainties in this global climate diagnosis.

The "ERA-40 reanalysis" paper describes a project, which, by using a data assimilation system, created a global four-dimensional state-of-atmosphere for the period 1957–2002. In the data assimilation, a prediction model was constrained by and navigated through all the historical observations using a variational analysis technique.

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The time series of the model states—each of which is a synthesis of a six-hour model prediction and all available observations—was archived every six hours on a regular grid and includes, in addition to the basic state variables pressure in atmospheric thermodynamic: temperature, wind, and humidity, the time integrals of quantities such as precipitation, evaporation, radiation, etc., as calculated from the various physical processes observed during the model.

The complete ERA-40 archive has been available for use by researchers and the general public since 2003, and the products have been used in an increasing number of studies and applications, from studies of bird migration to the atmospheric forcing of ocean models and detection of long-term climate variations.

Uncertainties are also present in the reanalysis products since the model and the observations used in the synthesis each have their own uncertainties. However, using multiple sources of information helps identify and handle systematic errors during the process.

In order to assess the results, users need to understand the characteristics and uncertainties involved. The high citation rate shows that atmospheric and climate researches are very active research areas. In particular, the citing indicates the good quality of the ERA-40 reanalysis products and the large number of applications using them.

The increasing citation rate is also an indication that the research community has been lacking a good quality reanalysis dataset and that the synthesis made by the data assimilation satisfies the quality criteria accepted by a wide range of scientists for their applications.

Does it describe a new discovery, methodology, or synthesis of knowledge?

The paper describes how, by applying the tools and techniques of modern everyday forecasting to the historical in-situ observations and satellite data, a very important dataset has been created for research. Many discoveries have been made on the long-term behavior of the atmosphere as well as on the strengths and weaknesses in the observing and the data assimilation systems.

The discoveries have lead to international efforts to improve the input observations and, at the European Centre for Medium Range Weather Forecasts (ECMWF), to improve the data assimilation system, which in the future will result in better reanalysis products.

The latest Intergovernmental Panel on Climate Change (IPCC) report, for the first time, refers to reanalysis, indicating their increasing importance in the understanding climate change and its prediction. The examination of the reanalyzed historical atmosphere "simultaneously" by thousands of scientists and also through many applications, exposes a realistic view of both certainties and uncertainties in our understanding of climate.

Would you summarize the significance of your paper in layman's terms?

The paper describes a very important public atmospheric dataset used by thousands of scientists and created by the use of very sophisticated methods. The paper is essentially a recipe book used for generating ERA-40 products, together with important quality considerations. It guides the users to interpret their results and to provide climate information in a physically consistent way for the global community.

How did you become involved in this research and were any particular problems encountered along the way?

First of all, reanalysis is and will always be a team effort and expertise of different fields is needed, as is seen from a glance at the ERA-40 paper author list. In 1979, I joined a group of scientists at ECMWF to create the first global atmospheric dataset for 1979, the FGGE year.

The First GARP Global Experiment (FGGE) observing system was both designed and implemented as a large international effort and its design has remained as a prototype for the modern observing system. The dataset was very popular and used extensively for diagnostic studies. Many scientific discoveries were made, especially in the tropical atmosphere.

"The First GARP Global Experiment (FGGE) observing system was both designed and implemented as a large international effort and its design has remained as a prototype for the modern observing system."

The FGGE analysis was a precursor of today's reanalysis, but applied an older analysis technique over a shorter period. Its products were also far from climate quality. Lack of processing power and difficulties in handling of "large data volumes" (equivalent to the capacity of a memory stick) made the assimilation process much more problematic than today. Since 1993 I have worked at ECMWF with reanalysis, first in the ERA-15 project and later as the project manager for ERA-40.

It is interesting to note that satellite data in the native radiance form has only been assimilated during the past 10 years. The ERA-40 reanalysis, for the first time, used the historical satellite data from 1972 onwards in its native form and was therefore able to extract more information from them than was possible at the time. Both scientific advances and improvements in computer power and data handling capabilities have contributed to this advance. Earlier disagreements on the impact of satellite data have long ago been forgotten and the use of satellite data has contributed enormously to the quality of the ERA-40 reanalysis.

Where do you see your research leading in the future?

The potential of reanalysis has been demonstrated very clearly for atmospheric and climate research. It is difficult to find any better way to digest and transform the vast information in the historical in-situ and satellite observations than can be done by a data assimilation system. Given these resources, both human and computer, we can see reanalysis becoming the main tool for climate research. When the assimilating model takes more of the processes and effects of atmospheric constituents into account—chemical reactions, aerosols, green house gases, etc.—and when it is combined with ocean data assimilation, reanalysis will become increasingly important.

If and when sophisticated data mining tools will be available for public use, reanalysis has all the potential to become as common a utility for environmental monitoring as daily weather forecasts are today.

Do you foresee any social or political implications for your research?

A very interesting and important question, but difficult to answer and quantify exactly. This is recommended as a good research topic to pursue. Since so many applications in various sciences are already using reanalyzed data, implications for both areas are evident.

Concerning the political implications, I hope that in the future it will become easier to find consensus on issues of climate change, because reanalysis is based on the best available observational and model information and these products are used by leading authorities around the world. This is helped by the fact that the uncertainties needed in global assessments will be narrowed further by each new reanalysis.

Although Sakari Uppala has recently left the European Centre for Medium Range Weather Forecasts, and is currently working as a freelance consultant from his home in Finland, he continues to be involved in reanalysis activities at ECMWF.

Sakari Uppala
ERA-40 Project Manager
European Centre for Medium Range Weather Forecasts (ECMWF)

Keywords: ERA-40 re-analysis, atmospheric research, global hydro-dynamical processes, climate research, remote sensing instruments, global climate diagnosis, global four-dimensional state-of-atmosphere, atmospheric forcing of ocean models, detection of long-term climate variations, european centre for medium range weather forecasts, intergovernmental panel on climate change, first garp global experiment observing system, processes and effects of atmospheric constituents, ocean data assimilation.

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2008 : December 2008 : Sakari Uppala