The Full Picture

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When the amount of precipitation is subtracted from the

amount of evaporation, areas of net evaporation and net

precipitation can be seen. Energy exchanged between the

atmosphere and the surface is an important component of

what is needed to understand how the Earth’s climate system

works, how the climate is changing and for what reason.

Comprehensive direct global measurements of such quan-

tities do not exist, and a key challenge for future reanalyses

is to provide improved estimates in support of studies of

climate change, its prediction and its implications.

Web and print versions of an ERA-40 Atlas have been

published by ECMWF. The Atlas describes the atmospheric

climate for 1979–2001. The climate is presented in maps

for the four seasons, the annual average and the inter-annual

variability. A selection of time series, showing the quasi-bien-

nial oscillation of stratospheric winds, is presented for the

full period 1958–2001. A complementary atlas of the ocean-

wave climate has been produced by the Royal Netherlands

Meteorological Institute, a partner in the ERA-40 project.

Outlook

An ECMWF ‘interim reanalysis’ is currently being produced

using a more advanced (four-dimensional variational) data

assimilation system and higher horizontal resolution than

ERA-40. It covers the period from 1989 onwards and

addresses several of the deficiencies (such as high tropical

oceanic precipitation) identified in products from the later

years of ERA-40. Production of ERA-Interimbegan in 2006

and will reach the present day in 2008, after which it will be

updated on a near-real-time basis. ECMWF is also develop-

ing plans for a new project, called ERA-75, to cover the

period from 1938 to the present using an even more recent

version of the reanalysis system, including provision to cope

better with the poorer data coverage of the pre-satellite years

and poorer quality of early satellite data. The project may

start in 2009, subject to establishing the appropriate funding

scheme and collaboration partnerships. When the new

reanalysis reaches the present day, it may in turn be contin-

ued in near real time, and supersede ERA-Interim.

Key to this effort will be coordinated international efforts

under the Global Climate Observing System (GCOS), the

World Climate Research Program (WCRP) and the wider

GEO umbrella to recover and combine historical observa-

tional data from a wide variety of sources, to manage these

data to modern standards, and to incorporate metadata on

the quality of these data, including that derived from any

use of the data in earlier reanalyses. Continued collabora-

tion among reanalysis centres to exchange experience and

ensure complementary and shared research and develop-

ment programmes is equally vital to ensure that progress is

made in the most effective way, and that the needs of a large

and widening user community are met as fully as possible.

Open access to the products of reanalysis enables users

worldwide to benefit froma specialized and highly advanced

central processing of observations that have been gathered

throughout the world, and easier future access to more

comprehensive and higher-resolution product sets will be

essential if reanalysis is to realise its potential as a key compo-

nent of the Global Earth Observation System of Systems.

Weather analysis, an optimal combination

of observations and model

In a weather analysis each weather element in the free atmosphere is

described in three dimensions. Observations are used to adjust optimally

the representation of elements within an atmospheric forecast model, at

grid points that cover the globe in the horizontal and at a number of levels

disributed in the vertical from the surface to the top of the model

atmosphere. The density of spacing defines the resolution of the model and

of the weather analysis.

Better reanalysis products can be produced by improving the model, the

observations and/or the weather analysis algorithm that uses the

observations to adjust forecast model values. Reanalysis is thus an

iterative process. A new reanalysis can extract new and more accurate

information from observations relating to the Earth’s climate than a

previous reanalysis

ERA-15 1979-1993 125km 31

32km 1995

Resolution

Horizontal

Vertical

Number of

levels

Top level

The design

year of the

system

ERA-40 1957-2002 125km 60

65km 2001

Operations 2006-now 25km 91

81km 2005

North Sea flood of 1953

Near-surface wind (maximum speed about 30ms-1) and ocean-wave height

(metres) from a reanalysis for 00UTC, 1 February 1953, the night of a

devastating North-Sea storm.

The strong winds from the north have a long fetch (the distance over which

wind blows without changing direction) to develop more than nine-metres

high waves over a large area.

The combination of high tide and the high waves had devastating effect. In

order to better understand the effects of the storm, the global reanalysed

fields have also been used as boundary conditions for high resolution limited

area atmospheric models, which in turn drive detailed storm surge models

OCIETAL

ENEFIT

REAS

– W

EATHER