Use of satellites at ECMWF

In order to accurately forecast the weather we need to know the state of the current weather in as much detail as possible.

Since the first very simple observations from satellites arrived over 50 years ago it has become clear that only satellites can give a truly global picture of the world’s weather. Current satellite instruments are very complex, often making many thousands of individual observations simultaneously at the same location, and requiring sophisticated techniques to interpret the information they contain. The European Centre for Medium-Range Weather Forecasts (ECMWF) has taken a world-leading role in the interpretation of observations from satellites, and now uses data from more than 50 sensors, and is testing many more. As recently as 2000 this number was just 10, reflecting a large rise in the value and importance of satellite observations in weather forecasting in recent years. Whilst satellite measurements can never give all the information a weather forecast model needs, their impact has grown to become the most important source of data for weather prediction. ECMWF works in close partnership with space agencies to make best use of the data and prepare for future instruments.

0 40 60 80 100 120 140Carbon monoxide (parts per billion)

160 180 200 500 1000 2000

Assimilation of satellite observations to improve weather forecasts

Imagers

Global PositioningSystem sounder

Microwavesounder

Infrared sounder

Satellitewinds

Upper-air

Surface

P

P

P

G

G/P

G = GeostationaryP = Polar-orbiting

ECMWF extracts meteorological information from satellite measurements using a technique called data assimilation, which interprets the often complicated relationship between what satellite observations actu-ally measure and the atmosphere itself. Advances in data assimilation have made it possible to extract much more information from satellite observations.

Satellite data are indirect observations. When we have a thermometer and measure temperature we call this a direct observation of temperature. Many weather observations are direct, whether they be from people’s back gardens, aircraft, ships or weather balloons. Satellite instruments do not directly measure temperature, wind or any weather information. They measure how much the weather changes things which we can measure from space. These indirect observations are just as important and valuable as the direct observations. Our data assimilation system allows us to use both direct and indirect observations.

Observations providing vertical profile information are known as soundings and are very important for weather forecasts. In this figure we show that the two most important types of observations for weather forecasts are soundings. There are direct observations, often simply referred to as ‘upper air’ (from weather balloons and aircraft) and satellite soundings. There are three types of satellite soundings. The Global Positioning System (GPS) sounder is unusual: the GPS navigation signals change very slightly depending on the vertical distribution of temperature and water. We can interpret these small changes to improve weather forecasts. Microwave and infrared satellite sounders are similar, but infrared soundings are difficult to use when there are clouds, whereas microwave soundings work in almost any weather conditions. We can also use satellites to learn about winds, the planet’s surface (e.g. its temperature) and rainfall. Although these observations from satellites are complex to interpret they still contribute to better weather forecasts.

0

10

20

30

40

50

60

70

1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 2016Sentinel 1 Cryosat Suomi-NPP Megha Tropiques Sentinel 3 GOSAT ADM Aeolus EarthCARE SMOS

TERRA/AQUA AMV GMS/MTSAT Rad GOES Rad METEOSAT Rad FY-2C/D AMV GMS/MTSAT AMV GOES AMV METEOSAT AMV HY-2A

Oceansat JASON-1/2/3 QuikSCAT FY-3A/B AURA AQUA TRMM GCOM-W/C GRACE

CNOFS COSMIC-2 COSMIC ENVISAT ERS-1/2 METOP DMSP POESS

Number of satellite data sources used

The number and type of satellite sensors providing observa tions assimilated at ECMWF 1996–2011 and projected to 2016.

How much different observations contribute to better forecasts for tomorrow’s weather.

Analysis and forecast of atmospheric composition is a growing area of activity which also uses satellite data. For example, global carbon monoxide analyses are one of ECMWF’s contributions to the European Union project “Monitoring Atmospheric Composition and Climate”. This uses observations from both European and American satellites and a version of ECMWF’s weather forecasting system developed to include atmospheric chemistry.

4000

10

0

5

5

5000 6000 7000 8000 9000

Hei

ght (

km)

4000

10

15

15

05000 6000 7000 8000 9000

Hei

ght (

km)

CloudSat – Averaged

Simulated re�ectivity

200–20Re�ectivity (dBZ)

Improving the forecast model using satellite observations

Observations not only improve the weather forecast, they can also tell us how reliable the forecasts are. One example is radar data from space. This provides very detailed vertical cross-sections through cloud and rain and ECMWF routinely makes comparisons between these satellite measurements and its predictions.

In the diagram observations from the radar on the CloudSat satellite on 2 January 2007 are compared with the ECMWF forecast. The radar data is sensitive to cloud, and comparisons such as this enable an improved understanding of cloud predictions. If we frequently see the same types of error our scientists use specialist knowledge to interpret the differences and improve the forecasts. This, in turn, eventually improves the model of the atmosphere so that more satellite data can be assimilated.

The role of ECMWF in calibration and validation of new observations

Chinese data before recalibration

45°S

0°

45°N

90°N

180°W 90°W 0° 90°E 180°EChinese data after recalibration

45°S

0°

45°N

90°N

180°W 90°W 0° 90°E 180°E

ECMWF works closely with satellite agencies to inter-pret and improve new observations to ensure the highest possible quality of data is available, not only for ECMWF, but also worldwide. ECMWF has a long history of working with EUMETSAT and ESA, agencies in Japan, in the United States and, more recently, with the China Meteorological Administration.

When new satellites are launched the initial work is to calibrate their instruments and ECMWF works with satellite agencies to establish the best calibration. Partnership between ECMWF and a satellite agency is extremely effective and beneficial. An example is the joint calibration of a new Chinese sounder. Comparison of the observations with the ECMWF forecasts before the post-launch calibration shows large differences. However, after joint research a new calibration is com-pleted and smooth fields with small differences from ECMWF forecasts are found, showing the data to be of very high quality. It is now ready to be assimilated and further improve the accuracy of weather forecasts. This type of analysis is commonly needed before new data is ready to be assimilated.

This plot shows the mean difference between satellite sounding observations from the Chinese microwave sounder and the ECMWF analysis. We expect very small differences with no obvious geographical variation. The ECMWF system can be used to show where there are problems with new observations and can confirm when the data is of good quality. In this example the quality of the data was improved when the China Meteorological Administration introduced a new processing system, and this improvement was quickly confirmed by comparisons at ECMWF.

ECMWF is an intergovernmental organisation supported by more than 30 States. It provides weather services with medium-range forecasts of global weather to 15 days ahead as well as with monthly and seasonal forecasts. ECMWF’s computer system at its headquarters in Reading, United Kingdom, is one of the largest for meteorology worldwide and contains the world’s largest archive of numerical weather prediction data. It runs a sophisticated medium-range prediction model of the global atmosphere and oceans. The National Meteorological Services of Member States and Co-operating States use ECMWF’s products for their own national duties, in particular to give early warning of potentially damaging severe weather.

European Centre for Medium-Range Weather Forecasts (ECMWF)

Shinfield Park, Reading RG2 9AX, United Kingdom

Tel: +44 (0) 118 949 9000

Fax: +44 (0) 118 986 9450

Web: www.ecmwf.int © 2012 ECMWF

Outlook

Some types of satellite data have been used operationally for many years. However, the recent five-fold increase in the number of instruments used at ECMWF means that for some instrument types there is still much research to be done to use all the information they contain. In the next ten years there will be further new instruments, exploiting new parts of the electromagnetic spectrum or making measurements in new ways. With so many instruments, many of which make millions of individual observations every day, new methods to extract the useful information are being developed.

Satellite data collected over the open ocean has always been used beneficially, but we aim to realise the same benefits from observations collected over land or sea ice. ECMWF aims to use more observations from cloudy areas, to improve the understanding of the satellite observations and to further develop the data assimilation system. In summary, satellite data is a crucial part of any modern weather prediction system, and there is vast potential to further expand the type and quantity of satellite data used.

In recent years increasing attention has been paid to observing the land surface in weather forecasting systems. One problem has been that satellite observa-tions can only tell us about the top few centimetres of the soil, whereas we really need to know about the top few metres. However, a dedicated processing chain that uses advanced techniques has been developed by EUMETSAT and ECMWF and this can estimate soil moisture much deeper down. Here we show how much moisture there is in the soil at depths up to one metre. The system uses observations from EUMETSAT’s ASCAT instrument and in the near future will also use data from the European Space Agency instrument called SMOS.

ECMWF can also simulate satellite imagery from its forecasts to evaluate forecasts of cloud. Comparing real imagery (left) with simulated imagery (right) shows that, although there are differences in the detail, the forecast cloud fields have many realistic features.

Copyright © Eumetsat 0

0.2

0.4

0.6

0.8

1

EUMETSAT/ECMWF operational root zone (28–100 cm) soil moisture produced by assimilation of ASCAT surface soil moisture at ECMWF. The colours show how wet the soil is from dark brown (dry) to dark blue (saturated).