New climatology on clouds and radiation from weather satellite data

The properties and distribution of clouds around the globe are important to climate research. There is now a 28-year compilation of clouds and radiation – a cloud climatology – based on data from polar satellites.

The Earth’s climate is largely determined by incoming solar radiation. Clouds help to reflect some of the radiation away.

“Roughly 30 per cent of the sun’s radiation is reflected away from the Earth, and most of that radiation is reflected away by clouds. Less than 10 per cent of the total radiation is reflected by the Earth’s surface,” says Karl-Göran Karlsson, a remote analysis scientist at SMHI and one of the people taking part in the project.

Cloud types and climate effects

In climate research, the ability to describe clouds and their properties is one of the most significant factors for the results from the various climate models.

“For climate research, it is important to know what kind of cloud you are looking at, as different cloud types affect the climate in different ways. Low clouds, which are made up of water drops, help reflect solar radiation away and cool the Earth, while thin, high cirrus clouds let solar radiation through and strengthen the greenhouse effect,” says Karl-Göran Karlsson.

Graphics: A compilation of the Earth’s mean cloudiness 1982-2009
A compilation of the Earth’s mean cloudiness during the period 1982-2009. Dark blue indicates clear weather, while dark red indicates completely overcast.

Clouds in climate models

For climate scenarios to be trustworthy, it is important that climate models describe current and past conditions as realistically as possible in terms of mean conditions and variability. The new global cloud climatology improves the opportunity to produce reasonable simulations of cloud and surface reflection of solar radiation over the Arctic and Antarctica during the polar summer.

The cloud climatology also adds high-quality data for medium and high latitudes, and for the tropics. The entire compilation is unique in that it encompasses the entire available time span that can be compiled – 28 years – based on the US National Oceanic and Atmospheric Administration’s (NOAA) complete archive of historical polar weather satellite data.

There are still some limitations in the cloud climatology of subtropical land areas and over the polar areas during the winter. A further improved dataset will be produced within four years (by 2016).

Dataset freely available

The CLARA-A1 dataset is now available to research groups and national climate services. To facilitate evaluation of climate models using these data, a special tool called the CLARA-A1 simulator is also under development.

About the dataset CLARA-A1

The CLARA-A1 dataset (the CMSAF cLoud, Albedo and RAdiation dataset) is a global dataset of cloud, surface albedo and surface radiation products derived from measurements of the Advanced Very High Resolution Radiometer (AVHRR) on board the polar orbiting NOAA and Metop satellites. Monthly and daily mean products have been compiled over a time period of 28 years starting in 1982 and ending in 2009.

The dataset was compiled by the EUMETSAT Climate Monitoring SAF project (CM SAF) where SMHI has been responsible for the work with cloud products in co-operation with scientists from KNMI and DWD.

CLARA-A1can be accessed and ordered at