Climate processes

Thunderstorms and Climate
Strong systems of precipitation, normally associated with lightning, affect the general structure of the atmosphere. In particular the tropical circulation patterns of the atmosphere are determined by the magnitude of the regions of precipitation. Strong precipitation regions also affect the vertical distribution of temperature and water vapor and thereby the strength of the greenhouse effect. The influence on the atmosphere is not limited to the troposphere, but reaches up into the lower stratosphere at 20 km altitude, where particularly strong tropical thunderstorms can deposit water vapor.

At any given time about 1000 thunderstorms are active somewhere on our planet, with on average 50 lightning discharges per second. ASIM will measure the electrical properties of thunderstorms. Thunderstorms do not only pump water vapor, but also dust and chemical constituents high into the atmosphere where they are distributed over large distances. A warmer climate is expected to foster more and more violent thunderstorms. Although we know they are important for the climate, we know too little of their effects, in particular in the atmosphere above the storms.

Giant lightning and climate
Lightning high above thunderstorms were first discovered in 1989. Today we know several different variations. The discharges most commonly observed from the ground are the Red Sprite, Blue Jets and the Giants. Our knowledge of these discharges is still limited. It is suggested that they will appear more frequently in a warmer climate and that they could modify the chemistry of the upper atmosphere, thereby affecting the radiation balance.

Lightning in the stratosphere (12-50 km) and mesosphere (50-80 km) can be seen with the naked eye, for instance from a mountain top, looking over a thunderstorm system on the horizon. The discharges are truly large, reaching 80 km in altitude and 100 kilometers horizontally as for the case of clustered sprites. It is unknown how much energy the discharges deposit in the atmosphere. Estimates range from 50 kJ to 1 GJ for one Red Sprite.

Red Sprites are observed from the Observatoire Midi Pyrenées, where the Danish Space Research Institute - DTU has fielded cameras. The first Red Sprites over France were photographed in 2000. Danish Space Research Institute heads a large international measurement campaign to observe these magnificent gigantic elelctric discharges over Europe.

The late discovery of Red Sprites, Blue Jets and Giants demonstrates our limited knowledge of the atmosphere above thunderstorms ASIM is equipped with instruments specially designed to measure gigantic discharges and their effect on the atmosphere.

X-and gamma-rays from thunderstorms
Some years ago it was discovered that X- and gamma-radiation is generated within thunderclouds as they charge up. It was also discovered that bursts of radiation were emitted from regions above thunderstorms. These observations motivated a search for radiation also in ordinary lightning and in high-voltage laboratory discharges. Contrary to expectations, radiation has been observed also for these conditions. Measurements in- and above thunderstorms have then led to new insights into the very fundamental process, the "spark".

X- and gamma-radiation is a sign of ionisation in thunderclouds and ions are considered important for condensation of water vapor into cloud drops. The formation and development of thunderclouds are then affected by the electrical processes in the clouds.

There are still no simultaneous observations of lightning, giant lightning and X- and gamma-radiation. ASIM will be the first dedicated scientific mission to try to observe all these at the same time.

The water cycle
Water in the atmosphere can take several forms such as drops in clouds and rain, ice crystals in clouds and snow or just water vapor. Water vapor is important for the climate because it is a greenhouse gas. Since the content of water vapor is strongly dependent on the temperature, water vapor acts as a positive feedback reinforcing a warming trend of the atmosphere. It is estimated that about 40% of the global anthropogenic temperature increase is due to increased water vapor content in the upper part of the atmosphere. This is why we need the best possible understanding of the water cycle in the atmosphere.

The measurements taken by ASIM of thunderstorm clouds and electrical activity can be used to estimate the amount of water vapor pumped high into the atmosphere. These measurements will be compared to measurements of water vapor in the upper atmosphere taken by other instruments on the space station (GPS occultation). Together, the observations will improve our understanding of the atmospheric water cycle, which will lead to improved climate models.

Formation of clouds
Clouds are important for the climate. Some clouds tend to warm the atmosphere and some cool the atmosphere. It is of great importance to understand the physics of clouds and how human activities affect the formation of clouds. For drops to form, water molecules must have something to stick to. This can be small particles (aerosols) in particular of salt, sulfates and dust. The formation of clouds is then dependent of many processes which bring the ingredients up in the atmosphere. These include dust storms, forest fires, industrial pollution, volcanoes and ionising radiation from space.

ASIM will study cloud formation by measuring cloud properties, water vapor content and selected sources of aerosols in the atmosphere. These can be forest fires in South America or dust storms from the Arabian Peninsula blowing westward over Africa.

Noctilucent clouds
Noctilucent clouds can be seen at high latitudes during the summer when it gets particularly cold in the mesosphere at 80 km altitude. They are best seen just after sunset or before sunrise, when the sun still illuminates the clouds at this altitude. It is suggested that noctilucent clouds are crystals of water, but it is unknown where the water comes from.

During the last years, noctilucent clouds occur more frequently and are sighted at lower latitudes reaching down to ~40 degrees latitude, which brings it within range of the ASIM instruments. The increase of the occurrence of noctilucent clouds could be a signature of a colder mesosphere, brought about by the increase of CO2 in the troposphere below, which trap heat at low altitudes. It could also be a signature of increased water vapor transport into the upper regions of the atmosphere, again brought about by a warmer climate.

The dynamics of the upper atmosphere
The solar UV radiation changes the composition of the upper atmosphere. Molecules of oxygen and nitrogen are broken apart into atoms and excited to higher energy states. When the energy is released again, the atmosphere glows, a phenomenon called "airglow". In the night, when the sun sets, the atmosphere continues to glow in narrow layers 10 km thick. This is called "nightglow". From airglow and nightglow observations it is possible to estimate properties of the atmosphere from about 80 to 300 km altitude. Sometimes waves are seen in the nightglow, which are really waves in the neutral atmosphere. Nightglow is like a television monitor that shows the dynamics of the atmosphere at the altitude of the nightglow layer. This technique of studying the atmosphere becauese nightglow occurs at altitudes that cannot pe measured directly by satellites or balloons.

Gravity waves are generated when winds meet mountain ranges or by the powerful updraft in thunderstorms. The waves propagate up into the stratosphere and may reach the mesosphere at 80 km altitude where they modulate the nightglow. It is suggested that gravity waves affect significantly atmospheric circulation in the mesosphere and thus the transport of trace gasses important for the radiation balance of the atmosphere.

ASIM will measure gravity waves in the upper atmosphere At the same time as the electrical activity of thunderstorms below. From the measurements we will be able to quantify the effects of thunderstorms and topological features on gravity wave generation and their transport of energy and momentum to the higher regions of the atmosphere.