The atmosphere is a layer of gases surrounding our planet, kept in place by its own weight under gravity.
Where weather happens
Most of our weather takes place in the lower layer of the atmosphere, the first 16 km (about 9 km at the poles), known as the troposphere. The layer above the troposphere is known as the stratosphere. To forecast the weather, we need to first understand the processes which affect the atmosphere and then quantify and simulate them in a numerical forecast model.
Atmosphere processes are complex
The atmosphere is a fluid and its motions are governed by the effects of pressure, rotation, gravity, friction and microphysical processes such as condensation, evaporation and precipitation. Atmospheric processes occur on a very wide range of spatial scales - from the smallest gust of wind to weather patterns as large as continents.
In numerical models, the processes are partitioned into two categories:
Large-scale: those that are large enough to be represented by the model directly
Small processes: those that are too small and whose effects must be represented indirectly by further equations and approximations known as parameterization.
The large-scale motions are dominated by the effects of pressure, rotation and gravity. These can be represented by fluid dynamics and thermodynamic equations in the forecast model.
There are some processes which are too small to be resolved by the model and whose effects must be represented by further equations:
Orography - the effects of hills and mountains
The boundary layer, also called the atmospheric or planetary boundary layer, is the part of the atmosphere which is directly influenced by the surface.
The characteristics of the boundary layer depend on the underlying surface and the time of day.
They vary in two main ways:
Unstable boundary layers - the surface transfers heat to the atmosphere, usually during the day over land.
Stable boundary layers - the atmosphere transfers heat to the surface, usually during the night over land.
Understanding the state of the boundary layer is crucial in forecasting weather conditions at, or near, the surface, such as the formation and dissipation of fog, or extremes of temperature.
Clouds significantly affect how nature balances the energy of the atmosphere around the Earth. This energy distribution helps determine how weather systems move and develop around the globe. For accurate predictions of both weather and climate, clouds must be well represented in our computer models.
To understand clouds and how precipitation (rain, snow and hail) forms within them we use both observed evidence and theories.
A lot of evidence is gained from the specialised aircraft FAAM (Facility for Airborne Atmospheric Measurement) BAe 146. This plane has instruments for measuring the number, size and chemical composition of aerosol particles, cloud droplets and ice crystals within clouds.
Undulations on the surface of the Earth of any size and shape, from small hills to major mountain ranges that span continents, are known collectively as the orography of the Earth.
Orography affects the weather in a variety of ways, both on a local and global scale, including:
enhancement of precipitation and increased wind speeds over mountain summits;
large-scale effects on the global circulation via a drag force exerted on the flow.
Understanding and representing the effects of orography is crucial for weather forecasting and climate prediction since the phenomena usually occur on scales too small to be resolved by the computer models.
Radiative transfer describes the transfer of electromagnetic energy in the atmosphere. The Earth, atmosphere and oceans are driven by the energy reaching the Earth from the sun. This incoming solar radiation is balanced by thermal emission to outer space.
However, the distribution of the incoming solar radiation on the planet is not uniform because the Earth is a sphere and it's axis of rotation is tilted.
Both the incoming radiation and the thermal emission from the Earth atmosphere system to space can also be affected by clouds, aerosols and the albedo of the surface (the extent to which it diffusely reflects light from the Sun) - all of which vary significantly across the globe.
In order to make weather predictions and conduct climate simulations our forecast models need to represent the processes of radiative transfer and their correct distribution across the globe, and with height in the atmosphere.