Core Physical Atmos.&weather, 2.1 Diurnal energy budgets

Diurnal Energy Budgets

• An energy budget quantifies energy entering, leaving, and transferring within a system, typically at global (macro) and local (micro) scales.
• Microclimate describes regional climates like those in urban, coastal, or mountainous areas.
• Climate and weather phenomena vary in spatial and temporal scales, from small turbulence to large anticyclones and jet streams.
• The Eyjafjallajökull glacier volcanic dust jet stream in 2010 is an example of jet stream activity.

Daytime and Night-time Energy Budgets

• The daytime energy budget includes incoming solar radiation (insolation), reflected solar radiation, surface absorption, sensible heat transfer, long-wave radiation, and latent heat (evaporation and condensation).
• The night-time energy budget includes long-wave Earth radiation, latent heat transfer (condensation), absorbed energy returned to Earth (sub-surface supply), and sensible heat transfer.
• Energy available at the surface = incoming solar radiation - (reflected solar radiation + surface absorption + sensible heat transfer + long-wave radiation + latent heat transfers).

Incoming Solar Radiation

• Incoming solar radiation (insolation) is affected by latitude, season, and cloud cover.
• For stratocumulus clouds when the Sun is low, about 23% of the total radiation is received at the Earth's surface (250 watts per m²).
• When the Sun is high, about 40% is received (just over 450 watts per m²).
• More radiation reaches the Earth’s surface if there is less cloud cover/higher clouds.

Reflected Solar Radiation

• Albedo is the proportion of energy reflected back to the atmosphere.
• Lighter materials are more reflective than dark materials.
• Grass has an average albedo of 20-30%.

Surface and Sub-Surface Absorption

• Energy reaching the Earth's surface can heat it, depending on its nature.
• Surfaces that conduct heat to lower layers remain cooler.
• Energy transferred to the soil and bedrock may be released at night, offsetting surface cooling.

Sensible Heat Transfer

• Sensible heat transfer involves the movement of air parcels into and out of an area.
• Warmed air rises via convection and is replaced by cooler air.
• Cold air moving in can reduce temperatures, while warm air can supply energy and raise temperatures.

Long-Wave Radiation

• Long-wave radiation is the radiation of energy from the Earth into the atmosphere and space with a downward movement of long-wave radiation from particles in the atmosphere.
• The difference between outgoing and incoming flows is the net long-wave radiation balance.
• During the day outgoing long-wave radiation transfer > incoming, so, there is a net loss of energy.
• During cloudless nights, large loss of long-wave radiation --> little return of long-wave radiation from the atmosphere and a net loss of energy from the surface.
• In hot desert areas the loss of energy at night is maximised (lack of cloud cover).
• In cloudy areas the loss of energy is less noticeable.

Latent Heat Transfer

• Heat energy used up when liquid water turns into water vapor.
• Heat released when water vapor condenses into liquid.
• Water at a surface uses energy to evaporate, reducing energy available to raise temperature.
• At night, water vapor can condense to form water, releasing latent heat.

Dew

• Dew refers to condensation on a surface.
• Occurs when air is saturated due to surface temperature drops or increased moisture like a sea breeze.

Absorbed Energy Returned to Earth

• Insolation reradiated as long-wave radiation.
• Some is absorbed by water vapor and greenhouse gases, increasing temperature.

Temperature Changes Close to the Surface

• Ground-surface temperatures vary between day and night.
• During the day, the ground heats air by radiation, conduction, and convection.
• Air near the ground warms due to more radiation received than emitted.
• Air movement is slower due to surface friction.

Annual Surface Energy Budget of an Arctic Site - Svalbard, Norway

• In summer, net short-wave radiation is the main energy source.
• Sensible heat transfers and surface absorption in the ground cause cooling.
• 15% of net radiation is used for seasonal thawing of the active layer (the topsoil layer that thaws in summer).
• During the polar night in winter, net long-wave radiation is the main energy loss, compensated by sensible heat transfer and ground heat transfer from refreezing of the active layer.
• Evaporation occurs during snowmelt and the snow-free period, while latent heat fluxes are minimal when the ground is snow-covered.