Chapter 2 Solar and Infrared Radiation

Chapter overview:

• Fluxes • Energy transfer • Seasonal and daily changes in radiation • Surface radiation budget

Fluxes

Flux (F): The transfer of a quantity per unit area per unit time (sometimes called flux density). A flux can be thought of as the inflow or outflow of a quantity through the side of a fixed volume.

Fluxes can occur in all three directions - Fx, Fy, and Fz What is the convention for the sign of a flux?

We can consider fluxes of mass or of heat. What are the units for a mass flux or a heat flux? The amount of a quantity transferred through a given area (A) in a given time (Δt) can be calculated as: Amount = F ⋅A ⋅ Δt For a heat flux, the amount of heat transferred is represented by ΔQH. Note: The textbook discusses kinematic fluxes, but we will not discuss fluxes in these terms in ATOC 3050. Unlike the textbook, we will use the symbol F to represent fluxes, not kinematic fluxes. What processes can cause a heat flux? Radiant flux: The radiant energy per unit area per unit time. Radiant energy: Energy transferred by electromagnetic waves (radiation).

Radiation emitted by the sun is referred to as solar or shortwave radiation. Shortwave radiation – refers to the wavelength band (< 4 µm) that carries most of the energy associated with solar radiation Solar constant (or total solar irradiance) (S0): The solar radiative flux, perpendicular to the solar beam, that enters the top of the atmosphere S0 = 1366 W m-2

Radiation emitted by the earth is referred to as longwave, terrestrial, or infrared radiation. Longwave radiation – refers to the wavelength band (> 4 µm) that carries most of the energy emitted by the Earth What wavelengths correspond to the peak shortwave emission from the Sun and longwave emission from the Earth? Advective flux: Caused by wind blowing through an area and carrying colder or warmer air with it. What are two ways that you could get a positive advective flux in the meridional direction? Turbulent flux: Caused by transport of warm or cold air by turbulent eddies. Both the advective and turbulent fluxes are caused by the movement of air. The advective flux is caused by the mean wind and the turbulent flux is caused by turbulent fluctuations of the wind.

Conductive flux: Flux caused by energy transfer due to molecules bouncing into each other.

Conduction: The transfer of energy due to physical contact between objects. Energy transferred by conduction always goes from the warmer to the colder object.

Which of the fluxes discussed above can occur in a vacuum? Seasonal and Daily Changes in Solar Radiation What role does solar radiation play in driving the Earth’s weather?

• Atmospheric circulation • Daily changes • Seasonal changes

What is the ultimate reason that the Earth has seasons? The ultimate cause of seasons is a change in the amount of solar energy received at the Earth’s surface.

The amount of solar energy received at the earth’s surface depends on:

• Angle that sunlight enters the atmosphere and strikes surface

• Length of daylight

What causes these factors to vary throughout the year?

Does the Earth’s distance from the sun cause the seasons? Note: The textbook discusses orbital factors that affect the amount of solar radiation in additional detail. Earth tilt angle (Φr): The tilt of the Earth’s axis relative to a line perpendicular to the ecliptic (orbital) plane of the Earth around the Sun. Currently Φr = 23.44° This angle determines the latitude of the Tropics of Cancer and Capricorn and of the Arctic and Antarctic circles. How is the Earth tilt angle related to these geographic locations?

Solar declination angle (δs): The angle between the ecliptic and the plane of the Earth’s equator. When is the solar declination angle at its maximum, minimum, and zero?

How can we describe the position of the sun relative to a location on Earth? Local elevation angle (Ψ): The angle of the sun above the local horizon Azimuth angle (α): The angle of the sun clockwise from north See the textbook for additional information on changes in the sun’s position throughout the daily and annual cycles. Equations are provided to calculate the solar declination angle, the local elevation angle of the sun, and the azimuth angle of the sun.

Average daily insolation: The average incoming solar radiation over an entire day. This accounts for the solar elevation angle (which varies seasonally and diurnally) and the length of day.

During which season does the average daily insolation vary most between the pole and equator? Remember: The ultimate cause of seasons is a change in the amount of solar energy received at the Earth’s surface.

Surface Radiation Budget

Why doesn’t the maximum daily temperature occur at noon, when the sun is highest in the sky and the surface is receiving the maximum amount of solar radiation? When during the day does the temperature begin to decrease?

Net radiative flux (F*): The sum of the incoming and outgoing radiative fluxes F* = K ↓+K ↑+I ↓+I ↑ Downwelling solar radiation (K ↓): solar radiation that enters the top of the atmosphere and passes through the atmosphere to the surface

What factors influence the amount of downwelling solar radiation that reaches the surface of the Earth?

Reflected upwelling solar radiation (K ↑): Solar radiation reflected from the surface K ↑= −A ⋅K ↓ Albedo (A): The ratio of the total reflected solar radiation to the total incoming solar radiation.

A =Ereflected

Eincoming

What is the average global albedo of the Earth? Downwelling longwave radiation ( I ↓): Radiation emitted from the atmosphere What factors influence the amount of downwelling longwave radiation? Upwelling longwave radiation ( I ↑): Radiation emitted from the Earth I ↑= eIRσ SBT

4 eIR emissivity in the IR (longwave) portion of the radiative spectrum (typically 0.9 to 0.99 for most surfaces) σSB - Stefan-Boltzman constant (=5.67x10-8 W m-2 K-4)

In the tropics more radiant energy is gained than lost and in the polar regions more radiant energy is lost than gained.

If the tropics and polar regions are not in energy balance why don’t their temperatures either continually warm (tropics) or cool (polar regions)?