Lecture 1-4:

What is the Moisture Equation?

Deals with the concepts of evaporation, condensation and mixing of moisture in the atmosphere. Despite a change of state, the mass of moisture remains the same. All moisture losses from the land surface and oceans will be balanced by gains due to precipitation and condensation. The equation is:

change in moisture = change due to evaporation + change due to condensation - change due to vertical mixing

*how much water added to atmosphere from underlying surface, associated with cooling/cloud formation, dispersion or dilution of moisture in atmosphere

What is the effect of global warming on precipitation?

• Hydrological balance has been thrown out of kilter as a result of atmospheric warming

• Extreme rain events will increase in intensity by 3-15% depending on region for every °C that the planet warms

• The water-holding capacity of the atmosphere increases by about 7% for every 1°C rise in temperature (Clausius-Chapeyron relationship)

What is the Equation of Continuity?

It describes the principal of conservation of mass that ‘no matter how much air is compressed or stretched, air will not change its mass’. By compressing air at one place, it must be balanced by expansion at another. The equation is:

vertical convergence = horizontal divergence + a density change

horizontal convergence = vertical divergence + a density change

Electromagnetic radiation

• The higher the temperature of the object emitting radiation, the shorter the wavelength of radiation emitted.

• The higher the temperature of the object emitting radiation, the greater the amount of radiation emitted

• Radiation from the Sun (0.15 - 3 um) and from the Earth (3 - 100 um) are
  of different wavelengths. Why? Because of their distinctly different temperatures, i.e. Sun 6000°K –Earth 300°K

Tropical cyclones divergence convergence

For a tropical cyclone to intensify the rate of divergence (loft) has to exceed the rate of convergence. A more rapid rate of divergence than convergence at the surface will draw in more ‘fuel’.

Electromagnetic radiation

• The higher the temp of the radiation emitting object, the shorter the wavelength and the greater amount of radiation emitted

• Radiation from the Sun (0.15 - 3um) and from the Earth (3 - 100 um) are different wavelengths

  • due to their distinct temperatures (Sun 6000°K and Earth 300°K)

What is the amount of solar radiation absorbed at the surface?

• The mean solar constant (amount of shortwave) received at the top of the atmosphere from the Sun is ~1368 Wm-2

• Net shortwave at the surface equals diffuse shortwave radiation (scattered and reflected) and direct short-wave radiation (without being absorbed/diffused)

• Amount of shortwave radiation:

  • that reaches the surface is dependent on atmospheric turbidity (e.g. aerosols, water vapour etc.)

  • absorbed at surface is dependent on surface albedo (e.g. fresh snow = 95%, lava = 10%, thick cloud = 70-95%, urban = 15%)

Solar cycles

• We are coming off the top of a solar cycle!

• Radiation coming in through the atmosphere is not constant, it oscillates every 10-11 years

• Auroras (solar flares caused by higher increase in frequency and incidence of high energy electromagnetic particles hitting top of atmosphere) at peak of cycle

What cloud properties influence the radiation budget?

• Cloud height

• Coverage of sky

• Cloud thickness

• Moisture content

• Persistence

• Cloud composition

• Layers of cloud

Measurement of radiation transfers

• Pyranometers measure short wave; pyrgeometer measure long wave

  • CM21/CG4

  • CNR 4

  • Radiation sensor on a flux tower

What is the driving force behind our weather and climate?

• The Radiation Balance

  • the global distribution of net radiation (Q*) primarily drives atmospheric circulation (note Q* can be position or negative)

  • causes change in air density at surface to set up pressure gradient and drive sea breeze convection etc.

• Q* = K ↓ - K ↑ + L ↓ - L ↑

  • (K = shortwave, L = longwave)

  • *incoming longwave is determined by cloud coverage and GHGs

  • note the variables display, diurnal, seasonal, and latitudinal variability also influenced by surface type (albedo)

Energy receipt

Energy receipt equals energy loss for the Earth-Atmosphere system as a whole (in radiative equilibrium).