Energy and Moisture Budgets of Deserts Energy Transfer in the Climate System

7/2/21

• What are three ways that energy can be transferred in the climate system?

• Conduction • The transfer of energy due to physical contact between objects • Energy is transferred from the warmer to the colder object • Convection • The transfer of energy due to the movement of a fluid • Often we think of this energy transfer occurring as warm, less dense fluid rises and cold, more dense fluid sinks but this can also apply to horizontal energy transfer due to horizontal movement of the fluid • Radiation • The transfer of energy by electromagnetic waves

1 2

Radiation Radiation

• Shortwave (or solar) radiation • We can determine the amount of energy emitted by an object • Radiation emitted by the sun with wavelength less than 4 µm based on its temperature (Steffan-Boltzmann Law) • Longwave radiation • Radiation with wavelength greater than 4 µm 4 • This is primarily radiation emitted by the Earth and atmosphere Emitted energy = esT e - emissivity s=5.67x10-8 W m-2 K-4 - Steffan-Botlzman constant

• We can determine the peak wavelength (lmax) emitted by an object using Planck’s Law -3 lmax = 2.88 x 10 / T • Sun peak wavelength: 0.5 µm • Earth peak wavelent: 10 µm

• For both of these equation the temperature (T) must be in K

3 4

1 7/2/21

What determines the amount of solar energy Radiation received at the surface of the Earth over a 24 h period? • How does the amount of energy and peak wavelength of radiation emitted differ between the sun (T = 6000 K) and the Earth (T = 288 •Angle of the sun K)? • Zenith angle •Length of day •Scattering / reflection / absorption in atmosphere

What causes these factors to change over the course of a year?

(µm) (µm)

5 6

Earth’s Orbit Around the Sun Average Orbital Characteristics

• Elliptical Orbit: 365.25 day period • Average distance (radius) = 150 x106 km • Perihelion - Earth is closest to sun (January - 147x106 km) • Aphelion - Earth is furthest from sun (July - 152x106 km) • Earth rotates on its axis 1 spin per 24 h • Average “Sunlight Day length” = 12 h • The Earth is tilted 23.5 deg from perpendicular relative to the orbital plane • The only reason we have seasons is because of this tilt

7 8

2 7/2/21

How does the tilt of Earth influence the Important Latitude Bands intensity of solar radiation? N °

• Equator: central latitude Arctic circle 66.5 • “Tropics”: 23.5°N and S N • Latitudes of maximum “displacement of the sun” ° • Tropic of Cancer: 23.5°N (Summer Northern Hemisphere) • Tropic of Capricorn: 23.5°S (Summer Southern Hemisphere) Tropic of Cancer 23.5 • Arctic/Antarctic Circles: 66.5°N and S ° • Latitudes where there is 24 h of daylight (nighttime) at summer S (winter) solstices Equator 0 ° • Equinox: Point(s) in the Earth’s orbit where the sun is directly over the equator • 12 h light and 12 h of darkness everywhere • Solstice: Point in orbit where the sun is “displaced” farthest (N Tropic of Capricorn 23.5 or S)

9 10

The “Directness” of Insolation How does the tilt of Earth influence the length of day? • N Insolation – Incoming solar radiation ° • The higher the sun is in the sky the more direct the insolation Arctic circle 66.5 N °

Tropic of Cancer 23.5 ° S Equator 0 °

Tropic of Capricorn 23.5

11 12

3 7/2/21

Solar Zenith Angle Daily total radiant • Solar zenith angle – angle of the sun from directly overhead energy received on a • What does a zenith angle of 90° mean? horizontal surface

This depends on: Zenith angle of sun Length of day

Noon zenith angle = |Sun latitude – local latitude|

13 14

Insolation – summer vs winter The Diurnal Cycle of Temperature and Radiation

• Solar radiation observed at CU weather station • How does peak solar radiation compare between summer and winter? • How does sunrise / sunset and length of day compare between summer and winter? • How does the total amount of solar radiation compare between summer and winter?

June December

15 16

4 7/2/21

Daytime Heating: Solar Radiation and Surface Air Temperature (from insolation)

• The daily change in incoming and outgoing radiation (the surface radiation budget) plays a large role in controlling the daily changes in temperature we observe near the surface of the Earth.

!"#$%&'

During the daytime temperature almost always decreases with height since solar heating of the ground is the primary source of heating of the atmosphere -> air near the ground is warmest 17 18

Nighttime Cooling: (from longwave radiational cooling) Radiation Inversion: Temperatures increase with height

During the night temperature almost always increases with height since the ground is radiatively cooling and this cools the lowest part of the atmosphere 19 20

5 7/2/21

Atmospheric Windows: Absorption of shortwave and longwave radiation in the atmosphere

SW and LW radiation can be absorbed by different gases in the atmosphere

Certain wavelengths of radiation are strongly absorbed while others can pass through the atmosphere with little absorption.

Wavelength bands with little absorption are known as atmospheric windows. Wavelength (µm)

21 22

Atmospheric Windows: Absorption of shortwave Atmospheric Radiation Budget and longwave radiation in the atmosphere Solar Constant: incoming radiation at top of atmosphere = 1367 W/m2

Albedo : Percent of incoming radiation that is reflected

23 24

6 7/2/21

Atmospheric Radiation Budget: Effects of Radiation Without the “Greenhouse Effect”

Driver of Weather Climate

(255 K) With No H2O or CO2 It’s COLD!

25 26

Atmospheric Radiation Budget: Atmospheric Energy Budget Components With the “Greenhouse Effect”

H2O

CO2

With greenhouse effect (288 K)

27 28

7 7/2/21

Why Do We Care About the How do desert energy budget components differ from the global energy Atmospheric Energy Budget? budget components?

• Determines overall climate • We will consider each component of the energy budget and • Variations in the energy budget from one location to another determine how it would differ in a desert compared to the drives weather and climate global average (next slides) • Determines local variations in climate • Determines response of climate to human changes (e.g., • You should know which terms are larger or smaller and why agriculture, urbanization) and natural changes they differ in this way.

29 30