Energy and Moisture Budgets of Energy Transfer in the Climate System

• 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 Radiation

• Shortwave (or solar) radiation • Radiation emitted by the sun with wavelength less than 4 µm • Longwave radiation • Radiation with wavelength greater than 4 µm • This is primarily radiation emitted by the Earth and atmosphere Radiation

• We can determine the amount of energy emitted by an object based on its temperature (Steffan-Boltzmann Law)

Emitted energy = esT4 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 Radiation

• How does the amount of energy and peak wavelength of radiation emitted differ between the sun (T = 6000 K) and the Earth (T = 288 K)?

(µm) (µm) What determines the amount of solar energy received at the surface of the Earth over a 24 h period?

• Angle of the sun • Zenith angle • Length of day • Scattering / reflection / absorption in atmosphere

What causes these factors to change over the course of a year? 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 “ 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 Important Latitude Bands

• Equator: central latitude • “Tropics”: 23.5°N and S • 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) • Arctic/Antarctic Circles: 66.5°N and S • Latitudes where there is 24 h of daylight (nighttime) at summer (winter) solstices • 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 or S) How does the tilt of Earth influence the intensity of solar radiation? N °

Arctic circle 66.5 N °

Tropic of Cancer 23.5 ° S Equator 0 °

Tropic of Capricorn 23.5 The “Directness” of Insolation

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

Arctic circle 66.5 N °

Tropic of Cancer 23.5 ° S Equator 0 °

Tropic of Capricorn 23.5 Solar Zenith Angle

• Solar zenith angle – angle of the sun from directly overhead • What does a zenith angle of 90° mean?

Noon zenith angle = |Sun latitude – local latitude| Daily total radiant energy received on a horizontal surface

This depends on: Zenith angle of sun Length of day Insolation – summer vs winter

• 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 The Diurnal Cycle of Temperature and Radiation Solar Radiation and Surface Air Temperature

• 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. Daytime Heating: (from insolation)

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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

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

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

Solar Constant: incoming radiation at top of atmosphere = 1367 W/m2

Albedo : Percent of incoming radiation that is reflected Effects of Radiation

Driver of Weather Climate Atmospheric Radiation Budget: Without the “Greenhouse Effect”

(255 K) With No H2O or CO2 It’s COLD! Atmospheric Radiation Budget: With the “Greenhouse Effect”

H2O CO2

With greenhouse effect (288 K) Atmospheric Energy Budget Components Why Do We Care About the Atmospheric Energy Budget?

• Determines overall climate • Variations in the energy budget from one location to another drives weather and climate • Determines local variations in climate • Determines response of climate to human changes (e.g., agriculture, urbanization) and natural changes How do energy budget components differ from the global energy budget components?

• We will consider each component of the energy budget and determine how it would differ in a desert compared to the global average (next slides)

• You should know which terms are larger or smaller and why they differ in this way. Energy Budget Components

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L Moisture Budget Components Why Care About Details of the Moisture Budget? Determines how much is available for • People • Plants and animals • • Replenishment of water table • Runoff • Streamflow • To fill reservoirs • Flash floods The Moisture Budget Has Implications for: • Agriculture • Cloud seeding • Emergency response (flooding) • Water management – dams, wells, water rights Why Care About Details of the Moisture Budget?

• Compare precipitation to water need to identify periods of water surplus or deficit • What happens when water need exceeds precipitation? • What happens when precipitation exceeds water need? Why Care About Details Tropical wet-dry Dry tropical of the Moisture Budget?

• Periods of moisture surplus or deficit vary over the annual cycle

• When a moisture surplus or deficit occurs depends on the local climate • Amount of precipitation Semi-arid Dry mid-latitude • Timing of precipitation (rainy season) • When vegetation is active

Desert Desert How do desert moisture budget components differ from the global moisture budget components?

• We will consider each component of the moisture budget and determine how it would differ in a desert compared to the global average (next slides)

• You should know which terms are larger, smaller or uncertain and why they differ in this way. Moisture Budget Components

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M/L Focus on Budgets at the Surface to Understand Conditions Where We Live Surface Radiation Budget R = (Q+q)(1-a) – I­ + I¯

R – net radiation Q – direct solar radiation q – diffuse solar radiation a- I – longwave radiation Components of surface radiation budget measured at a location in the Great Basin. (cloud-free, September) How is net radiation different in the desert? R = (Q+q)(1-a) – I­ + I¯

•Q – ? How is net radiation different in the desert? R = (Q+q)(1-a) – I­ + I¯

•Q – Larger because of less cloud and often sub-tropical latitude How is net radiation different in the desert? R = (Q+q)(1-a) – I­ + I¯

•Q – Larger because of less cloud and often sub-tropical latitude •q – ? How is net radiation different in the desert? R = (Q+q)(1-a) – I­ + I¯

•Q – Larger because of less cloud and often sub-tropical latitude •q – Smaller because of less cloud How is net radiation different in the desert? R = (Q+q)(1-a) – I­ + I¯

•Q – Larger because of less cloud and often sub-tropical latitude •q – Smaller because of less cloud •a - ? How is net radiation different in the desert? R = (Q+q)(1-a) – I­ + I¯

•Q – Larger because of less cloud and often sub-tropical latitude •q – Smaller because of less cloud •a - Larger because of less vegetation

How is net radiation different in the desert? R = (Q+q)(1-a) – I­ + I¯

•Q – Larger because of less cloud and often sub-tropical latitude •q – Smaller because of less cloud •a - Larger because of less vegetation •I¯ - ? How is net radiation different in the desert? R = (Q+q)(1-a) – I­ + I¯

•Q – Larger because of less cloud and often sub-tropical latitude •q – Smaller because of less cloud •a - Larger because of less vegetation •I¯ - Probably less due to fewer clouds How is net radiation different in the desert? R = (Q+q)(1-a) – I­ + I¯

•Q – Larger because of less cloud and often sub-tropical latitude •q – Smaller because of less cloud •a - Larger because of less vegetation •I¯ - Probably less due to fewer clouds •I­ - ? How is net radiation different in the desert? R = (Q+q)(1-a) – I­ + I¯

•Q – Larger because of less cloud and often sub-tropical latitude •q – Smaller because of less cloud •a - Larger because of less vegetation •I¯ - Probably less due to fewer clouds •I­ - Larger because T-surface is larger Surface Energy Budget 0 R = LE + H + G Annually

R – net radiation LE – latent heat flux (evaporation & ) H – heat flux to atmosphere G – heat flux into ground How would LE, H, and G be different in the desert?

• LE? • Depends on amount of energy for evaporation • But it also depends on the moisture available • LE – Small or zero in desert How would LE, H, and G be different in the desert?

• H? • Depends on temperature difference between ground and air, but is mostly driven by ground temperature • What does ground temperature depend on? • H is usually higher in the desert How would LE, H, and G be different in the desert?

• G? • Depends on - Amount of energy available -Temperature of ground and temperature gradient in substrate - Thermal properties of substrate (sand, rock, playa)

• Will discuss this in more detail in next chapter The diurnal variation of of the surface energy budget components for three different surfaces

Why are the energy budget components different at these three sites? The annual variation of the surface energy budget components at three different locations

Why are the energy budget components different at these three sites? Surface Energy Budget in Different Climatic Regimes

• How does the annual cycle of H and LE differ across these different climatic regimes?

• What causes LE (H) to be large or small? The Oasis Effect – An energy budget peculiar to the desert

What causes LE to be larger than R? Oasis – Death Valley What causes deserts to be hot?

• More solar energy received at the surface because of less cloud (connection between water budget and energy budget) and often sub-tropical locations • Drier substrate, so less solar energy lost to evaporation and more available for heating surface (and atmosphere) • Hotter surface means more heat input to atmosphere and substrate