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Microwave Interactions with the Atmosphere

This document explores the interactions of microwaves with the atmosphere, detailing how different atmospheric constituents absorb and emit microwave radiation across various frequencies. It covers the composition and profile of the atmosphere, the behavior of gases like water vapor and oxygen, and their effects on microwave transmission. Key aspects include absorption bands, brightness temperature, and atmospheric profiles. Understanding these elements is crucial for applications in telecommunications, remote sensing, and atmospheric science.

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Microwave Interactions with the Atmosphere

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  1. Microwave Interactions with the Atmosphere S. L. Cruz Pol Electrical and Computer Engineering University of Puerto Rico

  2. Atmospheric Constituents

  3. Introduction • Up to now, we have assumed lossless atmosphere • For 1 GHz< f< 15 GHz ~lossless • For higher freqs., =>absorption bands O2 H2O • 22.235 GHz • 183.3 GHz • IR & visible • 50-70GHz • 118.7GHz • IR & visible

  4. Outline I. The atmosphere: composition, profile II. Gases: many molecules 1. Shapes(G, VVW, L): below 100GHz, up to 300GHz e.g.H2O , O2 2. Total Atmospheric Absorption kg, opacity tq, and atm-losses Lq 3. TB: Downwelling Emission by Atmosphere

  5. U.S. Standard Atmosphere Thermosphere (or Ionosphere) 1000-3000oF! Mesopause Mesosphere no aircrafts here too cold ~-90oF Stratopause Stratosphere- no H2O or dust ozone absorption of UV warms air to ~40oF Tropopause Troposphere – clouds, weather P= 1013 HPa T= 300K

  6. Atmospheric ProfilesUS Standard Atmosphere 1962 • Temperature • Density • Pressure P= nRT/V=rairRT/M or Poe-z/H3 or

  7. Water Vapor Profile • Depends on factors like weather, seasons, time of the day. • It’s a function of air temperature. • Cold air can’t hold water • Hot air can support higher humidities.(P dependence) rv(z)= roe-z/H4[g/m3] where ro averages 7.72 in mid latitudes and the total mass of water vapor in a column of unit cross section is

  8. Water vapor molecule emissions

  9. EM interaction with Molecules • Total internal energy state for a molecule • electronic energy corresponding to atomic level • vibration of atoms about their equilibrium position • rotation of atoms about center of molecule • E = Ee + Ev + Er • Bohr conditionfml= (Em - El ) /h • Values for energy differences for • electronic: 2 to 10 eV • vibrational-rotational: 0.1 to 2 eV • pure rotational: 10-4 to 5 x 10-2 eV ( microwaves)

  10. Line Shapes where, • Slm is the line strength • F(f,flm) is the line shape LINE SHAPES One molecule frequency Absorption Many molecules: pressure broaden* frequency *caused by collision between molecules

  11. Line shapes • Lorentz • Gross • Van-Vleck-Weisskopt

  12. Absorption Bands • Mainly water and oxygen for microwaves Brightness Temperature [K] Frequency [GHz]

  13. Total Atmospheric • Absorption kg, • Opacity tq, • Loss factor Lq

  14. Atmospheric Emission • For clear atmosphere where Also there is some background radiation Tcos=2.7K from the Big Bang and Tgal~0 above 5GHz

  15. Relative Humidity

  16. Aviris

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