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More on Water Vapor/Clouds

More on Water Vapor/Clouds. Satellite Meteorology/Climatology. January. (M. King, our changing planet). July. Water vapour evaluated in multiple infrared window channels where absorption is weak, so that  w = exp[- k w u] ~ 1 - k w u where w denotes window channel and

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More on Water Vapor/Clouds

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  1. More on Water Vapor/Clouds Satellite Meteorology/Climatology

  2. January (M. King, our changing planet)

  3. July

  4. Water vapour evaluated in multiple infrared window channels where absorption is weak, so that w = exp[- kwu] ~ 1 - kwu where w denotes window channel and dw = - kwdu What little absorption exists is due to water vapour, therefore, u is a measure of precipitable water vapour. RTE in window region us Iw = Bsw (1-kwus) + kw Bwdu o us represents total atmospheric column absorption path length due to water vapour, and s denotes surface. Defining an atmospheric mean Planck radiance, then _ _ us us Iw = Bsw (1-kwus) + kwusBw with Bw =  Bwdu /  du o o Since Bsw is close to both Iw and Bw, first order Taylor expansion about the surface temperature Ts allows us to linearize the RTE with respect to temperature, so _ Tbw = Ts (1-kwus) + kwusTw , where Tw is mean atmospheric temperature corresponding to Bw. (courtesy: Paul Mentzel)

  5. For two window channels (11 and 12um) the following ratio can be determined. _ Ts - Tbw1 kw1us(Ts - Tw1) kw1 _________ = ______________ = ___ _ Ts - Tbw2 kw1us(Ts - Tw2) kw2 where the mean atmospheric temperature measured in the one window region is assumed to be comparable to that measured in the other, Tw1 ~ Tw2, Thus it follows that kw1 Ts = Tbw1 + [Tbw1 - Tbw2] kw2 - kw1 and Tbw - Ts us = . _ kw (Tw - Ts) Obviously, the accuracy of the determination of the total water vapour concentration depends upon the contrast between the surface temperature, Ts, and _ the effective temperature of the atmosphere Tw

  6. Basic interpretation of water vapor imagery • Comes from 6-7mm, part of the water vapor absorption channel • Usually receiving radiation from 600-300 mb • The drier the column of air, the lower in the atmosphere that WV channel “sees” • For very dry air may see some radiation from below 800mb • Does not depict low level moisture

  7. Water vapor imagery • Acts as a tracer of atmospheric motion!! • Upper-level ridge/troughs • Mid-troposhperic vorticity maxima (rotation center) • Jet stream (sharp moisture gradient)

  8. Water vapor imagery Upper Low Louie Grasso and Eric Hilgendorf, GOES-8 Channel 3 (6.7 micrometer water vapor channel ) http://www.cira.colostate.edu/ramm/picoday/980805.html

  9. Where did it go?? GOES-8 Visible GOES-8 channel 4 (10.7 micron IR)

  10. Water vapor imagery • A lower brightness temperature -- light tones -- means that the water vapor (the “optically thick layer”) is higher than a neighboring region with a higher brightness temperature -- dark tones.

  11. Water vapor imagery Considered Saturated

  12. Water vapor imagery • However, must also account for the temperature of the water vapor! • Low temperatures may also appear bright in water vapor imagery

  13. Water vapor imagery Bright = moist in mid trop?? Dark = dry?? Eric Hilgendorf and Jack Dostalek http://www.cira.colostate.edu/ramm/picoday/121297.html

  14. Minnesota Florida

  15. Water vapor imagery • Minnesota was not more humid, just colder

  16. Water vapor imagery • Another way to think about “the vapor” • A warmer temperature means that the water vapor (the “optically thick layer”) is lower than a neighboring cooler region at a fixed time. • So… If the temperature of a region is cooling (warming) that could imply that upward (downward) vertical motion is occurring. • However, another reason could be that the cooler/warmer region simply advected in from somewhere else.

  17. Water vapor imagery http://www.cira.colostate.edu/ramm/picoday/102397.html

  18. Increased “dark” (red) implies downward motion. No dry air apparently advected.

  19. WV gray shade interpretation • Very Light to Nearly White (lightest 10%) • Cirrus clouds are likely • Pattern often recognizable by a textured appearance and relatively distinct edges • Exceptions: • Very cold winter continental air (rare) • Nearly saturated upper tropospheric moisture at tropical latitudes (relatviely common at low latitudes)

  20. WV gray shade interpretation • Light to Very Light (75-90% of white) • Significant moisture is present in the upper middle troposphere • typically with single digit DD through a layer of at least 100 mb between 500 and 300 mb • May be dry in the lower troposphere • Exceptions: • Very cold winter season continental air • Combinations of middle clouds and moisture

  21. WV gray scale interpretation • Medium Gray Shades (40 to 75% of whiteness scale) • The vertical location of the moisture cannot be determined from the gray shades alone • Numerous combinations of moisture conditions and clouds may produce the medium gray shades

  22. WV grey shade interpretation • Very Dark to Black (lower 40% of shades) • Very dry air in the middle and upper trop. • Dry air probably extends down to 700 mb • Deep dry conditions most likely if the dark area is becoming darker or expanding • Exceptions: • Dark area is decreasing or becoming lighter with time- thin high moisture may be present

  23. WV climatology from MODIS: example 1: Tibet

  24. Global WV vs. Tskin (over land only)

  25. Is cirrus related to air traffic?

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