More on Water More on Water Vapor/CloudsVapor/Clouds
Satellite Satellite Meteorology/ClimatologyMeteorology/Climatology
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
dw = - kwduWhat 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
ous 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)
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
Basic interpretation of Basic interpretation of water vapor imagerywater vapor imagery
Comes from Comes from 6-76-7m,m, part of the water part of the water vapor absorption channelvapor absorption channel
Usually receiving radiation from 600-Usually receiving radiation from 600-300 mb300 mb
The drier the column of air, the lower in The drier the column of air, the lower in the atmosphere that WV channel “sees”the atmosphere that WV channel “sees”• For very dry air may see some radiation For very dry air may see some radiation
from below 800mbfrom below 800mb Does not depict low level moistureDoes not depict low level moisture
Water vapor imageryWater vapor imagery
Acts as a tracer of atmospheric Acts as a tracer of atmospheric motion!!motion!!
Upper-level ridge/troughsUpper-level ridge/troughs Mid-troposhperic vorticity maxima Mid-troposhperic vorticity maxima
(rotation center)(rotation center) Jet stream (sharp moisture Jet stream (sharp moisture
gradient)gradient)
Water vapor imageryWater vapor imagery
UpperLow
Louie Grasso and Eric Hilgendorf, GOES-8 Channel 3 (6.7 micrometer water vapor channel )http://www.cira.colostate.edu/ramm/picoday/980805.html
Water vapor imageryWater vapor imagery
A A lower lower brightness temperature -- brightness temperature -- light tones -- means that the water light tones -- means that the water vapor (the “optically thick layer”) vapor (the “optically thick layer”) is is higherhigher than a neighboring than a neighboring region with a region with a higherhigher brightness brightness temperature -- dark tones.temperature -- dark tones.
Water vapor imageryWater vapor imagery
However, must also account for However, must also account for the temperature of the water the temperature of the water vapor!vapor!• Low temperatures may also appear Low temperatures may also appear
bright in water vapor imagerybright in water vapor imagery
Water vapor imageryWater vapor imageryBright = moist in mid trop??
Dark = dry??
Eric Hilgendorf and Jack Dostalek
http://www.cira.colostate.edu/ramm/picoday/121297.html
Water vapor imageryWater vapor imagery
Minnesota was not more humid, Minnesota was not more humid, just colderjust colder
Water vapor imageryWater vapor imagery
Another way to think about “the vapor”Another way to think about “the vapor”• A warmer temperature means that the water A warmer temperature means that the water
vapor (the “optically thick layer”) is lower than a vapor (the “optically thick layer”) is lower than a neighboring cooler region at a fixed time.neighboring cooler region at a fixed time.
• So… If the temperature of a region is So… If the temperature of a region is coolingcooling ((warmingwarming) that could imply that ) that could imply that upwardupward ((downwarddownward) vertical motion is occurring.) vertical motion is occurring.
• However, another reason could be that the However, another reason could be that the cooler/warmer region simply advected in from cooler/warmer region simply advected in from somewhere else.somewhere else.
Increased “dark” (red) implies Increased “dark” (red) implies downward motion. No dry air downward motion. No dry air apparently advected.apparently advected.
WV WV gray shade gray shade interpretationinterpretation
Very Light to Nearly White (lightest 10%)Very Light to Nearly White (lightest 10%)• Cirrus clouds are likelyCirrus clouds are likely
• Pattern often recognizable by a textured Pattern often recognizable by a textured appearance and relatively distinct edgesappearance and relatively distinct edges
Exceptions: Exceptions: • Very cold winter continental air (rare)Very cold winter continental air (rare)
• Nearly saturated upper tropospheric Nearly saturated upper tropospheric moisture at tropical latitudes (relatviely moisture at tropical latitudes (relatviely common at low latitudes)common at low latitudes)
WV gray shade WV gray shade interpretationinterpretation
Light to Very Light (75-90% of white)Light to Very Light (75-90% of white)• Significant moisture is present in the upper Significant moisture is present in the upper
middle troposphere middle troposphere – typically with single digit DD through a layer of typically with single digit DD through a layer of
at least 100 mb between 500 and 300 mbat least 100 mb between 500 and 300 mb
• May be dry in the lower troposphereMay be dry in the lower troposphere Exceptions: Exceptions:
• Very cold winter season continental air Very cold winter season continental air
• Combinations of middle clouds and moistureCombinations of middle clouds and moisture
WV gray scale WV gray scale interpretationinterpretation
Medium Gray Shades (40 to 75% of Medium Gray Shades (40 to 75% of whiteness scale)whiteness scale)• The vertical location of the moisture The vertical location of the moisture
cannot be determined from the gray cannot be determined from the gray shades aloneshades alone
• Numerous combinations of moisture Numerous combinations of moisture conditions and clouds may produce conditions and clouds may produce the medium gray shadesthe medium gray shades
WV grey shade WV grey shade interpretationinterpretation
Very Dark to Black (lower 40% of shades)Very Dark to Black (lower 40% of shades)• Very dry air in the middle and upper trop.Very dry air in the middle and upper trop.
• Dry air probably extends down to 700 mbDry air probably extends down to 700 mb
• Deep dry conditions most likely if the dark Deep dry conditions most likely if the dark area is becoming darker or expandingarea is becoming darker or expanding
Exceptions: Exceptions: • Dark area is decreasing or becoming lighter Dark area is decreasing or becoming lighter
with time- thin high moisture may be presentwith time- thin high moisture may be present