More on Water More on Water Vapor/CloudsVapor/Clouds

Satellite Satellite Meteorology/ClimatologyMeteorology/Climatology

January

(M. King, our changing planet)

July

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

Where did it Where did it go??go??

GOES-8 channel 4 (10.7 micron IR)

GOES-8 Visible

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

Considered SaturatedConsidered Saturated

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

MinnesotaFlorida

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.

Water vapor imageryWater vapor imagery

http://www.cira.colostate.edu/ramm/picoday/102397.html

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

WV climatology from MODIS: WV climatology from MODIS: example 1: Tibetexample 1: Tibet

Global WV vs. TGlobal WV vs. Tskinskin (over land (over land only) only)

Is cirrus related to air traffic?