 # METR180: Remote Sensing Lecture 14 – On Water Vapor.

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METR180: Remote Sensing Lecture 14 – On Water Vapor

January (M. King, our changing planet) Q: Where are the most and least water vapor abundant regions?

July Q: Where are the most and least water vapor abundant regions?

Earth emitted spectra overlaid on Planck function envelopes CO2 H20 O3 CO2

Radiation interacts with the atmosphere in 4 different ways

o R =  sfc B (T s )  (p s ) +  B (T(p)) [d  (p)/ dp] dp p s + (1-  sfc ) R  (p s ) + ρ H  (p s ) The first term is the spectral radiance emitted by the surface and the second term is the spectral radiance emitted to space by the atmosphere directly. Third term is the downwelling atmospheric radiation reflected by the surface and transmitted to space. Fourth term is the downwelling radiation from the sun, reflected by the surface and transmitted to space The atmospheric contribution is the weighted sum of the Planck radiance contribution from each layer, where the weighting function is [ d  (p) / dp ]. This weighting function is an indication of where in the atmosphere the majority of the radiation for a given spectral band comes from. Radiative Transfer Equation

Transmittance Transmission through an absorbing medium for a given wavelength is governed by the number of intervening absorbing molecules (path length u) and their absorbing power (k ) at that wavelength. Beer’s law indicates that transmittance decays exponentially with increasing path length - k u (z)  (z   ) = e  where the path length is given by u (z) =   dz. z k u is a measure of the cumulative depletion that the beam of radiation has experienced as a result of its passage through the layer and is often called the optical depth . Realizing that the hydrostatic equation implies g  dz = - q dp where q is the mixing ratio and  is the density of the atmosphere, then p - k u (p) u (p) =  q g -1 dp and  (p  o ) = e. o

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 d  w = - k w du What little absorption exists is due to water vapour, therefore, u is a measure of precipitable water vapour. RTE in window region u s I w = B sw (1-k w u s ) + k w  B w du o u s represents total atmospheric column absorption path length due to water vapour, and s denotes surface. Defining an atmospheric mean Planck radiance, then _ _ u s u s I w = B sw (1-k w u s ) + k w u s B w with B w =  B w du /  du o o Since B sw is close to both I w and B w, first order Taylor expansion about the surface temperature T s allows us to linearize the RTE with respect to temperature, so _ T bw = T s (1-k w u s ) + k w u s T w, where T w is mean atmospheric temperature corresponding to B w. (courtesy: Paul Mentzel)

For two window channels (11 and 12um) the following ratio can be determined. _ T s - T bw1 k w1 u s (T s - T w1 ) k w1 _________ = ______________ = ___ _ T s - T bw2 k w1 u s (T s - T w2 ) k w2 where the mean atmospheric temperature measured in the one window region is assumed to be comparable to that measured in the other, T w1 ~ T w2, Thus it follows that k w1 T s = T bw1 + [T bw1 - T bw2 ] k w2 - k w1 and T bw - T s u s =. _ k w (T w - T s ) Obviously, the accuracy of the determination of the total water vapour concentration depends upon the contrast between the surface temperature, T s, and _ the effective temperature of the atmosphere T w

Basic interpretation of water vapor imagery Comes from 6-7  m, part of the water vapor absorption channel Comes from 6-7  m, part of the water vapor absorption channel n Usually receiving radiation from 600-300 mb n 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 800mbFor very dry air may see some radiation from below 800mb n Does not depict low level moisture

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

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

Water vapor imagery n 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.

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

WV climatology from MODIS: example 1: Tibet

Summer a)b) c)d) Figure 6. Seasonal land surface temperature, cloud fraction, water vapor, and NDVI Suzi

FIGURE 20. AQUA TSkin Vs. water vapor from August 2002 to August 2009 James

Total Column Water Vapor and Surface Temperature n Top Figure: Scatter plot showing the relation between land surface temperature and total column water vaporScatter plot showing the relation between land surface temperature and total column water vapor n Bottom Figure: Spatial plot of correlation between land surface temperature and total column water vaporSpatial plot of correlation between land surface temperature and total column water vapor n In general total column water vapor will increase as temperature increases (exponentially). This is because of the clausius clapeyron relationship This can be observed in the top figureThis can be observed in the top figure n This relationship is somewhat more complicated over the Tibetan Plateau (bottom figure) Most of the domain shows a strong positive correlation between water vapor and temperatureMost of the domain shows a strong positive correlation between water vapor and temperature There is a slight negative correlation between water vapor and temperature observed over the Indian sub continentThere is a slight negative correlation between water vapor and temperature observed over the Indian sub continent This negative correlation is likely a result of higher water vapor leading to clouds and precipitation in this region that would work to cool the temperatureThis negative correlation is likely a result of higher water vapor leading to clouds and precipitation in this region that would work to cool the temperature Pat

Correlation n The effect of water vapor seems to be uniformly positive towards higher latitudes and negative towards the equator. n Perhaps the negative correlation has to do with the fact that the tropics are saturated and continuously transport heat and moisture out of the area. n In lower humidity, any amount of moisture has a noticeable effect on temperature. Fig. 13 Tegan

Global WV vs. T skin (over land only)