SOFIE Water Vapor Channel

The H2O channel is located near 2.6 microns.  PMCs will have no effect on H2O measurements in this band. 

Water Vapor Signal Simulations

Figures 1 and 2 shows simulated limb transmission spectra for H2O,  CH4,  and CO2 using a climatological atmosphere for 60N during summer.  Also shown is a PMC spectra is based on an average PMC size distribution and a path length of 250 km.  The proposed new H2O band is in a region of very little PMC absorption.  A region of strong CO2 absorption exists towards longer wavelengths than the H2O region.  CH4 interference exists at wavenumbers greater than about 3800 1/cm.  CO2 interference can be minimized by locating the absorbing H2O band at wavenumbers greater than about 3780 1/cm.  This choice will result in some CH4 interference,  however,  the CH4 signal is far weaker than the H2O signal.  Locating the non-absorbing H2O band toward greater wavenumbers than the H2O would eliminate the PMC induced difference signal.  

For design reasons, the H2O bands were narrowed and separated.

Suggested band locations (see Figure 1):
strong:  3800 - 3880 1/cm
weak:  4040 - 4120 1/cm

Figure 3 shows simulated transmission profiles for the absorbing and non-absorbing bands listed above,  using a climatological atmosphere for 60 degrees N latitude during summer.  The PMC profile is from an average of HALOE measurements,  shifted in wavelength.  While there is gaseous interference in the weak band,  it is small.  In fact,  H2O dominates the total difference signal at altitudes down to the tropopause.   PMCs comprise less than 1% of the difference signal and thus should have little or no effect on SOFIE H2O retrievals using this new band.   

As seen in Figures 2 and 3,  H2O dominates the signal even in the stratosphere.   The sulfate aerosol signals shown represent background conditions.  Sulfate aerosols will have little effect on the SOFIE H2O measurements.  This band should provide H2O retrievals from the tropoause to 95 km.  

In Figures 1 and 2 transmissions (t) are shown as log(1-t),  so that t = 0.9 corresponds to -1, t = 0.99 to -2,  etc...  

1     Figure 1


2   Figure 2

3       Figure 3

Simulated SOFIE Water Vapor Retrievals

Simulated water vapor retrievals were conducted for a proposed SOFIE H2O band located from 3800 to 3880 1/cm.  These retrievals used a climatological atmosphere for summer at 60 deg. N latitude to simulate a water vapor transmission profile.  This profile was then used as input for a limb transmission onion peeling gas mixing ratio retrieval.  The retrieval assumed nothing about the instrument,  except an estimate of the measurement noise (1e-6).  While the simulated retrieval operates on transmission alone,  SOFIE measurements will yield difference signals.  However,  a transmission retrieval assuming difference signal precission gives similar results to those obtained from the inversion of difference signals.  The simulated retrieval is shown in Figure 4.   These results indicate that measurements using the proposed SOFIE H2O band can be used to infer H2O mixing ratios at altitudes from the tropopause to about 95 km.  

       Figure 4.  

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