Issue 6: Observed refractivity (or delay) vs. retrieved meteorological profiles

 Because it is more closely connected to the measured delay, refractivity has some advantages over derived temperature and humidity profiles.  But is refractivity itself a useful climate parameter to monitor?  How would one use changes in refractivity as a climate indicator (e.g., indictor of what)?  Do any other observing systems depend on precise refractivity data for calibration?  Does a time series of refractivity tell us anything useful about the evolution of the climate system? 

Response from Kevin Trenberth:
Refractivity is similar to radiances used widely in NWP.  Retrievals from radiances alone are no longer used although the MSU channels have been used as a satellite temperature, but with more uncertainty that using refractivity.  I think the refractivity could become a benchmark temperature measurement when estimates of humidity from elsewhere are included above about 6 km.  Below that level, the confounding influences of water vapor an its uncertainties make refractivity alone the useful quantity, but given that temperature and moisture are positively correlated, a useful index may emerge. 

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5 Comments

  1. Tom Yunck

    Agreed. Refractivity has a promising future as a climate indicator above 6 km.

  2. Tony Mannucci

    I am not an expert in this area. I expect refractivity is useful as a way to discriminate between alternative models of how the atmosphere is responding to global change. The fact that refractivity depends on water vapor is not a significant disadvantage. Models that estimate future climate trends must be capable of accurately predicting refractivity trends.

  3. Seth Gutman

    Consolidate Comments from Gutman, Yoe and Reale
    Seth Gutman's Response
    I have mixed feelings about this issue. Chris Rocken made a well reasoned argument for using refractivity as a benchmark observation at the GCOS Reference Upper-Air Network (GRUAN) meeting in Seattle. On the pro side, refractivity measurements carry information about path-integrated temperature, pressure and water vapor in the neutral atmosphere, and direct assimilation of refractivity requires no a priori assumptions about how these parameters are parsed. On the con side, you have to convert refractivity estimates into geophysical parameters (or convert appropriate geophysical parameters into N units) at some point in order to compare them with other (more conventional) observations, and to do so requires additional information that may be accurate in a statistical sense but have low precision. In the absence of additional information of known accuracy, there will always be inherent ambiguity when parsing refractivity into its wet and dry components. So in my mind, the problem comes down to observation verification, and this is not going to be easy for a GPS-RO refractivity measurement. The author of this issue raises an extremely good point when they ask "How would one use changes in refractivity as a climate indicator (e.g., indictor of what)?" One can legitimately ask the question "does the PBL get warmer because the amount of water vapor is increasing, or does the amount of water vapor in PBL increase because near surface temperatures are rising?" I'm afraid the answer is yes. With regard to the question about which other observing systems requiring refractivity data for calibration, the answer is radar. With regard to the question about whether a time series of refractivity can tell us anything useful about the evolution of the climate system, the answer is decidedly yes. A time series of refractivity data can help verify climate models and climate predictions.

    Jim Yoe's Response
    My opinion about this issue is decidedly less mixed, and I agree with Chris Rocken's argument. Just because there is an "absence of additional information of known accuracy", is all the more reason to insist on having the one piece of information for which the physical accuracy may be relied upon. Used in conjunction with other data, refractivity will provide a constraint on the possible sets of solutions - how much wetter given how much warmer (or not) etc.

  4. Ulrich Foelsche

    Response from Ulrich Foelsche

    Refractivity itself is already a very useful parameter, since it is directly proportional to density in the upper troposphere and stratosphere (in the presence of significant amounts of atmospheric water vapor the relation is more complicated). Projected trends in bending angle are very similar to trends in refractivity (which is only a scaled representation of the index of refraction). As bending angle is even closer to the original measurement than refractivity, bending angle climatologies could be very useful. Bending angle (and refractivity) is expected to show significant changes in a changing climate, and the signal can be decomposed into contribution from changes in temperature, pressure, and humidity.

    Nevertheless, derived parameters like temperature should not be completely disregarded. RO temperatures are, e.g., very accurate around the tropical tropopause and any potential influence from high altitude initialization or uncorrected ionospheric errors has vanished at these altitudes. Given the good height resolution of RO data, temperature climatologies can provide valuable insight in changes in tropopause height and temperature as well as in the transition region between tropospheric warming and stratospheric cooling.

    A detailed response can be found in the attached pdf

    Response by Ulrich Foelsche 

  5. Jens Wickert

    Response from Jens Wickert

    I attach two papers on some investigations to use refractivity as indicator for climate change (Henrik Vedel, DMI). May be there are some helpful suggestions regarding this issue.