Example of Limb Effects on NESDIS SFR Product

SPoRT continues to work with select NWS WFOs in evaluating the NESDIS SFR product.

One thing to take into consideration when using data from “whisk-broom” instruments on polar-orbiting satellites, such as the Advanced Microwave Sounding Unit (AMSU) used to generate the SFR product, is that data at the edge of the swath (i.e. limb) may provide misleading or erroneous observations. As the instrument scans farther from nadir, it is looking through more of the atmosphere, creating both a bigger observation field of view (i.e., larger pixel) and having the signal attenuated by more atmospheric constituents (e.g., in-cloud and falling snow).  As a result, when interpreting the SFR product, it is important to look for the extent of the swath (outlined in gray in the product in AWIPS) to determine whether the observed SFR is going to be limited by these limb effects.

Let’s take a look at an example over the NY Tri-State area for the post Super Bowl snow event.  In the first image, from Metop-A valid at 1458 UTC, there is a large area of snowfall across the area.  The heaviest SFRs appear to be around 1.2-1.5 in/hr (when multiplying the liquid equivalent by 10) across central and southern New Jersey.  However, an hour later (see second image from Metop-B valid at 1554 UTC), the shape of the heaviest SFR has expanded north and west and there are now readings over 2.0 +in/hr.  Other areas that had a SFR of less than 0.5 in/hr in the 1458 UTC image, appear to have a SFR of around 1.0 in/hr just an hour later, which is a large jump in intensity.

While it is certainly possible that the snow evolved and intensified in less than an hour, it is more likely that instrument limb effects are likely to blame for the larger SFRs in the second image.  Make sure to check that swath edge when using polar-orbiting satellite data!

NESDIS SFR Product from 1458 UTC on 3 February 2014 showing snow detected near nadir for Metop-A

NESDIS SFR Product from 1458 UTC on 3 February 2014 showing snow detected near nadir for Metop-A.

NESDIS SFR Product from 1554 UTC on 3 February 2014 depicting what are likely erroneous higher intensity SFR values along the swath edge from Metop-B.

NESDIS SFR Product from 1554 UTC on 3 February 2014 depicting what are likely erroneous higher intensity SFR values along the swath edge from Metop-B.

5 thoughts on “Example of Limb Effects on NESDIS SFR Product

  1. Thanks for the observation of the limb effect in the product. The SFR algorithm indeed takes the instruments’ scanning geometry into consideration. However, it is possible that the compensation is inadequate to overcome the limb effect. This is something we need to pay more attention to in future development.

  2. I was surprised that the effects were this large. For this example, where there is that much snow on the scan edge, it might not be possible to fully account for the limb effects.

    I haven’t looked at ATMS much. Does it have similar limb issues to AMSU? Or have some of the effects been mitigated as was done with the design of VIIRS, where it retains pretty good fidelity on the limbs?

    • We haven’t done any focused study on limb effect in SFR because we haven’t noticed such large limb effect before. As far as I know, AMSU and ATMS do not have any special design to reduce limb effect.

  3. So one of the advantages of a MW sounder is the wider swath width as compared to the MW images; so we are always faced with proving more data coverage at some potential degradation in performance if we restrict the retrievals to more nadir viewing angles. Also remember the larger field of view sizes at the limb also increases the uncertainty in the retrievals there.

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