Sawtooth Update June 15

The experiment testing the impact of not using AMSUA for land snow conditions progressed several more days overnight. The results show that a weaker version of the Sawtooth shows up as we get deeper into winter, even without AMSUA. AIRS radiances are still being assimilated. Further analysis shows that in Siberia (where the Sawtooth is worst) the synoptic conditions change, decreasing clouds, and the Sawtooth simultaneously appears. The figure below shows the original Sawtooth (b5_b10p9), the latest experiment restricting AMSUA (exp06g) and NCEP Operational analysis for Jan 1-11, 2006. It is important to note that the weakened Sawtooth in exp06g somewhat compares to a semi-diurnal cycle in the NCEP operational analysis (between Jan 9-11, should have been still using the SSI analysis).

The working hypothesis for why the weakened sawtooth appears is that AIRS is still prevalent in this region. Some are looking into the radiative transfer model. Also, we are not using the latest NCEP CRTM. There was some discussion on updating the CRTM to the latest, however, it quickly determined that this would require a complete update of the GSI in the GEOS5 system. Of course, that would incur significant delays.

For now, we are going to continue to monitor the experiment. But the major portion of the problem seems to be under control. Since theGEOS5 is in range of NCEP operational analyses, plans for validation will go forward. In validation we will examine further whether this feature adversely affects the science quality of the data set. The good news would be that the problem has finite bounds in, only over land snow, and only occur then when AIRS is available (starting 2002).

Just a quick note and reminder of the positive aspects of this system we have previously reported (we tend to hammer at the problems). The precipitation from the configuration for MERRA seems to be well in line with GPCP, and a good contribution compared to other reanalyses. TPW looks reasonable compared to NVAP data (though those obs may have bias). There will be data supporting the stratosphere community that should be very unique. At one-hourly intervals, the surface diagnostics should be very useful for diurnal cycle studies. These and a few other aspects should help make MERRA a reasonable contribution.