NAM upgrade in December

A major NAM implementation was made in December. Highlights are summarized on the COMET Operational Models Matrix page at
http://www.meted.ucar.edu/nwp/pcu2/
in the "What's New!" section at the top of the page.

The most important change with a consistent positive forecast impact in the 2-3 day time range is "partial cycling". The NAM uses a data assimilation cycle that starts 12 hours before the forecast initial time, such as 00 UTC the previous night for the 12 UTC morning forecast cycle. At the 12-hours-prior time (for example, 00 UTC), an analysis is performed using a first guess plus observations valid around that time. It is this 12-hour-old first guess which has changed. Then a 3-hour "forecast" is run, serving as the first guess to an analysis using more observations at 9-hours-prior (for example, 3 UTC), and repeated in 3-hourly increments up to the time of the forecast cycle (for example, 12 UTC). Thus, in total, there are four 3-hour "forecasts", each serving as as the first guess to the next analysis, culminating in the initial analysis for the 84-hour NAM forecast. This allows the model to utilize late data from previous cycles and spin up a dynamically consistent field across the entire domain over the 12-hour pre-forecast time period. Previously, the initial guess at the 12-hour-prior time came from the previous NAM assimilation cycle, so the NAM was essentially cycling on itself. Now, the atmospheric fields (e.g., not soil moisture and other non-atmospheric conditions) are taken from the GFS, using the final GDAS cycle which includes late-arriving data. Thus, now, the NAM forecast has a direct link to the GFS analysis from 12 hours ago, though it has been modified by 12 hours of NAM assimilation cycling.

Example:
12 UTC 84-hour NAM forecast
----------------------------------
1. 00 UTC first guess from GFS final analysis (includes data arriving later than from 00 UTC GFS run)
2. observations + regional GSI --> NAM analysis valid 00 UTC
3. 3-hour NAM "forecast" 00-03 UTC --> first guess for 03 UTC analysis
4. observations + regional GSI --> NAM analysis valid 03 UTC
5. 3-hour NAM "forecast" 03-06 UTC --> first guess for 06 UTC analysis
6. observations + regional GSI --> NAM analysis valid 06 UTC
7. 3-hour NAM "forecast" 06-09 UTC --> first guess for 09 UTC analysis
8. observations + regional GSI --> NAM analysis valid 09 UTC
9. 3-hour NAM "forecast" 09-12 UTC --> first guess for 12 UTC analysis
10. observations + regional GSI --> NAM analysis valid 12 UTC
11. 84 hour NAM forecast from 12 UTC

Stephen

The December 2008 NAM changes combined with the March 2008 NAM changes have enabled the NAM WRF-NMM to overcome many of the shortfalls during the initial implementation of WRF-NMM in the NAM slot, replacing the Eta model, back in 2006, finally achieving the success that had been anticipated. The combination of these two excellent implementations places the NAM on an even par with the GFS in synoptic forecast skill all the way through 84 hours.

The March 2008 NAM changes, as well as the December 2008 NAM changes, each separately led to consistently large, positive forecast impacts, much more so than typical of model changes over the past 10 years. While many factors probably contributed to the March model improvement, the largest contribution seems to have come from the gravity wave drag and mountain blocking, which had not previously been included in the Eta model nor its successor in the NAM, the WRF-NMM model. It is explained on the page
http://www.meted.ucar.edu/nwp/pcu2/namturb2b.htm
or you can find it navigating the COMET Operational Models Matrix at
http://www.meted.ucar.edu/nwp/pcu2
scroll down to the Turbulence row at the bottom of the Model Physics section and click on the link in the NAM WRF-NMM column. Then, click on the Scheme Implementation tab at the top of the page and then click on the Topography tab among the expanded choices at the top of the page.

Stephen