Status July 10

The January 2006 validation experiment is underway. This period has been rerun enough in recent weeks that much of the preparatory work had already been done. The primary validation experiment for 2004, is still being prepared. Scripts, code and data need to be in place and working together, soon was the latest update.

There has been much work on the mid- to late- 80s experiment (referred to as the SSMI experiment). Originally, this was established to investigate the impact of the availability of SSMI observations in July 1987 on the time series. The experiment was initialized in Mid-Dec 1983, and run almost through 1990. The spatial resolution is coarse (2 x 2.5), and the version of the system is slightly behind the expected version for MERRA validation (a subsequent test shows that the physics/statistics difference do not change the main results of the SSMI experiment).

The main points to be discussed on the SSMI experiment are:

1. Global time mean precipitation bias
2. Water cycle time series
3. Impact of SSMI

1.) As stated on the previous post, global time mean precipitation, where the SSMI experiment (version b10p9 read - beta 10 patch 9) is 2.2 mm/day compared to 2.6 mm/day for GPCP and ~3mm/day for JRA25 and ERA40. An experiment at the full resolution and fallback MERRA version of GEOS5 (b10p14) shows that the precipitation to be ~0.2mm/day higher than the coarse resolution experiment. Most of this increase is a result of the spatial resolution. Much of the difference in the precipitation is over the tropical oceans where reanalyses are typically much to high. As the system stands now, global mean precipitation is lower than GPCP in the mid to late 80s.

2.) The time series of the SSMI experiment also showed some features that are currently being investigated more closely. Figure 1 shows the time series of precipitation anomalies (mean annual cycle from 1984-1987 removed) for the GEOS5 SSMI experiment, JRA25, ERA40 and GPCP.

Figure 1 (click on the figure to expand it)

Both GEOS5 and ERA40 show a decreasing tendency in the precipitation from 1984 throught the end of 1985. In Jan 1986, ERA40 tendencies reverse and start increasing. In Nov 1986, GEOS5 drops sharply, but stabilizes. NOAA10 data begins in Nov 1986, SSMI begins in Jul1987, NOAA11 begins in Jan 1989. The two issues being investigated are the sudden downward jump of precipitation with NOAA10, and the slight downward tendency early in the experiment. Also the introduction of SSMI is noticeable in the ocean only average (and increase for GEOS5, and a decrease for JRA25).

Figure 2

Figure 2 shows the time series of GEOS5 monthly analysis increments of water vapor (the incremental analysis updates that drive the diagnostic ouput, such as precipitation) at 4 levels in the lower troposphere for latitudes 60S-60N. At the lowest model level (not shown) the shipborne observations of moisture lead to positive increments almost every where and when. Above the surface layer, the lower tropospheric analysis is largely negative increments, acting to dry the atmosphere. The negative increments appear correlated to the precipitation anomalies, though there is also a period between Jul1985 and Jan1988 where TPW increases (see Figure 3). A large jump in the increments and precipitation (Figure 1) is associated with the introduction of NOAA10 (and shutdown of NOAA6 MSU). These are the focus of some ongoing evaluations.

First, an experiment with the latest MERRA system has been run over the start of NOAA10, and it shows less sensitivity in the increments than Figure 2. However, the precipitation in these experiments is similar. To test strictly the sensitivity of the system to NOAA10, a new experiment is being run forward but constraining the NOAA10 observations to passive mode (an analysis is made, but the increments will not contribute back to the system). Secondly, NOAA NESDIS is generating new coefficients for the historical periods polar orbiting satellites (to which we are grateful). It is not clear at this point what the impact of that will be, but will be thoroughly tested prior to MERRA production. Lastly, bias corrections are being made in the system (e.g. for view angle). The procedures for initializing and carrying these bias corrections are being reviewed. This is one possible source of error, but it is not yet clear a problem exists.

In summary for point 2, it seems we have some sensitivity to the observing system (regarding precipitation) on the same order of magnitude as previous reanalyses in the 80s. We are using this opportunity to flesh out an problems in the system that may be exacerbating the discontinuity of the analysis during observing system changes. These tests are intended as checks on the system before production, though, we expect that there will be noticiable changes in the MERRA time series as a function of the observing system.

Figure 3 Monthly anomalies from the Jan84-Dec87 mean annual cycle.

3.) The impact of SSMI was not immediately apparent in GEOS5 global precipitation (Figure 1), and ocean only average precipitation increases. The SSMI impact on evaporation and surface wind speed over the ocean is also apparent (Figure 3), likely related to the SSMI wind speed. The GEOS5 anomalies and JRA25 seem to be tracking really closely. In the mean, JRA is ~0.2m/s higher wind speed than GEOS5 (of 4.4m/s). So, it seems that in some ways GEOS5 is sensitive to SSMI, like JRA. However, in ocean precipitation, GEOS5 is more sensitive to the NOAA transitions than JRA (Figure 1).

Just a caveat regarding these results. In comparisons of the 2 degree resolution with 1/2 degree, the monthly precipitation increases at finer resolution, especially in mid-latitudes. We have not yet run multiple years of the 1/2 degree system, and don't have a grasp on the interannual variations. It should be interesting to see how similar the coarse and fine resolution analyses are over long periods.


Figure 4 Zonal time series of precipitation anomalies (after ENSO removal) for JRA and the GEOS5 SSMI experiment. The impact of SSMI on JRA is apparent in the southern hemisphere (30-60S). GEOS5 low frequency decreasing precipitation is focused in the tropics.