In late 2005 a MERRA review group was formed from experts in various aspects of Earth system science and users of existing reanalyses. Their charge was to review the GMAO strategy for MERRA and the validation experimentation and results, possibly contributing some of their own analysis. The goal for GMAO was to gain a preliminary assessment of the scientific merit of the GEOS-5 data assimilation system for MERRA prior to full production. In September 2007, the validation experiments began, and on November 7, the user review group met to discuss the results of the validation experiments with the GMAO and NASA HQ representatives.
The GMAO started the day, presenting a summary of the system and critical improvements in recent months (Rienecker), the dynamical circulation, clouds and radiation (Suarez and Bacmeister), climate variability features (monsoons, hurricanes, low level jets (LLJ) and diurnal cycle - Schubert) and precipitation statistics and land hydrology (Bosilovich and Koster). Key points from the presentations are summarized below.
Michele Rienecker reviewed some major and critical changes to the system since the inception of the Review Group. These include improvements in the use of retrieved wind speed over the ocean, improvement in the radiance assimilation (through the latest CRTM radiative transfer coefficients), corrections to bias and jumps in the radiosonde observations and a fix for diurnal cycle of glacier surface temperatures.
In looking at zonal circulation, Max Suarez showed the differences between the GEOS-5 and other reanalysis systems for winds, temperature and humidity. For example, the GEOS-5 eddy heights compare with ECMWF operational analysis both in a mean sense, and in the interannual variability. With small contour intervals in the zonal cross-sections, differences in tropopause height can be identified among all the reanalyses. In addition, GEOS-5 reproduction of stratospheric ozone profiles is reasonable, and a limited comparison of the beginning of a quasi-biennial oscillation looks promising. One possible systematic problem is high upper troposphere humidity (as compared to ECMWF and NCEP operational analyses). The radiation fluxes have some bias, as well, but these are somewhat reduced compared to the existing reanalyses (Figure 1).
Siegfried Schubert reviewed some evaluations of monsoonal circulations, including the North American monsoon and Indian monsoon. GEOS-5 reproduces the low level winds (e.g. the Somali jet and the Great Plains LLJ) as well as the subseasonal breaks observed in the monsoonal precipitation. There are some apparent regional biases in the precipitation, but this is also true among all the existing reanalyses. The GEOS-5 North American monsoon circulation and precipitation compare well with the North American Regional Reanalysis (NARR) (for July 2004, Figure 2). Globally, the interannual variations of precipitation compares well with observations, and better than existing reanalyses. In addition, the monthly average water budget shows globally averaged analysis tendencies to be a small value (Figure 3). However, the diurnal amplitude of continental precipitation is large and the phase is shifted to a daytime maximum compared to observations. This is a problem for all reanalyses, and it persists in the GEOS-5 system.
Mike Bosilovich reviewed monthly mean precipitation, where GEOS-5 generally produces good fields compared with GPCP and CMAP, not only in the global mean, but also spatial correlation. In addition global P-E is generally small (near zero) indicating that the global analysis is relatively well balanced (but will be non zero). The GEOS-5 precipitation is reasonable in many regions and latitude bands. Comparisons for the Mississippi River basin precipitation against daily gauge data show the GEOS-5 was able to produce the daily precipitation events, and the no-rain days for Jan-Oct 2004 (Figure 4). However, maximum intensities in the warm season are underestimated, leading to an underestimate of the total basin precipitation. Randy Koster’s analysis of the time series of precipitation shows that the occurrence of rain during the day coincident with solar forcing causes high interception loss of water vapor, and then the runoff water is underestimated. The transition of the observing system to include SSM/I was tested in a data withholding experiment. GEOS-5 tropical precipitation increases with the inclusion of SSM/I, but the increase is less than 10% of the tropical precipitation (in contrast, JRA25 has a change in extratropical precipitation). There is also a small increase of total column water, ocean surface winds and ocean evaporation.
The overall conclusion is that the GEOS-5 system can produce many aspects of the Earth system as well or better than existing reanalyses. The quality of the data coupled with the fine temporal and spatial scale of the data should make the GEOS-5 reanalysis useful for many purposes. While there were spirited discussions among all the participants, the external user group members’ sentiment reflects this conclusion as well. As of November 2007, the reanalysis data streams are being spun up, and data should start flowing to the scientific community early in 2008. The full MERRA data product will take approximately 18 months to generate.
Figure 1 Monthly mean (Jan 2004) TOA Longwave radiation differences between CERES ERBE-like observations and several reanalyses and operational analyses.
Figure 2 Comparison of the seasonal evolution of the North American monsoon between the North American Regional Reanalysis (NARR) and GEOS5.
Figure 3 Global vertically integrated water vapor budget for July 2004 including the physical components, the analysis increment and residual.
Figure 4 Mississippi River basin area-average (over all sub-basins) daily precipitation for January – September 2004. The figures show the scatter of the daily data, the daily time series, and the accumulated precipitation. The observations are CPC daily ¼ degree gridded gauge data.
