MERRA FAQ

MERRA began production in March 2008. After data is produced, the GMAO examines the data files in several ways to test the quality of the data. When we are confident that the data files are within the tolerance of the uncertainty we have seen in our validation experiments, we release it to the MDISC. At that point, they provisionally release it for download. We are not holding the data for any substantial length of time, before it can be accessed by the community.

When will my case study be produced and available? Or, when will X number of continuous years be available? Each stream is moving forward at ~10 days/real day, slightly faster for the early period as there are less observations to deal with. So, let's say ~2 months each week, so maybe in 7 months we can have 5 years.

These are best case conditions that will not happen over long periods. The computers go down from time to time, scheduled and unscheduled. We estimate more than a year to finish any one stream, and likely 18 months to finish it all. Also, there is a chance that a severe bug may occur that affects the scientific integrity of the data. In this case, any time period may need to reprocessed delaying overall production. Check: http://gmao.gsfc.nasa.gov/operations/merra_status_production.gif for the current progress of the data streams. This provides the current date of each stream. Use 10 days per day, and add 25-50% slack to estimate when the date you are interested in will be produced.

The MERRA File Specification Document provides extensive information on the collections of variables, units and data files.

MERRA production will continue into the future, and does not have a set stopping time. However,version of the GSI analysis used by MERRA is not the latest,not be capable of ingesting new instrument data. This will become a serious issue if the new instrument is a replacement for an important older insturment that has failed or gone offline. So,feasible to do so.

The plan is to catch the first and second streams up to the beginning of the subsequent streams. The initial release of MERRA data will simply transition there. However, we plan to continue the first and second streams for two years,and variance between the two. Significant differences are not expected in most meteorological fields and diagnostics. Should any differences arise, we will revise the plan for the transition of streams.

MERRA's land parameterization is Randy Koster's Catchment model, but other surfaces, such as inland water, ocean surface and glaciers are also accounted for as sub-grid tiles. In the LND collection of variables, all the data are derived from the land model, and are not weighted according to the land fraction at that grid point. This data is provided to better compute land budgets for soil water and land energy.

The data in FLX, RAD or any other collection of variables represent the gridbox average of all the different tiles weighted by their fractional cover. This is where you would use evaporation to compute the atmospheric energy balance. The important distinction here is that LND is land only, while all other collections are representative of the whole grid box.

Fractional land cover in GEOS5 and MERRA is discussed more here: http://gmao.gsfc.nasa.gov/research/merra/land_fractions.php

The GEOS5 data assimilation system used to produce MERRA does not (or did not at the time of production) extrapolate data to pressure levels greater than the surface pressure. These grid points are marked by undefined values. The result is that area averages that include these points will not be representative compared to other data sets without additional screening. Time averages, such as monthly means, may also have substantial differences at the edges of topography. The lowest model level data and surface data are available so that users can produce their own extrapolation. A page discussing this issue is available. See http://gmao.gsfc.nasa.gov/research/merra/pressure_surface.php