Output Data Review

We are currently reviewing the MERRA File Specification Document, and considering new variables and diagnostics that should be incorporated into future experiments and reanalyses. This is directed toward information that needs to be captured during run time, as opposed to diagnostics that can be post processed from existing data. For example, to get Max/Min temperature from MERRA would be the maximum/minimum hourly averaged temperature, in the current configuration. We will implement for future analyses hourly maximum and hourly minimum temperatures, so that the instantaneous Max/Min T can be captured from the system.

Any suggestions would be welcome. Please include any computations that may be non standard, and reasons or applications of the suggested diagnostic.

Apologies for letting the blog become stagnant. While I have seen a lot of interesting work with reanalyses and MERRA, finding the time to work up a worthwhile post is remarkably challenging.

Extrapolation to P > Ps

As many have found, MERRA pressure level data does not provide values for pressure surfaces when they are greater than the surface pressure (e.g. high topography).  Other reanalyses extrapolate the data using the surface meteorology and assumed lapse rates. This data may be useful in some cases such as zonal averaging, stream functions and thickness calculations.

A recent post at reanalysis.org provides user developed codes to fill these undefined grid points. This should be useful as one could adapt the codes to the filling method applied in other reanalyses to better match their extrapolated data.

See Extrapolation of MERRA Reanalyses to obtain continuous fields for more information.

MERRA real time production schedule

Stream three has finished March 2010, signifying a transition to real time production schedule. Production will lag real time by approximately 3 weeks, allowing collection of all the observations used in the retrospective periods. We are planning one week to review and quality assure each new month's data files, before being sent to the GES DISC. So, Each new month of MERRA data will be available online roughly 1 month behind present time, and will be released in 1 month segments.

Jan-Mar 2010 are currently being reviewed and should be available on the DISC within the next couple of days.

Pressure levels greater than surface pressure

Some questions have come in regarding differences between MERRA and other reanalysis at pressure 1000mb and 850mb pressure levels. It is very important to note that MERRA does not extrapolate pressure level data vertically greater than the surface pressure. The result is that there is undefined data points in much of the 1000mb fields. This will affect the representativeness of both time and area averages of MERRA data compared with reanalyses or other data that extrapolates gridpoints to pressure levels greater than the surface pressure. The GMAO provides a summary of the impact that this has on averaging. In addition, the FAQ will be updated to call out this difference with other data sets. The main GMAO www page also has other pages with useful practical information about the data and assimilation system.

Hurricane Andrew, Aug 1992

MERRA Stream 2 has completed through 1993. We have been looking at various weather and climate events. Hurricane Andrew was a powerful, but fairly small hurricane. MERRA's 1/2 degree resolution is likely too coarse to adequately resolve the circulation, and there is no bogus or center relocation being done in the system. Still, assimilation of observations will show some circulation or feature.

There isn't much of a circulation prior to landfall in Southern Florida. Landfall was at 9Z24Aug1992. Figure 1 shows the sea level pressure and wind barbs from 6Z the closest analysis time before landfall. The pressure center is much higher than the observed center pressure (955 mb). the center of the pressure is located south of the best track at that time. The feature that really attracted attention is the offset of the wind circulation, even further south than the pressure center, and crossing the isobars at the center.

Figure 1 MERRA Sea Level Pressure and 1000 mb wind barbs from the 6Z 24 AUG92 analysis. The blue line shows the best track befre and after landfall, with red markers at 00Zs.

A closer look at the observations being assimilated shows that ERS1 did track over the center of circulation around 6Z. Figure 2 shows all the observations accepted into the analysis. There are ERS1 wind vectors crossing the center of the circulation, and the assimilation system accepted the data. The vectors closest the center are likely contaminated by precipitation, and should have been rejected. At this point, it's not clear how often this kind of problem occurs, or what might be done to detect and reject the bad data. It's under investigation.
An important point for reanalysis users, especially as resolutions are increased to better resolve weather and smaller scale circulations, is that reanalyses are assimilating vast quantities of observations. sometimes poor quality data does make it into the analyses. While quality of data and analyses are improving, users still need to consider that features may or may not be realistic. Likewise, we are providing some information on the accepted observations in MERRA. More difficult is providing access to the users on the actual observations.

Figure 2. MERRA analysis sea level pressure, analysis streamlines and windbarbs showing the accepted observations (red is buoy or ship, black is ERS1). Most of the ERS1 vectors seem to agree with the mass field, however, close to the center of the low pressure the ERS1 vectors are crossing the isobars. The wind analysis is drawing to the observations, even when they disagree with the mass field.

MERRA FAQ

We have started to put together a list of frequently asked questions regarding MERRA. Obviously, it will grow with the frequency of questions. Comments or questions posted here will also contribute to the list.

http://gmao.gsfc.nasa.gov/research/merra/faq.php