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.
What data are being produced?
The MERRA File Specification Document provides extensive information on the collections of variables, units and data files.
How long will MERRA be? Will MERRA continue as a climate analysis?
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.
MERRA has 3 separate streams, how is the transition made from one to the other?
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.
Is there a way I can subset the data?
yes. You can currently subset the monthly and hourly data through the Data Subsetter application. It is a user friendly interface that allows you to subset the data spatially, temporally, and vertically, as well as compute means or subset out a time in the hourly files.
How can I get access to the data?
You can find access to the data and documenation on the products by going to "Data Holdings" on the MDISC portal. From a drop down you can select the product of interest and the page will provide all the data access methods with links, as well as visualization options.
Is LWGAB the same as LWGDWN and is LWGEM the same as LWGUP?
yes, ab=absorbed, em= emitted.
I was wondering if I am correct in assuming LWGEM=LWGUP and LWGNT=LWGNET?
For the LW fluxes, there are variables such as LWGEM,LWGAB, and LWGNT which concern the ground. In the full file specification documentation, however, these are not mentioned and instead, there are LWGDWN, LWGUP, and LWGNET. To get LWGDWN you can take LWGNT+LWGEM, but also Absorbed Longwave at the ground LWGAB would be the LWGDWN. The net is net down, so LWGNT=LWGAB-LWGEM. The documentation is currently being revised to correct this inconsistency between the documentation and the data files.
Does MERRA have rainfall data?
Yes. You can look at the 2 dimensional surface turbulent flux product tavg1_2d_flx_Nx (MAT1NXFLX). You can get more information on this data product and ways to access it from:
In the 2 dimensional surface turbulent flux product tavg1_2d_flx_Nx (MAT1NXFLX) there is a variable FRSEAICE that has the fractional cover of sea ice. From that you can determine the extent of sea ice. This data is on the native grid. You can get more information on this data product and ways to access it from:
What is the difference between the flux data included in the FLX collection and the LND collection?
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.
Why are there such large discrepancies at 1000mb and 850mb bewtweenMERRA and other reanalyses?
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. http://gmao.gsfc.nasa.gov/research/merra/pressure_surface.php
Radiation calculations with MERRA's outgoing shortwave radiation variable are not correct
In older versions of MERRA's File Specification Document and the original HDF meta data, radiations variables called SWTNT* have long_name descriptions that state they are outgoing shortwave radiation. The long_name is in error, as is the older File Specification Documents. Variables that start with SWTNT are really net shortwave radiation at the top of the atmosphere.
Both MERRA and NCEP CFSR use the GSI data assimilation, are they identical analysis routines?
No. MERRA development and validation proceeded before NCEP CFSR, and so uses an older version of the GSI. The release date of the MERRA GSI is December 2005. The MERRA system is documented in the GEOS-5 Tech memo.
Are the units of OMEGA documented correctly in MERRA?
The units for OMEGA are Pa/s. Only in the metadata for the 3d_asm_Cp products are the units for omega erroneously listed as Pa/day.
How can I obtain the data in ASCII format? / How do I use OPeNDAP?
To obtain the files in ASCII format you can use OPeNDAP. I am using the MAI3CPASM product as an example since it includes the 3rd dimension. A sample url for Jan 01, 1979 for the wind components u and v would be: http://goldsmr3.sci.gsfc.nasa.gov/opendap/MERRA/MAI3CPASM.5.2.0/1979/01/MERRA100.prod.assim.inst3_3d_asm_Cp.19790101.hdf.ascii?EOSGRID.Data%20Fields.U[0:1:7][0:1:4][0:1:143][0:1:287],EOSGRID.Data%20Fields.V[0:1:7][0:1:41][0:1:143][0:1:287]
A breakdown of the url is as follows: Dataurl.ascii?EOSGRID.Data%20Fields.Parameter1[time array][vertical level][latitude array][longitude array]
If you look at your url you will see the parameters you put in your email then the time array [0:1:7], which means you want all the hours for that day, then you have [0:1:143][0:1:287] which means you want the global region, and [0:1:41] for all the vertical levels. You will want to copy into notepad the Data URL example above, and change the year and filename for each year, month and day you want. Since you would have to construct a url for each data file its best to do with a script to create all the urls if you want a lot of data.
You can build the url easily if you want different parameters, vertical levels or a different spatial region by going to the opendap page for the individual data products: CHEM data : http://goldsmr1.sci.gsfc.nasa.gov/opendap/ 2D Data : http://goldsmr2.sci.gsfc.nasa.gov/opendap/ 3D Data : http://goldsmr3.sci.gsfc.nasa.gov/opendap/ Click on the check boxes next to the variables you want to download and at the top of the screen the Data URL will update with the variables. NOTE that this is for an individual day but serves as a good guideline on what the url structure would need to be.
If you want to download a lot of data files then it is reccommended to write a little script to generate all the urls and output to a txt file. In the example below it is named myfile.txt and it contains 5 files. Notice the txt file contains "-O MERRA100.prod.assim.inst3_3d_asm_Cp.19790101.SUB.hdf.ascii" which would be what the the downloaded data file would be named, with "SUB" in the filename so it is distinguishable from the original data file name.
What you would do is create a txt file named whatever you like and run the command for wget: On your command line: wget -i myfile.txt
You can also download wget for windows on PC through: http://gnuwin32.sourceforge.net/packages/wget.htm
You can also download with CURL: a UNIX curl example: On your command line: xargs -n 1 curl -O -u anonymous:curl@example.com < myfile.txt
The myfile.txt file would look like the following if using on unix with curl (same without the first line for wget): #!/bin/sh
I have noticed high frequency noise (2 delta x) in MERRA height and other 3D fields. Is there a problem with the data or the analysis?
We are aware of noise in height fields in GEOS-5 products, including MERRA. They are arise from orographic gravity waves in the model because we are not filtering the topography. Because of the association with topography, the features are present even in monthly means. They are not a result of the analysis.
To our knowledge, they are merely an annoyance and do not affect the quality of the product otherwise. We suggest smoothing before diagnostic analysis, especially if taking derivatives.
An alternative would be to use the fields from the coarser resolution products (inst3_3d_asm_Cp (288x144)) rather than those on the native grid. The coarser resolution products bin the data from the native grid and so smooth out fields with 2 delta_x noise.
I need the surface wind speed in MERRA, and the File Specification document mentions the variable SPEED. Is this what I need?
The SPEED variable is the wind speed used for computing turbulent surface fluxes. It is the magnitude of the 2D wind at the top of the surface layer plus a gustiness term based on a diagnosed vertical velocity scale plus a ventilation term related to precipitation. The definition of SPEED is valid for the MERRA data, but has changed in subsequent versions of the GEOS5 data assimilation system.
If you are looking for the horizontal wind speed at the surface you will likely be better served by downloading the two meter wind speeds ( u2m and v2m ).
What is Displacement Height, DISPH?
The choice for a more complete derivation and discussion is a micrometeorology text book, for example, "Boundary Layer Meteorology" by Roland Stull.
Briefly, elements of the Earth's surface, grass, shrubs, crops, trees and buildings, all cause some friction and perturbation to the wind profile. The displacement height (or depth, or zero-plan displacement) accounts for their effect in the calculation of the surface layer log wind profile. The displacement height is the height at which the log wind profile projects the wind to be zero for purposes of computing the surface later turbulent fluxes. At heights less than displacement, different physical processes and theory take over from the log profile methods. For practical purposes, MERRA 2m and 10m output are intended to compare with screen level meteorology stations.
