MERRA Data

MERRA U-WIND in June 1979
(Credit: Image from NASA)

MERRA is an abbreviation for Modern Era Retrospective-Analysis for Research and Applications. The dataset originates from the Global Modeling and Assimilation Office of NASA / Goddard Space Flight Center. The MERRA analysis are being conducted with the GEOS-5 Atmospheric Data Assimilation System(ADAS).

Resolution and Coverage

The model grid is 0.5 degree latitude and 2/3 degree longitude.
It holds 1 hourly values in a period from 1979 until present.
Coverage is global for land-areas and coastal regions.
Offshore coverage is only to be expected up to approximately 50 km from the coastline.

Data Availability

MERRA data is commonly available within WindPRO with a time-lag of 1-2 months. This is due to the processing time of the data at NASA and the subset-processing at EMD. When the data have been processed at EMD, then the following parameters are available from within the online-data services within WindPRO. Please note that the displacement height is the height at which the log wind profile projects the wind to be zero. See more at the MERRA-FAQ.

Table: Overview of MERRA Dataset Parameters.

Parameter	Description
time	UTC time stamp
pressure	Sea level pressure
temperature	Temperature at 2 m above the displacement height
wDir	Wind direction at 50 m above surface
wSpeed	Wind speed at 50 m above surface
dispHeight	Displacement height

Note from 2014-06-03:
50 m temperature records are now available in WindPRO for new MERRA downloads. This 50 m temperature is modelled from the 2 m native temperature data - by invoking the global temperature lapse rate from the Standard Atmosphere Model (essentially 0.65 degree C per 100 m, i.e. an adjustment of 0.312 degree C from 2 m to 50 m). For completeness, please note that any height difference due to the displacement height is presumed negligible due to other model components and has therefore not been taken into account. For information on the standard atmosphere model see here.

Data Experiences

Generally, the MERRA data is always considered when seeking for candidate reference wind datasets for a particular site. Correlation between local measurements and MERRA data is often high, see the figure below. For his particular study made in 2010, MERRA data was compared against NCEP/NCAR, 3Tier (commercial data) and SYNOP data.

Comparing Tall Mast data (Risø) to NCEP/NCAR, 3Tier, MERRA and SYNOP data

Usage Notes

MERRA data downloaded in WindPRO before April 21, 2014 may contain up to 10 single-day gaps in the years 2000-2012. If the data is processed solely in WindPRO, this is no problem due to the small amount of missing data and the distribution over the years. If the data is to be processed outside WindPRO, it may however lead to problems synchronizing data from different sources. In these cases, it is recommended to re-download the required datasets.

External Links

• The full MERRA data-set is available at http://gmao.gsfc.nasa.gov/research/merra/
• The MERRA-FAQ is available at: http://gmao.gsfc.nasa.gov/research/merra/faq.php
• FTP download directory for daily MERRA files: [1]

Acknowledgement

Credits: The Global Modeling and Assimilation Office (GMAO) for the MERRA-analysis and the GES DISC for the dissemination of MERRA.