MERRA-2 Data

Example of MERRA-2 mean wind speeds at 50 m. Screen shot from http://www.windprospecting.com/

Observations assimilated in MERRA2 (top) and MERRA (bottom) datasets for the period 01.1980 until 12.2014. Units are millions per 6 hours. From Bosilovich et al.

Introduction

MERRA-2 is the most recent version of the popular MERRA dataset modelled and distributed by the Global Modeling and Assimilation Office of NASA / Goddard Space Flight Center. The original MERRA dataset is discontinued in early 2016. After this point MERRA-2 is the most recent and updated MERRA dataset. MERRA-2 uses a newer version of the modelling system and also incorporates newer data sources and observations (see the lower-figure to the right).

At EMD, we have done a comprehensive evaluation of the performance of the MERRA2 dataset for wind-energy applications. Please consult the paper by Thøgersen et al, "Is MERRA2 Able to Replace MERRA as a Trusted Reference Wind Dataset?". The paper is available from the windPRO knowledgebase, here.

Dataset Overview

• Content:: Renewable energy focused data subset of MERRA2 data.
• Spatial Resolution:
  • 0.5 degree latitude
  • 0.625 degree longitude
• Temporal Resolution: Hourly values.
• Coverage:
  • Coverage is global for land-areas and coastal regions.
  • In windPRO, offshore coverage is at a distance of approximately 300 km from any known coastlines.
• Period: 25+ years. The EMD databases will hold data back to 1992.
• Update schedule:
  • EMD updates the dataset once a month - with data being available with a 1-2 month delay from real-time due to NASA/MERRA data availability.

Usage Notes and User Notifications

2021-11-30: NASA has notified us about an issue with the MERRA2 data from a 4-month period ranging from June 2021 to September 2021. The issue is caused by an error in the MERRA2 pre-processing of ocean buoys observations of near-surface temperatures – resulting in significant near-surface temperature bias’s in various regions. It is currently not clear to EMD if other parameters than near-surface temperatures are also influenced, and if so, to which degree. As such, MERRA2 data from June-September 2021 has now been removed from the windPRO online-data services. For users who have downloaded these MERRA2 containing the 3 months above, we recommend that you consider taking the following actions with your windPRO project: 1) Remove the MERRA2-data for the months of June 2021 - September 2021. 2) Redo any calculations or reports where you have used these months. We will add a new notification here when corrected MERRA2 data for June 2021 - September 2021 has become available from NASA sources. According to NASA, the corrected data is likely to be released during mid-December 2021. 2020-11-05: NASA has released a revised MERRA-2 dataset covering the month of September 2020. This revised data is released with windPRO on November 6th 2020. It is confirmed that the 50m wind-speeds are affected, see our map that visualizes the difference between the two batches of data - here. We have also a plot that visualizes the time-series differences in 50m wind speed for a single node at location N56.0 E9.4, here. If you have downloaded the September 2020 data in the period from October 24th to November 3rd, we recommend that you: 1) Delete the month of September 2020. 2) Download the most recent data. 3) Re-do your calculations. Read the final newsletter about this issue here
2020-11-03: NASA has notified us about an issue with the September 2020 data (released from NASA on October 20th 2020). The MERRA-2 science team at NASA do not recommend using the data - and are currently reprocessing the data. In the windPRO online-data services, September 2020 data has now been removed as of November 3rd. If you have downloaded the MERRA-2 data in the period between October 24th to November 3rd, we recommend that you disable or remove the MERRA2-data for the month of September 2020. The actual nature of the issue is unknown to the windPRO online-data team. We will send out a notification when new data has been released from NASA. Read the original statement from NASA here. Read the windPRO newsletter about this issue, here.

Data Availability

MERRA-2 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.

Table: Overview of MERRA-2 Dataset Parameters.

Parameter	Description	Unit
time	UTC time stamp	YYYY-MM-DD HH:MM
psfc	Surface pressure	Pa
msl	Mean Sea Level pressure	Pa
wSpeed.50	Wind speed at 50 m above surface	m/s
wDir.50	Wind direction at 50 m above surface	deg
wSpeed.850hpa	Wind speed at 850 hpa	m/s
wDir.850hpa	Wind direction at 850hpa	deg
temperature.10	Temperature at 10 m above surface	celcius
temperature.50	Temperature at 50 m above surface	celcius
dispHeight	Displacement height	m
precIceWater	Total precipitable ice water	g/m^2
precLiqWater	Total precipitable liquid water	g/m^2
precWaterVapor	Total precipitable water vapor	g/m^2
specHumidity.10	Specific humidity at 10 m	g/kg

External Links and References

• The MERRA-2 project page - HERE
• Bosilovich et al "MERRA-2: Initial Evaluation of the Climate" - HERE
• Thøgersen et al, "Is MERRA2 Able to Replace MERRA as a Trusted Reference Wind Dataset?" - HERE.

Acknowledgement

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