Merra - info
|Last update: Mar. 2017||Back to Web service >>|
MERRA dataset is a database hosted by NASA and generated by the Godard Space Flight Center. All results are produced by a numerical weather forecast model (GEOS-5).
To improve forecasts, data are assimilated in the numerical model; it means data from meteorological ground stations and satellites for example are compared to the forecast at the right time. If forecast is far from 'reality' it is corrected in accordance to observations. Such corrected forecasts with a maximal number of observations are called re-analysis.
- Provider: NASA (http://gmao.gsfc.nasa.gov/reanalysis/MERRA-2/)
- Spatial coverage: worldwide
- Spatial resolution: 0.625° in latitude and 0.5° in longitude (approx. 50 km)
- Temporal coverage: Jan. 1980 until approx. one month before the current day
- Outputs parameters: Temperature in Kelvins (at 2 m above ground), Relative Humidity in % (at 2 m), Pressure in hPa (at 2 m), Wind speed in m/s and direction in degrees (at 10 m), Rainfall, Snowfall and Snow depth
- Time step: 1 month, 1 day, 1 hour, 15 min, 10 min, 5 min and 1 min. Please note that original time step of data is 1 h. As a consequence, below 1 h, received data result from an interpolation.
- Missing data: -999
- Manual access via the interface is for free. Manual requests with a time step below 10 min are limited to 1 month.
- Automatic access for pay: a fee is applied to buy and maintain the servers (annual subscription). Automatic requests with a time step below 10 min are limited to one month, and automatic requests with a time step below one day limited to one year (identical to HelioClim-3 automatic request behavior).
Don't hesitate to contact us to ask for a quotation.
Request characteristics / constraints:
- for a given location results from a bilinear interpolation of the values of the 4 closest MERRA pixels. Original time step is the hour.
- No altitude correction applied to the meteorological parameters retrieved by using this service.
- Wind speed and direction are not directly provided by MERRA, but computed from the "u" and "v" vectors of wind speed. As the formula are not linear, please note that the monthly (or daily) values computed from the hourly values of wind speed are different from the monthly values retrieved from the interface.
- Same remark for the relative humidity parameter, computed with a non-linear expression from pressure, temperature and specific humidity.
- Other variables (pressure and temperature) are linear.
- For values with a time step below the hour, the hypothesis corresponds to the end of the interval.
- Latitude and longitude: enter the values in decimal degrees, or directly click on the map to select your point, or you can also use the "Search Address" tool on the top of the map.
- Start date and End date: format yyyy-mm-dd. Please pay attention to ask for complete months when asking for monthly values. If incomplete, the given monthly value results from an interpolation based on the selected days.
- Time step: 1 month, 1 day, 1 hour, 15 min, 10 min, 5 min and 1 min
- Output format: CSV only
The output file format is CSV. Right click on the link to retrieve your file.
- Temperature: temperature at 2 m above ground in Kelvins (K)
- Relative humidity: Relative humidity at 2 m above ground in percent (%)
- Pressure: Pressure at ground level in hPa
- Wind speed: Wind speed at 10 m above ground in m/s
- Wind direction: Wind direction at 10 m above ground in degrees (0 means from North, 90 from East...)
- Rainfall: Rainfall in kg/m2 (Rain depth in cm can be obtained by dividing the values by 10)
- Snowfall: Snowfall in kg/m2
- Snow depth: snow depth in meters
Even if the data provided by NASA are for free, we need to dimension our servers to welcome the large number of automatic requests. A fee is thus applied depending on the number of time series retrieved every day / month. Please for a quotation.
Automatic requests with a time step below 10 min are limited to one month, and automatic requests with a time step below one day limited to one year (identical to HelioClim-3 automatic request behavior).
Here is an example of automatic access using the wget command:
wget -O SoDa_Merra_Carpentras.csv --header="soda-user: guest" --header="soda-passwd: guest" ...
More information on the automatic access (a few parameters are similar to those of HelioClim-3)