Documentation on IEM generated NEXRAD Mosaics
Example composite from 1 March 2007.
Quick Information:
| N0R | N0Q | |
|---|---|---|
| Product: | 4 bit Base Reflectivity 5dbz | 8 bit Base Reflectivity 0.5dbz |
| Format: | PNG image (GeoTiff available) | |
| Size: | 6000x2600 pixels | 12000x5200 pixels 12200x5400 after 8 Aug 2014 |
| Time: | Every 5 minutes since 1 Jan 1995 | Every 5 minutes since 13 Nov 2010 16:25 UTC |
| Download: | Current and Archive for today | |
| Precipitation | N0R (4bit) Reflectivity | N0Q (8bit) Reflectivity | |||||
|---|---|---|---|---|---|---|---|
| Ramp | Index values [inch] | Ramp | Index values [dBZ] | Ramp | Index values [dBZ] | ||
 |
0
0.25 0.5 0.75 1.0 1.25 1.5 1.75 2.0 2.25 2.5 2.75 3.0 3.25 3.5 3.75 4 |
4.8
5.6 6.4 7.2 8.0 8.8 9.6 10.4 11.2 |
12
13 14 15 16 17 18 19 20 21 22 23 24+ |
 |
NODATA
-30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 |
 |
NODATA
-32 dBZ through 90 dBZ 0.5 dBZ increment |
IEM Applications using this product
- Static Maps
- Interactive RADAR
- VTEC Product Browser
- Max Reflectivity Mosaic 0z-0z
- Max Reflectivity Mosaic 6z-6z
- NEXRAD Mosiacs by Year
Since 2003, the IEM has been generating and archiving nationwide mosaics of NEXRAD base reflectivity every 5 minutes. These mosaics are generated by combining the 130+ individual RADARs that comprise the NEXRAD network. This page provides background information on this dataset and highlights various ways this dataset can be and is used. Please note: Errors can and do occur during the automated generation of these images, please use them with care.
Important changes to this product.
- 1 August 2003: 5 minute interval archive begins. The initial pixel resolution is roughly ~2 km.
- 14 November 2005: The pixel resolution is increased to ~1 km thanks to the support of a Unidata Equipment grant.
- 10 May 2007: Implemented a clutter suppression algorithm to remove most anomalous propogation during the summertime.
- 11 Nov 2008: Implemented a crude check against RUC model surface temperature analysis for near freezing areas to stop the clutter suppression algorithm from removing areas of winter precipitation.
- 14 May 2009: Completed a 7 month process of backfilling the mosaics back to 1 Jan 1995. Also regenerated mosaics prior to 2006 to produce them on a ~1 km grid as well. Big thanks to Steve Ansari at NCEI for providing the level3 archive.
- 25 Aug 2010: Reprocessed 2005 + 2006 data to improve product quality and to backfill the archive with clutter suppression enabled during the summer months (Apr-Sep).
- 16 July 2014: Modified the clutter removal routine to use the higher resolution EET level III product. Previously NET was used.
- 8 Aug 2014: The spatial domain of the N0Q composite was expanded 1 degree east and south. The reason is that the clipping was occurring on the eastern and southern borders of the domain.
- 7 Sep 2016 1940 UTC: The original intent of the N0Q mosaics was to provide imagery at a resolution of 0.5 dBZ. After some investigation, it was discovered that the N0Q mosaics only contained data at values of whole integer values of dBZ. After some debugging of GEMPAK, the issue was hopefully fixed within GEMPAK. The good news is that this bug did not impact any processing of the file. The conversion of color index and RGB colors to dBZ was OK. There just was not any data at those 0.5 dBZ, non-int values.
- 5 May 2021: The NWS discontinued the dissemination of the N0R product. The IEM is now generating a pseudo composite derived from the N0Q composite.
- 18 April 2022: The NWS discontinued the dissemination of the N0Q product. The IEM is now using the N0B product as the source for N0Q.
- 28 September 2023: The NWS discontinued the dissemination of N1P (1 Hour Precip) and NTP (Storm Total Precip) products. The IEM is now producing composites of DAA and DTA instead with no plans of creating a psuedo composite.
Product Generation:
Composite of mosaics for 18 Oct 2007. Click image for regular size.
The IEM receives a feed of NEXRAD Level III products from the Unidata Internet Data Distribution. This feed includes the Base Reflectivity (N0R) and Net Echo Top (NET) products which are saved to disk for later processing. Every 5 minutes, a script runs a GEMPAK program called nex2img. nex2img searches the on-disk data for current (within 15 minutes of runtime) products and then mosaics them into a large image.
The result of the nex2img process is a simple raster image with size of 6000 pixels in width by 2600 pixels in height. This raster image is then compared with a similiar raster of NET to remove any suspected false echos. In the wintertime, this comparison is skipped for most of the country. The final result is a PNG formatted image looking much like the example image shown above.
The final image is shipped to the IEM webfarm for archival and use. You can
always find the 10 most recently generated rasters here (n0r_0* is the newest).
You can also find current and historical images in the archive. The format
for the URL is:
https://mesonet.agron.iastate.edu/archive/data/YYYY/mm/dd/GIS/uscomp/
For example, here are the images generated for today:
https://mesonet.agron.iastate.edu/archive/data/2024/11/21/GIS/uscomp/
All dates in the archive are in UTC time.
Product Format:
The images are stored in PNG format. They have a 4-bit colormap and are non-interlaced. Images for the past 50 minutes are also available in TIFF, GeoTIFF, and gzipped GeoTIFF format. Most people want to use these images in a GIS, so world files (.wld or .tfw) are available in the same current and archive folders. The projection of these images is in EPSG:4326 (WGS84 longlat).
For some people, the PNG format is not readable by their GIS, so the
IEM provides a server-side conversion of the PNG files for delivery of a
GeoTIFF. This application is called like so:
https://mesonet.agron.iastate.edu/request/gis/n0r2gtiff.php?dstr=200711010025
where dstr specifies the {year}{month}{day}{hour}{minute} in UTC
of the requested raster. Rasters are only available at minutes modulo 5.
Colormap Information:
The raster image has a colormap associated with it. The raw pixel values increase monotonically from -30 dbz to 75 dbz every 5 dbz. All values below 0 dbz are assigned black (rgb 0 0 0). We have tried to match the commonly used NEXRAD display as what you see on National Weather Service page. Please note: We do not include a concept of missing data or "no-data". So spots with beem blockage in the mountains or out away from the CONUS are just "black".
Web Map Service (WMS)
A special way you can interface with this data archive is via the Web
Map Service that also supports time based retrieval (WMS-T). Point your WMS-T aware app at:
https://mesonet.agron.iastate.edu/cgi-bin/wms/nexrad/n0r-t.cgi?
The nexrad-n0r-wmst layer is time aware!
Example WMS-T Application
Here is an example interface utilizing the WMS-T NEXRAD service.
The interface code is a part of the
OpenLayers toolkit.
The default time display shows Hurricane Katrina making landfall.