Introduction
The Microwave Imager (MWI) is the primary instrument on the first Weather System Follow-on Microwave satellite (WSF-M1), launched on April 11, 2024. WSF-M is the Department of Defense follow-on to the environmental sensing provided by the Defense Meteorological Satellite Program (DMSP) and WindSat. MWI was designed to provide ocean surface vector winds and tropical cyclone intensity, along with other environmental data.
Instrument Description
MWI is a fully polarimetric, conical-scanning, passive microwave radiometer with a 1.8 m antenna. It has 17 channels at six frequencies from 10.85 to 89 GHz. At 10.85, 18.85 and 36.75 GHz, MWI measures the third and fourth Stokes parameters in addition to vertical and horizontal polarization, and these polarimetric measurements are used to retrieve ocean surface vector winds. WSF-M1 is in a sun-synchronous orbit, crossing the equator at about 6 PM local time going north (ascending) and 6 AM going south (descending). Some details of the MWI instrument are listed here:
| MWI | |
| Satellite Platform | WSF-M1 |
| Equator Crossing Time (Local time zone) |
6:00 PM Asc 6:00 AM Desc |
| Antenna Size | 1.8 m |
The 17 instrument channels are summarized in the following table. V and H denote vertical and horizontal polarization; 3rd and 4th denote the third and fourth Stokes parameters.
| Band [GHz] | Polarization |
Spatial Resolution |
| 10.85 | V, H, 3rd, 4th | 38 x 23 |
| 18.85 | V, H, 3rd, 4th | 23 x 15 |
| 23.8 | V | 21 x 13 |
| 36.75 | V, H, 3rd, 4th | 15 x 10 |
| 37.3 | V, H | 15 x 10 |
| 89.0 | V, H | 15 x 10 |
Data Processing
We obtain MWI sensor data records (SDRs) from NOAA NESDIS. We recalibrate the MWI brightness temperatures at RSS, intercalibrating them with historical MCDC validated satellite observations. This in-house calibration keeps MWI consistent with the other microwave radiometers in our climate data record. We then produce the typical RSS microwave radiometer ocean product suite: Sea Surface Temperature (SST), Surface Wind Speed, Atmospheric Water Vapor, Cloud Liquid Water, and Rain Rate.
MWI wind speed is a blend of our two standard rain-free radiometer winds, the low-frequency (LF) and medium-frequency (MF) winds. Where the LF wind is available, the reported wind speed is the average of the LF and MF winds; elsewhere, the MF wind is used.
Because of its polarimetric channels, MWI also measures wind direction. The wind direction retrieval algorithm finds wind vector solutions for observed TBs, these solutions are called ambiguities. We select the ambiguity whose direction is closest to the NCEP wind direction, which is included in the data files. The wind speed of the selected ambiguity is provided as a separate vector wind speed. Wind vectors are provided only in rain-free conditions.
RSS MWI Data Products
The current version of the RSS MWI ocean products is V6.1. MWI V6.1 data are available from April 13, 2024 to the present.
We produce daily and time-averaged (3-day, weekly and monthly) netCDF data files. The daily files consist of over-ocean measurements mapped to a regular 0.25-degree grid complete with data gaps between orbits. Two maps exist for each parameter, one of ascending orbit segments (local evening passes) and the other of descending orbit segments (local morning passes). Data on each of the segment maps are overwritten at both the high latitudes where successive orbits cross and at the "seam" or region where the last orbit of the day overlaps the first orbit of the day. An observation overwrites a cell when it is more than 50 minutes later; observations closer in time, from the same overpass, are averaged. Daily data files contain time maps consisting of the UTC observation time for each set of passes (ascending and descending). Time-averaged data files do not contain any time information.
Gridded data are organized according to observation date. All dates and times are Coordinated Universal Time (UTC). Data products include daily and time-averaged geophysical data as follows:
| Daily | orbital data mapped to 0.25 degree grid divided into 2 maps based on ascending and descending passes, early data may be overwritten by later data at high latitudes and daily "seam" |
| 3-Day | average of 3 days ending on and including file date |
| Weekly | average of 7 days ending on and including the Saturday file date |
| Monthly | average of all data within the calendar month |
netCDF Data Files
The data files are in netCDF-4 format and follow the CF-1.11 and ACDD-1.3 metadata conventions. Each file contains the geophysical variables below on a 0.25 x 0.25 degree grid (1440, 720). Daily files add a pass dimension: index 1 is the ascending pass and index 2 is the descending pass. Missing values use the standard netCDF fill value, which most netCDF readers mask automatically.
| Variable | Product Name |
Product Description |
Units | Valid Data Range | Reason for No Data |
|---|---|---|---|---|---|
| time | Time | UTC observation time, seconds since the start of the day (daily files only) |
s | 0 to 86400 | no data |
| SST | Sea surface temperature | Temperature of top layer (skin) of water ~1 mm thick |
°C | -3 to 34.5 | high winds (>20 m/s), sun glint, rain, RFI, near sea ice or land |
| wind_speed | 10-m wind speed | Blend of the low- and medium- frequency wind speeds |
m/s | 0 to 50 | sun glint, rain, RFI, near sea ice or land |
| wind_speed_vector | 10-m vector wind speed | Wind speed of the selected wind vector |
m/s | 0 to 50 | sun glint, rain, RFI, near sea ice or land |
| wind_direction_vector | 10-m wind direction | Meteorological-convention wind direction, relative to north |
deg | 0 to 360 | sun glint, rain, RFI, near sea ice or land |
| water_vapor | Columnar atmospheric water vapor | Total gaseous water contained in a vertical column of atmosphere |
kg m-2 (= mm) | 0 to 75 | heavy rain or near land |
| cloud_liquid_water | Columnar cloud liquid water | Total cloud liquid water contained in a vertical column of atmosphere |
kg m-2 (= mm) | -0.05 to 2.45 | near land |
| rain_rate | Rain rate | Rate of liquid water precipitation | mm/hr | 0 to 25 | near land |
Each file also contains a masks group (land, sea ice, no observations, and observations that exist but did not pass the surface quality check) and an ancillary group with collocated NCEP wind speed, wind direction and water vapor, and CMC sea surface temperature. Time-averaged files add a counts group giving the number of daily-pass values averaged for the vector wind, water vapor, cloud liquid water and rain rate.
The files are located in the ncmaps_v6.1 directory. The daily and 3-day files are stored in year subdirectories. The file names have the following naming conventions:
| Time | Directory Path | File Name |
| Daily | daily/[year]/ | RSS_MWI_ocean_L3_daily_yyyy-mm-dd_v6.1.nc |
| 3-Day | 3day/[year]/ | RSS_MWI_ocean_L3_3day_yyyy-mm-dd_v6.1.nc |
| Weekly | weekly/ | RSS_MWI_ocean_L3_weekly_yyyy-mm-dd_v6.1.nc |
| Monthly | monthly/ | RSS_MWI_ocean_L3_monthly_yyyy-mm_v6.1.nc |
The center of the first cell of the 1440 column and 720 row map is at 0.125 E longitude and -89.875 latitude. The center of the second cell is 0.375 E longitude, -89.875 latitude.
Missing Data
There are gaps within these data. Missing data generally affect Daily and 3-Day products, but can also reduce the number of observations in Weekly and Monthly averages.
When browsing imagery, the navigation may skip dates with no data, or you may see a blank map stating that no data are available for that time.
Data files for dates with completely missing data are not produced; they will be absent from our data server.
Data gaps are generally due to missing data from our upstream provider. Occasionally, there are delays in obtaining and/or processing recently recorded data; beyond several weeks, it is unlikely that missing data will become available. For information on a current outage, be sure to look at the web page announcements.
Dates for which MWI data are completely missing include:
| date range | # days |
| 2024.05.14 | 1 |
| 2025.06.29 - 2025.06.30 | 2 |
| 2026.02.15 | 1 |
| 2026.08.12 | 1 |
| 2026.08.30 - 2026.08.31 | 2 |
Browse Images
Each daily, 3-day, weekly and monthly browse image map displays one geophysical parameter: Sea Surface Temperature (SST), 10 meter Wind Speed (WIND), Columnar Water Vapor (VAPOR), Cloud Liquid Water (CLOUD), or Rain Rate (RAIN). The daily maps display the morning (descending) and evening (ascending) passes separately. The date of the data displayed is the UTC date when the data were collected. The scale for each browse image is located next to the map for reference. Though the valid data range (minimum to maximum) is given in the table above, the scale bars in the browse images are set to visually enhance the data and may vary. MWI wind vectors are shown in our wind vector browse images.
Related Data
NOAA and NESDIS distribute the official WSF-M sensor data records. Near-real-time MWI intercalibrated brightness temperatures (Level 1C) and GPROF precipitation retrievals are available from the NASA Precipitation Processing System.
Acknowledgements
MWI data are produced by Remote Sensing Systems and sponsored by the NASA Earth Science MEaSUREs Program. We are grateful to NOAA NESDIS for making the MWI data available.
How to Cite These Data
Continued production of this data set requires support from NASA. We need you to be sure to cite these data when used in your publications so that we can demonstrate the value of this data set to the scientific community. Please include the following statement in the acknowledgement section of your paper:
"MWI data are produced by Remote Sensing Systems and sponsored by the NASA Earth Science MEaSUREs Program. Data are available at www.remss.com."
RSS MWI data are licensed under CC BY 4.0.
Data Citations:
MWI Daily:
Wentz, F., R. Lindsley, T. Meissner, L. Ricciardulli, C. Mears, M. Densberger, K. Nelson, 2026: RSS MWI Daily Air-Sea Essential Climate Variables (AS-ECV) on 0.25 deg grid V6.1, Remote Sensing Systems, Santa Rosa, CA, USA. Available at www.remss.com
MWI 3-Day:
Wentz, F., R. Lindsley, T. Meissner, L. Ricciardulli, C. Mears, M. Densberger, K. Nelson, 2026: RSS MWI 3-Day Air-Sea Essential Climate Variables (AS-ECV) on 0.25 deg grid V6.1, Remote Sensing Systems, Santa Rosa, CA, USA. Available at www.remss.com
MWI Weekly:
Wentz, F., R. Lindsley, T. Meissner, L. Ricciardulli, C. Mears, M. Densberger, K. Nelson, 2026: RSS MWI Weekly Air-Sea Essential Climate Variables (AS-ECV) on 0.25 deg grid V6.1, Remote Sensing Systems, Santa Rosa, CA, USA. Available at www.remss.com
MWI Monthly:
Wentz, F., R. Lindsley, T. Meissner, L. Ricciardulli, C. Mears, M. Densberger, K. Nelson, 2026: RSS MWI Monthly Air-Sea Essential Climate Variables (AS-ECV) on 0.25 deg grid V6.1, Remote Sensing Systems, Santa Rosa, CA, USA. Available at www.remss.com