MODIS

Last Updated: Tuesday, 15-Jul-2008 11:05:00 PDT

MODIS Winds Become Operational at FNMOC

Fleet Numerical Meteorology and Oceanography Center (FNMOC) began using MODIS polar-orbiter feature-track winds operationally in the U.S. Navy's global data assimilation and modeling systems on 20 October 2004, after testing by the Naval Research Laboratory-Monterey (NRL) and beta-testing by FNMOC showed positive impacts. These winds are produced in near real time by the Cooperative Institute for Meteorological Satellite Studies (CIMSS) at the University of Wisconsin-Madison, using 11 µm infrared and 6.7 µm water vapor imagery from the MODIS (MODerate-resolution Imaging Spectroradiometer) instrument on board the NASA EOS Terra and Aqua polar orbiting satellites.

MODIS winds are superobbed before they are assimilated by the NRL Atmospheric Variational Data Assimilation System (NAVDAS). The superobbing strategy for MODIS winds is essentially the same as that developed at NRL for geostationary satellite feature-track winds. Observations are first binned into 2° "prisms" with a depth of 50 mb. Each prism has a height of 2° latitude and a width that varies by latitude to give both roughly square areas and an integer number of prisms in a latitude band. Superobs are required to have two or more observations from the same satellite at the same time and in the same channel (i.e., infrared or water-vapor) that are consistent with each other after possibly rejecting one or two outliers. Consistency is defined here as wind speeds within approximately 7 m/s, and either u and v components within 5 m/s or wind directions within 20°. The superobs are formed by averaging the available innovations (observation minus background) and are used at the average location. A kinetic energy adjustment is applied to ensure that the resultant speed and mean speed are the same.

A number of quality control checks are also applied prior to superobbing. The MODIS winds meeting any of the following criteria were rejected:

• Quality Index < 0.60
• Pressure = 725 mb in the Northern Hemisphere
• Pressure = 525 mb in the Southern Hemisphere
• Pressure < 275 mb
• Latitudes equatorward of 65°
• Northern Hemisphere land points over Western Europe, North America, and Greenland (100°W to 45°E)
• Vector innovations greater than 8-12 m/s (as a function of pressure)

Testing of MODIS winds was accomplished by running NAVDAS using all operational data with and without the addition of MODIS winds, then running the Navy Operational Global Atmospheric Prediction System (NOGAPS) to generate forecasts out to five days. A positive impact was seen in the 500 mb height anomaly correlation, especially in the Southern Hemisphere and in the Arctic for forecast ranges greater than 48 hr. The improvement in the Arctic was also accompanied by a reduction in RMS errors for not only vector winds but also heights and temperatures. Further details about the MODIS tests can be found in Pauley and Pauley (2005).

Pauley, P.M., and R.L. Pauley, 2005: Operational testing of MODIS winds in NAVDAS. Preprints, 9th Symposium on Integrated Observing and Assimilation Systems for Atmosphere, Ocean, and Land Surface, American Meteorological Society, San Diego, California.