Increasing Occurrence of Severe Weather Justifies Substantial Upgrades to Infrared Sounding from the Polar Orbits of Future Numerical Weather Prediction (NWP) Observing Systems

This presentation is based on the tenet that the significant observed increase in climate-induced, extreme weather events warrants added societal investment in improved global NWP observing systems. For the current international observing system for global NWP, it is widely acknowledged that the primary impact of satellite observations comes from the combination of microwave and infrared sounding sensors. Overall, microwave data has the largest impact, while on a per-instrument basis, the impact of infrared data is higher. However, planning for future NWP polar observing systems in Europe and the United States appear to be emphasizing new microwave capabilities, with the infrared being limited to baseline capabilities similar to current systems.

We argue here that a viable path forward for optimizing NWP impacts should be based on adding more of the proven infrared observing capabilities. The scientific basis for such an improved system is the discovery that the NWP benefits of LEO Infrared and microwave sounder observations do not saturate as more orbits are added, at least up to 1 to 2-hour sample intervals (Tony McNally’s group at ECMWF and William L. Smith’s work with the U of Wisconsin). Therefore, a logical next step is to add more orbits to the baseline international observing system. By adding three orbits equally spaced in time between the current baseline system orbits (0530, 0930 and 1330), the frequency of observations can be doubled, upgrading from the current 4-hour to a 2-hour time spacing.

For this example of doubling the number and frequency of observations in a 3-orbit baseline system, the NWP impact is expected to also double over that of the baseline alone, while the cost of each sensor goes down when reproducing significant numbers. So the cost/benefit arguments for providing a capable polar baseline system with 4-hour sampling intervals apply even more strongly for providing more frequent observations with the same basic capabilities for achieving substantial forecast improvements.

In some sense, this is the argument that is being used in considering new microwave observing capabilities at EUMETSAT (STERNA) and at NOAA (NEON-SMBA). However, at this point both are missing the infrared component, and in doing so, would sacrifice about half of the potential improvement over the current baseline system. We strongly implore NOAA to commit to the implementation of 3 new sun-synchronous orbits with advanced infrared sounders. A key for the impact of the infrared is that the higher vertical resolution provided by advanced infrared sounders yields very significant wind information when high frequency data is assimilated by NWP models.

The current NOAA JPSS Cross-track Infrared Sounder (CrIS) sensor provides a well-proven path for this type of augmented 2-hour interval infrared observation. It has been demonstrated at EUMETSAT and UK Met-Office that CrIS data offers approximately equal global NWP impact to data from the much larger EUMETSAT IASI sounder. And CrIS sets a very high standard for absolute radiometric calibration that is leading to research in treating the data as “anchor data” for assimilation. In addition to NWP, this high accuracy also provides highly valuable climate data in support of well-defined decadal trending studies and trace gas data for atmospheric chemistry.

We highly recommend that higher frequency, well calibrated, CrIS-like infrared capabilities be included in planning for future polar orbiting global NWP observing. Such a small constellation would provide the highest value for assimilation by using sensors that can be treated as a single sensor and not need separate bias tuning.

The challenges of climate-induced, extreme weather changes should be met with improved forecasts from improved global NWP observations as described here, in addition to crucial now-casting improvements from the new US GXS GEO sounder. We need both.