WP-1000 Review of EDA approach and recommendations for small satellite configurations

The start of October 2020 marked the beginning of a project to investigate potential future constellations of small satellites carrying microwave (MW) sounding instruments. Observations from MW instruments  currently provide the greatest impact of the satellite data used in the ECMWF assimilation system (Bormann et al., 2019). We are currently in a fortunate era of having nine and eleven MW instruments with temperature and humidity sounding capabilities respectively in operational use from a variety of polar orbiting platforms. Continued positive impact is still found from the latest additional sounders showing that a saturation point, where new data brings no significant benefit, has not yet been reached (Duncan and Bormann, 2020). This prompts the question of how much further benefit could be achieved with even better temporal sampling from additional instruments. At the same time, many of these satellites are well past their design-life or will be decommissioned in coming years, with only a few new MW sounding instruments scheduled for launch. So in the immediate future, the constellation as it stands now is expected to decline in the coming years.
Recent advances in technology have allowed the possibility of launching MW sounding instruments on small satellites with a performance that is expected to be adequate for Numerical Weather Prediction (NWP). While some compromises remain with the compact instruments, such as the potential loss of lower frequency wavelengths (below 50GHz) or marginally poorer noise performance, the small satellites provide a cost-effective means to enhance the temporal sampling provided from MW sounding instruments. Such constellations are expected to become an important presence in the future observing system, as a complement to fully specified instruments on a back-bone constellation of larger platforms. The higher temporal sampling will allow us to better observe faster evolving cloud and humidity features and to reduce the random component of noise through higher measurement numbers.
This project will consider various options for a possible new constellation of small satellites in order to make recommendations for the design that balance the benefit to NWP with practicalities such instrument limitations and cost. Key questions to consider include:
 What is the optimal number of satellites and their orbit type (e.g. polar or low inclination orbits)?
 What sets of channels (humidity and/or temperature sounding) provide the most benefit?
 What are the effects from limitations in performance and geo-location accuracy?
The benefit to NWP of different configurations of small satellites carrying MW sounding instruments will be assessed using an Ensemble of Data Assimilations (EDA) method. EDA experiments concerning simulated satellite data have recently been used at ECMWF to evaluate the impact of increasing numbers of Global Navigation Satellite System (GNSS) Radio Occultation (RO) observations (Harnisch et al., 2013) and the benefit from Aeolus (Tan et al., 2007).
In this report we will start by providing a short description of the EDA method (section 2) and review the planned approach for this project, considering the experience from previous studies (section 3). This includes considering the required length of study period and necessary adaptations for the observation error modelling. Section 3.3 considers the key themes we would like to address concluding with an initial recommendation of a list of scenarios. The exact list of parameters required from ESA as part of the data simulation step is given in Appendix B.