Final Report: Study to assess earth observation with small satellites and their prospects for future global numerical weather prediction

Small satellites are expected to become an important component of the future observing system and will complement a continued backbone of larger, high-performance platforms. An Ensemble of Data 
Assimilations (EDA) method is employed to evaluate the impact of different potential future small satellite constellations carrying MW sounding instruments to explore the optimal design of a constellation for global Numerical Weather Prediction (NWP). Six different potential constellations of small satellites are chosen to probe key aspects of the constellation design: the number of satellites, types of orbits and the trade-off between having 183-GHz humidity sounding channels (complemented with window channels at 89-GHz and 165-GHz) or with additional temperature sounding capability in the 50-GHz band. The small satellite data and accompanying observation errors are simulated, using an all-sky framework, and the benefit from adding the data to the observing system is measured by reducing the spread of the ensemble members which reflects improvement to the uncertainties in analyses and forecasts.
The reduction in EDA spread for different variables (e.g. wind and geopotential height) and pressure levels is already significant using a smaller constellation of eight satellites added to a 4-satellite baseline. The EDA spread reduction continues as further observations are added although the rate of reduction slows, especially where scenarios only include use of humidity sounding channels. The use of temperature sounding channels gives a significant added benefit over humidity sounding only generally in the order 2-3 and 1.5-2 times larger in the extra-tropics and tropics respectively. Different behaviour in the relative magnitudes and rates of EDA spread reduction is seen generally between the extra-tropics and tropics which can be attributed to different physical processes and different spatial and temporal growth of errors in the EDA. Advantages of using small satellites in polar orbits only, mid-inclination orbits only or a mix of orbits are difficult to distinguish especially due to a suboptimal interaction between the chosen satellite phasing and current thinning strategies.