Assessing forecast benefits of future constellations of microwave sounders on small satellites using an ensemble of data assimilations

Abstract

The ensemble of data assimilations (EDA) method is employed to evaluate the expected impact of a wide range of potential future constellations of passive microwave (MW) sounders on small satellites for global numerical weather prediction. Such constellations are expected to become an important component of the future observing system, complementing a backbone of larger, high‐performance platforms and allowing unprecedented temporal sampling. Twelve constellations are investigated to probe key aspects of the constellation design: number of satellites (ranging from 8 to 20), types of orbits (sun synchronous, mid‐inclination), and channel complement (183 GHz humidity‐sounding capabilities only versus combining these with temperature‐sounding capability around 50 GHz). The small‐satellite data and accompanying errors are simulated, using an all‐sky framework, and the relative benefits from adding the different constellations to the observing system are measured by the reduction in the spread of the ensemble members, reflecting improvement to the forecast uncertainties. Results suggest continued benefit from adding MW‐sounding observations beyond currently available observations. The EDA spread reduction for different variables (e.g., wind and geopotential height) and different pressure levels is already significant using the smallest constellation considered, adding eight satellites to a four‐satellite baseline of existing MW‐sounding instruments. The EDA spread reduction continues as further observations are added, although the rate of reduction slows significantly, especially where scenarios use only humidity‐sounding channels. Use of temperature‐sounding channels gives significant added benefit over humidity sounding only, generally 2–3 and 1.5–2 times larger in the extratropics and Tropics respectively. Different behaviour in the relative magnitudes and rates of EDA spread reduction is seen between the extratropics and Tropics, which can be attributed to different physical processes and different error growth. This study was unable to provide conclusive results on the choice of polar orbits only, mid‐inclination orbits only, or a mix of orbit types.