Small Satellite Humidity Sounding for Global NWP: First Evaluation of CAMS on TY-16 in the CMA-GFS 4D-Var system

Polar-orbiting microwave humidity sounders with 6-h revisit intervals cannot resolve rapidly evolving lower-tropospheric moisture. Cost-effective small-satellite constellations promise shorter revisit times, yet their real-data impact on global numerical weather prediction (NWP) has not been quantified. For the first time, clear-sky radiances from the Compact Atmospheric Microwave Sounder (CAMS) on the 500-km small satellite TY-16 (launched December 2023) were cycled in the CMA-GFS global 4D-Var system during 25 March–25 April 2024. Four cycling experiments were conducted: a control run (CTRL) assimilating conventional and operational satellite observations (excluding FY-3E MWHS-2); CTRL plus CAMS radiances (EXP1); CTRL plus FY-3E MWHS-2 as a benchmark (EXP2); and CTRL plus both CAMS and MWHS-2 (EXP3). Twelve of eighteen channels (seven temperature, five humidity) were bias-corrected off-line against CMA-GFS backgrounds. Compared to CTRL, EXP1 reduces the standard deviation of background departures for humidity-sensitive channels of independent sounders by 0.5–1.1% (95% confidence), peaking at 1.1% for FY-3D MWHS-2 channel 15. RMSE of 850-hPa specific-humidity analyses falls by 4.2%, 2.4% and 1.1% over the Southern Hemisphere, tropics and Northern Hemisphere, respectively; tropical temperature RMSE at 850 hPa drops 1.8%. CAMS alone yields 4–6% lower RMSE in 1–3-day moisture forecasts below 700 hPa within 20°S–20°N. Joint assimilation with MWHS-2 extends the moisture improvement throughout the 10-day forecast and produces statistically significant reductions in meridional-wind error up to day 5. These first real-data results demonstrate that CAMS on a small satellite can match the quality of legacy large platforms and materially improve global humidity analyses and short- to medium-range forecasts, supporting the deployment of the planned 45-satellite constellation for operational NWP.