The stakes are high. Nocturnal heavy precipitation events strike when public awareness and emergency response capacity are at their lowest, increasing the risk of casualties and economic losses. Recent research indicates that both the frequency and intensity of these events have increased over many regions of China under a warming climate. Because the initiation and rapid intensification of convective precipitation are strongly controlled by the pre-onset thermodynamic environment, particularly the vertical distributions of temperature and water vapor, extending the lead time of heavy-rainfall forecasts hinges on monitoring atmospheric thermodynamic structure before the first drops fall.

Two complementary instruments anchor the new approach. Ground-based microwave radiometers passively measure naturally emitted microwave radiation from atmospheric gases and cloud liquid water at multiple frequencies, delivering continuous temperature and humidity profiles roughly every two minutes, with vertical resolution better than 100 meters in the planetary boundary layer and several hundred meters higher up. Mie–Raman lidars, by contrast, actively probe the atmosphere with laser pulses at 354.7 nanometers, detecting nitrogen Raman returns near 386.7 nanometers and water-vapor Raman returns near 407.5 nanometers. This yields moisture profiles with vertical resolution better than 45 meters, fine enough to resolve the moisture gradients and turbulent mixing processes closely tied to convection initiation. The catch is that Raman lidar signals are strongly attenuated by clouds, precipitation, and aerosols, which is precisely why combining the two sensors is so attractive.

The innovation lies in how the retrieval cost function treats each observation. Conventional variational frameworks assign static weights, effectively assuming that every channel or vertical bin contributes equally regardless of conditions. The team generalized their ratio-based cost function, which uses virtual potential temperature as the control variable, by introducing adjustable weights between zero and one for each observation. These coefficients are optimized during the cost-function minimization, performed with the L-BFGS-B algorithm, so the framework continuously re-evaluates the relative influence of individual microwave radiometer channels, individual lidar range bins, and the two instruments in the synergistic configuration. The retrieval domain extends from the surface to 10.2 kilometers above ground level on a non-uniform grid, with 30-meter spacing near the surface.

To test the method, the researchers assembled 107 nocturnal heavy-precipitation cases, defined as hourly accumulated precipitation of at least 10 millimeters, drawn from 26 of the 56 stations where China Meteorological Administration has deployed Mie–Raman lidars alongside radiosondes and microwave radiometers since 2021. Most cases, 82 events or about 76.6 percent, fell in the 10-to-20-millimeter category, while 14 events reached 20 to 30 millimeters and 11 exceeded 30 millimeters. Retrievals were verified against independent radiosonde soundings using mean bias and root-mean-square error, with ERA5 reanalysis providing the a priori atmospheric state.

The results show a consistent edge for adaptive weighting, with the most dramatic gains in water vapor mass mixing ratio profiles. For microwave-radiometer-only retrievals, the adaptive scheme reduced the mean bias of moisture retrievals by up to roughly 0.08 grams per kilogram near 2700 meters, with root-mean-square error improvements of about 0.05 grams per kilogram at the same altitude, concentrated between 600 and 4800 meters. Temperature improvements were more modest, with root-mean-square error reductions of approximately 0.02 kelvin between 900 and 4800 meters. For lidar-only retrievals, adaptive weighting cut both bias and error of the moisture profiles by up to about 0.1 grams per kilogram, a physically sensible outcome given that Raman lidars observe water vapor directly while temperature information enters only indirectly.

The weight diagnostics themselves proved revealing. Among the nine microwave channels common to all instrument models, the water-vapor channels at 22.235, 23.035, and 23.835 gigahertz and the oxygen absorption channels at 53.85, 54.94, and 56.66 gigahertz consistently received median and mean weights exceeding 90 percent, marking them as the dominant observational constraints. The 26.235, 51.25, and 52.28 gigahertz channels showed lower and more variable weights, suggesting their contributions depend strongly on environmental conditions. For the lidar, adaptive weights increased with height, stabilizing above roughly 1200 meters, a pattern the authors attribute to the greater variability introduced near the surface by moisture transport, turbulent mixing, and precipitation-related processes.

Stratifying the cases by rainfall intensity exposed an important limitation. For weak-to-moderate events the adaptive advantage was robust, but for the most intense category, exceeding 30 millimeters per hour, the benefit over static weighting became much less apparent in microwave-radiometer retrievals, likely because heavy rainfall increases observational uncertainty and limits the effectiveness of dynamically adjusting error statistics. Even then, the adaptive framework continued to deliver measurable moisture improvements for lidar-based and synergistic retrievals, indicating that its primary strength lies in extracting humidity information under challenging pre-storm conditions.

When both instruments were combined, the synergistic retrievals generally achieved the best overall performance of all experiments, with further moisture-profile error reductions of roughly 0.02 grams per kilogram relative to either instrument alone. Yet the improvement was smaller than the sum of the single-instrument gains, a sign of overlapping, redundant water-vapor information between the two sensors. The weight diagnostics showed the framework actively redistributing influence: several microwave channels gained contribution when lidar data were added, while lidar weights decreased between roughly 1800 and 3000 meters once radiometer observations entered the mix, with the pattern shifting as precipitation intensity changed.

The authors are careful to note that the optimized weights should be interpreted primarily as adaptive scaling factors within the retrieval framework rather than direct measures of formal information content, though their diagnosed characteristics hint at deeper information-theoretic meaning that future work could quantify. They also flag open questions, including whether residual information in the underweighted channels could be exploited by future algorithms, how the framework would perform with hyperspectral infrared spectrometers or rotational Raman lidars that measure temperature directly, and how sensitive the results are to background uncertainty specifications. For now, the study demonstrates a practical path toward more reliable thermodynamic soundings in the critical hours before nocturnal floods, exactly when every improvement in moisture accuracy can translate into earlier warnings and safer communities.

Subject of Research: Adaptive observation weighting for ground-based multi-sensor thermodynamic profile retrievals prior to nocturnal heavy precipitation

Article Title: Adaptive observation weighting in TCKF1D-Var for ground-based multi-sensor thermodynamic retrievals prior to nocturnal heavy precipitation over China

Article References: Zhang, Q., Chen, T., Guo, J., Deng, B., Li, H., & Wu, Y. (2026). Adaptive observation weighting in TCKF1D-Var for ground-based multi-sensor thermodynamic retrievals prior to nocturnal heavy precipitation over China. Geoscientific Model Development, 19(19), 9555-9580. https://doi.org/10.5194/gmd-19-9555-2026

Image Credits: AI Generated

DOI: 10.5194/gmd-19-9555-2026

Keywords: TCKF1D-Var, adaptive observation weighting, microwave radiometer, Mie–Raman lidar, thermodynamic profiling, water vapor retrieval, nocturnal heavy precipitation, data assimilation, 1D-Var, ERA5 reanalysis, convection initiation, China Meteorological Administration