Yuan et al. (2026) Diurnal variation of summer precipitation in the middle and lower Yangtze River Basin and associated mechanisms

Identification

• Journal: Climate Dynamics
• Year: 2026
• Date: 2026-01-13
• Authors: Xueqing Yuan, Anning Huang, Guangtao Dong, Daokai Xue, Zhu Xc, Rongchang Wu
• DOI: 10.1007/s00382-025-08025-8

Research Groups

• School of Atmospheric Sciences, Nanjing University, Nanjing, China
• Shanghai Climate Center, Shanghai, China
• Nanjing Institute of Environmental Sciences, Ministry of Ecology and Environment of the People’s Republic of China, Nanjing, China

Short Summary

This study systematically reveals the detailed features and associated mechanisms of summer precipitation diurnal variation in the middle and lower Yangtze River Basin (MLYRB) from 1980 to 2022, identifying a distinct bimodal pattern with primary afternoon and secondary morning peaks driven by different dynamic and thermal processes. The research further delineates four spatial patterns of diurnal variation and their sub-seasonal evolution, highlighting the interplay between large-scale moisture transport and thermal instability.

Objective

• To identify the differences in environmental conditions among various regional diurnal precipitation regimes.
• To characterize the fine-scale spatial heterogeneity of summer precipitation diurnal variation over the MLYRB and its underlying causes.
• To understand how the dynamical competition between large-scale moisture transport and thermal forcing governs the sub-seasonal evolution of the dominant diurnal phase of precipitation.

Study Configuration

• Spatial Scale: Middle and lower Yangtze River Basin (MLYRB)
• Temporal Scale: Summers (June to August) from 1980 to 2022 (43 years)

Methodology and Data

• Models used:
  • Spectral clustering analysis (for spatial patterns)
  • Harmonic analysis (Fourier method for diurnal cycles)
  • Moisture budget analysis (Trenberth 1991 framework)
• Data sources:
  • High-resolution hourly rain gauge precipitation data from 300 stations in the MLYRB (China Meteorological Administration - CMA).
  • ERA5 reanalysis dataset (0.25° spatial resolution, 1-hour temporal resolution) for specific humidity, precipitable water, evaporation, convective available potential energy (CAPE), and zonal/meridional wind components.

Main Results

• Summer precipitation in the MLYRB exhibits a distinct bimodal diurnal cycle, with a primary peak at 16:00–18:00 Beijing time (BJT) and a secondary peak at 07:00–09:00 BJT. The morning peak is dominated by long-duration precipitation, while the afternoon peak is driven by short-duration precipitation.
• Three regional precipitation day types (morning peak, afternoon peak, dual-peak) show fundamentally different environmental configurations:
  • Morning type: Dominated by nocturnal large-scale dynamic lifting (e.g., low-level jet) consuming pre-stored moisture, with weak CAPE (below 600 J/kg) and maximum ascent at 400–600 hPa (up to -0.11 Pa/s).
  • Afternoon type: Governed by local thermal processes, relying on daytime surface evaporation and energy storage, characterized by persistent weak moisture flux convergence (0.09 mm/h daily mean) and high afternoon CAPE (exceeding 1200 J/kg).
  • Dual-peak type: Reflects synergistic effects of strong persistent dynamic forcing (0.27 mm/h daily mean moisture flux convergence) and diurnal thermal processes, with deep upward motion throughout the day (maximum ascent near 400 hPa, -0.13 Pa/s).
• Four distinct spatial patterns of precipitation diurnal variation are identified: northwestern morning-dominant, mountainous afternoon-enhanced, transitional bimodal equilibrium, and southeastern coastal afternoon-dominant. These differences are primarily reflected in afternoon peak precipitation, governed by thermally forced ascent due to mountain-plain and land-sea contrasts.
• A distinct southeastward propagation delay of morning precipitation peaks is observed within the 110°E–119°E and 28°N–30°N range, linked to long-duration precipitation influenced by large-scale weather systems.
• Sub-seasonal variation shows a morning-to-afternoon transition in precipitation dominance, shifting from large-scale moisture transport during pre-Meiyu and Meiyu periods to thermal instability dominance in the post-Meiyu period. Differences are mainly driven by variations in moisture flux convergence, particularly the wind divergence term.

Contributions

• Provides a substantially refined spatial differentiation of precipitation diurnal patterns, identifying four distinct regimes compared to the traditional two or three.
• Systematically reveals the fundamentally different environmental configurations (moisture, dynamics, thermodynamics) for morning, afternoon, and dual-peak precipitation days.
• Documents a previously unrecognized distinct southeastward propagation delay of morning precipitation peaks within the 110°E–119°E and 28°N–30°N range, expanding understanding beyond traditional eastward propagation.
• Offers essential references for evaluating and improving numerical models and enhancing the accuracy of refined precipitation forecasts in the MLYRB.

Funding

• National Natural Science Foundation of China (Grants U2342207)
• Postgraduate Research and Practice Innovation Program of Jiangsu Province (No. 206 in 2024)
• Jiangsu Collaborative Innovation Center for Climate Change
• Science and Technology Talents and Platform Plan: Technology Innovation Center of Yunnan Province for Digital Water Engineering (No. 202305AK34003)

Citation

@article{Yuan2026Diurnal,
  author = {Yuan, Xueqing and Huang, Anning and Dong, Guangtao and Xue, Daokai and Xc, Zhu and Wu, Rongchang},
  title = {Diurnal variation of summer precipitation in the middle and lower Yangtze River Basin and associated mechanisms},
  journal = {Climate Dynamics},
  year = {2026},
  doi = {10.1007/s00382-025-08025-8},
  url = {https://doi.org/10.1007/s00382-025-08025-8}
}