Characteristics of the observed diurnal cycle of precipitation during long-rain season over Tanzania

The diurnal cycle (DC) of precipitation is a critical component for society and climate systems, yet it remains poorly characterised in Tanzania, a region uniquely influenced by strong lake/ocean-land interactions and complex topography that modulate convective processes and enhance spatial variability. This study provides a comprehensive analysis of the DC during the primary wet season (March-May; MAM) over 1990–2025 using high-resolution hourly ERA5 reanalysis data. A hierarchical waveform modeling approach, including the First Diurnal Harmonic (FDH), skew-permitting, and spike-permitting waveforms, was applied to quantify the amplitude, phase, and shape of DC characteristics. Results show a dominant late-afternoon peak (~ 16:00 local time) over inland regions associated with convective rainfall, while Lake Victoria and coastal areas exhibit nocturnal to early morning maxima driven by lake-land and sea-breeze circulations. The spike-permitting waveform explains approximately 81% of the variance (R² ≈ 0.81) and outperforms both the FDH (R² ≈ 0.62) and skew-permitting waveform (R² ≈ 0.68), demonstrating a clear improvement in representing sharp, intermittent rainfall peaks. Furthermore, about 76.7% of the domain is characterised by intermediately complex (1.0 < W ≤ 2.0) non-sinusoidal cycles, highlighting the limitations of simple harmonic approaches. These findings establish a robust DC climatology for Tanzania and provide quantitative benchmarks for evaluating regional climate and weather models. They also identify deficiencies in representing mesoscale circulations and precipitation intermittency, thereby offering guidance for improving model parameterization and the simulation of rainfall processes in regions with strong surface heterogeneity.