# The Summertime Diurnal Cycle of Precipitation Derived from IMERG
## 5 Conclusions  
This study has analysed the summertime diurnal cycles of precipitation occurrence, intensity and accumulation globally at \(2^{\circ}\times 2^{\circ}\) resolution using four years of IMERG final run version-5 data for the period June 2014 to May 2018. A harmonic analysis was conducted on the data and their uncertainties to extract the diurnal (24 h) and semi-diurnal (12 h) harmonics from the diurnal cycle, and to quantify their amplitudes, phases and respective errors. The results of the analysis can be summarised as follows:1. The amplitudes of the diurnal cycles of occurrence and accumulation are typically greater over land (>25%) than over ocean (<25%) (Figures 4 and 6). The amplitude of the diurnal cycle of intensity is typically comparable over land and over ocean (~15-35%). 2. The diurnal cycles of occurrence and accumulation over land are mainly contributed to by the 24 h harmonic, which tends to peak across late afternoon and evening (~16-24 LST), compared to the 12 h harmonic, which tends to peak in the early to mid morning and early afternoon to early evening (~2-8 LST and 14-20 LST) (Figures 4 and 6). However over ocean, the contributions are comparable for the 12 h harmonic, which tends to peak between late evening to early morning and late morning to early afternoon outside of the convergence zones (~21-2 LST and 9-14 LST), and the 24 h harmonic, which peaks in the morning in open regions (~0-10 LST) and between late morning and afternoon in the coastal regions (~8-15 LST). The diurnal cycle of intensity is similarly contributed to by the 24 h harmonic, which typically peaks between mid evening and early morning (~20-4 LST) over land (45\({}^{\circ}\) N-S) and early to late morning (~3-12 LST) over ocean, and the 12 h harmonic, which peaks in the mid morning and the early evening (~5-8 LST and 17-20 LST) everywhere (Figure 5). 3. The diurnal cycle of occurrence dictates the diurnal cycle of accumulation as opposed to intensity. This is supported by strong correlations in diurnal means between occurrence and accumulation (Figure 3a,c), in increased amplitudes in the 24 h harmonic regions (Figures 4a and 6a), and in the time at which the 24 h harmonic peaks (Figures 4c and 6c). 4. Fitting errors on the 24 h and 12 h harmonics representing the diurnal cycle suggest that results over land are generally more reliable than over ocean, phases are generally more reliable than amplitudes, and that results for accumulation are generally more reliable than for occurrence and intensity (Figure 7). In particular, amplitudes and phases for occurrence and intensity should be treated with caution (except for the 24 h harmonic for occurrence over land). Furthermore, amplitudes and phases for accumulation in the 45\({}^{\circ}\)-60\({}^{\circ}\)N/S region should be treated with caution. 5. The use of 24 h and 12 h harmonics is not sufficient to capture the diurnal cycle of intensity (see the distribution of black dots in Figure 5). Low correlations between the harmonic function and the diurnal cycle derived from IMRG data and/or high error estimates for most of the globe suggest that higher-order harmonics are required to better capture the diurnal cycle of intensity. 6. IMRG precipitation estimates in the 45\({}^{\circ}\)-60\({}^{\circ}\)S region present some anomalies, and results from these regions should be treated cautiously. This is specifically marked by pronounced precipitation occurrences, intensities and accumulations in this zonal region of the Pacific Ocean compared to the Indian and Atlantic Oceans (Figure 3). These artifacts are under investigation. 7. Variations in the diurnal cycle of accumulation across the seasons are minimal in the tropics, where diurnal amplitudes are stronger (mostly exceeding 60% over land) throughout the seasons, and more evident in the mid-latitudes (Figure 8). Summertime mid-latitude diurnal amplitudes tend to exceed those in winter.