# The Summertime Diurnal Cycle of Precipitation Derived from IMERG
## 4 Discussion  
Aspects of agreement and disagreement with previous studies [2; 3; 4; 14] are highlighted, as well as novelties of this study. 1. As previously suggested [3; 4; 14], the diurnal cycles of occurrence and accumulation are largely captured by the 24 h harmonic over land, though the 12 h harmonic is comparable in contribution with the 24 h harmonic over the ocean (Figures 4a,c and 6a,c). A novelty of this study is that the 24 h and 12 h amplitudes for intensity are found to contribute comparably over most regions, with the superposition of the two harmonics leading to enhanced and muted peaks in the diurnal cycle (e.g., Figure 1b,c). 2. As previously found [14], occurrence is clearly the main driver of the diurnal cycle of accumulation, as opposed to intensity. This is supported in this study by a strong correlation between occurrence and accumulation (Figure 3a,c), as well as coincidence of the regions where the 24 h amplitudes are pronounced (Figures 4a and 6a) and similar phases of the 24 h harmonic peaks (Figures 4c and 6c) corroborate this strong relationship. 3. The 24 h harmonic is the main contributor to the diurnal cycle of accumulation with large amplitudes (mostly \(>25\%\)) over land, peaking in the late afternoon and evening (Figure 6). Figure 8: Seasonal diurnal cycle of accumulation at \(2^{\circ}\times 2^{\circ}\) resolution derived from four years of IMERG data, represented by (**a,c,e,g**) the amplitude of the 24 h harmonic normalized to the diurnal mean accumulation shown in Figure 3c; (**b,d,f,h**) the phase of the 24 h harmonic. Meteorological seasons are (**a,b**) December-January-February (DJF), (**c,d**) March-April-May (MAM), (**e,f**) June-July-August (JJA), and (**g,h**) September-October-November (SON). Refer to Section 2.2 for explanation of the blank and the black-dotted grid boxes. Furthermore, 24 h amplitudes over ocean are again shown to be smaller (mostly \(<25\%\)) with mostly morning peaks as previously found [2; 14], though some early afternoon peaks in the coastal regions should be noted. 4. Whilst seasonal variations in the diurnal cycle of accumulation over the ocean are small in general [14], strong diurnal variations in the North Atlantic Ocean should be noted (Figure 8). 5. Large disagreements between the peak time of the 12 h harmonic for accumulation north of 30\({}^{\circ}\) have been identified between IMERG and a selection of other satellite precipitation products (TRMM TMPA, CMORPH, PERSIANN). Whilst TRMM TMPA and PERSIANN tend to maximise at \(\sim\)3-7 LST and 15-19 LST and CMORPH tends to maximise at \(\sim\)5-8 LST and 17-20 LST (see Figure 6 of [14]), IMERG tends to maximise at \(\sim\)21-1 LST and 9-13 LST (Figure 6d). Errors on the IMERG phases exceeding 1 h can be found in such regions and may explain some of the discrepancies. 6. When comparing the diurnal harmonic of accumulation between IMERG and TRMM TMPA, it is clear that the IMERG amplitude is much larger (\(\sim\)30-60%, Figure 6a) than the corresponding TRMM amplitude (\(\sim\)20-40%, see Figure 5j of [14]) over the North Atlantic and Pacific Oceans. Small errors in the IMERG amplitudes in these regions do not appear to explain such differences. The results determined from the 4-year period are unaffected by sampling issues or the strong El Nino event of 2015-2016. Comparison of the results with those deduced using the same methodology but with a 2-year period (excluding 2015-2016, not shown) are found to be nearly indiscernible, which highlights that El Nino events do not strongly impact the diurnal cycle of precipitation though there are perceptible differences in accumulations. Furthermore, it suggests that a 4-year period is sufficient for analysing the diurnal cycle. Future studies could investigate the impact of El Nino onto the regional diurnal cycle. This study only accounts for random errors of IMERG precipitation estimates when deducing the diurnal cycle of precipitation (accumulation and intensity only).