# The Summertime Diurnal Cycle of Precipitation Derived from IMERG
## 3 Results
### IMERG Summertime Climatology  
Figure 3 exhibits the summertime diurnal mean precipitation (term \(f_{0}\) in Equation (4)). As previously noticed, the diurnal mean of occurrence, intensity and accumulation is generally highest in the tropics in correspondence with the Intertropical Convergence Zone (ITCZ), especially over land. Either side of the ITCZ, arid regions (where diurnal mean accumulation is less than 0.275 mm) are ever-present in the Sahara and oceans adjacent to the west coasts of Southern Africa and South America. Outside of the tropics, occurrences over the ocean exceed those over land with a sharp increase at higher latitudes (45\({}^{\circ}\)-60\({}^{\circ}\)N/S). Conversely, the most intense rainfall is mostly found over land with solar heating and the consequent land-based convection being the dominant driver of precipitation intensity. The most intense regions outside of the tropics tend to be found in Eastern US, South Asia, South America, South-East Africa, and their eastward adjacent coastal oceans. Unexpected precipitation trends are found in the southern oceans in the 45\({}^{\circ}\)-60\({}^{\circ}\)S region with diurnal mean occurrence, intensity and accumulation significantly higher for the Pacific Ocean than for the Indian and Atlantic Oceans. The global pattern of diurnal mean accumulation is clearly correlated strongly with occurrence (correlation = 0.94), which highlights that occurrence (and not intensity) is the dominant contributor to accumulation. Figure 3: Summertime diurnal mean precipitation (\(f_{0}\)) for (**a**) occurrence, (**b**) intensity, and (**c**) accumulation at 2\({}^{\circ}\times 2^{\circ}\) resolution derived from four years of IMERG data. The diurnal cycle of occurrence is mostly dominated by the 24 h harmonic over land, especially in the 45\({}^{\circ}\) N-45\({}^{\circ}\)S region with most amplitudes exceeding 25%. Within this region, a pronounced 24 h amplitude is found in areas such as the US and South America, Southern/Eastern Africa, South-East and East Asia, and Northern Australia with 24 h amplitudes exceeding 60%. The 24 h harmonic tends to maximise over land across the evening (~16-24 LST). A pronounced 24 h amplitude is also found over the ocean adjacent to the west of the US and Western Europe. The 12 h amplitude is only comparable with the 24 h amplitude for some oceanic regions where results are reliable (i.e., adjacent to the west coasts of the US, South America, Southern Africa, and Australia), with the harmonic amplitudes unreliable over most of the ocean (see the distribution of black dots). The 24 h maximum occurs in the morning (~3-9 LST) in the oceanic regions where the 24 h and 12 h harmonics are comparable, whilst the maximum occurs earlier in the morning (~0-4 LST) north of 30\({}^{\circ}\)N. The 12 h maxima occur at 23-2 LST and 11-14 LST over the ocean. For intensity, the amplitudes of the 24 h and 12 h harmonics are comparable (~15-35%) over land and ocean, with the 12 h harmonic generally slightly stronger over land except for specific regions with quite pronounced 24 h amplitude enhancement (e.g., central US, South American coasts, African coasts, Australia, East and South-East Asia). The 24 h harmonic typically reaches its maximum between evening and early morning over land (~20-4 LST), apart from above 45\({}^{\circ}\)N where it maximises in the afternoon (~14-18 LST) and in Eastern Asia where it peaks later in the morning (~4-9 LST). Over ocean, the 24 h harmonic tends to maximise in the early to late morning (~3-12 LST), apart from in the 45\({}^{\circ}\)-60\({}^{\circ}\)S region where the peak is between late evening and early morning (~20-2 LST). The 12 h harmonic reaches its peak everywhere in the mid morning and the early evening (~5-8 LST and 17-20 LST). The diurnal cycle of accumulation is dominated by the 24 h harmonic over land with prominent amplitudes in most regions (mostly > 25% and > 60% in many regions), whilst the 24 h harmonic still tends to dominate over ocean but with lesser amplitudes (mostly < 25%).