# The Summertime Diurnal Cycle of Precipitation Derived from IMERG
## 1 Introduction  
Characterising precipitation is a key challenge for the research community and is generally important for many aspects related to society (e.g., flooding, agriculture, water availability, etc.). Characteristics such as occurrence, intensity and duration of precipitation are expected to be more strongly influenced by climate change than accumulation, and models are expected to represent such characteristics accurately for better simulations and forecasts [1]. A test of model-representation of precipitation characteristics is how well they capture the diurnal cycle of precipitation, with observations paramount in outlining model deficiencies [1]. In fact, model simulations have been found to precede the observed peak in precipitation by several hours [1, 2], and have difficulty in representing the diurnal cycle [1]. Obtaining the necessary spatio-temporal resolution and coverage to accurately observe the diurnal cycle globally is challenging without the use of multiple satellites. Previous observational studies using weather stations [3, 4, 5] have been restricted by spatial coverage, whilst those using single sun-asynchronous orbit satellites (i.e., the Tropical Rainfall Measuring Mission, TRMM) have been susceptible to sampling issues [6, 7, 8]. Constellations of satellites offer the obvious advantage of closely repeated observations and coverage over remote regions but require proper cross-calibration between the different sensors. Radars indisputably provide the best precipitation estimates as they directlyobserve the vertical structure of precipitating systems. However, spaceborne precipitation radars provide limited spatial coverage as typically only one has been in space since 1997 [9; 10]. Consequently, satellite constellations must include passive microwave (PMW) and infrared (IR) sensors, of which there are many in space (see a subset in Table 1 of [11]), though they provide measurements (e.g., cloud top temperatures) that are less directly related to precipitation. The TRMM constellation was the first precipitation-oriented satellite constellation, from which the TRMM Multi-satellite Precipitation Analysis product (TMPA) was generated. TRMM TMPA, a merged PMW-IR product with ground-based land gauge adjustment, provides 50\({}^{\circ}\)N-S coverage at 0.25\({}^{\circ}\times\) 0.25\({}^{\circ}\) and 3-hour resolution [12] and is commonly used for assessing the diurnal cycle of precipitation [2; 13; 14]. The product was calibrated by precipitation products from the TRMM satellite's precipitation radar and microwave radiometer [12] until October 2014, when the precipitation radar measurements became unsuitable as a consequence of the de-orbiting of the TRMM satellite [9]. The TRMM TMPA product, without radar-radiometer calibration from the TRMM satellite, is expected to be produced until mid-2019 [9]. The TRMM follow-up, the NASA-JAXA Global Precipitation Measurement (GPM) Mission Core Observatory (CO), was launched in February 2014 with the first space-borne dual-frequency precipitation radar (DPR) and the GPM microwave imager (GMI) onboard [10; 15; 16]. The GPM-CO instruments act as calibrators for the Integrated Multi-satellite Retrievals for GPM (IMERG) product, another multi-satellite merged PMW-IR precipitation product with gauge input, with unprecedented spatial (0.1\({}^{\circ}\times\) 0.1\({}^{\circ}\)) and temporal (half-hour) resolution for 60\({}^{\circ}\)N-S coverage [11]. Also, IMERG replaced TMPA as the only radar-radiometer calibrated gridded precipitation product in December 2014 [9]. The GPM-CO offers several improvements over TRMM as a calibrator for the multi-satellite product including: Ka-band (35.5 GHz) measurements accompanying Ku-band (13.6 GHz) measurements, which better constrains the precipitation retrieval; additional microwave imager channels at 165.5 GHz and 183.3 GHz, which aid in detecting light and solid precipitation; and an inclination that is no longer constrained to the tropics (GPM: 65\({}^{\circ}\); TRMM: 35\({}^{\circ}\)) [11].