Evaluation of diurnal variation of GPM IMERG-derived summer precipitation over the contiguous US using MRMS data

Abstract

This paper investigates the accuracy of the Integrated Multi-satellitE Retrievals for Global Precipitation Measurement (GPM), which provides merged microwave and infrared satellite precipitation estimates, over the contiguous US. The study focuses on diurnal variations in precipitation during a two-year summer period (June–August, 2014–2015). The normalized amplitude and phase of the diurnal cycle of IMERG are evaluated against those of ground reference from the Multi-Radar/Multi-Sensor system in terms of precipitation amount, frequency and intensity on a 1°/1 hr scale. IMERG well captures large-scale regional features of the diurnal cycle of precipitation and overall agrees well with the reference for both diurnal and semidiurnal variations. The comparison results indicate that the IMERG precipitation estimates can be a reliable alternative to ground-based measurements even at the subdaily scale; however, region-specific data discrepancies are still observed. For instance, we reveal that IMERG substantially overestimates normalized amplitude of diurnal precipitation in the central US, while IMERG tends to underestimate diurnal variations over the mountain regions in the western and eastern US. In terms of phase, we find a significant difference in the timing of peak precipitation between convective and stratiform regions of mesoscale convective systems (MCSs) over the Great Plains. This time shift is more apparent during the mature and dissipation stages of MCSs, which lead to relatively early peaks in the diurnal cycle of precipitation from IMERG. This phase bias implies a higher sensitivity of IMERG towards the convective regions of MCSs, supposedly because of the brightness temperature depression coming from ice particles aloft sampled by spaceborne passive microwave sensors. Such discrepancy between the actual and satellite-estimated precipitation timing can be challenging, e.g. when the satellite data are used to study subdaily precipitation processes or to validate numerical simulations. Consequently, our assessment of the IMERG performances highlights the need for improvements in the IMERG system.