Assessment of GPM IMERG Satellite Precipitation Estimation and Its Dependence on Microphysical Rain Regimes over the Mountains of Central Chile

Central Chile is characterized by complex topography with a coastal mountain and the Andes. It also lacks long-term precipitation observations over high terrain and operational radars. Satellite data can potentially provide vital information to supplement the lack of ground-based observations over this region and others with similar observational gaps. This study focuses on investigating the skill of the Global Precipitation Measurement (GPM) products Integrated Multi-Satellite Retrieval for GPM (IMERG) quantitative precipitation estimation (QPE), and its dependence of microphysical regimes.

We compare the IMERG “late run” 0.1º horizontal resolution and 30-minute temporal resolution QPE against ground-based observations from two field campaigns that took place near ~36ºS: the Chilean Coastal Orographic Precipitation Experiment (CCOPE; winter 2015), which collected data over the coastal mountain range, and the Chilean Orographic and Mesoscale Precipitation Study (ChOMPS; winter 2016), which collected observations in a transect from the coast to the Andes. To expand the full campaign analyses, we additionally used daily precipitation data from 80 operational rain gauges during the ChOMPS period. To evaluate the dependence of IMERG performance on microphysical regime, for each campaign we classified 30-min rainfall periods into regimes that include “ice-initiated rain” or “warm rain” based on the presence or absence of a well-defined melting layer in the profiling radar data. Rain gauges were used to evaluate performance of IMERG QPE for the entire campaign period and for these two regimes.

For both field campaigns, IMERG was able to reproduce the spatial pattern of orographic precipitation enhancement, but underestimated its magnitude. For CCOPE, over the coastal range the precipitation for the entire period was underestimated by IMERG by 34% over the windward slope and by 50% at higher elevation. For the ChOMPS full campaign, the average of underestimation over all the Andes sites was 19%. During CCOPE, over the high elevations of the coastal mountains, IMERG underestimated ice-initiated rain by 37% while underestimating warm rain by 70%. For ChOMPS the underestimation at an Andes site was 32% for ice-initiated rain and 48% for warm rain. Overall, IMERG showed a better performance for ice-initiated periods and coastal and valley sites than for warm rain and mountain locations. This analysis is unique in an area that lacks operational weather radar coverage providing a microphysical insight of orographic enhancement in this region and it may useful to improve satellite-based QPE over complex terrain.