Abstract
<title>Abstract</title> <p>We Investigate temporal and spatial properties of extreme daily rainfall (ExtD) from 75 rain gauges across Thailand and three gridded estimates (0.05° CHIRPS v2, 0.25° ERA5 and 0.1° MSWEP v2.8). ExtD is defined locally from all available days as a day reaching the 90th (p90), 95th (p95), 99th (p99) and 99.9th (p99.9) percentiles, corresponding to spatially-averaged daily amounts between 14 (p90) and 121 (p99.9) mm in the observed network. Overall, the most accurate gridded product to reproduce daily rainfall and ExtD occurrence is MSWEP. The spatial scale is estimated through the horizontal distance at which the spatial autocorrelation of ExtD occurrence falls below 1/e ~ 0.37. p90 has an e-folding distance of 77 km in Thailand Meteorological Department (TMD) gauges and respectively 158, 208 and 274 km for the co-located MSWEP, CHIRPS and ERA5 grid-points. The three gridded products tend to underestimate the percentiles and overestimate the spatial and temporal scales of ExtD. 55% (p90) to 90% (p99.9) TMD ExtD are time-isolated from similar ExtD. Accumulated rainfall amounts on non-overlapped 3-day to 11-day spells tend to enlarge the spatial scales, but this effect is modest for p95, and almost absent for p99 and p99.9 over a 3-day summation. Lastly, the observed yearly frequency of ExtD from p95 increases faster than the mean amount of rainfall at the country scale from 1979 to 2024, suggesting a long-term intensification of rainfall, which is reasonably well reproduced by ERA5, but not by MSWEP suggesting it is not reliable for trend studies over this area.</p>