The relationship between incoming solar radiation and diurnally-forced precipitation has been used to explain phase differences in precipitation over the land and sea with the onset of active Madden-Julian Oscillation conditions in the Maritime Continent. However, few direct observations are available to test this relationship, and most studies have relied on modelling or satellite-derived results.

Here, the relationship between incoming solar radiation and diurnally-forced precipitation is examined at three forested, low-lying AsiaFlux sites in the Maritime Continent with 2-7 years of continuous sub-daily observations. Although designed to study terrestrial ecosystem exchanges, the quality multi-year, sub-daily records are ideal for studying interactions between intraseasonal and mesoscale variability. Short-wave incoming radiation can be considered a proxy for cloudiness, which varies strongly with intraseasonal variability, while precipitation is indicative of both large-scale and mesoscale processes.

Relationships between morning solar insolation and relative humidity and the onset-time and amount of diurnally-forced precipitation were examined, suggesting that solar insolation is at least as important as moisture availability in dictating amount and timing of afternoon rainfall.  A clustering algorithm was used to delineate clear-sky days, heavy afternoon precipitation days and persistent rain and cloudy days. The occurrence of these days shows expected variation with intraseasonal variability. Comparison with mesoscale modelling results suggests that simulated incoming solar radiation is too large, possibly relating to insufficient stratiform precipitation.

The importance of incoming solar radiation, and therefore cloudiness, in dictating afternoon rainfall highlights that radiation, microphysics and convection all need to be properly represented in order to simulate realistic interscale variability.

The author would like to thank the data providers from AsiaFluxDB for permission to use the data: Takahisa Maeda for the Bukit Soeharto site, Takashi Hirano for the Palangkaraya drained forest site and Yoshiko Kosugi, Satoru Takanashi and Shinjiro Ohkubo for the Pasoh Forest Reserve site.