Characterisation and model representation of the diurnal cycle of precipitation over the Maritime Continent

The diurnal cycle is one of the major modes of weather variability over the islands and coastal seas of the Maritime Continent. As observed in gridded data from the GPM satellite, the mean diurnal cycle of precipitation over land comprises a relatively rapid increase in rainfall rate through the afternoon followed by a more gradual decrease in intensity through the evening and night. Characterisation of this cycle using a small number of independently intuitive parameters helps in the assessment of model performance, and a best-fit waveform approach is often favoured for its conversion from discrete data to a continuous approximation. The phase of the peak of the best-fit first diurnal harmonic (a pure sine wave of 24-hour period) is systematically later over land than the observed time of peak precipitation by the order of one to three hours. Fitting of the mean diurnal cycle of precipitation using a skew-permitting waveform is trialled to capture its characteristics more accurately. A positive skewness, indicating a sharp rise and gradual decrease, is observed across most land area while skewness over the coastal seas is regionally dependent. Positive skewness over land displaces the best-fit peak to earlier time than the first diurnal harmonic peak, resulting in improved alignment between the observed peak and the best-fit peak, generally within one hour.

 

The skew-permitting approach to characterisation is further applied to data from regional hindcast model runs to assess model skill at capturing the diurnal cycle. It suggests reasonable skill over mountainous land area where diurnal deep convection tends to initiate, but poor skill over land away from such convection centres. The skewness parameter exposes poor skill over the coastal seas; immediately offshore southwest of Sumatra the amplitude and phase parameters from the model align well with observed values, however model skewness infers a gradual intensification of mean precipitation up to the peak while observations indicate a rapid intensification in this area. These findings will contribute to our understanding of the deficiencies of rainfall propagation mechanisms in models.