Abstract/Summary

The diurnal cycle of precipitation over the Central Himalaya is governed by a complex interaction between the diurnal cycle of tropical convection and local orographic flow. Understanding this interaction is crucial for model evaluation, where the simulation of such processes is highly sensitive to model resolution and choice of parameterisation schemes. In this study, the mean diurnal cycle is computed using GPM-IMERG data and is shown to be bimodal, with one peak in the late afternoon (1700 IST) and a stronger one in the early morning (0200 IST). This structure is an artefact of compositing, as individual days are associated with single peaks. The late afternoon `convective' peak is shown to be linked to the diurnal cycle of tropical convection, whereas the nocturnal `katabatic' peak is shown to be triggered by nocturnal downslope flow converging with the background monsoon circulation. As such, the katabatic peak is strongly favoured by an active monsoon trough, which provides greater southeasterly moisture flux to the foothills, resulting in increased low-level moisture flux convergence upon interaction with the katabatic northerlies. When the trough is less active, precipitation is brought to the region by mesoscale convective systems, ranging in scale from tens to thousands of kilometres, resulting in convective peaks. We hypothesise that these convective afternoon peaks are enhanced by anabatic flow. It is shown that the Boreal Summer Intraseasonal Oscillation (BSISO) does not play a significant role in modulating either the timing or amplitude of the diurnal cycle; however, low-pressure systems do: either by intensifying the trough (and hence the katabatic peak), or, when further north, by providing deep convection (hence supporting the convective peak). Two reanalyses and a separate 17-km model with parameterised convection capture both peaks, but overestimate the magnitude of the convective peak and underestimate the magnitude of the katabatic peak.