Katabatic and convective processes drive two preferred peaks in the
precipitation diurnal cycle over the Central Himalaya The diurnal cycle
of precipitation over the Central Himalaya
Abstract
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 LT) and a stronger one in the early morning (0200 LT).
This structure is an artefact of compositing, as individual days are
typically associated with single peaks. The late afternoon ‘convective’
peak is shown to be linked to the diurnal cycle of tropical convection,
whereas the early morning ‘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. In
contrast, 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 peaks may be enhanced by anabatic flow. It is
shown that the 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). Reanalyses and a
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.