Katabatic and convective processes drive two preferred peaks in the precipitation diurnal cycle 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 parametrisation schemes. In this study, the mean diurnal cycle is computed using gridded satellite data–Integrated Multi‐satellitE Retrievals for Global Precipitation Measurement (GPM) (IMERG) data and is shown to be bimodal, with one peak in the late afternoon (1700 India Standard Time (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 does not play a significant role in modulating either the timing or the 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 parametrised convection capture both peaks, but overestimate the magnitude of the convective peak and underestimate the magnitude of the katabatic peak.

Analysis of Himalayan summer monsoon rainfall characteristics using Indian High‐Resolution Regional Reanalysis

“…The shuttle radar topography mission with a 90‐m spatial resolution digital elevation model is used to derive terrain and stream network distribution (Figure 1b). It is worth noting that the complex terrain, different climate systems such as seasonal migration of intertropical convergence zone, upper‐tropospheric jet streams, mesoscale convective systems and highly localized weather and climate conditions potentially have a significant impact on this region's rainfall distribution (e.g., Gadgil, 2003; Hunt et al, 2022; Nandargi & Dhar, 2011; Shrestha et al, 2012; Zandler et al, 2019) Furthermore, the Himalayan region is well known for giving birth to several major rivers, including the Ganges, Brahmaputra and Indus. Millions of people in this region rely on the glacier and snow‐fed rivers for freshwater.…”

Seasonal and Semi‐Diurnal Variations in Cloud‐Phase Characteristics Over the Southern Himalayas and Adjacent Regions as Observed by the Himawari‐8 Satellite

“…The semi‐diurnal cycle of cloud during daytime is mainly controlled by orography interacting with solar heating (Hunt et al., 2022; Yu et al., 2020). Due to the large‐scale topography, the TP absorbs solar radiation strongly and its considerable sensible heating warms and moistens the boundary layer, resulting in the convergence of the surrounding air toward the TP, forming an ascending movement over the TP.…”

“…This can explain the results displayed in Figures 7–10. As we know, there is no solar radiation at nighttime, and the katabatic mountain‐valley breezes interacting with the large‐scale circulation are the main factor driving the convective activities over the study region (Hunt et al., 2022). Therefore, this may reduce the uphill water vapor transport, and strong cooling over the TP may contribute to the weaker convection than that during the daytime, but produce more convection over the plains and slope regions.…”

“…It has formed a climatic boundary that affects the circulations of the atmosphere and waters (Barros et al., 2004; Gao et al., 2019; Shrestha et al., 2012). Owing to their unique thermodynamic properties and topographic distribution, the southern Himalayas and adjacent regions exhibit a remarkable regional difference in cloud and precipitation (Q. Chen et al., 2019; Fu et al., 2018; Hunt et al., 2022; Kukulies et al., 2019; Luo et al., 2011; X. Pan et al., 2021; Qie et al., 2014). Therefore, for understanding the relationship between cloud and terrain, it is important to quantitatively reveal the different cloud‐phase characteristics over the southern Himalayas and adjacent regions.…”

Recent tangible interannual variability of monsoonal orographic rainfall in the Eastern Himalayas

“…Indian monsoon has different flavors such as decadal, interannual, annual, semi-annual, seasonal, and diurnal. Rainfall over the south-facing terrain of the Himalayas is distinguished by an orographic diurnal cycle (Bhatt and manuscript submitted to AGU publications Nakamura, 2006;Hunt et al, 2022). However, a recent study highlights drying due to amplification in the diurnal cycle (Norris et al, 2020).…”