Abstract
The characteristics of intense diurnal precipitation occurring beneath the South Asian High (SAH) are diagnosed in the summer monsoon season from 2010 to 2015 using observational data. The diagnostics indicate that summer nighttime rainfall events in the northeastern Tibetan Plateau can intensify towards the end of the monsoon period. By defining a transition index to identify the transition day during which the episodes of diurnal convection start to decline, daily thermodynamic properties and precipitation from each year were composited before and after the transition date. The analysis reveals that warmer air, increased moisture, and stronger upward velocity are present in the atmosphere before the transition day, potentially elevating nighttime convective precipitation. Enhanced upward velocity that is present through the two months prior to transition date coincides with the timing of the peak SAH, while weakened upward velocity afterwards coincides with its subsequent retreat. The large-scale lift due to terrain-ambient air interaction underneath the SAH and the increased moisture content can enhance the potential for diurnal convection, which lends support to the nighttime peak of rainfall. This feature persists until the transition date, after which the SAH starts to retreat.
Highlights
On 20–21 August 2013, two severe diurnal rainfall events occurred consecutively in the cities of Chaka and Datong near Lake Qinghai on the Tibetan Plateau.The two events had a clear diurnal signal with both showing peak rainfall rates around 22:00 local time before steadily weakening in the morning hours (Figure 1b)
During the summer monsoon season, low-level convergence over the Tibetan Plateau associated with surface heating produces what is known as the “sensible heat driven air-pump” (SHAP) [30,31]
The SHAP mechanism suggests that summer precipitation should predominantly be daytime convective rainfall, though most of the severe events occurred during the night
Summary
The two events had a clear diurnal signal with both showing peak rainfall rates around 22:00 local time before steadily weakening in the morning hours (Figure 1b). Many studies have focused on the impacts of SAH spatial afternoon and reaches its peak in late evening and through the night [3,9,10]. The extreme nature and high cost of the August 2013 events call for an examination of the occurrence, duration, and intensity of diurnal rainfall events over the Tibetan Plateau and how the related convection interacts with the SAH lifecycle. This study is intended to analyze the characteristics of the warm season diurnal precipitation variation, its daily amplitude change over the northeastern Tibetan Plateau, and the thermodynamic properties related to the SAH evolution that play a role in enhancing precipitation
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