Abstract
ABSTRACTWe investigate trends in monsoon and extreme precipitation in Nepal based on rain gauge measurements. We find that precipitation amounts in Nepal vary considerably in space and time. The number of occurring extremes and the amount of precipitation are controlled mainly by the Indian summer monsoon. Almost all extreme precipitation events, recorded by 98 considered meteorological stations, occur during the Indian summer monsoon with the maximum in July. For Nepal in general, we find that the amount of precipitation and number of extreme events per station are neither significantly increasing nor decreasing between 1971 and 2010. However, on a regional scale we identify areas with positive and negative trends. A comparison of the combined precipitation time series with the ENSO 3.4 index reveals a connection between ENSO and the variability in monsoon precipitation. The correlation with the number of monsoon extremes vanishes for increasing percentiles. We investigate trends of upper percentiles of daily precipitation which pinpoint regions of increasing and decreasing extremes. These patterns are similar to spatial patterns in mean monsoon precipitation trends, whereas the median of the precipitation distribution undergoes only minor changes. Further analysis using extreme value theory confirm the prevailing trends from quantile regression for most stations and depict strong changes in return levels. Especially for Far‐West Nepal, we find robust evidence for a systematic increase in extreme precipitation.
Highlights
1.1 Setting the sceneOver the recent years, extreme weather events and extreme precipitation events have become increasingly prevalent in scientific literature
I identified the main gaps as follows: there was no coherent picture of climatic trends in extremes in Nepal based on measurements, a systematic study on synoptic conditions leading to extreme precipitation in Nepal was missing, direct moisture sources and their role for extreme precipitation events had not yet been investigated in that region, and a comprehensive case analysis as existing for neighboring regions was not yet conducted
We identify several synoptic scale conditions that lead to the intense development of the convective system: anonamously high moisture sources along the path of a low-level flow characteristic for monsoon break periods, this lead to moist airmasses feeding into the convective system, the additional available moisture was created by foregone precipitation events, and airmasses were destabilized by upslope flow and quasi-geostrophic forcing where the upslope flow was likely the main trigger mechanism
Summary
1.1 Setting the sceneOver the recent years, extreme weather events and extreme precipitation events have become increasingly prevalent in scientific literature. The Special Report on Extreme Events of the Intergovernmental Panel on Climate Change (Field et al, 2012) and the Fifth Assessment Report (Stocker et al, 2013) state that the magnitude of precipitation extremes and their frequency of occurrence have been increasing over most of the globe. They further conclude that a shift in the distribution, for instance a shift in the mean, can affect the extremes. We assess the role of synoptic conditions and moisture sources, and how they effect extreme precipitation in Nepal
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