Changing lateral boundary conditions for probable maximum precipitation studies: A physically consistent approach

Abstract This article presents a conceptual study toward establishing a new method for altering lateral boundary conditions in numerical model based estimates for probable maximum precipitation (PMP). We altered an extreme event in a physically and dynamically consistent way in a regional convective-scale weather prediction model (AROME-MetCoOp) by applying fields from a global ensemble climate model approach based on EC-EARTH. Ten ensemble members are downscaled with the regional model, which results in 10 different realizations of an extreme precipitation event for the west coast of Norway. We show how the position and orientation of the moisture flow is different between the individual ensemble members, which leads to relatively large changes in precipitation values for a selected catchment. For example, the modification of the moisture transport on scales of several hundred kilometers impacts the extreme precipitation amount by about 75% among the model members. Compared with historical rainfall records, precipitation changes of 62% and 71% are found for two selected catchments. Although the present study is restricted to one particular extreme event that is modified 10 times with the ensemble approach, there is a considerable spread of the moisture transport compared to the spread of the moisture transport of extreme precipitation events of the past 40 years. We conclude that the described approach is a step toward a new method to derive PMP values for a given catchment; however, a larger amount of events and larger ensembles would have to be considered to estimate PMP values

Large-scale flow patterns associated with extreme precipitation and atmospheric rivers over Norway

A climatology of extreme cold season precipitation events in Norway from 1979 to 2014 is presented, based on the 99th percentile of the 24-h accumulated precipitation. Three regions, termed north, west, and south are identified, each exhibiting a unique seasonal distribution. There is a proclivity for events to occur during the positive phase of the NAO. The result is statistically significant at the 95th percentile for the north and west regions. An overarching hypothesis of this work is that anomalous moisture flux, or so-called atmospheric rivers (ARs), are integral to extreme precipitation events during the Norwegian cold season. An objective analysis of the integrated vapor transport illustrates that more than 85% of the events are associated with ARs. An empirical orthogonal function and fuzzy cluster technique is used to identify the large-scale weather patterns conducive to the moisture flux and extreme precipitation. Five days before the event and for each of the three regions, two patterns are found. The first represents an intense, southward-shifted jet with a southwest-northeast orientation. The second identifies a weak, northward-shifted, zonal jet. As the event approaches, regional differences become more apparent. The distinctive flow pattern conducive to orographically enhanced precipitation emerges in the two clusters for each region. For the north and west regions, this entails primarily zonal flow impinging upon the south-north-orientated topography, the difference being the latitude of the strong flow. In contrast, the south region exhibits a significant southerly component to the flow

Recent changes in circulation patterns and their opposing impact on extreme precipitation at the west coast of Norway

Understanding recent and future changes of extreme precipitation is essential for climate change adaptation. Here, we use 3800 extreme precipitation events produced by an ensemble seasonal prediction system. The ensemble represents the climate from 1981 to 2018 and we analyse 3-day maximum precipitation events in September–October–November for the west coast of Norway. Two dominant atmospheric patterns, described by an empirical orthogonal function (EOF) analysis, are related to the results of the extreme value statistics. The principal components of the second and third mode of EOFs have significant trends over the last 40 years, but with an opposing impact on the return values of extreme precipitation. This explains the observed stationarity of extreme precipitation over recent decades at the west coast of Norway, which was also found in previous studies. The second mode of EOFs also shows a relation to the sea-ice coverage in the Barents and Kara Seas, which suggests a connection between the decline of sea-ice to the changes in the atmospheric pattern

Dimensjonerende korttidsnedbør for Telemark, Sørlandet og Vestlandet

Korttidsnedbør for Telemark, Sørlandet og Vestlandet er analysert ved bruk av foreliggende pluviometer-målinger, både fra stasjoner med vippepluviometer (tidsoppløsning 1 minutt) og vektpluviometer (tidsoppløsning 1 time). Pluviometerdataene er brukt til studier av høyeste observerte og dimensjonerende nedbørmengder for ulike varigheter, for å studere trender og geografiske forskjeller og til å produsere Intensitet-Varighet-Frekvens (IVF) statistikk. Data fra værradar og pluviometre er brukt til detaljerte studier av romlige mønstre i episoder med kraftig nedbø

Ekstrem korttidsnedbør på Østlandet fra pluviometer og radar data

Korttidsnedbør på Østlandet er studert ved bruk av pluviometer-målinger og radardata. Pluviometerdataene er brukt til studier av ekstreme nedbørmengder for ulike varigheter, for å studere trender og geografiske forskjeller og til å produsere Intensitet-Varighet-Frekvens (IVF) statistikk. Data fra værradar og pluviometre i Østlands-området er brukt til detaljerte studier av romlige mønstre i episoder med kraftig nedbør og til å vurdere sammenheng mellom 1-times nedbørintensitet fra radar og bakkemålinger