Research interests
- Hydroclimatology
- Stochastic and large-scale hydrology
- Land surface modelling
- Climate impacts on the physical system
Background
Education
Positions held
Tags:
Hydrology,
Hydrological modelling,
Climate change
Publications
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Dyrrdal, Anita Verpe; Isaksen, Ketil; Jacobsen, Jens Kristian Steen & Nilsen, Irene Brox
(2020).
Present and future changes in winter climate indices relevant for access disruptions in Troms, northern Norway.
Natural Hazards and Earth System Sciences.
ISSN 1561-8633.
20(6),
p. 1847–1865.
doi:
10.5194/nhess-20-1847-2020.
Full text in Research Archive
Show summary
A number of seaside communities in Troms, northern Norway, are vulnerable to sudden weather-induced access disruptions due to high-impact weather and dependency on one or few roads. In this paper we study changes in winter weather known to potentially cause access disruptions in Troms, for the present climate (1958–2017) and two future periods (2041–2070; 2071–2100). We focus on climate indices associated with snow avalanches and weather that may lead to for example slippery road conditions. In two focus areas, the most important results show larger snow amounts now compared to 50 years ago, and heavy snowfall has become more intense and frequent. This trend is expected to turn in the future, particularly at low elevations where snow cover during winter might become a rarity by 2100. Strong snow drift, due to a combination of snowfall and wind speed, has slightly increased in the two focus areas, but a strong decrease is expected in the future due to less snow. Events of heavy rain during winter are rather infrequent in the present winter climate of Troms, but we show that these events are likely to occur much more often in all regions in the future.
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Rizzi, Jonathan; Nilsen, Irene Brox; Stagge, James Howard; Gisnås, Kjersti Gleditsch & Tallaksen, Lena M.
(2018).
Five decades of warming: impacts on snow cover in Norway.
Hydrology Research.
ISSN 1998-9563.
49(3),
p. 670–688.
doi:
10.2166/nh.2017.051.
Show summary
Northern latitudes are experiencing faster warming than other regions in the world, which is partly explained by the snow albedo feedback. In Norway, mean temperatures have been increasing since the 1990s, with 2014 being the warmest year on record, 2.2 °C above normal (1961–1990). At the same time, a concurrent reduction in the land area covered by snow has been reported. In this study, we present a detailed spatial and temporal (monthly and seasonal) analysis of trends and changes in snow indices based on a high resolution (1 km) gridded hydro-meteorological dataset for Norway (seNorge). During the period 1961–2010, snow cover extent (SCE) was found to decrease, notably at the end of the snow season, with a corresponding decrease in snow water equivalent except at high elevations. SCE for all Norway decreased by more than 20,000 km2 (6% of the land area) between the periods 1961–1990 and 1981–2010, mainly north of 63° N. Overall, air temperature increased in all seasons, with the highest increase in spring (particularly in April) and winter. Mean monthly air temperatures were significantly correlated with the monthly SCE, suggesting a positive land–atmosphere feedback enhancing warming in winter and spring.
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Nilsen, Irene Brox; Stagge, James Howard & Tallaksen, Lena M.
(2017).
A probabilistic approach for attributing temperature changes to synoptic type frequency.
International Journal of Climatology.
ISSN 0899-8418.
37(6),
p. 2990–3002.
doi:
10.1002/joc.4894.
Show summary
To understand the cause of regional temperature change, it is common to separate the temperature change signal
into changes in atmospheric synoptic circulation and other factors, so-called within-type changes. In this study, we suggest a novel probabilistic approach that allows detection of months and regions where temperature changes can mainly be attributed to
changes in synoptic circulation and where within-type changes also play a role. By combining resampling with a Monte Carlo
test, we assess the likelihood that the observed warming can be explained by synoptic circulation changes alone. This method
is applicable for any variable, and in any region of the world. We applied it to an example case using gridded WATCH Forcing
Data ERA-Interim (WFDEI) temperature data and synoptic types derived from the SynopVis Grosswetterlagen catalogue
(1981–2010). For this European example, the most widespread warming was found in summer, with up to 60% of the land
area experiencing signicant warming during August, notably in Eastern and Northern Europe. In spring and autumn, this area
was reduced to 10–30%. In December and January, only about 5% of the land area experienced signicant warming, most
pronounced in northern Scandinavia. The probabilistic approach revealed that changes in synoptic circulation c ould not account
for all the observed (WFDEI) warming, with the exception of regions in southeastern Europe in February and Western Europe
in May. Signicant warming in other months and regions, such as the large-scale warming in April, June, July, August, and
November, must also be caused by other factors. Within-type changes were conrmed for the Black Sea region in November,
where the magnitude of a widespread temperature trend was strongest. This European example contributes to an improved
understanding of the causes of recent temperature change by assessing the relative role of synoptic circulation changes and
within-type changes on regional-scale warming.
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Nilsen, Irene Brox; Fleig, Anne K.; Tallaksen, Lena M. & Hisdal, Hege
(2014).
Recent trends in monthly temperature and precipitation patterns in Europe.
IAHS-AISH publication.
ISSN 0144-7815.
363,
p. 132–137.
View all works in Cristin
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Nilsen, Irene Brox; Dalen, Thea Grobstok & Bryn, Anders
(2021).
Vil du være med og registrere tregrensa i sommer?
www.forskning.no.
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Bryn, Anders; Dalen, Thea Grobstok; Finne, Eirik Aasmo; Heiberg, Hanne; Keetz, Lasse Torben & Nilsen, Irene Brox
[Show all 30 contributors for this article]
(2021).
Natur i endring - samspillet mellom klima og økosystemene.
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Nilsen, Irene Brox; Tallaksen, Lena M. & Stordal, Frode
(2017).
Potential feedbacks between snow cover, soil moisture and surface energy fluxes in Southern Norway.
Show summary
At high latitudes, the snow season has become shorter during the past decades because snowmelt is highly
sensitive to a warmer climate. Snowmelt influences the energy balance by changing the albedo and the partitioning
between latent and sensible heat fluxes. It further influences the water balance by changing the runoff and soil
moisture. In a previous study, we identified southern Norway as a region where significant temperature changes in
summer could potentially be explained by land-atmosphere interactions. In this study we hypothesise that changes
in snow cover would influence the summer surface fluxes in the succeeding weeks or months. The exceptionally
warm summer of 2014 was chosen as a test bed. In Norway, evapotranspiration is not soil moisture limited, but
energy limited, under normal conditions. During warm summers, however, such as in 2014, evapotranspiration
can be restricted by the available soil moisture. Using the Weather Research and Forecasting (WRF) model
we replace the initial ground conditions for 2014 with conditions representative of a snow-poor spring and a
snow-rich spring. WRF was coupled to Noah-MP at 3 km horizontal resolution in the inner domain, and the
simulations covered mid-May through September 2014. Boundary conditions used to force WRF were taken from
the Era-Interim reanalysis. Snow, runoff, soil moisture and soil temperature observational data were provided
by the Norwegian Water Resources and Energy Directorate for validation. The validation shows generally good
agreement with observations. Preliminary results show that the reduced snowpack, hereafter “sim1” increased
the air temperature by up to 5 K and the surface temperature by up to 10 K in areas affected by snow changes.
The increased snowpack, hereafter “sim2”, decreased the air and surface temperature by the same amount. These
are weekly mean values for the first eight simulation weeks from mid May. Because of the higher net energy
available (∼ 100 Wm-2) in sim 1, both the evapotranspiration and sensible heat fluxes increased. In sim 2, they
decreased because of lower net energy. The ground heat flux decreased in sim1 (and increased in sim2). Large
increases were seen in runoff, both surface and underground runoff, during the first weeks of sim2 (from mid
May), but the timing of snowmelt was only slightly affected. This study contributes to a greater understanding of
land-atmosphere interactions in a wet, temperate climate, in particular the role of snow cover (and snowmelt) and
its feedback to the atmosphere.
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Nilsen, Irene Brox; Stagge, James Howard & Tallaksen, Lena M.
(2016).
A probabilistic approach to attribute warming to changes in atmospheric circulation
.
Show summary
Europe has been warming over the past decades, especially in southern Europe in summer and northern Europe
in winter. To understand the causes of regional warming, it is common to separate the temperature change signal
into changes in atmospheric circulation (or dynamic causes) and other factors, so-called within-type changes (or
thermodynamic causes). For example, increasing temperatures due to greenhouse gases may alter the position
and strength of the polar jet stream, thus causing a change in the atmospheric circulation signal. On the other
hand, warming may be entirely independent of circulation, occurring as a general increase in surface temperature.
With the aim to detect regions and time of the year in Europe in which recent warming can either be explained
by changes in atmospheric circulation or by within-type changes, we suggest a novel probabilistic approach to
calculate the circulation-induced trend, the part of the temperature trend that is induced by changes in atmospheric
circulation. Through the use of resampling, in combination with a Monte Carlo test, we assessed the likelihood
that the observed temperature trend can be explained entirely by changes in atmospheric circulation frequency. The
temperature data originate from the gridded 0.5◦ Watch Forcing Data Era-Interim (WFDEI), and cover the period
1981–2010. The SynopVis Grosswetterlagen catalogue of circulation types was used to detect circulation-induced
trends in the same time period. We analysed trends on the monthly time scale to reveal short-term responses, such
as those related to snow or greening. The most wide-spread observed warming was found in summer, with up to
60% of the European land area experiencing a significant warming trend during August, most notable in eastern
and northern Europe. In spring and autumn, the percent area with significant temperature increases reduced to
10–30%. In December and January, only 5% of the land area experienced significant warming, most pronounced
in northern Scandinavia. The probabilistic approach revealed that changes in atmospheric circulation could not
account for all the observed warming. Regions where the observed trend likely can be explained entirely by changes
in atmospheric circulation include western Europe in May, eastern Europe in August and Scandinavia in September.
In most of the regions and months experiencing significant trends, however, warming must be caused by other
factors as well; such within-type changes potentially driven by feedbacks between the land surface and atmosphere.
For two cases where the magnitude of the temperature trend was strongest, northern Scandinavia in December and
in the Black Sea region in November, circulation types warmed over time. This confirms the role of within-type
change, i.e., that circulation types changed their properties over time. This study provides an important contribution toward improved understanding of the causes of synoptic-scale temperature change in Europe, more specifically, the relative role of circulation-induced changes and within-type changes.
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Nilsen, Irene Brox; Tallaksen, Lena M. & Stagge, James Howard
(2015).
A probabilistic method of calculating circulation-induced trends.
Show summary
The water cycle in Europe has changed substantially over the past three decades. Increasing
runoff is observed during winter and at northern latitudes in particular. Spring and summer
months, as well as southern latitudes, are facing drier conditions. To understand what is driving
large-scale changes in runoff, we look into changes in precipitation and temperature and link
these to changes in atmospheric circulation. Previous studies have used the method of trend
ratios (Cahynová and Huth, 2009) to attribute precipitation and temperature trends to changes
in the frequency of circulation types. A trend ratio is the ratio of hypothetical trend, i.e., the
trend that would result due to changes in circulation type frequency only, to the observed trend.
However, the method of trend ratios has two drawbacks. First, if the observed trend is small,
division by a very low value results in a meaningless trend ratio and thus requires a cut-off
value to keep the trend ratio within meaningful boundaries. Second, the method does not allow
a comparison of the observed trend to the spread of possible outcomes, because the method of
hypothetical trends is based on a deterministic model. We propose a new, more robust method
for detecting the importance of circulation-induced changes in explaining the observed trends,
which has the benefit of being a non-parametric statistical test that assesses the entire range
of hypothetical trends. Instead of creating a hypothetical series by replacing the observation
on a given day with the long-term climatic mean of a certain month and circulation type (as
in the existing trend ratio method), the new approach replaces the observation on a given day
with a random sample from the distribution of the variable for the given month and circulation
type. The method introduces the possibility to assign a rejection rate, thus allowing statistical
significance to be assessed. We apply the method on time series of precipitation and temperature
from the gridded 0.5 degree WFDEI dataset, covering Europe (40-65N, 10W-30E). The SynopVis
Grosswetterlagen catalogue of circulation types for the time period 1981-2010, the same period
as the climatic data, is used. The new approach is used to map in which regions and months
changes in atmospheric circulation is the dominating factor controlling changes in precipitation
and temperature in Europe.
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Nilsen, Irene Brox; Tallaksen, Lena M.; Hisdal, Hege & Fleig, Anne Kristina
(2014).
Regional precipitation and temperature trend patterns in Europe (presentasjon av Paper I: Recent trends in monthly temperature and precipitation patterns in Europe)
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Nilsen, Irene Brox
(2014).
Does NOAH-LSM perform well for extremes, regarding heat fluxes?
Show summary
As the climate and land surface in certain regions of Europe is drying or becoming wetter, certain feedback mechanisms may contribute to enhancing or reducing the effects on climate. For instance, soil moisture–precipitation feedbacks may act as a driving force to dry southern parts of Europe and to moisten northern parts. Understanding the partitioning of net radiation into turbulent fluxes into sensible and latent heat is important to represent these processes. In this project, NOAH will be run using ERA-Interim forcing data for four years covering both extremely wet and dry periods for regions in southern as well as northern Europe. In the first part, maps of the Bowen ratio will be plotted to establish the distribution of turbulent fluxes for wet and dry conditions. Second, the results will be compared with precipitation and temperature trends found in my PhD study to identify if possible concurrent changes in heat fluxes can explain the trends. The project contributes to understanding the driving forces controlling water availability which is important for water shortage mitigation.
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Nilsen, Irene Brox
(2014).
Landoverflatemodellering og varmeflukser under hydrologiske ekstremer.
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Nilsen, Irene Brox
(2014).
Hva gjør en klimaforsker?
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Nilsen, Irene Brox & Tallaksen, Lena M.
(2014).
Temperature changes in Norway: Recent increases in WFDEI validated against interpolated observations.
Show summary
Increased global temperatures have been observed during the past decades, and trends are expected to be greatest
at high latitudes because of the polar amplification. Polar amplification may be caused by feedbacks between the
atmosphere and the surface, such as the ice–albedo feedback: when increased temperatures cause increased snow
melting, areas with a lower albedo are exposed and more radiation is absorbed. Thus, the temperature increase is
enhanced. In a previous study, we analysed precipitation and temperature trends for the period 1979–2009 using
gridded, bias-corrected re-analysis data from the WATCH Forcing Data Era-Interim (WFDEI). The data used
consisted of daily temperature and precipitation data from 1 January 1979 to 31 December 2009 (31 years) and
covered 34◦ –72◦ N and 13◦ W–32◦ E with a spatial resolution of 0.5◦ × 0.5◦ . Temperature and precipitation trends
covering all of Europe were mapped using the Theil-Sen slope as a robust trend estimator. We found the greatest
warming in Europe in the northernmost parts of Norway during December and January. The temperature trends
for certain months and regions were as high as 8 ◦ C. In this study, monthly temperature trends from WFDEI will
be compared with and validated against a daily interpolated dataset for Norway (presented at www.seNorge.no).
This dataset has a spatial resolution of 1 × 1 km and covers mainland Norway. Residual kriging is applied to
the data to ensure stationarity and remove bias. The daily data will be aggregated to the monthly time scale and
monthly temperature trends will be calculated using the Theil-Sen slope for the period 1979–2009. The results will
be compared visually with the WFDEI trends covering Norway. In addition, the distribution of trends as well as
the fraction of positive and negative trends in the two datasets will be compared.
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Nilsen, Irene Brox; Eisner, Stephanie & Heiberg, Hanne
(2022).
Databehov i landbruket : oppsummering av interessegruppemøter om klimainformasjon i landbruk, NVE Rapport nr. 31/2022.
Norges vassdrags- og energidirektorat.
Show summary
Denne rapporten oppsummerer to interessegruppemøter arrangert for landbruket. Noen brukerbehov kan innfris innenfor prosjektene, men det er sprik mellom hva forskningsprosjekter kan levere og hva interessegruppen har behov for, slik at ikke alle ønsker kan innfris. Enkelte ønsker har blitt videreformidlet til værvarslingstjenesten. Som et svar på brukerbehov etter interessegruppemøtet i 2022 beregnet vi tetraterm (varmekrav for ulike treslag i sommermånedene), inkludert endringer i tetraterm over tid. Tetratermen har økt i hele landet, og med det øker den potensielle utbredelsen av varmekjære trær. I den nettbaserte versjonen av rapporten (https://storymaps.arcgis.com/stories/d4bddb92349e4e4baf24b20fdaf2ad24) beskrives også eksisterende dataportaler mer inngående, noe interessegruppen har etterspurt.
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View all works in Cristin
Published
Dec. 10, 2013 7:55 PM
- Last modified
Aug. 2, 2022 11:45 AM