Norway's marine and terrestrial climate mapped with dynamical downscaling

Long-term numerical reconstructions on high spatial resolution of past weather are essential tools for studies of the local climate and climate extremes. The focus of this thesis has been to resolve the wind, precipitation, temperature and wave climate of Norway and Norwegian waters by high-resolution dynamical downscaling. Known as a hindcast archive, this is a well-known method to obtain local information based on more coarse atmospheric fields, typically reanalyses. Such reanalyses provide the state of the atmosphere as accurately as possible on meso-beta scale (20-200 km) whereas flow over complex terrain and along irregular coastlines requires resolutions on meso-gamma scale (2-20 km) or microscale (1 km or less) to be well represented. By using numerical weather prediction models tailored to high resolution modelling to downscale the reanalyses, we obtain far more detailed information than what a global reanalysis alone can give. In this thesis I focus on a convection-permitting non-hydrostatic downscaling and compare it to a hydrostatic hindcast as well as the host reanalysis. We see improvement in performance of the wind speed in both downscaling procedures, compared to the large scale reanalysis. However, extreme winds and precipitation are much better resolved by the convection-permitting non-hydrostatic model with better representation of convective features and the wind field in steep terrain and along irregular coastlines. We also find that the representation of polar lows is improved. Both atmospheric hindcasts are accompanied by wave hindcasts. We find that the wave field is sensitive to strong winds, and indeed the strongest winds (realistically) rendered by the non-hydrostatic NORA3 hindcast yields too strong wave growth. A new parameterization of the Charnock coefficient is explored and successfully used to generate a high-resolution wave hindcast based on the NORA3 atmospheric hindcast.Lange historiske rekonstruksjoner med høy romlig oppløsning av været som har vært, er et viktig verktøy for studier av det lokale klimaet og klima-ekstremer. Fokuset i denne avhandlingen har vært å oppløse vind-, nedbør-, temperatur- og bølge-klima i Norge og for norske farvann ved bruk av en høyoppløselig dynamisk nedskalering. Dette er en velkjent metode for å oppnå lokal informasjon basert på atmosfære-felter med grovere oppløsning (typiske reanalyser), og kjennetegnes som hindcast-arkiv. Reanalyser gir oss atmosfærens tilstand så nøyaktig som mulig på meso-beta skala (20-200 km), mens vind over komplekst terreng og langs irregulære kystlinjer krever oppløsninger på meso-gamma skala (2-30 km) eller mikroskala (1 km eller mindre) for å bli vel representert. Ved å bruke numeriske værvarslingsmodeller, som er skreddersydd for modellering med høy oppløsning til å nedskalere reanalysene, oppnår vi langt mer detaljert informasjon enn hva en global reanalyse alene vil gi. I denne avhandlingen fokuseres det på en konveksjons-oppløsende ikke-hydrostatisk nedskalering, som blir sammenliknet med en hydrostatisk hindcast i tillegg til den tilhørende reanalysen. Vi ser forbedring i ytelse av vindhastighet i begge nedskalerings-prosedyrene, sammenliknet med den storskala reanalysen. Ekstreme vinder og nedbør er mye bedre oppløst av den konveksjons-tillatende ikke-hydrostatiske modellen med bedre beskrivelse av de konvektive trekkene og vindfeltet i bratt terreng og langs den uregelmessige kystlinjen. Vi ser også at polare lavtrykk er bedre beskrevet. Begge de atmosfæriske nedskaleringene, etterfølges av bølge-nedskaleringer. Vi ser at bølge-feltet er sensitivt for sterke vinder, og at spesielt de sterkeste vindene i NORA3 gir for kraftig bølge vekst. En ny parametrisering av Charnock-koeffisienten er studert, og har vist seg svært vellykket i genereringen av en høyoppløselig bølge-hindcast basert på atmosfære-feltene fra NORA3.Doktorgradsavhandlin

The 3 km Norwegian reanalysis (NORA3) – a validation of offshore wind resources in the North Sea and the Norwegian Sea

We validate a new high-resolution (3 km) numerical mesoscale weather simulation for offshore wind power purposes for the time period 2004–2016 for the North Sea and the Norwegian Sea. The 3 km Norwegian reanalysis (NORA3) is a dynamically downscaled data set, forced with state-of-the-art atmospheric reanalysis as boundary conditions. We conduct an in-depth validation of the simulated wind climatology towards the observed wind climatology to determine whether NORA3 can serve as a wind resource data set in the planning phase of future offshore wind power installations. We place special emphasis on evaluating offshore wind-power-related metrics and the impact of simulated wind speed deviations on the estimated wind power and the related variability. We conclude that the NORA3 data are well suited for wind power estimates but give slightly conservative estimates of the offshore wind metrics. In other words, wind speeds in NORA3 are typically 5 % (0.5 m s−1) lower than observed wind speeds, giving an underestimation of offshore wind power of 10 %–20 % (equivalent to an underestimation of 3 percentage points in the capacity factor) for a selected turbine type and hub height. The model is biased towards lower wind power estimates due to overestimation of the wind speed events below typical wind speed limits of rated wind power (u11–13 m s−1). The hourly wind speed and wind power variability are slightly underestimated in NORA3. However, the number of hours with zero power production caused by the wind conditions (around 12 % of the time) is well captured, while the duration of each of these events is slightly overestimated, leading to 25-year return values for zero-power duration being too high for the majority of the sites. The model performs well in capturing spatial co-variability in hourly wind power production, with only small deviations in the spatial correlation coefficients among the sites. We estimate the observation-based decorrelation length to be 425.3 km, whereas the model-based length is 19 % longer.publishedVersio

Nora3: A nonhydrostatic high-resolution hindcast of the North sea, the Norwegian sea, and the Barents sea

The 3-km Norwegian Reanalysis (NORA3) is a 15-yr mesoscale-permitting atmospheric hindcast of the North Sea, the Norwegian Sea, and the Barents Sea. With a horizontal resolution of 3 km, the nonhydrostatic numerical weather prediction model HARMONIE–AROME runs explicitly resolved deep convection and yields hindcast fields that realistically downscale the ERA5 reanalysis. The wind field is much improved relative to its host analysis, in particular in mountainous areas and along the improved grid-resolving coastlines. NORA3 also performs much better than the earlier hydrostatic 10-km Norwegian Hindcast Archive (NORA10) in complex terrain. NORA3 recreates the detailed structures of mesoscale cyclones with sharp gradients in wind and with clear frontal structures, which are particularly important when modeling polar lows. In extratropical windstorms, NORA3 exhibits significantly higher maximum wind speeds and compares much better to observed maximum wind than do NORA10 and ERA5. The activity of the model is much more realistic than that of NORA10 and ERA5, both over the ocean and in complex terrain.publishedVersio

The Impact of a Reduced High-Wind Charnock Parameter on Wave Growth With Application to the North Sea, the Norwegian Sea, and the Arctic Ocean

As atmospheric models move to higher resolution and resolve smaller scales, the maximum modeled wind speed also tends to increase. Wave models tuned to coarser wind fields tend to overestimate the wave growth under strong winds. A recently developed semiempirical parameterization of the Charnock parameter, which controls the roughness length over surface waves, substantially reduces the aerodynamic drag of waves in high winds (above a threshold of 30 m s−1). Here, we apply the formulation in a recent version of the wave model WAM (Cycle 4.7), which uses a modified version of the physics parameterizations by Ardhuin et al. (2010, https://doi.org/10.1175/2010jpo4324.1) as well as subgrid obstructions for better performance around complex topography. The new Charnock formulation is tested with wind forcing from NORA3, a recently completed nonhydrostatic atmospheric downscaling of the global reanalysis ERA5 for the North Sea, the Norwegian Sea and the Barents Sea. Such high-resolution atmospheric model integrations tend to have stronger (and more realistic) upper-percentile winds than what is typically found in coarser atmospheric models. A 2-year comparison (2011–2012) of a control run against the run with the modified Charnock parameter shows a dramatic reduction of the wave height bias in high-wind cases. The added computational cost of the new physics and the reduction of the Charnock parameter compared to the earlier WAM physics is modest (14%). A longer (1998–2020) hindcast integration with the new Charnock parameter is found to compare well against in situ and altimeter wave measurements both for intermediate and high sea states.publishedVersio

A high-resolution hindcast of wind and waves for The North Sea, The Norwegian Sea and The Barents Sea

A combined high-resolution atmospheric downscaling and wave hindcast based on the ERA-40 reanalysis covering the Norwegian Sea, the North Sea and the Barents Sea is presented. The period covered is from September 1957 to August 2002. The dynamic atmospheric downscaling is performed as a series of short prognostic runs initialized from a blend of ERA-40 and the previous prognostic run to preserve the fine-scale surface features from the high-resolution model while maintaining the large-scale synoptic field from ERA-40. The nested WAM wave model hindcast consists of a coarse 50 km model covering the North Atlantic forced with ERA-40 winds and a nested 10-11 km resolution model forced with downscaled winds. A comparison against in situ and satellite observations of wind and sea state reveals significant improvement in mean values and upper percentiles of wind vectors and the significant wave height over ERA-40. Improvement is also found in the mean wave period. ERA-40 is biased low in wind speed and significant wave height, a bias which is not reproduced by the downscaling. The atmospheric downscaling also reproduces polar lows, which can not be resolved by ERA-40, but the lows are too weak and short-lived as the downscaling is not capable of capturing their full life cycle.Comment: 34 pages, 12 figures, 6 table

Norway's marine and terrestrial climate mapped with dynamical downscaling

Long-term numerical reconstructions on high spatial resolution of past weather are essential tools for studies of the local climate and climate extremes. The focus of this thesis has been to resolve the wind, precipitation, temperature and wave climate of Norway and Norwegian waters by high-resolution dynamical downscaling. Known as a hindcast archive, this is a well-known method to obtain local information based on more coarse atmospheric fields, typically reanalyses. Such reanalyses provide the state of the atmosphere as accurately as possible on meso-beta scale (20-200 km) whereas flow over complex terrain and along irregular coastlines requires resolutions on meso-gamma scale (2-20 km) or microscale (1 km or less) to be well represented. By using numerical weather prediction models tailored to high resolution modelling to downscale the reanalyses, we obtain far more detailed information than what a global reanalysis alone can give. In this thesis I focus on a convection-permitting non-hydrostatic downscaling and compare it to a hydrostatic hindcast as well as the host reanalysis. We see improvement in performance of the wind speed in both downscaling procedures, compared to the large scale reanalysis. However, extreme winds and precipitation are much better resolved by the convection-permitting non-hydrostatic model with better representation of convective features and the wind field in steep terrain and along irregular coastlines. We also find that the representation of polar lows is improved. Both atmospheric hindcasts are accompanied by wave hindcasts. We find that the wave field is sensitive to strong winds, and indeed the strongest winds (realistically) rendered by the non-hydrostatic NORA3 hindcast yields too strong wave growth. A new parameterization of the Charnock coefficient is explored and successfully used to generate a high-resolution wave hindcast based on the NORA3 atmospheric hindcast

NORA3. Part II: Precipitation and Temperature Statistics in Complex Terrain Modeled with a Nonhydrostatic Model

The 3-km Norwegian Reanalysis (NORA3) is a convection-permitting, nonhydrostatic hindcast for the North Sea, the Norwegian Sea, and the Barents Sea as well as the Scandinavian Peninsula. It has a horizontal resolution of 3 km and provides a full three-dimensional atmospheric state for the period 1995–2020 with a surface analysis and boundary conditions from ERA5, a global reanalysis. In complex terrain it is found to outperform both the host reanalysis ERA5 and also the earlier hydrostatic 10-km Norwegian Hindcast Archive (NORA10), in terms of 2-m temperature and daily precipitation. Of particular interest is the representation of extreme rainfall. It is found that the upper percentiles are much better represented than in ERA5, with very little bias up to 99.9%, suggesting that the new hindcast archive is well suited for hydrological mapping and extreme-value analysis of rainfall in complex terrain.publishedVersio

The 3 km Norwegian reanalysis (NORA3) – a validation of offshore wind resources in the North Sea and the Norwegian Sea

We validate a new high-resolution (3 km) numerical mesoscale weather simulation for offshore wind power purposes for the time period 2004–2016 for the North Sea and the Norwegian Sea. The 3 km Norwegian reanalysis (NORA3) is a dynamically downscaled data set, forced with state-of-the-art atmospheric reanalysis as boundary conditions. We conduct an in-depth validation of the simulated wind climatology towards the observed wind climatology to determine whether NORA3 can serve as a wind resource data set in the planning phase of future offshore wind power installations. We place special emphasis on evaluating offshore wind-power-related metrics and the impact of simulated wind speed deviations on the estimated wind power and the related variability. We conclude that the NORA3 data are well suited for wind power estimates but give slightly conservative estimates of the offshore wind metrics. In other words, wind speeds in NORA3 are typically 5 % (0.5 m s−1) lower than observed wind speeds, giving an underestimation of offshore wind power of 10 %–20 % (equivalent to an underestimation of 3 percentage points in the capacity factor) for a selected turbine type and hub height. The model is biased towards lower wind power estimates due to overestimation of the wind speed events below typical wind speed limits of rated wind power (u11–13 m s−1). The hourly wind speed and wind power variability are slightly underestimated in NORA3. However, the number of hours with zero power production caused by the wind conditions (around 12 % of the time) is well captured, while the duration of each of these events is slightly overestimated, leading to 25-year return values for zero-power duration being too high for the majority of the sites. The model performs well in capturing spatial co-variability in hourly wind power production, with only small deviations in the spatial correlation coefficients among the sites. We estimate the observation-based decorrelation length to be 425.3 km, whereas the model-based length is 19 % longer

NORA10EI: A revised regional atmosphere-wave hindcast for the North Sea, the Norwegian Sea and the Barents Sea

NORA10EI, a new atmosphere and wave hindcast for the Norwegian Sea, the North Sea and the Barents Sea is presented. The hindcast uses ERA‐Interim as initial and boundary conditions and covers the period 1979–2017. The earlier NORA10 hindcast used ERA‐40 as initial and boundary conditions before September 2002 and operational analyses from the European Centre for Medium‐Range Weather Forecasts (ECMWF) in the continuation. This change in initial and boundary conditions may lead to non‐stationarities in bias and random errors, and it is a question of some concern whether this also leads to spurious trends. We investigate this by comparing the two hindcasts. We find only minor differences in the statistics of means and upper percentiles, but somewhat larger differences in the extremes (100‐year return values) of significant wave height and 10‐m winds. Generally, NORA10EI outperforms NORA10 in the ERA‐40 period (before September 2002) since ERA‐Interim outperforms ERA‐40. Conversely, NORA10 outperforms NORA10EI after 2006, since the operational ECMWF analyses here outperform ERA‐Interim. Years 2002–2006 is a transition period with minor differences between the NORA10 and NORA10EI where the resolution of ERA‐Interim is lower than that of the ECMWF analyses, but its physics are from a more recent model (2006). An important finding is that the regional hindcasts appear quite insensitive to changes in the host reanalysis with no statistically significant differences in mean and upper percentile trends of wind speed and wave height. A comparison of four polar low cases confirms that using ERA‐Interim as host reanalysis yields a slightly better representation of evolution and intensity of polar lows than NORA10 in the ERA‐40 period and the opposite after 2006

NORA10EI: A revised regional atmosphere-wave hindcast for the North Sea, the Norwegian Sea and the Barents Sea

NORA10EI, a new atmosphere and wave hindcast for the Norwegian Sea, the North Sea and the Barents Sea is presented. The hindcast uses ERA‐Interim as initial and boundary conditions and covers the period 1979–2017. The earlier NORA10 hindcast used ERA‐40 as initial and boundary conditions before September 2002 and operational analyses from the European Centre for Medium‐Range Weather Forecasts (ECMWF) in the continuation. This change in initial and boundary conditions may lead to non‐stationarities in bias and random errors, and it is a question of some concern whether this also leads to spurious trends. We investigate this by comparing the two hindcasts. We find only minor differences in the statistics of means and upper percentiles, but somewhat larger differences in the extremes (100‐year return values) of significant wave height and 10‐m winds. Generally, NORA10EI outperforms NORA10 in the ERA‐40 period (before September 2002) since ERA‐Interim outperforms ERA‐40. Conversely, NORA10 outperforms NORA10EI after 2006, since the operational ECMWF analyses here outperform ERA‐Interim. Years 2002–2006 is a transition period with minor differences between the NORA10 and NORA10EI where the resolution of ERA‐Interim is lower than that of the ECMWF analyses, but its physics are from a more recent model (2006). An important finding is that the regional hindcasts appear quite insensitive to changes in the host reanalysis with no statistically significant differences in mean and upper percentile trends of wind speed and wave height. A comparison of four polar low cases confirms that using ERA‐Interim as host reanalysis yields a slightly better representation of evolution and intensity of polar lows than NORA10 in the ERA‐40 period and the opposite after 2006