Parameterisation of sea ice cover in short-range regional numerical weather prediction

With the ongoing climate change economic activity in the Arctic steadily increases and it is expected to grow further in the coming years. However, harsh weather conditions of the present-day Arctic place strong demands for accurate and timely weather forecasts, which nowadays are obtained by means of numerical weather prediction. Sea ice covers a considerable part of the Arctic Ocean and numerical weather prediction systems operating in the region require a reliable and computationally-efficient representation of the sea ice cover in the model. Traditionally, simplified one-dimensional parameterisation schemes are applied for this task. However, implications of utilising such schemes in the context of contemporary high-resolution regional operational numerical weather prediction are not well studied. The present work aims to assess these effects through a series of numerical experiments in the operational-like environment. A new one-dimensional parameterisation scheme, allowing for varying level of complexity, implemented in the HARMONIE-AROME numerical weather prediction system, is used as the main research tool. The findings show that applying an over-simplified parameterisation scheme can result in considerable deterioration of the ice surface temperature field in the model. Errors in the modelled ice surface temperature influence the turbulent exchange between the ice surface and the model atmosphere, and, as a result, the near-surface atmospheric variables, such as the screen-level air temperature. Thus, improving the ice surface temperature in the model results in a positive impact on the atmospheric forecast of these parameters. Therefore, a sea ice scheme within an operational numerical weather prediction system should preferably include an explicit representation of the snow layer to accurately represent the surface energy budget of sea ice. Applying a sea ice data assimilation procedure to assimilate a near real time satellite ice surface temperature product in HARMONIE-AROME further reduces the root mean square error of the ice surface temperature and improves the screen-level air temperature forecast over Svalbard and Franz Josef Land archipelagos, however the positive effect in the ice surface temperature is short-lived and greatly reduced already after three hours of model forecast. Spatial resolution of contemporary regional numerical weather prediction systems allows some of the fine-scale features of the sea ice cover to be explicitly represented in the model. Numerical experiments show that introducing irregular structures within the marginal ice zone of the model ice cover results in both local and non-local responses in the atmospheric model. These findings indicate potential benefits of applying high-resolution ice cover in regional numerical weather prediction systems, for example, as a surface perturbation in ensemble prediction systems.Klimatförändringarna gör att ekonomisk aktivitet i Arktis ökar och förväntas öka ytterligare under kommande år. Krävande väderförhållanden ställer stränga krav till pålitliga väderprognoser i god tid. Prognoserna tillhandahålls genom en numerisk vädermodell. En stor del av Norra ishavet är täckt av havsis och vädermodeller för området behöver en pålitlig och beräkningsmässigt effektiv representation av havsisen i modellen. Traditionellt sett har förenklade en-dimensionella parametriseringar använts, men det kan vara mindre lämpligt i moderna modeller med finare upplösning. Denna avhandling undersöker detta i en operationell väderprognosmiljö genom en serie numeriska experiment. En ny en-dimensionell rutin som möjliggör varierande grad av komplexitet är implementerad i en regional vädermodell och utgör huvudverktyget i forskningen. Resultaten visar att införandet av en alltför förenklad representation av havsis kan leda till betydlig försämring av isens yttemperatur i modellen, som också påverkar aspekter av atmosfären nära marknivån, t.ex. lufttemperaturen. Förbättringen av temperaturen på isytan ger således en positiv påverkan i väderprognosen av atmosfären för dessa parametrar. En havsisrutin i en vädermodell bör därför innehålla en explicit representation av snölager för att ge en korrekt beskrivning av havsisens yta. Användning av observationer i havsisrutinen har också en positiv effekt på isytans temperatur och förbättrar lufttemperaturen nära havsnivån över Svalbard och Franz Josefs land, men effekten är störst i början av prognoserna. Den rumsliga upplösningen i nutida regional vädermodellsystem gör det möjligt att representera fler och fler egenskaper hos havisen explicit. Numeriska experiment visar att genom att implementera finskala-struktur i havsismodellen påverkas atmosfären inte bara direkt över havsisen men också på mer avlägsna platser. Dessa resultat visar också potentiella fördelar med högupplöst havsis i regionala vädersystem, till exempel som ytterligare en källa till variabilitet i ensembleprognossystem

Boundary-layer height and surface stability at Hyytiälä, Finland, in ERA5 and observations

We investigate the boundary-layer (BL) height at Hyytiala in southern Finland diagnosed from radiosonde observations, a microwave radiometer (MWR) and ERAS reanalysis. Four different, pre-existing algorithms are used to diagnose the BL height from the radiosondes. The diagnosed BL height is sensitive to the method used. The level of agreement, and the sign of systematic bias between the four different methods, depends on the surface-layer stability. For very unstable situations, the median BL height diagnosed from the radiosondes varies from 600 to 1500 m depending on which method is applied. Good agreement between the BL height in ERAS and diagnosed from the radiosondes using Richardson-number-based methods is found for almost all stability classes, suggesting that ERAS has adequate vertical resolution near the surface to resolve the BL structure. However, ERAS overestimates the BL height in very stable conditions, highlighting the ongoing challenge for numerical models to correctly resolve the stable BL. Furthermore, ERAS BL height differs most from the radiosondes at 18:00 UTC, suggesting ERAS does not resolve the evening transition correctly. BL height estimates from the MWR are also found to be reliable in unstable situations but often are inaccurate under stable conditions when, in comparison to ERAS BL heights, they are much deeper. The errors in the MWR BL height estimates originate from the limitations of the manufacturer's algorithm for stable conditions and also the misidentification of the type of BL. A climatology of the annual and diurnal cycle of BL height, based on ERA5 data, and surface-layer stability, based on eddy covariance observations, was created. The shallowest (353 m) monthly median BL height occurs in February and the deepest (576 m) in June. In winter there is no diurnal cycle in BL height; unstable BLs are rare, yet so are very stable BLs. The shallowest BLs occur at night in spring and summer, and very stable conditions are most common at night in the warm season. Finally, using ERA5 gridded data, we determined that the BL height observed at Hyytiala is representative of most land areas in southern and central Finland. However, the spatial variability of the BL height is largest during daytime in summer, reducing the area over which BL height observations from Hyytiala would be representative.Peer reviewe

The HARMONIE–AROME Model Configuration in the ALADIN–HIRLAM NWP System

The aim of this article is to describe the reference configuration of the convection-permitting numerical weather prediction (NWP) model HARMONIE-AROME, which is used for operational short-range weather forecasts in Denmark, Estonia, Finland, Iceland, Ireland, Lithuania, the Netherlands, Norway, Spain, and Sweden. It is developed, maintained, and validated as part of the shared ALADIN–HIRLAM system by a collaboration of 26 countries in Europe and northern Africa on short-range mesoscale NWP. HARMONIE–AROME is based on the model AROME developed within the ALADIN consortium. Along with the joint modeling framework, AROME was implemented and utilized in both northern and southern European conditions by the above listed countries, and this activity has led to extensive updates to themodel’s physical parameterizations. In this paper the authors present the differences inmodel dynamics and physical parameterizations compared with AROME, as well as important configuration choices of the reference, such as lateral boundary conditions, model levels, horizontal resolution, model time step, as well as topography, physiography, and aerosol databases used. Separate documentation will be provided for the atmospheric and surface data-assimilation algorithms and observation types used, as well as a separate description of the ensemble prediction system based on HARMONIE–AROME, which is called HarmonEPS

Обчислення геометричних характеристик полігональних і поліедральних клітинних комплексів

Simple formulas are given which were obtained through analytical integration. They can be used for developing effective algorithms which exclude accumulation of calculation errors and do not depend on characteristics of polygons shapes and spatial position of surface facets and polyhedrons.Приведены простые формулы, которые получены путем аналитического интегрирования, пригодные для построения эффективных алгоритмов, исключающих накопление вычислительной ошибки и не зависящих от особенностей формы полигонов и пространственного расположения граней поверхностей и полиэдров.Наведені прості формули, які отримані аналітичним інтегруванням, придатні для побудови ефективних алгоритмів, що виключають накопичення обчислювальної похибки й не залежать від особливостей форми полігонів і просторового розташування граней поверхонь та поліедрів

Постійні та пульсаційні навантаження на гребному гвинті у розрахунках центрування та коливань валопроводів

The methods  for estimation of propeller hydrodynamic loads in the stationary non-uniform wake flow in relation to calculation of cargo ship propulsion shafting are discussed. Widely used approaches for propeller loads estimation which were implemented by authors in ShaftDesigner software are analyzed. Practical recommendations for setting of propeller hydrodynamic loads in the cases where not enough information for calculation are proposed.Рассмотрены методы определения гидродинамических нагрузок на гребном винте в стационарном неоднородном потоке применительно к задачам расчета валопроводов морских транспортных судов. Проанализированы наиболее распространенные схемы определения нагрузок, реализованные авторами в рамках автоматизированной системы расчетов валопроводов ShaftDesigner. Предложены практические рекомендации по назначению величин гидродинамических нагрузок на винт в случае отсутствия исходной информации для выполнения расчетов.Розглянуто методи визначення гідродинамічних навантажень на гребному гвинті у стаціонарному неоднорідному потоці стосовно задач розрахунку валопроводів морських транспортних суден. Проаналізовано найбільш розповсюджені схеми визначення навантажень, що були реалізовані авторами під час розробки автоматизованої системи розрахунків валопроводів ShaftDesigner. Запропоновано практичні рекомендації щодо призначення величин гідродинамічних навантажень на гвинт у випадках, коли бракує достатньо інформації для виконання розрахунків

A Kilometer-Scale Coupled Atmosphere-Wave Forecasting System for the European Arctic

Abstract Accurately simulating the interactions between the components of a coupled Earth modelling system (atmosphere, sea-ice, and wave) on a kilometer-scale resolution is a new challenge in operational numerical weather prediction. It is difficult due to the complexity of interactive mechanisms, the limited accuracy of model components and scarcity of observations available for assessing relevant coupled processes. This study presents a newly developed convective-scale atmosphere-wave coupled forecasting system for the European Arctic. The HARMONIE-AROME configuration of the ALADIN-HIRLAM numerical weather prediction system is coupled to the spectral wave model WAVEWATCH III using the OASIS3 model coupling toolkit. We analyze the impact of representing the kilometer-scale atmosphere-wave interactions through coupled and uncoupled forecasts on a model domain with 2.5 km spatial resolution. In order to assess the coupled model’s accuracy and uncertainties we compare 48-hour model forecasts against satellite observational products such as Advanced Scatterometer 10 m wind speed, and altimeter based significant wave height. The fully coupled atmosphere-wave model results closely match both satellite-based wind speed and significant wave height observations as well as surface pressure and wind speed measurements from selected coastal station observation sites. Furthermore, the coupled model contains smaller standard deviation of errors in both 10m wind speed and significant wave height parameters when compared to the uncoupled model forecasts. Atmosphere and wave coupling reduces the short term forecast error variability of 10 m wind speed and significant wave height with the greatest benefit occurring for high wind and wave conditions

Polar low variability and future projections for the Nordic and Barents Seas

Polar lows are intense mesoscale cyclones occurring during winter over open sea areas in certain polar sub‐regions. Due to their small size, they are not explicitly represented in present global climate models or Earth system models. In this study 18 members of the CESM Large Ensemble were dynamically downscaled to ∼12 km horizontal mesh width using the quasi‐hydrostatic ALARO model within the HARMONIE script system in climate mode (HCLIM‐ALARO). The domain covers the Nordic and Barents Seas. One historical and two future time‐periods were selected. For validation, the ERA‐Interim reanalysis was also downscaled. A cyclone‐tracking algorithm was used to identify tracks of individual polar lows. Their frequency of occurrence, lifetime, and maximum relative vorticity were estimated. Relative to ERA‐Interim, the historical frequency of occurrence of polar lows was slightly overestimated in the Nordic Seas and underestimated in the Barents Sea, which is likely due to positive biases in sea‐surface temperature and sea‐ice concentration. For future climate projections, the regions of polar low genesis are diagnosed to move northwards in accordance with the sea‐ice retreat. In the Nordic Seas, the number of polar lows decreases at the beginning of the season, while there is an increase in March. In the Barents Sea, a February–April increase in the occurrence of polar lows is seen

Evaluation of a sub-kilometre NWP system in an Arctic fjord-valley system in winter

Terrain challenges the prediction of near-surface atmospheric conditions, even in kilometre-scale numerical weather prediction (NWP) models. In this study, the ALADIN-HIRLAM NWP system with 0.5 km horizontal grid spacing and an increased number of vertical levels is compared to the 2.5-km model system similar to the currently operational NWP system at the Norwegian Meteorological Institute. The impact of the increased resolution on the forecasts’ ability to represent boundary-layer processes is investigated for the period from 12 to 16 February 2018 in an Arctic fjord-valley system in the Svalbard archipelago. Model simulations are compared to a wide range of observations conducted during a field campaign. The model configuration with sub-kilometre grid spacing improves both the spatial structure and overall verification scores for the near-surface temperature and wind forecasts compared to the 2.5-km experiment. The sub-kilometre experiment successfully captures the wind channelling through the valley and the temperature field associated with it. In a situation of a cold-air pool development, the sub-kilometre experiment has a particularly high near-surface temperature bias at low elevations. The use of measurement campaign data, however, reveals some encouraging results, e.g. the sub-kilometre system has a more realistic vertical profile of temperature and wind speed, and the surface temperature sensitivity to the net surface energy is closer to the observations. This work demonstrates the potential of sub-kilometre NWP systems for forecasting weather in complex Arctic terrain, and also suggests that the increase in resolution needs to be accompanied with further development of other parts of the model system

The winter central Arctic surface energy budget: A model evaluation using observations from the MOSAiC campaign

International audienceThis study evaluates the simulation of wintertime (15 October, 2019, to 15 March, 2020) statistics of the central Arctic near-surface atmosphere and surface energy budget observed during the MOSAiC campaign with short-term forecasts from 7 state-of-the-art operational and experimental forecast systems. Five of these systems are fully coupled ocean-sea ice-atmosphere models. Forecast systems need to simultaneously simulate the impact of radiative effects, turbulence, and precipitation processes on the surface energy budget and near-surface atmospheric conditions in order to produce useful forecasts of the Arctic system. This study focuses on processes unique to the Arctic, such as, the representation of liquid-bearing clouds at cold temperatures and the representation of a persistent stable boundary layer. It is found that contemporary models still struggle to maintain liquid water in clouds at cold temperatures. Given the simple balance between net longwave radiation, sensible heat flux, and conductive ground flux in the wintertime Arctic surface energy balance, a bias in one of these components manifests as a compensating bias in other terms. This study highlights the different manifestations of model bias and the potential implications on other terms. Three general types of challenges are found within the models evaluated: representing the radiative impact of clouds, representing the interaction of atmospheric heat fluxes with sub-surface fluxes (i.e., snow and ice properties), and representing the relationship between stability and turbulent heat fluxes

HCLIM38 : a flexible regional climate model applicable for different climate zones from coarse to convection-permitting scales

This paper presents a new version of HCLIM, a regional climate modelling system based on the ALADIN–HIRLAM numerical weather prediction (NWP) system. HCLIM uses atmospheric physics packages from three NWP model configurations, HARMONIE–AROME, ALARO and ALADIN, which are designed for use at different horizontal resolutions. The main focus of HCLIM is convection-permitting climate modelling, i.e. developing the climate version of HARMONIE–AROME. In HCLIM, the ALADIN and ALARO configurations are used for coarser resolutions at which convection needs to be parameterized. Here we describe the structure and development of the current recommended HCLIM version, cycle 38. We also present some aspects of the model performance. HCLIM38 is a new system for regional climate modelling, and it is being used in a number of national and international projects over different domains and climates ranging from equatorial to polar regions. Our initial evaluation indicates that HCLIM38 is applicable in different conditions and provides satisfactory results without additional region-specific tuning. HCLIM is developed by a consortium of national meteorological institutes in close collaboration with the ALADIN–HIRLAM NWP model development. While the current HCLIM cycle has considerable differences in model setup compared to the NWP version (primarily in the description of the surface), it is planned for the next cycle release that the two versions will use a very similar setup. This will ensure a feasible and timely climate model development as well as updates in the future and provide an evaluation of long-term model biases to both NWP and climate model developers.This research has been supported by Horizon 2020 (EUCP (grant no. 776613)) and the Maj and Tor Nessling foundation