Surface influence on the marine and coastal Antarctic atmosphere

The Antarctic region plays an important role in the global climate system, and it contributes to the future of global climate through changes in regional factors, such as sea ice, atmospheric circulation patterns and moisture distribution. The aim of this thesis is to improve the understanding of the influence of the Earth surface on the marine and coastal Antarctic atmosphere. The thesis outlines the characteristics of typical phenomena of the Antarctic environment both near the surface and higher in the atmosphere, and describes the challenges related to numerical modelling in the region. The work is based on combined use of several observational data sets and regional numerical modelling. Marine atmosphere and its reprensentation in regional modelling was studied based on observational data collected on two research projects called 'Short Timescale Motion of Pancake Ice', and 'Ice Station Polarstern'. The coastal atmosphere was studied based on data from the Movable Atmospheric Radar for Antarctica from Queen Maud Land and the Integrated Global Radiosonde Archive from 11 coastal stations. Model simulations were made using two regional-scale atmospheric models: the Fifth-Generation Pennsylvania State University/National Center for Atmospheric Research Mesoscale Model (MM5) and the Weather Research and Forecasting (WRF) model. The boundary layer over the marine Antarctic was found to be strongly governed by the presence or absence of sea ice. The major challenges related to the modelling of the atmosphere over Antarctic sea ice were associated with clouds, boundary layer processes and sea ice and snow description. The model sensitivity to different physical parameterisation schemes was most profound in the lowest parts of the atmosphere. The results confirm that numerical simulations reproduce relatively accurately the basic meteorological surface variables, such as temperature, humidity, air pressure, wind speed and wind direction, over ice-covered ocean, but the turbulent and radiative fluxes that affect those variables were not that well depicted. To minimise the modelling errors, the sea ice conditions should be updated frequently with advanced description of ice and snow processes. The modelling benefits also from advanced cloud schemes with prediction of the vertical distribution of cloud water and ice content. The vertical structure of the atmosphere over the coastal Antarctic was found to be featured by internal gravity waves and temperature and humidity inversions. Gravity waves generated by a small mountain peak emerging above the ice sheet were found to reach the lower stratosphere. This implies that even small mountains can generate strong gravity waves, and it is possible to simulate them when applying high horizontal resolution in the model. Humidity inversions were found to be, nearly all the time, present on multiple levels in the coastal Antarctic atmosphere. The presented statistics for humidity inversions can be used as a baseline for further studies addressing moisture changes in the Antarctic atmosphere.Antarktiksella on tärkeä rooli maapallon ilmastosysteemissä. Se vaikuttaa merkittävästi maapallon ilmaston tulevaisuuteen paikallisten tekijöiden, kuten merijään, ilmakehän virtausrakenteiden ja kosteusjaukauman muutosten kautta. Tämän väitöskirjatyön tavoitteena on lisätä ymmärrystä maapallon pinnan vaikutuksesta ilmakehään Antarktiksen meri- ja rannikkoalueilla. Väitöskirja selvittää alueelle tyypillisten ilmiöiden ominaisuuksia pinnan lähellä sekä ylempänä ilmakehässä, ja määrittää alueen numeeriseen mallinnukseen liittyviä haasteita. Työ perustuu useiden havaintoaineistojen hyödyntämiseen ja alueelliseen numeeriseen mallinnukseen. Merialueen ilmakehää ja sen kuvausta alueellisessa mallinnuksessa tutkittiin kahden tutkimusprojektin keräämän havaintoaineiston avulla. Rannikkoalueiden ilmakehän tutkimus perustui tutkahavaintoihin Kuningatar Maudin Maalta ja radioluotauksiin 11 rannikkoasemalta. Mallisimulaatiot tehtiin käyttäen kahta alueellista ilmakehämallia. Ilmakehän rajakerrosta Antarktiksen merialueella määrittää ensisijaisesti se, onko alueella merijäätä vai ei. Suurimmat haasteet merijään yläpuolisen ilman mallittamiseen liittyivät pilviin, rajakerrosprosesseihin ja merijään ja lumen kuvaukseen. Mallin herkkyys eri fysikaalisille parametrisointimenetelmille oli selkein ilmakehän alimmissa osissa. Tulokset osoittavat, että numeerinen mallinnus voi merijään peittämällä merellä kuvata suhteellisen tarkasti meteorologisia perussuureita kuten lämpötilaa, kosteutta, ilmanpainetta, tuulen nopeutta ja tuulen suuntaa, mutta turbulenttiset ja säteilyvuot, jotka noihin suureisiin vaikuttavat, ovat huonommin mallinnettuja. Mallinnusvirheiden pienentämiseksi merijää tulisi syöttää mallisysteemiin riittävän usein sisällyttäen mukaan jään ja lumen prosessit yksityiskohtaisesti kuvattuina. Lisäksi mallinnusta parantavat edistyneet pilvien parametrisointimenetelmät, joissa on mukana pilviveden ja jään pystyjakauman ennustaminen. Ilmakehän pystyrakenteelle Etelämantereen rannikkoalueilla on tyypillistä sisäiset gravitaatioaallot ja lämpötila- ja kosteusinversiot. Pienen jäätikön yläpuolelle nousevan vuorenhuipun kehittämien gravitaatioaaltojen osoitettiin ulottuvan stratosfäärin alaosiin. Tämä viittaa siihen, että myös suhteellisen pienet vuoret voivat synnyttää voimakkaita gravitaatioaaltoja. Mallinnuksella osoitettiin, että aallot on mahdollista simuloida käyttäen tarkkaa horisontaalista resoluutiota. Kosteusinversioiden havaittiin olevan lähes jatkuvasti läsnä useissa eri kerroksissa Etelämantereen rannikkoalueen ilmakehässä. Esitettyjä kosteusinversioiden ilmastotilastoja voidaan käyttää pohjana Antarktiksen ilmakehän kosteuden muutosten jatkotutkimuksille

On the utility of individual tendency output: Revealing interactions between parameterized processes during a marine cold air outbreak

Forecasts of marine cold air outbreaks critically rely on the interplay of multiple parameterization schemes to represent subgrid-scale processes, including shallow convection, turbulence, and microphysics. Even though such an interplay has been recognized to contribute to forecast uncertainty, a quantification of this interplay is still missing. Here, we investigate the tendencies of temperature and specific humidity contributed by individual parameterization schemes in the operational weather prediction model AROME-Arctic. From a case study of an extensive marine cold air outbreak over the Nordic seas, we find that the type of planetary boundary layer assigned by the model algorithm modulates the contribution of individual schemes and affects the interactions between different schemes. In addition, we demonstrate the sensitivity of these interactions to an increase or decrease in the strength of the parameterized shallow convection. The individual tendencies from several parameterizations can thereby compensate each other, sometimes resulting in a small residual. In some instances this residual remains nearly unchanged between the sensitivity experiments, even though some individual tendencies differ by up to an order of magnitude. Using the individual tendency output, we can characterize the subgrid-scale as well as grid-scale responses of the model and trace them back to their underlying causes. We thereby highlight the utility of individual tendency output for understanding process-related differences between model runs with varying physical configurations and for the continued development of numerical weather prediction models.publishedVersio

Fine-scale model simulation of gravity waves generated by Basen nunatak in Antarctica

Peer reviewe

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 subkilometre 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

A well-observed polar low analysed with a regional and a global weather-prediction model

The capability of a regional (AROME‐Arctic) and a global (ECMWF HRES) weather‐prediction model are compared for simulating a well‐observed polar low (PL). This PL developed on 3–4 March 2008 and was measured by dropsondes released from three flights during the IPY‐THORPEX campaign. Validation against these measurements reveals that both models simulate the PL reasonably well. AROME‐Arctic appears to represent the cloud structures and the high local variability more realistically. The high local variability causes standard error statistics to be similar for AROME‐Arctic and ECMWF HRES. A spatial verification technique reveals that AROME‐Arctic has improved skills at small scales for extreme values. However, the error growth of the forecast, especially in the location of the PL, is faster in AROME‐Arctic than in ECMWF HRES. This is likely associated with larger convection‐induced perturbations in the former than the latter model. Additionally, the PL development is analysed. This PL has two stages, an initial baroclinic and a convective mature stage. Sensible heat flux and condensational heat release both contribute to strengthen the initial baroclinic environment. In the mature stage, latent heat release appears to maintain the system. At least two conditions must be met for this stage to develop: (a) the sensible heat flux sufficiently destabilises the local environment around the PL, and (b) sufficient moisture is available for condensational heat release. More than half of the condensed moisture within the system originates from the surroundings. The propagation of the PL is “pulled” towards the area of strongest condensational heating. Finally, the sensitivity of the PL to the sea‐surface temperature is analysed. The maximum near‐surface wind speed connected to the system increases by 1–2 m·s−1 per K of surface warming and a second centre develops in cases of highly increased temperature

On the utility of individual tendency output: Revealing interactions between parameterized processes during a marine cold air outbreak

Forecasts of marine cold air outbreaks critically rely on the interplay of multiple parameterization schemes to represent subgrid-scale processes, including shallow convection, turbulence, and microphysics. Even though such an interplay has been recognized to contribute to forecast uncertainty, a quantification of this interplay is still missing. Here, we investigate the tendencies of temperature and specific humidity contributed by individual parameterization schemes in the operational weather prediction model AROME-Arctic. From a case study of an extensive marine cold air outbreak over the Nordic seas, we find that the type of planetary boundary layer assigned by the model algorithm modulates the contribution of individual schemes and affects the interactions between different schemes. In addition, we demonstrate the sensitivity of these interactions to an increase or decrease in the strength of the parameterized shallow convection. The individual tendencies from several parameterizations can thereby compensate each other, sometimes resulting in a small residual. In some instances this residual remains nearly unchanged between the sensitivity experiments, even though some individual tendencies differ by up to an order of magnitude. Using the individual tendency output, we can characterize the subgrid-scale as well as grid-scale responses of the model and trace them back to their underlying causes. We thereby highlight the utility of individual tendency output for understanding process-related differences between model runs with varying physical configurations and for the continued development of numerical weather prediction models

Der Einfluss der frühen operativen Versorgung von Wirbelsäulenverletzung bei polytraumatisierten Patienten - eine single center-Studie

Presentation at the European Geoscience Union General Assembly conference, Vienna, Austria, 07.04.19 - 12.04.19. (https://www.egu2019.eu/. </a

Interplay between large scale and local meteorological conditions in an Arctic fjord based on research aircraft measurements

A majority of the population of High Arctic lives at onshore locations, typically characterized by fjords and other features of complex topography. Therefore it is essential to better understand interaction of the large-scale at- mospheric flows and local conditions in such areas. In this study, we used data from a winter research aircraft campaign over a long (about 110 km) and wide (5-20 km) south to north oriented Arctic fjord which has an ideal straight axis but is bounded with complex topography varying in the scale of a hundred meters to kilometers. The research aircraft flights were part of the Spring Time Atmospheric Boundary Layer Experiment (STABLE). Anal- ysis of the data shows a channeling of the easterly large-scale flow along the fjord towards its bottom located in the south. This channeled large scale flow interacts in a complex manner with a locally driven shallow katabatic flow region propagating northward from the glaciers and mountains at the fjord bottom. The interaction of these two flow regimes is further complicated by large changes in the surface conditions; the surface type is glacier and snow-covered land in the south, complete sea ice cover in the most of the fjord and open water in the fjord mouth area in the north. The related changes in the surface temperature are driving a shallow strong convective boundary layer in the north. The detailed analysis of the aircraft measurements provides an excellent testbed for atmospheric mesoscale model simulations aiming e.g. at effects of model resolution over an Arctic fjord