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  • 1.
    Birman, Camille
    et al.
    Météo-France-CNRS, Toulouse .
    Mahfouf, Jean-François
    Météo-France-CNRS, Toulouse.
    Milz, Mathias
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Space Technology. IRV.
    Mendrok, Jana
    Chalmers University of Technology, Gothenburg, Sweden.
    Buehler, Stefan A.
    University of Hamburg, Hamburg.
    Brath, Manfred
    University of Hamburg, Hamburg .
    Information content on hydrometeors from millimeter and sub-millimeter wavelengths2017In: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 69, no 1, article id 1271562Article in journal (Refereed)
    Abstract [en]

    This study examines the information content on hydrometeors that could be provided by a future HYperspectralMicrowave Sensor (HYMS) with frequencies ranging from 6.9 to 874 GHz (millimeter and sub-millimeter regions). Through optimal estimation theory the information content is expressed quantitatively in terms of degrees of freedom for signal (DFS). For that purpose the Atmospheric Radiative Transfer Simulator (ARTS) and its Jacobians are used with a set of 25 cloudy and precipitating profiles and their associated errors from the European Centre for Medium-range Weather Forecasting (ECMWF) global numerical weather prediction model.

    In agreement with previous studies it is shown that frequencies between 10 and 40 GHz are the most informative ones for liquid and rain water contents. Similarly, the absorption band at 118 GHz contains significant information on liquid precipitation. A set of new window channels (15.37-, 40.25-, 101-GHz) could provide additional information on the liquid phase. The most informative channels on cloud icewater are the window channels at 664 and 874GHz and thewater vapour absorption bands at 325 and 448 GHz. Regarding snow water contents, the channels having the largest DFS values are located inwindow regions (150-, 251-, 157-, 101-GHz). However it is necessary to consider 90 channels in order to represent 90% of the DFS. The added value of HYMS has been assessed against current Special Sensor Microwave Imager/Sounder (SSMI/S) onboard the Defense Meteorological Satellite Program (DMSP) and future (Microwave Imager/Ice Cloud Imager (MWI/ICI) onboard European Polar orbiting Satellite – Second Generation (EPS-SG)) microwave sensors. It appears that with a set of 276 channels the information content on hydrometeors would be significantly enhanced: the DFS increases by 1.7 against MWI/ICI and by 3 against SSMI/S. A number of tests have been performed to examine the robustness of the above results. The most informative channels on solid hydrometeors remain the same over land and over ocean surfaces. On the other hand, the database is not large enough to produce robust results over land surfaces for liquid hydrometeors. The sensitivity of the results to the microphysical properties of frozen hydrometeors has been investigated. It appears that a change in size distribution and scattering properties can move the large information content of the channels at 664 and 874 GHz from cloud ice to solid precipitation.

  • 2.
    Edman, Jennifer
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering.
    Deriving characteristics of thin cirrus clouds from observations with the IRF lidar2019Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Cirrus clouds play an important role in radiative transfer, and thus have impact on the energy balance of the atmosphere and the climate of the Earth. Furthermore, they occur often and cover large areas globally at any time. Nevertheless, cirrus clouds are poorly studied, especially in the polar regions. Cirrus clouds are present in a large amount of the 14 years of data produced by the lidar at the Swedish Institude of Space Physics (IRF), but has not been studied to a large extent. A lidar is an active remote sensing instrument using a laser. This master's thesis develops and improves programs for analysis of cirrus clouds from this lidar data. It also performs analysis of six case studies chosen from the available data, and statistics of these six cases.

    The parameters calculated for each date are the cloud top, base and mean altitude, the geometrical thickness, the depolarisation ratio, the backscatter ratio (BSR), the backscatter coefficient, the extinction coefficient, the optical thickness and the number of cloud layers. No clear correlation between the optical thickness and the cloud top, base or mean altitude was found. There seems to be a weak correlation between increased optical thickness and increased geometrical thickness, which is not unreasonable. The mean cloud layer top altitude was 11.82 km and the mean cloud base was 10.36 km. The mean optical thickness for a cloud layer was 1.46 km, and the average of the cloud layer mean altitude was 11.09 km. It should be noted that the statistical analysis is based on only six cases with a total observation time of no more than 37 hours. A far larger dataset is needed in order to obtain any statistically signicant conclusions. The effect of averaging is studied, and it is concluded that averaging over altitude reduced the noise and facilitated the interpolation more than averaging over time did.

    Different approaches to obtain the molecular backscatter coefficient are compared, as well as the effect on the simulated molecular signal. Two of these approaches calculate the molecular backscatter coeffcient with input of the temperature and pressure either as continuously measured ground vales from the weather station at IRF or as radiosonde profiles for a specific time. In the other two, the molecular backscatter coeffcient is obtained from ECMWF data and from the standard atmosphere. Differences in the range 12-35% between the methods are found.

    Different approaches to calculate the backscatter ratio (BSR) are also compared. At cirrus altitudes, the decrease in the signal due to the molecular cloudfree part of the atmosphere is still strong, and finding the top and base separately by comparison with the standard deviation of the signal is proven a better method than interpolating between the point where the signal starts to increase and the point where it reaches the same signal value again. Height-normalising the signal provides a more robust method.

    For thin cirrus, the signal is not significantly attenuated above the cloud layer, and it is found that a method based on the ratios between the measured signal and the simulated molecular signal at cloud top and base did not produce reliable results for the optical thickness.

    In addition to analysing data and data processing methods, new data processing tools in MATLAB have been developed and existing functions have been improved. These will be valuable for continued studies with the IRF lidar, for cirrus as well as PSCs and thick and/or low-altitude clouds.

  • 3.
    Fonseca, Ricardo
    et al.
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Space Technology.
    Martín-Torres, Javier
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Space Technology. Instituto Andaluz de Ciencias de la Tierra (CSIC-UGR), 18100 Granada, Spain.
    High-Resolution Dynamical Downscaling of Re-Analysis Data over the Kerguelen Islands using the WRF Model2019In: Journal of Theoretical and Applied Climatology, ISSN 0177-798X, E-ISSN 1434-4483, Vol. 135, no 3-4, p. 1259-1277Article in journal (Refereed)
    Abstract [en]

    We have used the Weather Research and Forecasting (WRF) model to simulate the climate of the Kerguelen Islands (49° S, 69° E) and investigate its inter-annual variability. Here, we have dynamically downscaled 30 years of the Climate Forecast System Reanalysis (CFSR) over these islands at 3-km horizontal resolution. The model output is found to agree well with the station and radiosonde data at the Port-aux-Français station, the only location in the islands for which observational data is available. An analysis of the seasonal mean WRF data showed a general increase in precipitation and decrease in temperature with elevation. The largest seasonal rainfall amounts occur at the highest elevations of the Cook Ice Cap in winter where the summer mean temperature is around 0 °C. Five modes of variability are considered: conventional and Modoki El Niño-Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), Subtropical IOD (SIOD) and Southern Annular Mode (SAM). It is concluded that a key mechanism by which these modes impact the local climate is through interaction with the diurnal cycle in particular in the summer season when it has a larger magnitude. One of the most affected regions is the area just to the east of the Cook Ice Cap extending into the lower elevations between the Gallieni and Courbet Peninsulas. The WRF simulation shows that despite the small annual variability, the atmospheric flow in the Kerguelen Islands is rather complex which may also be the case for the other islands located in the Southern Hemisphere at similar latitudes.

  • 4.
    Hoskins, Brian
    et al.
    Department of Meteorology, University of Reading, Reading, UK Grantham Institute—Climate Change and the environment, Imperial College, London, U.
    Yang, G.Y
    Department of Meteorology, University of Reading, Reading, UK. Climate Directorate, National Centre for Atmospheric Science, University of Reading, Reading, UK.
    Fonseca, Ricardo
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Space Technology. Khalifa University of Science and Technology, Abu Dhabi, United Arab Emirates.
    The detailed dynamics of the June–August Hadley Cell2020In: Quarterly Journal of the Royal Meteorological Society, E-ISSN 1477-870XArticle in journal (Refereed)
    Abstract [en]

    The seminal theory for the Hadley Cells has demonstrated that their existence is necessary for the reduction of tropical temperature gradients to a value such that the implied zonal winds are realisable. At the heart of the theory is the notion of angular momentum conservation in the upper branch of the Hadley Cells. Eddy mixing associated with extra‐tropical systems is invoked to give continuity at the edge of the Hadley Cell and to reduce the subtropical jet by a factor of 3 or more to those observed. In this paper a detailed view is presented of the dynamics of the June–August Hadley Cell, as given by ERA data for the period 1981–2010, with an emphasis on the dynamics of the upper branch. The steady and transient northward fluxes of angular momentum have a very similar structure, both having a maximum on the equator and a reversal in sign near 12°S, with the transient flux merging into that associated with eddies on the winter sub‐tropical jet. In the northward absolute vorticity flux, the Coriolis torque is balanced by both the steady and transient fluxes. The overturning circulations that average to give the Hadley Cell are confined to specific longitudinal regions, as are the steady and transient momentum fluxes. In these regions, both intra‐seasonal and synoptic variations are important. The dominant contributor to the Hadley Cell is from the Indian Ocean and W Pacific regions, and the maxima in OLR variability and meridional wind in these regions have a characteristic structure associated with the Westward‐moving Mixed Rossby‐Gravity wave. Much of the upper tropospheric motion into the winter hemisphere occurs in filaments of air from the summer equatorial region. These filaments can reach the winter sub‐tropical jet, leading to the strengthening of it and of the eddies on it, implying strong tropical‐extratropical interaction.

  • 5.
    Nyman, Oscar
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering.
    Towards a quality control for cloud top pressure and cloud top height products2017Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Cloud top height plays an important roll in the energy budget and is also important for aviation. This thesis concerns the quality control of cloud top height (CTH) retrievals. The approach for quality controlling retrieved CTH has been using the forward simulating software RTTOV. An error estimation function has been developed as well as an investigation to what simplifications can be done regarding the forward simulations for CTH purposes at SMHI. The purpose of the error estimation function is to validate CTH output from CTH retrieval algorithms by giving a rough error estimate of the retrieved CTH compared to what forward simulations predict. For simplifying the forward simulations the most promising results have been shown for lower clouds. Further testing is still of interest and for future work suggestions are provided regarding the error estimation function as well as for simplifying the forward simulations. 

  • 6.
    Olsson, Patrik
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Space Technology.
    Martian Caves as Special Region Candidates: A simulation in ANSYS Fluent on how caves on Mars are, and what their conditions would be for being considered as special regions.2018Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    One of the most interesting questions about Mars is if life ever existed on it. One of the main requirements for life to exist as we know it is the presence of liquid water. It has been suggested by Martín-Torres et al. (2015a) that a daily-transient liquid water cycle takes place on the surface of Mars through deliquescence and efforescence (binding and releasing of water vapour) of perchloratic salts in the Martian soil. Given the right conditions regarding water activity and temperature, certain planetary areas have been defined as Special Regions where there is a chance of life-form reproduction to occur (Kminek et al. 2017). Sub-surface cavities and caves are defined as such and are still a relatively unexplored and not yet studied feature of the Martian surface. This report is an assessment of the environmental conditions in Martian subsurface cavities such as caves and how it can be considered as a Special Region. Based on observations of lava tubes made by Cushing and Titus (2010) with atmospheric and thermal data from REMS on board the Curiosity rover by Martín-Torres et al. (2015b), simulation models were set up in ANSYS Fluent to examine the behaviour of the temperature and relative humidity within these caves. Different properties of the studied models included size, shape, inclination, materials of the ground composition and air flow behaviour. The results showed that a cave roof with a thickness greater than 1-2 m prevents the ground temperature variation during the day to have any considerable impact on the air temperature in the cave which implies that the thermal waves are the main driving factor of the thermal environment in larger models. The average temperature and relative humidities throughout the entire models resulted in unfavourable conditions (relative humidity under 20% RH) to allow for any perchloratic salts to hydrate or form brines. The most interesting results were found in smaller models where different phenomena with higher relative humidity near the floor and in corners occurred for several hours during the same day. This happened at certain times during the day (LMST 7 and 17) when the inlet temperature surpassed the average temperature in the cave and resulted in relative humidities of up to 90% RH which potentially could allow perchloratic salts to stay in brine form, or at least in a hydrated state throughout the day. While the low temperatures in today's Martian caves may be too harsh for life forms to exist, a previous warmer climate might have allowed for extremophiles to thrive in highly saline solutions. This could be an implication that Martian caves should be defined as Special Regions and that further studies should be done on the subject.

  • 7.
    Singh, Shaktiman
    et al.
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Space Technology.
    Bhardwaj, Anshuman
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Space Technology.
    Singh, Atar
    Department of Environmental Science, Sharda University, Greater Noida, India.
    Sam, Lydia
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Space Technology.
    Shekhar, Mayank
    Birbal Sahni Institute of Palaeosciences, Lucknow, India.
    Martin-Torres, Javier
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Space Technology. Instituto Andaluz de Ciencias de la Tierra (CSIC-UGR), Armilla, Granada, Spain.
    Zorzano Mier, María-Paz
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Space Technology. Centro de Astrobiología (INTA-CSIC), Madrid, Spain.
    Quantifying the Congruence between Air and Land Surface Temperatures for Various Climatic and Elevation Zones of Western Himalaya2019In: Remote Sensing, ISSN 2072-4292, E-ISSN 2072-4292, Vol. 11, no 24, article id 2889Article in journal (Refereed)
    Abstract [en]

    The surface and near-surface air temperature observations are primary data for glacio-hydro-climatological studies. The in situ air temperature (Ta) observations require intense logistic and financial investments, making it sparse and fragmented particularly in remote and extreme environments. The temperatures in Himalaya are controlled by a complex system driven by topography, seasons, and cryosphere which further makes it difficult to record or predict its spatial heterogeneity. In this regard, finding a way to fill the observational spatiotemporal gaps in data becomes more crucial. Here, we show the comparison of Ta recorded at 11 high altitude stations in Western Himalaya with their respective land surface temperatures (Ts) recorded by Moderate Resolution Imagining Spectroradiometer (MODIS) Aqua and Terra satellites in cloud-free conditions. We found remarkable seasonal and spatial trends in the Ta vs. Ts relationship: (i) Ts are strongly correlated with Ta (R2 = 0.77, root mean square difference (RMSD) = 5.9 °C, n = 11,101 at daily scale and R2 = 0.80, RMSD = 5.7 °C, n = 3552 at 8-day scale); (ii) in general, the RMSD is lower for the winter months in comparison to summer months for all the stations, (iii) the RMSD is directly proportional to the elevations; (iv) the RMSD is inversely proportional to the annual precipitation. Our results demonstrate the statistically strong and previously unreported Ta vs. Ts relationship and spatial and seasonal variations in its intensity at daily resolution for the Western Himalaya. We anticipate that our results will provide the scientists in Himalaya or similar data-deficient extreme environments with an option to use freely available remotely observed Ts products in their models to fill-up the spatiotemporal data gaps related to in situ monitoring at daily resolution. Substituting Ta by Ts as input in various geophysical models can even improve the model accuracy as using spatially continuous satellite derived Ts in place of discrete in situ Ta extrapolated to different elevations using a constant lapse rate can provide more realistic estimates. 

  • 8.
    Sjöberg, Ludvig
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Space Technology.
    Wind Forecasts Using Large Eddy Simulations for Stratospheric Balloon Applications2019Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    The launch of large stratospheric balloons is highly dependant on the meteorological conditions at ground level, including wind speed. The balloon launch base Esrange Space Center in northern Sweden currently uses forecasts delivered through the Swedish Meteorological and Hydrological Institute to predict opportunities for balloon launches. However the staff at Esrange Space Center experience that the current forecasts are not accurate enough. For that reason the Weather Research and Forecasting model is used to improve the forecast. The model performs a Large Eddy Simulation over the area closest to Esrange Space Center to predict wind speed and turbulence. During twelve hypothetical launch days the improved forecast have an overall accuracy of 93% compared to the old forecast accuracy of 69%. With some improvements and the right computational power the system is thought to be operationally viable.

  • 9.
    Wolf, Veronika
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Space Technology.
    Analysis of Arctic ice cloud properties using in-situ and remote sensing measurements2019Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Cirrus clouds play an important role in the radiation balance of the atmosphere as theycan have a warming and cooling effect. The resulting net radiation effect depends ontheir micro- and macrophysical properties such as particle size, shape, and numberconcentration. The net warming or cooling effect of clouds is still one of the biggest uncertainties, for example in climate models. For weather and climate models and remotesensing retrievals, precise knowledge of micro- and macrophysical cloud propertiesis therefore necessary. This is true in particular for Arctic cirrus clouds, where we still lack data and need better understanding. Yet, climate change affects high latitudes stronger than other regions. Thus, more knowledge about micro- and macrophysicalproperties of Arctic cirrus clouds is needed urgently.

    The focus of this thesis is on the retrieval of physical particle properties of Arcticice clouds. Balloon-borne in-situ measurements with concurrent lidar measurementshave been performedinKiruna in winter. The Balloon-borne IceCloud Particle Imager(B-ICI) takes images of ice particles directly inside the cloud. After recovery, the imagesare analysed to gain information about particle shape, size, area, and number concentrationand to determine the extinction coefficient.

    Whenever possible, concurrent lidar measurements have supplemented the balloonbornemeasurements. Due to balloon drift, there is a spatial and temporal distance betweenB-ICI and lidar. Hence, both instruments do not sample a cloud at the same timeand place. Taking into account the wind speed, it is possible to determine the time of lidarobservation at which the cloud segment probed by balloon was closest to the lidar. However, cloud homogeneity has to be assumed. The results from B-ICI are comparedto extinction coefficients and depolarization ratios obtained fromlidar measurements. Measurement results from B-ICI and lidar measurements are, despite the spatial andtemporal distance, very similar and thus comparable. Clouds consisting of small andcompact particles have a smaller extinction coefficient and depolarization ratio thanclouds which consist of large, complex-shaped particles.

    For each cloud that has been measured, the cloud origin, i.e. its formation processis determined with the help of back-trajectory modelling. With that, it can be studiedif particle properties depend on cloud origin. This analysis reveals that particle sizeand shape exhibit strong differences with respect to the formation process. If ice particleshave been formed via the liquid droplet phase, they can grow to large sizes andinto complex shapes. If, however, they have been formed directly from vapour or supercooledsolutions, they are smaller and most often compact in shape. Hence, it ispossible to predict the formation process if size distribution and predominant particleiiishapes are known. Or inversely, size distribution and shapes can be predicted by knowingthe formation process. To account for these differences, a new parametrization forparticle size distribution is given that depends on the formation process.

    While the cloud formation process is depending on temperature, supersaturation,and updraught speed, it should not depend on latitude. In fact, comparing results fromArctic measurements with measurements from other latitudes similarities are recognizable. For example, the new parametrization for particle size distributions of Arcticcirrus clouds depending on formation processes and an established parametrizationfor midlatitude cirrus depending on particle size are very similar. Thus, this thesis andthe attached papers will not just provide better information about the particle propertiesof Arctic ice clouds, it can also be used to improve weather and climate models forall latitudes.

  • 10.
    Wolf, Veronika
    et al.
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Space Technology.
    Kuhn, Thomas
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Space Technology.
    Krämer, Martina
    On the Dependence of Cirrus Parametrizations on the Cloud Origin2019In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 46, no 21, p. 12565-12571Article in journal (Refereed)
    Abstract [en]

    Parametrizations of Arctic cirrus particle size distributions are presented depending on the cloud origin, either in situ or liquid. The data set used originate from balloon‐borne measurements above Kiruna during winter. The observed particle size distributions are represented by gamma functions. The gamma coefficients exhibit large differences with regard to cloud origin. The functions for Arctic cirrus confirm established parametrizations for continental cirrus sorted by two particle size modes but differ from others depending only on temperature. We suppose that the agreement between the parametrizations of the geographically different cirrus is because in situ and liquid origin cirrus also distinguish by particle size modes. Since cloud sorting by their origin is based on physical processes which are independent of geographical region, we further hypothesize that these cloud‐type‐based parametrizations might be generally valid for use in global models and satellite retrievals, given the distribution of the cloud types is known.

  • 11.
    Wolf, Veronika
    et al.
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Space Technology.
    Völger, Peter
    Kuhn, Thomas
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Space Technology.
    Gumbel, Jörg
    ANALYSIS OF PARTICLE SHAPE, DEPOLARIZATION AND LIDAR RATIO IN ARCTIC CIRRUS CLOUDS: A CASE STUDY2019Conference paper (Refereed)
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