Browsing by Subject "Geophysics"

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  • Ronkainen, Iina (Helsingfors universitet, 2013)
    In this Master's thesis the long-term changes in Baltic Sea ice characteristics were studied. In the Baltic Sea the length of ice season is 5-7 months. The amount of seasonal ice varies significantly from year to year. However, in the last 100 years there has been a decreasing trend in the ice occurrence, which has resulted mainly from climate warming. Both observations and model results were analyzed in order to find out the long-term ice statistics, changes in ice conditions and reasons behind these changes. Three stations along the Finnish coast were chosen, Kemi in the Bay of Bothnia, Utö in the Archipelago Sea and Loviisa in the Gulf of Finland. The time series were 120 years long including the dates of freezing and break-up, the length of ice season and the maximum annual ice thickness. The model used was NEMO/LIM-3 and the modeled time 1961-2007. The key questions for the thesis were the positive trend in ice thickness in Kemi station, the reasons for the 100 year long decreasing trends although the climate warming has not affected so long and the changes in drift ice thickness. The study results show that the probability of ice occurrence has been decreasing in Utö and is now 81 %. In Kemi and Loviisa the probability is still 100 %. The freezing date has become 7-24 days later, while the break-up date has taken place 11-20 days earlier in 100 years. Consequently, the observed length of ice season has become 18-46 days shorter in 100 years. The trend of the maximum annual ice thickness is not so uniform. In Kemi station, there is an increasing trend, whereas in Loviisa the trend is decreasing. According to the model, the maximum annual ice thickness has a decreasing trend also in Kemi. The maximum annual ice volume has a decreasing trend in the entire Baltic Sea and also in different basins (Bay of Bothnia, Bothnian Sea and Gulf of Finland). The modeled ice volume correlates well with the observed maximum annual ice extent even though the ice volume has higher inter-annual variations. The possible other reasons besides rise in air temperature were detected. Sea ice thickness depends in addition to air temperature also greatly on snow accumulation and ice dynamics. The observation sites are not documented, so the places may have changed. The observation sites are usually near harbors and the increasing shipping might have affected ice conditions. The land uplift has been over 1 meter in Kemi during the 120 years, so that might also have an influence on results. In Loviisa the nuclear power plant might have affected the past few decades.
  • Veikkolainen, Toni (Helsingfors universitet, 2010)
    The magnetic field of the Earth is 99 % of the internal origin and generated in the outer liquid core by the dynamo principle. In the 19th century, Carl Friedrich Gauss proved that the field can be described by a sum of spherical harmonic terms. Presently, this theory is the basis of e.g. IGRF models (International Geomagnetic Reference Field), which are the most accurate description available for the geomagnetic field. In average, dipole forms 3/4 and non-dipolar terms 1/4 of the instantaneous field, but the temporal mean of the field is assumed to be a pure geocentric axial dipolar field. The validity of this GAD (Geocentric Axial Dipole) hypothesis has been estimated by using several methods. In this work, the testing rests on the frequency dependence of inclination with respect to latitude. Each combination of dipole (GAD), quadrupole (G2) and octupole (G3) produces a distinct inclination distribution. These theoretical distributions have been compared with those calculated from empirical observations from different continents, and last, from the entire globe. Only data from Precambrian rocks (over 542 million years old) has been used in this work. The basic assumption is that during the long-term course of drifting continents, the globe is sampled adequately. There were 2823 observations altogether in the paleomagnetic database of the University of Helsinki. The effect of the quality of observations, as well as the age and rocktype, has been tested. For comparison between theoretical and empirical distributions, chi-square testing has been applied. In addition, spatiotemporal binning has effectively been used to remove the errors caused by multiple observations. The modelling from igneous rock data tells that the average magnetic field of the Earth is best described by a combination of a geocentric dipole and a very weak octupole (less than 10 % of GAD). Filtering and binning gave distributions a more GAD-like appearance, but deviation from GAD increased as a function of the age of rocks. The distribution calculated from so called keypoles, the most reliable determinations, behaves almost like GAD, having a zero quadrupole and an octupole 1 % of GAD. In no earlier study, past-400-Ma rocks have given a result so close to GAD, but low inclinations have been prominent especially in the sedimentary data. Despite these results, a greater deal of high-quality data and a proof of the long-term randomness of the Earth's continental motions are needed to make sure the dipole model holds true.
  • Vuorinen, Tommi (Helsingfors universitet, 2016)
    The ground motion induced by an earthquake and its attenuation as a function of magnitude and distance can be estimated by using various ground motion prediction equations (GMPE). From a historical perspective, the Fennoscandian shield has been a seismically quiet area with a scarcity of strong earthquakes. This has made the area nominally an area with a low seismic hazard. The lack of quantative strong motion data in an area with low seismic hazard and the subsequent lack of advanced theoretical models of seismic response have hindered the developement of GMPEs for the region. Using more direct empirical methods, which do not depend on pre-existing models and simulations of Fennoscandian seismicity, and by taking advantage of the comparatively large Fennoscandian shield ground motion database two GMPEs targeted for the Fennoscandian shield area were created. The created GMPEs are based on an existing attenuation relationship that targets Eastern North America, which is a stable continental area similar to the Fennoscandian shield. The current and historical stage of relative seismic quietness was preceded by a considerably more active phase during and after the deglaciation of the Weichselian continental ice shelf ca. 9000 - 15000 years ago. There is ample geological evidence that this postglacial faulting caused some of the largest known earthquakes that have ever occurred in a stable continental area. The moment magnitudes of the largest postglacial earthquakes have been estimated to have been in excess of 8.0. The magnitude contrast between the geologically recent postglacial seismicity and the available Fennoscandian ground motion database, which consists mostly of low magnitude events, implies that an empirical method can not provide a GMPE capable of reliably estimating the ground motion induced by a potential large postglacial faulting event. It is unlikely that such an event could occur today, but not completely impossible. The two GMPEs presented here are based on a ground motion database consisting of 6465 recordings from 1701 events with magnitudes between -1.0 – 5.2. The events were observed at 84 seismic stations around the Fennoscandian shield between 2003 and 2014. The first GMPE is an empirical model which takes an existing GMPE and uses a non-linear least squares regression method to refit the constant coefficients of the model to our regional ground motion database. The second GMPE is a referenced empirical model which works by multiplying the ground motion prediction of an existing GMPE with a function of certain seismological parameters. The multiplying function's coefficients are then fitted to the ground motion database. The resulting equations provide a reasonably good model of the peak ground acceleration (PGA) and spectral accelerations (SA) at 9 different frequencies: 0.5 Hz, 1.0 Hz, 2.5 Hz, 5 Hz, 10 Hz, 20 Hz, 25 Hz, 33.3 Hz, and 40 Hz. The GMPEs were further assessed by applying them to an independent regional earthquake and various higher magnitude external events that have originated in a presumably similar stable continental area. Based on these comparisons, the upper magnitude limits of applicability were independently estimated for PGA and each SA frequency. A tendency of the ground motion estimate to improve with increasing frequency at higher magnitudes can be seen. The distance ranges of the GMPEs were determined to be identical with the original base model.
  • Vähä-Piikkiö, Olga (Helsingfors universitet, 2015)
    The purpose of this study is to develop a method for optimizing the data assimilation system of the HIROMB-BOOS -model at the Finnish Meteorological Institute by finding an optimal time interval and an optimal grid for the data assimilation. This is needed to balance the extra time the data assimilation adds to the runtime of the model and the improved accuracy it provides. Data assimilation is the process of combining observations with a numerical model to improve the accuracy of the model. There are different ways of doing this, some of which are covered in this work. The HIROMB-BOOS -circulation model is a 3D-forecast model for the Baltic Sea. The variables forecast are temperature, salinity, sea surface height, currents, ice thickness and ice coverage. Some of the most important model equations are explained here. The HIROMB-BOOS -model at the Finnish Meteorological Institute has a preoperational data assimilation system that is based on the optimal interpolation method. In this study the model was run for a 2-month test period with different time intervals of data assimilation and different assimilation grids. The results were compared to data from five buoys in the Baltic Sea. The model gives more accurate results when the time interval of the data assimilation is small. The thicker the data assimilation grid is, the better the results. An optimal time interval was determined taking into account the time the assimilation takes. An optimal grid was visually determined based on an optimal grid thickness, for which the added time had to be considered as well. The optimized data assimilation scheme was tested by performing a 12-month test run and comparing the results to buoy data. The optimized data assimilation has a positive effect on the model results.
  • Koskinen, Paula (Helsingfors universitet, 2013)
    Finland is situated in an intraplate area of low seismicity. Seismic hazard analyses require an assessment of regional maximum earthquake magnitude. One of the methods for estimating maximum magnitude is relating it to the dimensions of active faults. Intraplate earthquakes usually occur when pre-existing zones of weakness are reactivated in response to the ambient stress field. Because minor earthquakes rarely cause surface ruptures, the reactivated faults have to be studied by indirect means. In this study structural lineaments are used as proxies for old shear zones, faults and fractures. Their orientation with respect to the crustal stress field is determined in order to find potentially unstable faults. Firstly the orientation of the stress field is determined by reviewing literature and available data on crustal stress in Finland. The main force causing the compressive stress field in Fennoscandia is the spreading of the mid-Atlantic ridge, which is why the plate motion of Finland relative to North America is also taken into account. An estimate of 115° to 135° is reached for the azimuth of the maximum horizontal stress in Finland. The stress regime is mostly reverse (minimum principal stress is vertical) according to stress indicators and focal mechanisms. The lineaments are split into straight segments for azimuth calculation. The segments are then divided into optimal orientation categories based on the horizontal angle between the segment and the maximum horizontal stress. Reverse faulting takes place perpendicular to and normal as well as transfer faulting takes place parallel to the maximum horizontal stress. The direction of strike-slip faulting depends on the coefficient of internal friction, which is around 0.6 for solid rock and as low as 0.2-0.4 for pre-existing fractures. With these values the Coulomb failure criterion gives an optimal angle of 30° to 40° to the maximum horizontal stress for strike-slip faulting. The lineament segments with the different faulting categories are shown with different colours in order to better visualise the regions hosting similar faulting directions. Some coefficients of internal friction were also calculated based on available stress magnitudes by assuming frictional equilibrium of pre-existing, optimally oriented zones of weakness. The data were scarce and only available for shallow depths. The calculated coefficients are quite high (0.7-0.8) near the surface and decrease with depth down to 0.4. A maximum earthquake magnitude of approximately 7 is suggested based on lengths of the lineament segments. An earthquake of such magnitude has a very low probability of occurring in Finland. This leads to the conclusion that the datasets used are too coarse for reliable estimates of fault length. Finally, the lineament orientations are compared with seismicity data. In western Lapland earthquakes are clearly linked to a reverse and transfer faulting system. In order to find differences in earthquake source mechanisms the earthquakes are divided into three depth categories. It seems that shallow earthquakes of depths less than 5 km most often occur near lineaments likely to reactivate as reverse and transfer faults, whereas earthquakes deeper than 15 km occur closer to lineaments optimal for strike-slip faulting. Earthquakes between the depths of 5 km and 15 km occur near lineaments of all orientations. The lineament orientation and seismicity maps will hopefully prove useful in further studies concerning the present structural framework of the Finnish crust. For reliable estimates of maximum earthquake magnitude based on fault length in Finland, faults should be studied in greater detail.
  • Leponiemi, Laura (Helsingfors universitet, 2014)
    In groundwater sites located in Sodankylä, Lumiaho, Taikkomäki and Tullinkangas data has been collected from natural state springs for quality and from groundwater pipes for groundwater level studies. Quality measurements and groundwater level changes has been studied graphically and statistically to understand the variation and the cause and effect phenomena of hydrological processes. Quality parameters and groundwater level results has been compared to a time series study: ‘Groundwater level and quality changes in Finland 1975-1999’ by Finnish Environmental Institute and to the regulation 401/2001 about quality requirements for water in household consumption by Ministry of Social Affairs and Health. Groundwater level fluctuation has been compared with changes in precipitation and evaporation rates close to research sites, and with the typical behaviour of groundwater level in related regimes. Precipitation and evaporation data has been measured for each groundwater site from the closest meteorological stations. Evaporation has also been calculated using the Hamon equation. The quality measurements include the following parameters: pH, electrical conductivity, nitrite-nitrate as nitrogen, ammonium as nitrogen, oxygen, phosphate as phosphorus, iron and manganese. Despite the fact that chosen research sites represent natural state areas, human behaviour has had a noticeable effect on groundwater level and quality. In all measurement sites the pH results are in most cases more acidic and conductivity results smaller than they should be in water with good quality. However, these results have improved within last ten years in most cases. The values of nitrite-nitrate and ammonium were mostly good and median values were mostly outstandingly lower than the results measured before. The values of oxygen and phosphate were good. Also, with the exception of Sodankylä, the values of iron and manganese were good. Groundwater levels in research sites followed more or less the water levels related to each regime. In the very Northern site, Sodankylä, the melting of snow and ice every Spring happened earlier in the year within the last ten years compared to period 1975-1999. There were no such significantly noticeable trends at the other station with relation to the other points of comparison. The first evaluation of graphs showed the clearest connection between groundwater level, precipitation and evaporation in the researched moraine soil; Taikkomäki and Lumiaho. The connection was discovered in other sites too, but not as clearly.
  • Matero, Ilkka Seppo Olavi (Helsingfors universitet, 2014)
    In this thesis I study the radiation balance and heat budget of a multiyear sea ice floe drifting in the central Arctic ocean. The objectives of the study were to quantify the vertical partitioning of shortwave- and longwave radiation and to quantify the different components of the heat budget of the floe in question, both inside and at its interfaces. The measurements were set up at 88 26.6N, 176 59.88W on 8th of August and carried out for ten days. The measurements were made as a part of the fourth Chinese National Arctic Expedition CHINARE2010. The measurement setup consisted of a net radiometer, four PAR-sensors, a pyrano-albedometer, three spectral radiometers, daily snow pit measurements, weather observations and six ice corings. With the data from these studies I was able to quantify the rate of melting and fluxes of heat both at the surface and at the bottom of the ice. The data allowed for examining the fraction of transmitted and conducted heat but were insufficient for properly quantifying the internal changes and spectral composition of the shortwave radiation at different depths. The surface was observed to be losing heat mainly in the longwave part of the spectrum. The average net radiation on top of the ice on wavelengths between 200 nanometers and 100 micrometers over the period was -25.0 Watts per square meter. The heat fluxes of the shortwave and longwave radiations were of opposite directions and the negative heat flux of the longwave radiation dominated until a distinct change in the radiative conditions on 17th of August. For the remainder of the period these heat fluxes nearly balanced each other and the average net radiation was -2.1 Watts per square meter. The latent and sensible heat fluxes were observed to have a minor role in the surface heat budget with averages of -1.5 Watts per square meter and -0.03 Watts per square meter respectively. The ice was observed to melt primarily at the bottom at a rate of 0.5 cm per day driven by the input of heat from the underlying ocean. Melting at the surface was not apparent until before the last two days of studies, when the upper layer of the snow cover melted. The changes in sea ice and snow cover were visually observed to exhibit significant spatial variability even on a single floe.
  • Moreau, Juulia-Gabrielle (Helsingin yliopisto, 2019)
    Ordinary chondrites are primitive materials of the solar system; they were subject to thermal and shock metamorphism during asteroid accretion and collision history. Shock-darkening is a shock metamorphic process which occurs in ordinary chondrites where iron sulphides and metals form a network of tiny melt veins, optically darkening the lithology. Together space weathering and shock-darkening can be a major factor in alteration of reflectance spectra, suppressing the 1 and 2 micron silicate absorption bands. S-complex asteroids, hosting ordinary chondrites, display silicate absorption bands. C/X-complex asteroids are either devoid of 1 and 2 micron silicate absorption bands or presenting a weak silicate absorption band at 1 micron. If shock-darkening can alter the spectra of S-complex asteroids, they can appear like C/X-complex asteroids and induce a mismatch in the asteroid distribution. This thesis provides an in-depth study of shock-darkening in order to determine the pressure-temperature conditions for shock melting of both iron sulphides and metals, in ordinary chondrites. In order to perform this study the following actions were required: I. observing shock wave interactions in heterogeneous mediums composed of silicates, metals, and iron sulphides, the principal components of ordinary chondrites II. quantifying post-shock heating and melting of the individual phases III. comparing my results with observations of shock metamorphism in ordinary chondrites IV. investigating on the best conditions to reproduce shock-darkening in shock-recovery experiments. In contrast to shock-recovery experiments, I adopted a numerical modelling method which calculated the post-shock heating and melting of individual phases and provided observation of shock wave interactions in heterogeneous mediums. The shock physics code iSALE was used on a mesoscale to study shock compression of ordinary chondrites. Using complex models, the numerical study lead to the following results: A) 40−60 GPa is the likely range for shock-darkening, dominated by melting of iron sulphides. B) Heterogeneous distribution of peak shock pressures and post-shock heating is caused by strong impedance contrasts between phases (with strong pressure increases through reflections from high density phases to lower density phases, e.g. metals to silicates). C) Special conditions, such as eutectic melting, hotspots from convergence of shock waves, or pore crushing, are necessary to melt metals. D) Porosity and pre-heating are important boundary conditions affecting shock metamorphism. E) Results from the mesoscale models are compatible to observations of shock metamorphism in ordinary chondrites. Finally, simulations of shock-recovery experiments showed that the reverberation technique may prevent shock-darkening from occurring. Compared to a single pressure load, the reverberation technique reduces the rise in entropy from super-imposing pressures, thus, if sufficient pressure for shock-darkening (40–60 GPa) is achieved, melting of iron sulphides or metals may not occur. Alternatively, I showed that spherical shock-recovery experiments, which use spherically induced shock waves to shock spherical samples, are ideal to study shock-darkening because the rise in entropy is directly related to the peak-shock pressure in the sample. With my results, a more in depth quantitative study of the volume of shock-induced darkened materials during asteroid collisions is now possible.
  • Honkanen, Henri (Helsingin yliopisto, 2017)
    Vuonna 2016 4. -- 9. lokakuuta toteutettiin mittausjakso kenttätyönä Intian Himalajalla sijaitsevalla Sunderdhunga-jäätiköllä. Tämä työ käsittelee jäätikön päällä tehtyjä lumikuoppa-, säteily- ja lumiasemamittauksia. Lumikuoppia kaivettiin kaksi Sunderdhunga-jäätikölle ja viereiselle Panuti-jäätikölle neljä. Lumikuopille toteutettiin tyypillisimmät lumikuoppamittaukset, joita ovat lämpötila-, tiheys-, kovuus- ja raekoon ja -muodon mittaukset. Lisäksi kosteus- ja tiheysmittauksia tehtiin neljässä lumikuopassa lumihaarukka-mittalaitteella. Säteilymittauksia toteutettiin Sunderdhungalla pyranometrillä, nettosäteilymittarilla ja PAR-sensoreilla, joilla mitattiin myös Panuti-jäätiköllä. Tässä työssä käsitellään mittausten perusteella jäätikön pinnan kerrostuneisuutta ja tarkastellaan pinnan ja ilman välistä lämmönvaihtoa, jossa suurimmaksi tekijäksi osoittautui pitkäaaltosäteily. Kenttämittauksissa havaittiin likainen kerros lumen ja ylivuotisen lumen välissä. Likaisessa kerroksessa oli paljon mustia partikkeleita ja tämän kerroksen vaikutuksia mm. lämmönjohtavuuteen ja tiheyteen tutkitaan. Tämä työ on osa Akatemian rahoittamaa Himalaja-projektia (toiminnassa 2013 -- 2017), jossa Helsingin yliopiston, Ilmatieteen laitoksen ja TERIn (The Energy and Resource Institute, Intiasta) oli tarkoitus yhdessä selvittää aerosolien kuten mustan hiilen määrä, kulkeutuminen ja vaikutukset jäätikön pintakerrokseen.
  • Smedberg, Ilmari (Helsingfors universitet, 2016)
    Yksi tapa tarkastella maankamarassa etenevien seismisten aaltojen vaimenemista on määrittää tutkittavalle alueelle seisminen Q-arvo, joka kuvaa maa- tai kallioperän keskimääräisiä vaimenemisominaisuuksia. Seismisellä Q-arvolla on myös taajuusriippuvuus, eli sillä on eri arvo eri taajuuksilla. Tässä työssä määritettiin taajuusriippuvat Q-arvot P- ja S-aallolle tutkimalla näiden ensisaapujien alusta alkavan kolmen sekunnin aikaikkunan maksimiamplitudeja. Kohdealue tutkimuksessa oli Pohjois-Suomi. Havaintoaineisto koostui Suomen seismiseen asemaverkkoon kuuluvien, viiden seismografiaseman maanjäristysrekisteröinneistä. Asemat sijaitsivat Hetan, Sodankylän, Oulun, Kuusamon ja Tornion alueilla ja maanjäristykset olivat tapahtuneet enintään 200 km:n episentrietäisyydellä. Eri asemien rekisteröintejä tutkittiin yhteensä 181 kpl. Tutkimuksessa käytetty menetelmä on nimeltään coda-normalisointimenetelmä, jossa P- ja S-aaltojen amplitudiarvot normalisoidaan coda-aaltojen amplitudiarvoilla. Eri hyposentrietäisyyksillä saaduista arvoista saadaan regression avulla määritettyä Q-arvo eri taajuuksille, joista linearisoinnin avulla saadaan määritettyä taajuusriippuva Q-arvo. Tässä työssä P-aallolle saatiin taajuusriippuva Q-arvo Qp(f ) = 183(±12)f^0.64(±0.15) sekä S-aallolle Qs(f )=288(±30)f^0.67 (±0.23 ) . Saatuja tuloksia verrattiin samalla menetelmällä eri puolilta maapalloa saatuihin tuloksiin. Pohjois-Suomen alueella S-aaltojen vaimeneminen oli kaikkein vähäisintä verrattuna tektonisesti aktiivisempiin tai geologisesti nuorempiin alueisiin. Tuloksia verrattiin myös sekä Fennoskandian alueelle että geologisesti samankaltaisille alueille määritettyihin Q-arvoihin. Tässä työssä määritetty Qp-arvo ja sen taajuusriippuvuus ovat hyvin samankaltaisia Norjan alueelle määritettyjen Qp-arvojen kanssa. Qs-arvo puolestaan osuu keskivaiheille muihin arvoihin vertailtaessa, ollen hyvin samankaltainen kuin Pohjois-Amerikkaan ja Australiaan määritetyt Qs-arvot. Coda-normalisointimenetelmä osoittautui riittämättömäksi alueelle, jolta on vähän ja epätasaisesti jakautunutta havaintoaineistoa maanjäristyksistä. Seismisten aaltojen vaimenemiseen liittyviä tutkimuksia suositellaan jatkettavan, mutta jollain toisella menetelmällä. Eräs mahdollinen tutkimuskohde tulevaisuudessa voisi olla geometrisen leviämistekijän ja sen etäisyysriippuvuden määrittäminen Suomen kallioperälle.
  • Vähä, Aki (Helsingin yliopisto, 2018)
    Lakes in the boreal region hold a significant importance in the global carbon cycle. They transport and store carbon and exchange it with the atmosphere. As turbulent transport is the most important process in transporting substances in water and air, special weight has been laid on studying turbulent processes in lakes. This work concentrates on turbulence in the surface boundary layer of Lake Kuivajärvi in Western Finland. A 16-day measurement campaign was carried out in Kuivajärvi in September 2014. An acoustic Doppler velocimeter (ADV) was used for high-frequency velocity measurements. The meteorological forcing had two distinct regimes during the campaign: a relatively calm and warm period during the first 13 days and a cold and windy period during the last three days. The two regimes were visible in the measured velocity as well as in the calculated turbulence parameters. The friction velocity in water was estimated to be 3*10^-4...2*10^-2 m/s during low winds and 2*10^-3...5*10^-2 m/s during high winds. In the low-wind regime, the friction velocity in water was generally smaller than the scaled friction velocity in air. In the high-wind regime, the situation was opposite. The common practice of estimating the water-side friction velocity from above-surface measurements isn't justified in all conditions. The viscous dissipation rate was calculated using the inertial subrange method and the neutral scaling. Dissipation rate estimates were from ~10^-10 to 10^-4 W/kg during the low-wind regime. During the high-wind regime, the estimates ranged from ~10^-7 to 10^-3 W/kg. It is likely that the highest dissipation rate estimates were erroneous as such high values have not been reported elsewhere in lakes. A simplified turbulent kinetic energy equation with dissipation rate, shear production and buoyancy production was tested. There was an imbalance of turbulence production and dissipation that was also related to the two meteorological regimes. Whether the equation should also include turbulent transport terms is an open question. The most important sources for errors were noise in the velocity time series and the effect of waves. It was shown that noise removal is an essential part of the ADV data analysis, however, noise-removal methods and methods for calculating the dissipation rate should be developed further. The installment of the ADV instrument should also be improved.