Sea-ice surface properties and their impact on the under-ice light field from remote sensing data and in-situ measurements

The surface properties of sea ice dominate many key processes and drive important feedback mechanisms in the polar oceans of both hemispheres. Examining Arctic and Antarctic sea ice, the distinctly different dominant sea-ice and snow properties in spring and summer are apparent. While Arctic sea ice features a seasonal snow cover with widespread surface ponding in summer, a year-round snow cover and strong surface flooding at the snow/ice interface is observed on Antarctic sea ice. However, substantial knowledge gaps exist about the spatial distribution and temporal evolution of these properties, and their impacts on exchange processes across the atmosphere/ocean interface. This thesis aims to overcome these limitations by quantifying the influence of surface properties on the energy and mass budgets in the ice-covered oceans. Remote sensing data and in-situ observations are combined to derive the seasonal cycle of dominant sea-ice surface characteristics, and their relation to the transfer of solar radiation from the atmosphere through snow and sea ice into the upper ocean. This thesis shows that characteristics of the solar radiation under Arctic sea ice can be described directly as a function of sea-ice surface properties as, e.g., sea-ice type and melt pond coverage. Using this parameterization, an Arctic-wide calculation of solar radiation through sea ice identifies the surface melt onset as the main driver of the annual sea-ice mass and energy budgets. In contrast, an analysis of the spring-summer transition of Antarctic sea ice using passive microwave satellite observations indicates widespread diurnal freeze-thaw cycles in the top snow layers. While the associated temporary thawing is identified as the predominant melt process, subsequent continuous melt in deeper snow layers is rarely found on Antarctic sea ice. Instead of directly influencing the snow depth on Antarctic sea ice, these melt processes rather modify the internal stratigraphy and vertical density structure of the snowpack. An additional analysis of satellite scatterometer observations reveals that snow volume loss on Antarctic sea ice is mainly driven by changes in the lower snowpack, due to the widespread presence of sea-ice surface flooding and snow-ice formation prior to changes in the upper snowpack. As a consequence, the largely heterogeneous and metamorphous Antarctic snowpack prevents a direct correlation between surface properties and the respective characteristics of the penetrating solar radiation under the sea ice. However, surface flooding is identified as the key process governing the variability of the under-ice light regime on small scales. Overall, this thesis highlights that the mass and energy budgets of Antarctic sea ice are determined by processes at the snow/ice interface as well as the temporal evolution of physical snowpack properties. These results are in great contrast to presented studies on Arctic sea ice, where seasonally alternating interactions at the atmosphere/snow- or atmosphere/sea-ice interface control both the energy and mass budgets. An improved understanding of the seasonal cycle of dominant sea-ice and snow surface characteristics in the Arctic and Antarctic is crucial for future investigations retrieving sea-ice variables, such as sea-ice thickness and snow depth, from recent microwave satellite observations

Menetelmä Suomen vesistöjen jääfenologian määrittämiseen perustuen Sentinel-1 IW -moodin satelliittidatan yhdistämiseen tuulennopeusmittauksiin

The subject of this thesis was to find a method for determining ice phenology, or ice freeze-up and break-up dates, in Finnish waterbodies using Sentinel-1 C-band synthetic aperture radar images. The method should minimize manual steps to make it suitable for automatic processing. Though possibilities for detecting these changes are presented by optical earth observation satellites, optical images are often limited by cloud cover and darkness over autumn-winter months in the northern hemisphere. A great advantage of synthetic aperture radars as active sensors is their capability to penetrate through cloud cover and their independence of sunlight to operate. For specific advantages of the Sentinel-1 constellation, data is gathered with a very high acquisition rate near polar regions and all data is openly accessible since 2014. As this thesis is written, few studies have been conducted on observing inland waterbodies’ ice using Sentinel-1 IW mode radar images. To gather data from local wind conditions, lake water / ice / snow surface and temperatures in preparation for the thesis to determine and develop the final method, an automatic sensor station was built to a lake shoreline in southern Finland. The method developed and presented in this thesis is based on the similarity of Sentinel-1 IW mode radar images produced by water surface waves in similar wind conditions. By classifying radar images using similar wind conditions determined by weather station measurements, then estimating a numerical value for the difference between the radar images, waterbodies with open water will feature higher similarity with each other than frozen waterbodies with open water. The difference in similarity is used to determine the dates when changes in ice phenology, freeze-ups and ice break-ups, occur. Calculated by the method, lake freeze-up and break-up periods were determined to be accurate to within few satellite flyovers for select four lakes of different sizes in southern Finland which included the lake with the sensor station. For river portions few hundred meters wide and long, the method was found to distinguish changes in ice phenology for inland river portions better than portions near the sea discharge location. As the method could be used for estimating ice phenology for a variety of waterbodies in Finland not being routinely observed, it will offer possibilities in expansion of freeze-up and break-up models for such waterbodies. There are also potential applications for other watershed models, as seasonal ice can affect certain types of data used to calibrate these models.Tämän diplomityön aiheena on löytää ja kehittää menetelmä Suomen sisävesistöjen jäätymis- ja sulamisajankohtien määrittämiseen hyödyntämällä Sentinel-1 C-taajuuden tutkasatelliittien keräämää dataa. Optisia satelliittikuvia voidaan käyttää jääfenologian määrittämisessä, mutta ovat pohjoisessa rajoittuneita etenkin loppuvuoden jäätymisajankohtien määrittämisessä johtuen pilvisyydestä sekä pimeydestä. Synteettisen apertuurin tutka (engl. SAR) on aktiivinen sensori mikä kykenee toimimaan ilman auringonvaloa sekä pilvipeitteiden läpi. Jääfenologian määrittämisen osalta Sentinel-1 tutkasatelliitit hyötyvät lisäksi korkeasta ylilentotaajuudesta pohjoisilla alueilla sekä vuodesta 2014 asti kerätystä avoimesti saatavilla olevasta tutkakuva-arkistosta. Työn kirjoitushetkellä tutkimuksia Sentinel-1:n IW -moodin tutkahavaintojen hyödyntämisestä sisävesistöjen jääfenologian tulkinnassa ei ole juurikaan laadittu. Tästä syystä työn yhteydessä on rakennettu mittausasema Etelä-Suomessa sijaitsevan järven rantaan, jonka kautta erilaisten menetelmien kehittämiseen vaadittavaa tietoa on kerätty sekä vaihtoehtoja karsittu johtaen nykyiseen versioon menetelmästä. Tässä diplomityössä esitetty menetelmä hyödyntää vesistöjen aaltojen samankaltaisuutta kun tuuliolosuhteet ovat vesistön osalta liki identtiset. Kun tutkakuvassa sulaa vesistöä samoissa tuuliolosuhteissa verrataan sulaan vesistöön, ne eroavat vain vähän toisistaan ja vastaavasti jäätynyt vesistö eroaa tyypillisesti sulasta vesistöstä. Tuuliolosuhteiden samankaltaisuus määritetään hyödyntämällä Ilmatieteen laitoksen sääasemien tuulihavaintoja. Menetelmällä määritettiin jäätymis- ja sulamisajankohdat neljälle järvelle Etelä-Suomessa mukaanlukien sensoriaseman mittaama järvi. Vertaamalla ajankohtia saatavilla oleviin manuaalisiin havaintoihin, menetelmä määritti ajankohdat oikein muutaman satelliittin ylilennon tarkkuudella. Menetelmän soveltuvuus muutaman sadan metrin pituisille ja leveille jokiosuuksille todettiin olevan parempi sisämaassa kuin sijainnissa jossa joki laskee mereen. Menetelmän todettiin tarjoavan mahdollisuuksia sellaisten sisävesistöjen jääfenologian seurantaan joissa jäätymis- ja sulamisajankohtaa ei mitata, tarjoten dataa kyseisten vesistöjen jääolojen mallintamiseen. On myös mahdollista että vesistömalleissa voidaan hyödyntää tietoa jääfenologiasta, koska kausiluontoinen jää vaikuttaa muunmuassa näiden mallien kalibroinnissa käytettyihin mittauksiin

Remote Sensing of Environmental Changes in Cold Regions

This Special Issue gathers papers reporting recent advances in the remote sensing of cold regions. It includes contributions presenting improvements in modeling microwave emissions from snow, assessment of satellite-based sea ice concentration products, satellite monitoring of ice jam and glacier lake outburst floods, satellite mapping of snow depth and soil freeze/thaw states, near-nadir interferometric imaging of surface water bodies, and remote sensing-based assessment of high arctic lake environment and vegetation recovery from wildfire disturbances in Alaska. A comprehensive review is presented to summarize the achievements, challenges, and opportunities of cold land remote sensing

Influence of Surface and Atmospheric Thermodynamic Properties on the Cloud Radiative Forcing and Radiative Energy Budget in the Arctic

The Arctic climate has changed significantly in the last decades, experiencing a dramatic loss of sea ice and stronger than global warming. The Arctic surface temperature and the growth or melt of sea ice is determined by the local surface energy budget. In this context, clouds are of essential importance as they strongly interact with the radiative fluxes and modulate the surface energy budget depending on their properties, the surface types, and atmospheric thermodynamics. For the quantification of changes in the radiative energy budget (REB) associated with the presence or absence of clouds, the concept of cloud radiative forcing (CRF) is commonly used. This concept is defined as the differences between the REB in cloudy and cloud-free conditions, two atmospheric states which can not be observed at the same location and time. Consequently, either radiative transfer simulations or observations in both states have to be related, both of which complicate the derivation of CRF. A review of available studies and their approaches to derive the CRF reveals conceptual differences as well as deficiencies in the handling of radiative processes related to the surface albedo. These findings call into question the current state of CRF assessment in the Arctic based on the few available studies, but also their comparability. By combining atmospheric radiative transfer simulations with a snow albedo model, two processes that control the surface albedo during the transition from cloud-free to cloudy conditions and their role in the derivation of CRF are discussed. The broadband surface albedo of snow surfaces typically increases in the presence of clouds due to a spectral weighting of downward irradiance toward shorter wavelengths. For more absorbing surface types such as white ice and melt ponds, which are common in summer, there is a strong shift between the albedo of direct and diffuse illuminated surface, which diminishes the surface albedo depending on the cloud optical thickness and solar zenith angle. In this thesis, a hypothesis on the impact of those surface-albedo--cloud interactions on the annual cycle of shortwave CRF is discussed, but an application to inner Arctic conditions remains an open issue. An improved method to derive the shortwave CRF is proposed and an application to two airborne campaigns in the marginal sea ice zone northwest of Svalbard (Norway) illustrates the role of surface-albedo--cloud interactions in the Arctic in spring and early summer. For the longwave CRF, conceptual differences and the general interpretation of the different CRF estimates are discussed and illustrated for a case study. Radiative transfer simulations of a rarely observed annual cycle of thermodynamic profiles in the inner Arctic are used to study both longwave CRF approaches and the impact of thermodynamic profiles on the longwave CRF. Making use of airborne low-level flights in the MIZ and other available datasets, common seasonal radiative states on sea ice and case studies of warm air intrusions and cold air outbreaks are illustrated. The CRF is analyzed as a function of the observed cloud/surface regime, which is extended by radiative transfer simulations characterizing the conditions in this region and seasons

Parameterisation of underwater light fields in the Arctic Ocean and associated impact on biological processes

Accurate characterisation of underwater light is an integral component in modelling the dynamics of marine ecosystems, particularly primary production and animal migration patterns. Existing methods of estimating light fields either rely on satellite data, in situ measurements or radiative transfer models that only operate when the sun is above the horizon. These methods are of limited use in Arctic waters, particular during Polar Night due to extended periods of extremely low light levels and prolonged periods when the sun remains below horizon. Estimating underwater light in the region is further hindered by the optical complexities introduced by widespread and seasonally varying snow and ice cover, and many current ecosystem models either simplify these under-ice light fields or excluding them entirely, potentially disregarding biologically significant light levels. This work presents a model of spectrally resolved underwater light that demonstrates the ability to simulate light levels over the full year into the period of Polar Night and is validated by in situ data. Downwelling spectral irradiance in the photosynthetically active radiation (PAR, 400 – 700nm) range is calculated in both open and ice-covered water columns and includes multiple reflection amplification effects of above surface irradiance between snow and cloud. Validation of downwelling broadband irradiance in open waters shows a mean absolute error of 20% of above surface irradiance to penetrate through thin ice ( 20% of total productivity. In open waters, calculations of primary production were found to be highly sensitive to the parameterisation of the diffuse attenuation coefficient of light. Comparing the results of various light field models designed for use in the Arctic showed a factor 12 difference in calculated water column productivity when using output irradiances to drive a model of primary production. Comparing modelled underwater spectral irradiance to the diel vertical migration (DVM) patterns of Arctic krill in early spring 2018 showed that the spectral distribution of light may act as a trigger mechanism for DVM. Results appear to indicate that although diurnal changes in the magnitude of downwelling irradiance largely drives bulk migration patterns, the population of krill also responded to changes in the ratio of green to blue light, driven by changes in lunar and solar elevations, preferring to occupy regions of the water column with a dominant blue colour of underwater light.Accurate characterisation of underwater light is an integral component in modelling the dynamics of marine ecosystems, particularly primary production and animal migration patterns. Existing methods of estimating light fields either rely on satellite data, in situ measurements or radiative transfer models that only operate when the sun is above the horizon. These methods are of limited use in Arctic waters, particular during Polar Night due to extended periods of extremely low light levels and prolonged periods when the sun remains below horizon. Estimating underwater light in the region is further hindered by the optical complexities introduced by widespread and seasonally varying snow and ice cover, and many current ecosystem models either simplify these under-ice light fields or excluding them entirely, potentially disregarding biologically significant light levels. This work presents a model of spectrally resolved underwater light that demonstrates the ability to simulate light levels over the full year into the period of Polar Night and is validated by in situ data. Downwelling spectral irradiance in the photosynthetically active radiation (PAR, 400 – 700nm) range is calculated in both open and ice-covered water columns and includes multiple reflection amplification effects of above surface irradiance between snow and cloud. Validation of downwelling broadband irradiance in open waters shows a mean absolute error of 20% of above surface irradiance to penetrate through thin ice ( 20% of total productivity. In open waters, calculations of primary production were found to be highly sensitive to the parameterisation of the diffuse attenuation coefficient of light. Comparing the results of various light field models designed for use in the Arctic showed a factor 12 difference in calculated water column productivity when using output irradiances to drive a model of primary production. Comparing modelled underwater spectral irradiance to the diel vertical migration (DVM) patterns of Arctic krill in early spring 2018 showed that the spectral distribution of light may act as a trigger mechanism for DVM. Results appear to indicate that although diurnal changes in the magnitude of downwelling irradiance largely drives bulk migration patterns, the population of krill also responded to changes in the ratio of green to blue light, driven by changes in lunar and solar elevations, preferring to occupy regions of the water column with a dominant blue colour of underwater light

Detection and classification of sea ice from spaceborne multi-frequency synthetic aperture radar imagery and radar altimetry

The sea ice cover in the Arctic is undergoing drastic changes. Since the start of satellite observations by microwave remote sensing in the late 1970\u27s, the maximum summer sea ice extent has been decreasing and thereby causing a generally thinner and younger sea ice cover. Spaceborne radar remote sensing facilitates the determination of sea ice properties in a changing climate with the high spatio-temporal resolution necessary for a better understanding of the ongoing processes as well as safe navigation and operation in ice infested waters.The work presented in this thesis focuses on the one hand on synergies of multi-frequency spaceborne synthetic aperture radar (SAR) imagery for sea ice classification. On the other hand, the fusion of radar altimetry observations with near-coincidental SAR imagery is investigated for its potential to improve 3-dimensional sea ice information retrieval.Investigations of ice/water classification of C- and L-band SAR imagery with a feed-forward neural network demonstrated the capabilities of both frequencies to outline the sea ice edge with good accuracy. Classification results also indicate that a combination of both frequencies can improve the identification of thin ice areas within the ice pack compared to C-band alone. Incidence angle normalisation has proven to increase class separability of different ice types. Analysis of incidence angle dependence between 19-47\ub0 at co- and cross-polarisation from Sentinel-1 C-band images closed a gap in existing slope estimates at cross-polarisation for multiyear sea ice and confirms values obtained in other regions of the Arctic or with different sensors. Furthermore, it demonstrated that insufficient noise correction of the first subswath at cross-polarisation increased the slope estimates by 0.01 dB/1\ub0 for multiyear ice. The incidence angle dependence of the Sentinel-1 noise floor affected smoother first-year sea ice and made the first subswath unusable for reliable incidence angle estimates in those cases.Radar altimetry can complete the 2-dimensional sea ice picture with thickness information. By comparison of SAR imagery with altimeter waveforms from CryoSat-2, it is demonstrated that waveforms respond well to changes of the sea ice surface in the order of a few hundred metres to a few kilometres. Freeboard estimates do however not always correspond to these changes especially when mixtures of different ice types are found within the footprint. Homogeneous ice floes of about 10 km are necessary for robust averaged freeboard estimates. The results demonstrate that multi-frequency and multi-sensor approaches open up for future improvements of sea ice retrievals from radar remote sensing techniques, but access to in-situ data for training and validation will be critical

Radiative Impact of Cryosphere on the Climate of Earth and Mars.

Snow- and ice-covered surfaces are the most reflective regions on Earth and Mars, and their extent can change substantially with small changes in climate. The presence of Earth’s cryosphere greatly alters the planet’s albedo and changes in cryospheric extent and reflectivity therefore partially determine the sensitivity of climate to anthropogenic and external forcings. Carbon dioxide ice is abundant on the Martian surface, and plays an important role in the planet’s energy budget. Firstly, we quantify the shortwave Cryosphere Radiative Effect (CrRE) on Earth. Relatively high resolution (0.05°×0.05°) MODIS data along with radiative kernel datasets are used to estimate the global land shortwave CrRE. We perform multiple analyses to determine the sensitivity of our estimates to the use of different thresholds for snow cover determinations, different climatologies for missing data, and radiative kernels generated with different distributions of clouds produced with various versions of the Community Atmosphere Model. We estimate a global land-based CrRE of about -2.6 W/m2 during 2001-2013, with about 59% of the effect originating from Antarctica. Secondly, we adapt the terrestrial Snow, Ice, and Aerosol Radiation (SNICAR) model to simulate CO2 snow albedo across the solar spectrum (0.2-5.0 μm). We apply recent laboratory derived refractive indices of CO2 ice, which produce higher broadband CO2 snow albedo (0.93–0.98) than previously estimated. We perform multiple analyses to determine the sensitivity of cryosphere spectral albedo to the amount and type of dust, co-presence of CO2 and H2O ices, ice grain size, snow layer thickness, and solar zenith angle. In addition, we also compare our simulations with observed Mars surface albedo, and achieved a reasonable fit between the two. Finally, SNICAR is implemented with the Laboratoire de Météorologie Dynamique Mars GCM to prognostically determine ice cap (both H2O and CO2) albedos interactively in the model. We then explore the impact of dust on surface cryosphere albedo and its impact on Mars’ shortwave energy flux. After integrating SNICAR into the Mars GCM, we find that the impact caused by dust is about 1.5 times the impact caused by the presence of snow itself on shortwave flux at the surface.PhDAtmospheric, Oceanic and Space SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/133296/1/sdeepak_1.pd

Ozone–climate interactions and effects on solar ultraviolet radiation

This report assesses the effects of stratospheric ozone depletion and anticipated ozone recovery on the intensity of ultraviolet (UV) radiation at the Earth's surface. Interactions between changes in ozone and changes in climate, as well as their effects on UV radiation, are also considered. These evaluations focus mainly on new knowledge gained from research conducted during the last four years. Furthermore, drivers of changes in UV radiation other than ozone are discussed and their relative importance is assessed. The most important of these factors, namely clouds, aerosols and surface reflectivity, are related to changes in climate, and some of their effects on short- and long-term variations of UV radiation have already been identified from measurements. Finally, projected future developments in stratospheric ozone, climate, and other factors affecting UV radiation have been used to estimate changes in solar UV radiation from the present to the end of the 21st century. New instruments and methods have been assessed with respect to their ability to provide useful and accurate information for monitoring solar UV radiation at the Earth's surface and for determining relevant exposures of humans. Evidence since the last assessment reconfirms that systematic and accurate long-term measurements of UV radiation and stratospheric ozone are essential for assessing the effectiveness of the Montreal Protocol and its Amendments and adjustments. Finally, we have assessed aspects of UV radiation related to biological effects and human health, as well as implications for UV radiation from possible solar radiation management (geoengineering) methods to mitigate climate change