• Characteristics And Variability Of Storm Tracks In The North Pacific, Bering Sea And Alaska

      Dos Santos Mesquita, Michel (2009)
      Storm activity in the North Pacific, Bering Sea and Alaska regions is investigated using various automated storm tracking and parameter extraction algorithms. Specific, novel details of storm activity throughout the year are presented. The influence of major climatic drivers is considered, including the Pacific/North American Index and sea ice variability. Details of synoptic-scale forcing on a specific, severe storm event are considered in the context of how different tracking algorithms are able to depict the event. New storm climatology results show that the inter-seasonal variability is not as large during spring and autumn as it is in winter. Most storm variables exhibited a maxima pattern that was oriented along a zonal axis. From season to season this axis underwent a north-south shift and, in some cases, a rotation to the northeast. Barotropic processes have an influence in shaping the downstream end of storm tracks and, together with the blocking influence of the coastal orography of northwest North America, result in high lysis concentrations, effectively making the Gulf of Alaska the "graveyard" of Pacific storms. Summer storms tended to be longest in duration. Temporal trends tended to be weak over the study area. Sea surface temperature did not emerge as a major cyclogenesis control in the Gulf of Alaska. Positive sea-ice anomalies in the Sea of Okhotsk were found to decrease secondary cyclogenesis, shift cyclolysis locations westward, and alter the North Pacific subtropical jet. In the Atlantic, a negative North-Atlantic-Oscillation-like pattern is observed; these results were confirmed by experiments on the ECHAM5 Atmospheric Global Circulation Model driven with sea-ice anomalies in the Sea of Okhotsk. The destructive west Alaska storm of autumn 1992, which flooded Nome, was investigated using two storm tracking algorithms: NOAA's (National Oceanic and Atmospheric Administration) current operational algorithm and the Melbourne algorithm. Manual tracking was performed as a control. The main storm location features were captured by both algorithms, but differed in the genesis and lysis location. The NOAA algorithm broke the event into two. This storm was shown to have been affected by a blocking high that influenced how the tracking algorithms handled the event.
    • Characteristics of Arctic storms and their influence on surface climate

      Yang, Yang; Zhang, Xiangdong; Danielson, Seth; Fochesatto, Javier; Hock, Regine (2020-05)
      Impacts of intense synoptic storms on Chukchi Sea and Beaufort Sea surface environmental conditions are examined, focusing on storms moving into the regions with northward and eastward pathways. Both storms alter the prevailing northeasterly wind to southerly and southwesterly wind. The storms moving from the East Siberian Sea that follow a west to east route are most active in summer and have the longest duration. Increasing southwesterly wind plays a key role in the decline of thin sea ice within the warm season. Storms traveling from the relatively warm Pacific Ocean into the Arctic over the Bering Strait are more common in winter, and are typically more intense than the summer storms that propagate west to east. Downward longwave radiation increases considerably with the passage of intense winter storms over the ice-covered Chukchi Sea; the sea ice concentration decreases accordingly. The impact of different sea ice conditions on Arctic synoptic storm systems in autumn are investigated in the North Pacific and Atlantic sectors, based on the ten ensembles of hindcast simulations from coupled regional climate model HIRHAM-NAOSIM. In both the Pacific and Atlantic sectors, greater transfers of heat and moisture fluxes from the open ocean to the atmosphere occur in low sea ice years than in high sea ice years. The largest increase of upward heat fluxes and baroclinicity occurs over the Laptev, southern Chukchi and Beaufort Seas in the Pacific sector, and over the southern Greenland and Barents Seas in the Atlantic sector. Enhanced baroclinity plays a dominant role in the development of intense storm systems. Therefore, storms in reduced sea ice years are more intense than in enhanced sea ice years in both Atlantic and Pacific sectors. The storm count also increases over locations exhibiting high baroclinicity. Sea ice volume anomalies are significantly correlated with synoptic storm counts based on maximum covariance analysis (MCA) leading modes of covariance between sea ice volume and storm count over Pacific and Atlantic sectors are identified respectively. The results are consistent with our findings in the composite analysis. In the Pacific sector, the first pattern of the MCA demonstrates that increasing storm counts over the Laptev Sea corresponds to decreasing sea ice volume over that region. In the Atlantic sector, the decrease of sea ice volume is highly correlated with decreasing storm counts over the northern Greenland Sea. Connection of storm activity over the North Pacific Ocean with the tropical stratosphere quasi-biennial oscillation (QBO) is investigated following a composite analysis of intense storm vertical cross sections. An observed stronger potential vorticity anomaly of intense storms is associated with the QBO west phase and results in enhanced warm air advection near the surface. A warm core structure forms over the east or northeast direction relative to the surface low center, which bows the isentropes downward. Upward motion following the isentropes reduces the surface low pressure, which in turn, facilitate storms to keep propagating in east and northeast directions. Under the QBO east phase, a weak surface warm core forms to the southeast of the storm center, resulting in a slow development of the storms, and these storms tend to move southeastward.
    • Climate and predictability of Alaska wildfires

      Bieniek, Peter A. (2007-12)
      Wildfires burn an average of 3,760km² each year in Alaska, but varies greatly from year to year. These fires, started by human and natural causes, can endanger life and property when they approach populated areas. The relationship between seasonal area burned and monthly and seasonal average mean sea level pressure, surface air temperature, total column precipitable water, 500hPa and 700hPa geopotential height, 700hPa specific humidity and 1000-500hPa layer thickness is examined. The assessment was done by examining the spring and summer seasonal composites associated with extreme high and low seasons. This showed the predominant anomalies from the climatology for seasons of both extremes. Point correlations were also made between seasonal area burned and the aforementioned climate variables for the entire Northern Hemisphere. Points of particularly high correlation with area burned were used in multiple regressions for both spring and summer, and for the preseason only to predict seasonal area burned. Results show correlations of about 0.78 for the preseason regression and 0.91 for the total period. The seasonal area burned in Alaska is intimately linked with the ongoing synoptic situation on monthly and seasonal scales before and during the fire season.
    • Climatology and forcing mechanisms of funnel clouds in Alaska

      Edwin, Stanley G.; Mölders, Nicole; Bhatt, Uma S.; Collins, Richard L. (2016-08)
      There are no forecasting systems for funnel clouds for Alaska. The inability of forecasting is problematic because funnel clouds pose a threat to aviation, which serves as Alaska’s main form of transportation. Motivated by the lack of research on the formation of funnel clouds in Alaska, this research investigated characteristics of funnel clouds and atmospheric conditions under which funnel clouds form using operational Doppler weather radars and radiosonde soundings as well as synoptic weather maps. In Alaska, funnel clouds usually occur during the summer months May to September with a maximum of occurrence in July and around 1500 Alaska Daylight Time as shown in the funnel cloud observational data. The observed funnel clouds are usually not associated with severe thunderstorms and do not occur with strong synoptic scale forcing. As such, it was hypothesized that local effects from sea breeze fronts and orographic circulations might be the main forcing. Operational soundings indicate that some, but not all funnel cloud events occurred under large Convective Available Potential Energy (greater than 500 J) and strong lowlevel wind shear. Funnel clouds were difficult to identify in routine operational Doppler weather radars because the funnel clouds display small cross-sectional area compared to the radar resolution. An algorithm to retrieve similar vertical profiles from the entire radiosonde data than those observed during documented funnel cloud events was developed. By using similarity between radiosonde profiles of days of the observed funnel clouds and the similar radiosonde profiles scanned over the years, an idea of funnel cloud or severe storm occurrence can be ascertained. The mechanisms for funnel cloud formation differ by region. In Interior Alaska, the Alaska Range’s katabatic slope winds and the Tanana Valley wind create the needed vorticity. Along the west coast of Alaska, air-sea interaction plays a role. In Cook Inlet, topography and land-sea play a role. All funnel cloud events have weak synoptic scale forcing.
    • Data analysis and data assimilation of Arctic Ocean observations

      Stroh, Jacob Nathaniel; Panteleev, Gleb; Mölders, Nicole; Weingartner, Thomas; Rhodes, John (2019-05)
      Arctic-region observations are sparse and represent only a small portion of the physical state of nature. It is therefore essential to maximize the information content of observations and bservation-conditioned analyses whenever possible, including the quantification of their accuracy. The four largely disparate works presented here emphasize observation analysis and assimilation in the context of the Arctic Ocean (AO). These studies focus on the relationship between observational data/products, numerical models based on physical processes, and the use of such data to constrain and inform those products/models to di_erent ends. The first part comprises Chapters 1 and 2 which revolve around oceanographic observations collected during the International Polar Year (IPY) program of 2007-2009. Chapter 1 validates pan- Arctic satellite-based sea surface temperature and salinity products against these data to establish important estimates of product reliability in terms of bias and bias-adjusted standard errors. It establishes practical regional reliability for these products which are often used in modeling and climatological applications, and provides some guidance for improving them. Chapter 2 constructs a gridded full-depth snapshot of the AO during the IPY to visually outline recent, previouslydocumented AO watermass distribution changes by comparing it to a historical climatology of the latter 20th century derived from private Russian data. It provides an expository review of literature documenting major AO climate changes and augments them with additional changes in freshwater distribution and sea surface height in the Chukchi and Bering Seas. The last two chapters present work focused on the application of data assimilation (DA) methodologies, and constitute the second part of this thesis focused on the synthesis of numerical modeling and observational data. Chapter 3 presents a novel approach to sea ice model trajectory optimization whereby spatially-variable sea ice rheology parameter distributions provide the additional model flexibility needed to assimilate observable components of the sea ice state. The study employs a toy 1D model to demonstrate the practical benefits of the approach and serves as a proof-of-concept to justify the considerable effort needed to extend the approach to 2D. Chapter 4 combines an ice-free model of the Chukchi Sea with a modified ensemble filter to develop a DA system which would be suitable for operational forecasting and monitoring the region in support of oil spill mitigation. The method improves the assimilation of non-Gaussian asynchronous surface current observations beyond the traditional approach.
    • Development of a parameterization for mesoscale hydrological modeling and application to landscape and climate change in the Interior Alaska boreal forest ecosystem

      Endalamaw, Abraham Melesse; Bolton, William R.; Young-Robertson, Jessica M.; Hinzman, Larry; Morton, Donald; Mölders, Nicole; Fochesatto, G. Javier (2017-08)
      The Interior Alaska boreal forest ecosystem is one of the largest ecosystems on Earth and lies between the warmer southerly temperate and colder Arctic regions. The ecosystem is underlain by discontinuous permafrost. The presence or absence of permafrost primarily controls water pathways and ecosystem composition. As a result, the region hosts two distinct ecotypes that transition over a very short spatial scale - often on the order of meters. Accurate mesoscale hydrological modeling of the region is critical as the region is experiencing unprecedented ecological and hydrological changes that have regional and global implications. However, accurate representation of the landscape heterogeneity and mesoscale hydrological processes has remained a big challenge. This study addressed this challenge by developing a simple landscape model from the hill-slope studies and in situ measurements over the past several decades. The new approach improves the mesoscale prediction of several hydrological processes including streamflow and evapotranspiration (ET). The impact of climate induced landscape change under a changing climate is also investigated. In the projected climate scenario, Interior Alaska is projected to undergo a major landscape shift including transitioning from a coniferous-dominated to deciduous-dominated ecosystem and from discontinuous permafrost to either a sporadic or isolated permafrost region. This major landscape shift is predicted to have a larger and complex impact in the predicted runoff, evapotranspiration, and moisture deficit (precipitation minus evapotranspiration). Overall, a large increase in runoff, evapotranspiration, and moisture deficit is predicted under future climate. Most hydrological climate change impact studies do not usually include the projected change in landscape into the model. In this study, we found that ignoring the projected ecosystem change could lead to an inaccurate conclusion. Hence, climate-induced vegetation and permafrost changes must be considered in order to fully account for the changes in hydrology.
    • Doppler sodar observations of the winds and structure in the lower atmosphere over Fairbanks, Alaska

      Kankanala, Pavan Kumar Reddy (2007-12)
      Fairbanks, Alaska (64°49ʹ N, 147°52ʹ W) experiences strong temperature inversions which when combined with the low wind speeds prevailing during the winter cause serious air pollution problems. The SODAR (Sound Detection And Ranging) or acoustic sounder is a very useful instrument for studying the lower atmosphere as it can continuously and reliably measure the vertical profiles of wind speed and direction,vertical motions, turbulence and the thermal structure in the lower part of the troposphere. A Doppler sodar was operated from December 2005 to April 2006 at the National Weather Service site in Fairbanks. The wind observations from the sodar indicate that the majority of the winds during the winter months were from the North, Northeast or the East, which is in good agreement with the radiosonde measurements and the long term trends in the wind patterns over Fairbanks area. Case studies were carried out using the sodar data depicting drainage winds, low-level jets, formation and breakup of inversions and estimation of the mixing layer height.
    • Emergent impacts of rapidly changing climate extremes in Alaska

      Lader, Rick T.; Walsh, John E.; Bhatt, Uma S.; Rupp, T. S.; Zhang, Xiangdong (2018-08)
      The frequency and intensity of certain extreme weather events in Alaska are increasing, largely due to climate warming from greenhouse gas emissions. Future projections indicate that these trends will continue, potentially leading to billions of dollars in climate-related damages this century. Expected damages arise from increases in extreme precipitation, severe wildfire, altered ocean chemistry, land subsidence from permafrost thaw, and coastal erosion. This dissertation applies new downscaled reanalysis and climate model simulations from the fifth phase of the Coupled Model Intercomparison Project to enhance current understanding of climate extremes in Alaska. Model output is analyzed for a historical period (1981-2010) and three projected periods (2011-2040, 2041-2070, 2071-2100) using representative concentration pathway 8.5. Unprecedented heat and precipitation are expected to occur when compared to the historical period. Maximum 1-day and consecutive 5-day precipitation amounts are expected to increase by 53% and 50%, respectively, and the number of summer days per year (Tmax > 25°C) increases from a statewide average of 1.5 from 1981-2010 to 29.7 for 2071-2100. Major alterations to the landscape of Alaska are anticipated due to a decreasing frequency of freezing temperatures. Growing season length extends by 48-87 days by 2071-2100 with the largest changes in northern Alaska. In contrast, projections indicate a reduced snow season length statewide and many locations in southwest Alaska no longer have continuous winter snow cover. Changes to these metrics indicate that a climate-warming signal emerges from the historical inter-annual variability, meaning that future distributions are entirely outside of those previously observed. The largest changes to extremes may be avoided by following a lower emissions trajectory, which would reduce the impacts and associated costs to maintain infrastructure and human health.
    • An evaluation of reanalysis products for Alaska to facilitate climate impact studies

      Lader, Rick T.; Bhatt, Uma; Walsh, John E.; Polyakov, Igor V.; Rupp, T. Scott (2014-08)
      Alaska is experiencing effects of global climate change due, in large part, to the positive feedback mechanisms associated with polar amplification. The major risk factors include loss of sea ice, glaciers, thawing permafrost, increased wildfires, and ocean acidification. Reanalyses, which are weather forecast models that assimilate observations, are integral to understanding mechanisms of Alaska's past climate and to help calibrate future modeling efforts. This study evaluates five reanalyses using monthly gridded datasets of temperature, precipitation, and snowwater equivalent, as well as daily station data of maximum and minimum temperature, precipitation, and snow depth across six climate regions in Alaska, and at eight stations from 1979-2009. The reanalyses evaluated in this study include the: NCEP-NCAR Reanalysis (NCEP-R1), North American Regional Reanalysis (NARR), Climate Forecast System Reanalysis (CFSR), ERA-Interim, and Modern-Era Retrospective Analysis for Research and Applications (MERRA). MERRA was the top-performing reanalysis for the station-based assessment, has the lowest statewide precipitation bias, and is the most reliable model for snow-water equivalent. NARR and ERA-Interim have the lowest near-surface air temperature biases across Alaska. The quality of reanalysis data varies by region, season, and variable. This thesis provides guidance for reanalysis users to make informed decisions.
    • Evaluation of the hydro-thermodynamic soil-vegetation scheme on the basis of observations and a Galerkin type finite element numerical scheme

      Narapusetty, Balachandrudu (2005-08)
      The Hydro-Thermodynamic Soil- vegetation Scheme (HTSYS), coupled in a two-way mode with the PennState/National Center for Atmospheric Research (NCAR) Mesoscale Meteorological Model generation 5 (MM5), has been evaluated for a 5 day typical snow-melt period using the Baltic Sea experiment meteorological data center's soil temperature, snow depth, and precipitation datasets. The HTSVS-MM5 evaluation investigates the coupled system's sensitivity to two cloud models and two radiation models, with their cross effects presented along with skill scores for snow depth changes. The coupled model satisfactorily predicts the soil temperature diurnal course cycles, changes in the snow depths, and accumulated precipitation. HTSVS's soil model has been further tested and evaluated in an offline mode for the advanced numerical treatment for the Partial Differential Equations (PDEs) using soil temperature datasets from three sites at Council, Alaska. A Galerkin Weak Finite Element (GWFE) method was tested and evaluated for the numerical treatment of PDEs and the predictions were compared against the existing Crank-Nicholson finite differences scheme (CNFD). GWFE solutions exhibit a remarkable soil temperature predictability, better capture the temperature peaks, and yield non-diffuse and non-oscillatory solutions for relatively high convection dominated regimes, while CNFD performs comparably well in diffusion dominated regimes with a lower computational burden.
    • Exploration of mesospheric metal layers from Chatanika, Alaska

      Martus, Cameron M.; Collins, Richard L.; Mölders, Nicole; Sassen, Kenneth (2013-08)
      Layers of free metal atoms exist in the mesopause region of the atmosphere, generally between 80 and 100 km altitude. Resonance fluorescence lidar provides the best way to measure the structure and dynamics of these metal layers. Resonance lidar observations using tunable dye lasers are conducted at the Lidar Research Laboratory (LRL) of Poker Flat Research Range (PFRR), Chatanika, Alaska (65°N, 147°W). In this thesis, we present three fundamental aspects of mesospheric resonance lidar studies: lidar system commissioning and operation, analysis of temporal variations in the metal layers based on observations, and observations of the nickel layer. An excimer-pumped dye laser system has been used in the past for observations at LRL-PFRR, and we report on the alignment and testing of this system as well as the deployment of a new Nd:YAG-pumped dye laser system. Both of these systems are tested with observations of the sodium layer at 589 nm. Through an analysis of simultaneous observations of the sodium and iron layers taken in the past at LRL-PFRR, we study the common motion of the two layers and suggest an external forcing mechanism for the motions. We find that the motion of the layers is, in most of the observations, consistent with downward propagating gravity waves. Based on elemental abundances in meteors, the most likely source of mesospheric metal layers, we expect to find a nickel layer, yet such a layer has not been reported. We attempt to detect the layer using resonance lidar at the 337 nm nickel absorption line. We make several likely detections of the layer and present an initial estimate of the nickel profile. Signals received in the nickel measurements are as expected from a simulation based on signals received in past observations of sodium and iron.
    • Global and local contributors to the historical and projected regional climate change on the North Slope of Alaska

      Cai, Lei; Alexeev, Vladimir A.; Arp, Christopher D.; Bhatt, Uma S.; Liljedahl, Anna K. (2018-05)
      This thesis includes four studies that explore and compare the impacts of four contributing factors resulting in regional climate change on the North Slope of Alaska based on a numerical simulation approach. These four contributing factors include global warming due to changes in radiative forcing, sea ice decline, earlier Arctic lake ice-off, and atmospheric circulation change over the Arctic. A set of dynamically downscaled regional climate products has been developed for the North Slope of Alaska over the period from 1950 up to 2100. A fine grid spacing (10 km) is employed to develop products that resolve detailed mesoscale features in the temperature and precipitation fields on the North Slope of Alaska. Processes resolved include the effects of topography on regional climate and extreme precipitation events. The Representative Concentration Pathway (RCP) 4.5 scenario projects lower rates of precipitation and temperature increase than RCP8.5 compared to the historical product. The increases of precipitation and temperature trends in the RCP8.5 projection are higher in fall and winter compared to the historical product and the RCP4.5 projection. The impacts of sea ice decline are addressed by conducting sensitivity experiments employing both an atmospheric model and a permafrost model. The sea ice decline impacts are most pronounced in late fall and early winter. The near surface atmospheric warming in late spring and early summer due to sea ice decline are projected to be stronger in the 21st century. Such a warming effect also reduces the total cloud cover on the North Slope of Alaska in summer by destabilizing the atmospheric boundary layer. The sea ice decline warms the atmosphere and the permafrost on the North Slope of Alaska less strongly than the global warming does, while it primarily results in higher seasonal variability of the positive temperature trend that is bigger in late fall and early winter than in other seasons. The ongoing and projected earlier melt of the Arctic lake ice also contributes to regional climate change on the Northern coast of Alaska, though only on a local and seasonal scale. Heat and moisture released from the opened lake surface primarily propagate downwind of the lakes. The impacts of the earlier lake ice-off on both the atmosphere and the permafrost underneath are comparable to those of the sea ice decline in late spring and early summer, while they are roughly six times weaker than those of sea ice decline in late fall and early winter. The permafrost warming resulted from the earlier lake ice-off is speculated to be stronger with more snowfall expected in the 21st century, while the overall atmospheric warming of global origin is speculated to continue growing. Two major Arctic summer-time climatic variability patterns, the Arctic Oscillation (AO) and the Arctic Dipole (AD), are evaluated in 12 global climate models in Coupled Model Intercomparison Program Phase 5 (CMIP5). A combined metric ranking approach ranks the models by the Pattern Correlation Coefficients (PCCs) and explained variances calculated from the model-produced summer AO and AD over the historical period. Higher-ranked models more consistently project a positive trend of the summer AO index and a negative trend of summer AD index in their RCP8.5 projections. Such long-term trends of large-scale climate patterns will inhibit the increase in air temperature while favoring the increase in precipitation on the North Slope of Alaska. In summary, this thesis bridges the gaps by quantifying the relative importance of multiple contributing factors to the regional climate change on the North Slope of Alaska. Global warming is the leading contributing factor, while other factors primarily contribute to the spatial and temporal asymmetries of the regional climate change. The results of this thesis lead to a better understanding of the physical mechanisms behind the climatic impacts to the hydrological and ecological changes of the North Slope of Alaska that have been become more severe and more frequent. They, together with the developed downscaling data products, serve as the climatic background information in such fields of study.
    • Hydroclimate in Eurasia from the Arctic to the Tropics

      Majhi, Ipshita; Bhatt, Uma S.; Zhang, Xiangdong; Molders, Nicole; Walsh, John; Krishnamurthy (2018-05)
      Hydrometeorology in Eurasia connects the Arctic with lower latitudes through exchanges in moisture and teleconnections influencing climate variability. This thesis investigates the role of dams on the Kolyma basin, of precipitation and temperature change on a pristine permafrost lined basin of the Yana, and of changing snow cover over Eurasia on the Indian Monsoon. These three pieces of work illustrate different aspects of a changing climate that impact Eurasian hydrometeorological variations. The Kolyma is one of the large rivers which flows into the Arctic Ocean where there has been a large winter increase and summer decrease in flow over the 1986-2000 period. Winter months are characterized by low flow while summer months by high flow. Reservoir regulation was identified as the main cause of changes in the discharge pattern, since water is released in winter for power generation and stored in summer for flood control. The overall discharge to the Arctic Ocean has decreased for Kolyma basin, despite the increase during winter. This study documents how human activities (particularly reservoirs) impact seasonal and regional hydrological variations. The Yana Basin is a small pristine basin that has experienced minimal human impact and is ideal for investigating the role of climate variability on discharge. The precipitation discharge and temperature discharge analysis for Ubileinaya suggests that increased precipitation and higher temperatures resulted in higher discharge, but other parameters also come into play since greater precipitation does not always yield higher discharge. Overall our analysis for this station has increased our understanding of natural basins and how the climate variables like precipitation and temperature play a role. Recent increases in May-June Indian monsoon rain fall were investigated in the context of Eurasian snow cover variations since the onset of the monsoon has long been linked to Himalayan snow cover. Himalayan snow cover and depth have decreased and this study argues that this is the driver of increased rainfall during May-June, the pre-monsoon and early monsoon period. In addition, there has been an increase in snow water equivalent in Northern part of Eurasia and decrease in Southern part, suggesting that the anomalies are large-scale. Storm track analysis reveals an increase in the number of storms in northern and a decrease in southern Eurasia. The large-scale Eurasian snow increases have been shown by other studies to be linked to Arctic sea ice decline. The direct linkage between fall Arctic sea ice decline and an increase in May-June Indian monsoon rainfall is proposed in this work but the exact climate mechanism is tenuous at this point. This study is focused on understanding changing Arctic rivers and the connection of the Arctic with the Indian monsoon. Our study has shed some light into the connection between the Arctic and the tropics. This study could benefit from modeling study where we could have case study with and without sea ice to understand better how that could impact the monsoon and the hydrological cycle in the present and the future. Better understanding of the mechanism would help us take steps towards better adaptation policies.
    • Ice clouds over Fairbanks, Alaska

      Kayetha, Vinay Kumar; Sassen, Kenneth; Mölders, Nicole; Collins, Richard (2014-05)
      Arctic clouds have been recognized long ago as one of the key elements modulating the global climate system. They have gained much interest in recent years because the availability of new continuous datasets is opening doors to explore cloud and aerosol properties as never before. This is particularly important in the light of current climate change studies that predict changing weather scenarios around the world. This research investigates the occurrence and properties of a few types of ice clouds over the Arctic region with datasets available through the Arctic Facility for Atmospheric Remote Sensing (AFARS; 64.86° N, 147.84° W). This study exclusively focuses on ice clouds that form in the upper (cirrus clouds) and midlevels of the troposphere, and that are transparent to laser pulses (visible optical depth τ< 3.0 - 4.0). Cirrus clouds are icedominated clouds that are formed in the upper levels of the troposphere and are relatively thin such that their visual appearances range from bluish to gray in color. Mid-level ice clouds are those clouds primarily composed of ice crystals forming in the midlevels of the troposphere. It is hypothesized that unlike the basic midlevel cloud type (altostratus), other varieties of midlevel ice clouds exist at times over the Arctic region. The midlevel ice clouds studied here are also transparent to laser pulses and sometimes appear as a family of cirrus clouds to a surface observer. Because of their intermediate heights of occurrence in the troposphere, these could have microphysical properties and radiative effects that are distinct from those associated with upper level ice clouds in the troposphere. A ground-based lidar dataset with visual observations for identifying cloud types collected at AFARS over eight years is used to investigate this hypothesis. Cloud types over AFARS have been identified by a surface observer (Professor Kenneth Sassen) using established characteristics traits. Essential macrophysical properties of the clouds are derived from the lidar data, which serves as a climatological representation for the visually identified cirrus and mid-level ice clouds over a typical sub-Arctic location. Synoptic-scale weather patterns conducive for such cloud type formations are derived using a clustering technique applied to a re-analysis dataset. The cloud properties derived from ground-based lidar over AFARS are used to assess the cloud observations from the CALIPSO satellite.
    • Impacts of storm on sea ice: from case study to climate scale analysis

      Peng, Liran; Zhang, Xiangdong; Collins, Richard; Fochesatto, Javier; Polyakov, Igor (2019-12)
      Recent studies have shown that intense and long-lasting storms potentially facilitate sea ice melting. Under the background of extratropical storm tracks poleward shift, significant reductions of Arctic sea ice coverage, and thinning of sea ice thickness over the last several decades, a better understanding on how storms impact sea ice mass balance is obviously of great importance to better predict future sea ice and the Arctic climate changes. This thesis presents a multi-scale study on how storms impact sea ice, consisting of three different parts of the effort. In the first part, we examined the impacts of the 2016 summer intense storm on sea ice changes over the Chukchi Sea using ship-borne observations. The results show that the intense storm can accelerate ice melt through enhanced upper-ocean mixing and upward heat transport. The satellite-observed long-term sea ice variations potentially can be impacted by many factors. In the second part, we first explore key physical processes controlling sea ice changes under no-storm condition. We examined and compared results from 25 sensitivity experiments using the NCAR's Community Earth System Model (CESM). We found that sea ice volume, velocity, and thickness are highly sensitive to perturbed air-ice momentum flux and sea ice strength. Increased sea ice strength or decreased air-ice momentum flux causes counter-clockwise rotation of the transpolar drift, resulting in an increase in sea ice export through Fram Strait and therefore reduction of the pan-Arctic sea ice thickness. Following four tracers released over the Arctic, we found the sea ice thickness distributions following those tracers are broader over the western Arctic and becomes narrower over the eastern Arctic. Additionally, thermodynamic processes are more dominant controlling sea ice thickness variations, especially over periphery seas. Over the eastern Arctic, dynamic processes play a more important role in controlling sea ice thickness variation. Previous studies show that thin ice responds to external perturbations much faster than the thick ice. Therefore, the impacts of storms on sea ice are expected to be different compared with the western/eastern Arctic and the entral/periphery seas. In the third part, we conduct a new composite analysis to investigate the storm impact on sea ice over seven regions for all storms spanning from 1979 to 2018. We focused on sea ice and storm changes over seven regions and found storms tend to have different short-term (two days before and after storm passage), mid-term (one-two weeks after storm passage), and long-term (from 1979 to 2018) impact on sea ice area over those regions. Over periphery seas (Chukchi, East Siberian, Laptev, Kara, and Barents Seas), storms lead to a short-term sea ice area decrease below the climatology, and a mid-term sea ice increase above the climatology. This behavior causes sea ice area to have a small correlation with the storm counts from 1979 to 2018, which suggest that storms have a limited long-term impact on sea ice area over periphery seas. Both the short term and mid-term storm impacts on sea ice area are confined within a 400 km radius circle with maximum impacts shown within a 200 km radius circle. Storms over the western Arctic (Chukchi, East Siberian, and Laptev Seas) have a stronger short-term and mid-term impact on sea ice area compared with the Eastern Arctic (Barents and Kara Seas). Storms over both Atlantic and Pacific entrance regions have a small impact on sea ice area, and storms over the Norwegian, Iceland, and Greenland Seas have the smallest impact on the sea ice area. Compared to the periphery seas, storms tend to have a stronger long-term impact on sea ice area over the central Arctic. The correlation coefficients between the storm count and sea ice area exceed 0.75.
    • An improved method of ice nucleus measurement

      Shih, Chi-Fan G. (1982-09)
      Ice nuclei, which initiate the ice nucleation process at a higher temperature than the homogeneous nucleation temperature, are essential for the initiation of the ice phase in clouds. Unfortunately, no standard method has been established for the measurement of ice nucleus concentration. The filter technique is a promising candidate if the tendency for ice nucleus concentrations to decreases as the volume sampled increases can be explained. For this study, an improved ventilation method for the development of exposed filters was developed and tested. The results were compared with results obtained in a static diffusion chamber. The volume effect was observed to be less with the new dynamic system. Further work needs to be done to find the optimum flow rate in order to reduce the vapor depletion problem to a minimum. The ratio of total counts of dynamic and static system appears to be a promising evaluation index.
    • Investigation of North Pacific sea ice anomalies in the context of atmospheric and oceanic variability

      Tivy, Adrienne (2001-08)
      This study investigates the main mode of variability in North Pacific sea ice and examines the relationship between sea ice concentration and northern hemispheric climate variability for the period 1968-1997. Through empirical orthogonal function (EOF) analysis, correlations, and composite analysis, it was found that the seesaw pattern (first EOF of wintertime sea ice concentrations) between ice concentrations in the Bering Sea and the Sea of Okhotsk, generally used to characterize North Pacific sea ice, does not adequately address variability in the Sea of Okhotsk. Relationships between the sea ice dipole and the large-scale circulation were investigated and were found to change with the 1977 and 1989 regime shifts in the North Pacific climate. Before 1977 the sea ice dipole is strongly related to tropical variability while after 1977 the dipole is more strongly related to mid-latitude variability.
    • Investigation of thin midlevel ice clouds in the Arctic using calipso data and radiative transfer modeling

      Kayetha, Vinay Kumar; Collins, Richard; Meyer, Franz; Prakash, Anupma; Bhatt, Uma (2015-08)
      In this research we investigate the global occurrence and properties of optically thin midlevel ice clouds. These clouds are difficult to detect with passive radiometric techniques and are under-represented in current studies. We use the Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) data set to identify thin midlevel ice clouds and determine their global occurrence and distribution. For the first time, we find that the global mean occurrence of these clouds is at least 4.5%, being at least 7.3% of all the tropospheric clouds detected at a horizontal scale of 10 km. Seasonally, these clouds are found most commonly in the polar regions. These clouds occur most commonly in the Arctic in winter and least commonly in the summer. In winter these clouds can occur up to 19% of the time. The occurrence of these clouds decreases with increasing spatial scale and are most commonly found at spatial scales of 25 km or less. We found five large distinct clouds over the Arctic and investigated them for their meteorological conditions and radiative effects. These thin midlevel ice clouds are formed along the frontal zones in weakly ascending air masses. Our model simulations show that thin midlevel ice clouds have a net warming effect on the surface of 23-48 W/m². We conclude that these clouds have a significant impact on the radiation budget in Arctic winters. Our study highlights the importance of active satellite-based remote sensing in globally detecting and characterizing optically thin clouds. Our estimates of occurrence and fraction of clouds represents a lower bound, as these clouds can be obscured by optically thicker clouds. The volume of measurements provided by the satellite allowed us to identify a small but consistent set of large clouds with which we could conduct a contemporary radiative analysis. These findings can be used to improve the representation of clouds and their impacts in regional and global climate models.
    • Investigation On Cirrus Clouds By The Cloud-Aerosol Lidar And Infrared Pathfinder Satellite Observation Data

      Zhu, Jiang; Sassen, Kenneth (2011)
      Understanding and describing the role of clouds in the climate system need intensive and extensive research on cloud properties. The albedo and greenhouse effects of clouds and their relations with the physical properties of clouds are analyzed. Cloud-top height and ice water content are key factors in impacting the longwave and shortwave radiation, respectively. Lidar and infrared radiometer measurement technologies are introduced. Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) level 1 Lidar profile, level 2 cloud layer, and level 2 Lidar/IIR track products are briefly reviewed. The algorithms for identification of cirrus clouds, Linear Depolarization Ratio (LDR), and effective diameter are presented. An average LDR profile is calculated by using the sum of total attenuated backscattering profiles and the sum of perpendicular attenuated backscattering profiles. A weight-average method is applied to calculate the average LDR. A split-window method is applied to estimate the effective diameters of clouds. A set of bulk ice crystal models and a radiative transfer model are applied to produce a look-up table that includes the radiative transfer simulation results. The macro-physical properties of cirrus clouds are analyzed. The frequency of occurrence of cirrus clouds varies with latitude, and strongly relates to the atmospheric circulation. Cirrus clouds are few in high-pressure zones and abundant where seasonal monsoonal circulation occurs. Cloud-top height decreases with increasing latitude. Cloud-top temperature is lower in the tropical regions than in the midlatutude and the polar regions. The measured cloud thickness shows a great diurnal variation.
    • Investigation on the impacts of low-sulfur fuel used in residential heating and oil-fired power plants on PM₂.₅₋ concentrations and its composition in Fairbanks, Alaska

      Leelasakultum, Ketsiri; Mölders, Nicole; Bhatt, Uma; Collins, Richard (2013-08)
      The effects of using low-sulfur fuel for oil-heating and oil-burning facilities on the PM₂.₅-concentrations at breathing level in an Alaska city surrounded by vast forested areas were examined with the Weather Research and Forecasting model coupled with chemistry packages that were modified for the subarctic. Simulations were performed in forecast mode for a cold season using the National Emission Inventory 2008 and alternatively emissions that represent the use of low-sulfur fuel for oil-heating and oil-burning facilities while keeping the emissions of other sources the same as in the reference simulation. The simulations suggest that introducing low-sulfur fuel would decrease the monthly mean 24h-averaged PM₂.₅-concentrations over the city's PM₂.₅-nonattainment area by 4%, 9%, 8%, 6%, 5% and 7% in October, November, December, January, February and March, respectively. The quarterly mean relative response factors for PM₂.₅ of 0.96 indicate that with a design value of 44.7 µg/m³ introducing low-sulfur fuel would lead to a new design value of 42 .9µg/m³ that still exceeds the US National Ambient Air Quality Standard of 35µg/m³ . The magnitude of the relation between the relative response of sulfate and nitrate changes differs with temperature. The simulations suggest that in the city, PM₂.₅-concentrations would decrease more on days with low atmospheric boundary layer heights, low hydrometeor mixing ratio, low downward shortwave radiation and low temperatures. Furthermore, a literature review of other emission control measure studies is given, and recommendations for future studies are made based on the findings.