Collections in this community

Recent Submissions

  • 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.
  • Response of major modes of eastern Arctic Ocean variability to climate change

    Baumann, Till M.; Polyakov, Igor V.; Bhatt, Uma S.; Walsh, John E.; Weingartner, Thomas J. (2019-12)
    The Arctic Ocean plays a central role in ongoing climate change, with sea ice loss being the most prominent indicator. Recent observations showed that Atlantic inflows play an increasingly important role in the demise of sea ice. This encroaching atlantification of the eastern Arctic Ocean impacts the mean state and the variability of hydrography and current dynamics throughout the basin. Among the most energetic modes of variability are the seasonal cycle and high frequency semidiurnal (∼12-hourly) dynamics in the tidal and inertial frequency band. Limited observations indicated a substantial increase of both, hydrographic seasonal cycles as well as semidiurnal current dynamics in the eastern Arctic over the last decade. Using a uniquely comprehensive data set from an array of six moorings deployed across the eastern Eurasian Basin (EB) continental slope along the 125°E meridian between 2013 and 2015 within the NABOS project, we assess the state of hydrographic seasonal cycles in the eastern EB. Results show a complex pattern of seasonality with a remarkably strong (∆T=1.4°C), deep reaching (∼600 m) temperature signal over the continental slope and large-scale seasonal displacements of isopycnal interfaces. Seasonally changing background conditions are also the main source of variability of semidiurnal frequency band currents: During winter, vigorous baroclinic tidal currents whose amplitudes by far exceed predictions follow the vertical evolution of the pycnocline. During summer, extensive open-water periods additionally lead to strong wind-driven inertial currents in the upper ocean, routinely exceeding 30 cm/s far offshore in the deep basin. In order to obtain an Arctic-wide perspective on the impact of baroclinic tidal currents, a pan-Arctic tidal current atlas has been developed that synthesizes all available observations from the last 20 years. This atlas allows for in-depth studies of regional baroclinic tidal current variability as well as for validation of ocean and climate models, an essential step towards more accurate projections of the future Arctic Ocean state. Our findings from the eastern EB region already indicate a new, more dynamic state of the eastern Arctic Ocean with direct implications for the ecosystem and further sea-ice reduction.
  • Lidar and radar studies of turbulence, instabilities, and waves in the Arctic middle atmosphere

    Li, Jintai; Collins, Richard L.; Newman, David E.; Simpson, William R.; Thorsen, Denise L.; Williams, Bifford P. (2019-08)
    This dissertation presents new studies of gravity waves and turbulence in the Arctic middle atmosphere. The studies employ lidars and radar to characterize wave activity, instability and turbulence. In the lidar-based studies, we analyze turbulence and wave activity in the MLT based on lidar measurements of atmospheric temperature, density and sodium density, temperature and wind. This combination of measurements provides simultaneous characterization of both the atmospheric stability as well as material transport that allow us to estimate the eddy diffusion coefficient associated with turbulence. We extend the scope of previous studies by developing retrievals of potential temperature and sodium mixing ratio from the Rayleigh density temperature lidar and sodium resonance density lidar measurements. We find that the estimated values of turbulent eddy diffusion coefficients, K, of 400-2800 m²/s, are larger than typically reported (1-1000 m²/s) while the values of the energy dissipation rates, ε, of 5-20 mW/kg, are more typical (0.1-1000 mW/kg). We find that upwardly propagating gravity waves accompany the instabilities. In the presence of instabilities, we find that the gravity waves are dissipating as they propagate upward. We estimate the energy available for turbulence generation from the wave activities and estimate the possible turbulent energy dissipation rate, εGW. We find that the values of εGW are comparable to the values of ε. We find that the estimate of the depth of the layer of turbulence are critical to the estimate of the values of both ε and εGW. We find that our method tends to overestimate the depth, and thus overestimate the value of ε, and underestimate the value of εGW. In the radar-based study, we conduct a retrieval of turbulent parameters in the mesosphere based on a hypothesis test. We distinguish between the presence and absence of turbulence based on fitting Voigt-based and Lorentzian-based line shapes to the radar spectra. We also allow for the presence and absence of meteoric smoke particles (MSPs) in the radar spectra. We find examples of Poker Flat Incoherent Scatter Radar (PFISR) spectra showing both the presence and absence of turbulence and the presence and absence of MSPs in the upper mesosphere. Based on the analysis, we find that relatively few of the radar measurements yield significant measurements of turbulence. The significant estimates of turbulence have a strength that is over a factor of two larger than the average of the estimates from all of the radar measurements. The probability of true positives increases with the quality factor of the spectrum. The method yields significant measurements of turbulence with probabilities of true positives of greater than 30% and false positives less than 0.01%.
  • 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.
  • Modelling investigation of interaction between Arctic sea ice and storms: insights from case studies and climatological hindcast simulations

    Semenov, Alexander; Zhang, Xiangdong; Bhatt, Uma; Hutchings, Jennifer; Mölders, Nicole (2019-05)
    The goal of this study is to improve understanding of atmosphere, sea ice, and ocean interactions in the context of Arctic storm activities. The reduction of Arctic sea ice extent, increase in ocean water temperatures, and changes of atmospheric circulation have been manifested in the Arctic Ocean along with the large surface air temperature increase during recent decades. All of these changes may change the way in which atmosphere, sea ice, and ocean interact, which may in turn feedback to Arctic surface air warming. To achieve the goal, we employed an integrative approach including analysis of modeling simulation results and conducting specifically designed model sensitivity experiments. The novelty of this study is linking synoptic scale storms to large-scale changes in sea ice and atmospheric circulation. The models were used in this study range from the regional fully coupled Arctic climate model HIRHAM-NAOSIM to the ocean-sea ice component model of the Community Earth System Model CESM and the Weather Research and Forecasting (WRF) model. Analysis of HIRHAM-NAOSIM simulation outputs shows regionally dependent variability of storm count with a higher number of storms over the Atlantic side than over the Pacific side. High-resolution simulations also reproduce higher number of storms than lower resolution reanalysis dataset. This is because the high-resolution model may capture more shallow and small size storms. As an integrated consequence, the composite analysis shows that more numerous intense storms produce low-pressure systems centered over the Barents-Kara-Laptev seas and the Chukchi-East Siberian seas, leading to anomalous cyclonic circulation over the Atlantic Arctic Ocean and Pacific Arctic Ocean. Correspondingly, anomalous sea ice transport occurs, enhancing sea ice outflow out of the Barents-Kara-Laptev sea ice and weakening sea ice inflow into the Chukchi-Beaufort seas from the thick ice area north of the Canadian Archipelago. This change in sea ice transport causes a decrease in sea ice concentration and thickness in these two areas. However, energy budget analysis exhibits a decrease in downward net sea ice heat fluxes, reducing sea ice melt, when more numerous intense storms occur. This decrease could be attributed to increased cloudiness and destabilized atmospheric boundary layer associated with intense storms, which can result in a decrease in downward shortwave radiation and an increase in upward turbulent heat fluxes. The sea ice-ocean component CICE-POP of Community Earth System Model (CESM) was used to conduct sensitivity experiment to examine impacts of two selected storms on sea ice. CICE-POP is generally able to simulate the observed spatial distribution of the Arctic sea-ice concentration, thickness, and motion, and interannual variability of the Arctic sea ice area for the period 1979 to 2011. However, some biases still exit, including overestimated sea-ice drift speeds, particularly in the Transpolar Drift Stream, and overestimated sea-ice concentration in the Atlantic Arctic but slightly underestimated sea ice concentration in the Pacific Arctic. Analysis of CICE-POP sensitivity experiments suggests that dynamic forcing associated with the storms plays more important driving role in causing sea ice changes than thermodynamics does in the case of storm in March 2011, while both thermodynamic and dynamic forcings have comparable impacts on sea ice decrease in the case of the August 2012. In case of March 2011 storm, increased surface winds caused the reduction of sea ice area in the Barents and Kara Seas by forcing sea ice to move eastward. Sea ice reduction was primarily driven by mechanical processes rather than ice melting. On the contrary, the case study of August 2012 storm, that occurred during the Arctic summer, exemplified the case of equal contribution of mechanical sea ice redistribution of sea ice in the Chukchi - East Siberian - Beaufort seas and melt in sea ice reduction. To understand the impacts of the changed Arctic environment on storm dynamics, we carried out WRF model simulations for a selected Arctic storm that occurred in March 2011. Model output highlight the importance of both increased surface turbulent heat fluxes due to sea ice retreat and self-enhanced warm and moist air advection from the North Atlantic into the Arctic. These external forcing factor and internal dynamic process sustain and even strengthen atmospheric baroclinicity, supporting the storm to develop and intensify. Additional sensitivity experiments further suggest that latent heat release resulting from condensation/precipitation within the storm enhances baroclinicity aloft and, in turn, causes a re-intensification of the storm from its decaying phase.
  • 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.
  • Assessing River Ice Breakup Date, Coastal Tundra Vegetation And Climate Divisions In The Context Of Alaska Climate Variability

    Bieniek, Peter A.; Bhatt, Uma (2012)
    In Alaska, there exists a substantial knowledge gap of key climate drivers and filling these gaps is vital since life and the economy are inexorably linked with climate in the state. This thesis identifies and investigates three topics that advance the understanding of Alaska climate variability: the role of large-scale climate in Interior river ice breakup, the link between climate and arctic tundra vegetation, and climate divisions based on objective methods. River ice breakup in the Yukon-Kuskoswim watershed is occurring earlier by 1.3 days decade-1 1948-2008 and displays large year-to-year variability. April-May Interior Alaska air temperatures are the best predictor of river ice breakup and were linked to El Nino Southern Oscillation (ENSO). During the warm phase of ENSO, fewer storms track into the Gulf of Alaska during Boreal Spring, resulting in reduced April-May cloudiness over Alaska, increased solar insolation at the land surface, warmer air temperatures and consequently earlier breakup. Northern Alaska tundra vegetation productivity has increased 1982-2011, based on the Normalized Difference Vegetation Index (NDVI), a satellite measure of vegetation correlated with above ground biomass. Vegetation productivity was linked to the Beaufort High circulation as well as snowfall, in addition to land surface temperatures and coastal sea ice extent. NDVI has decreased from 1982-2011 over the coastal tundra along the Bering Sea and was correlated with delayed springtime warming due to enhanced coastal sea ice and a delayed snowmelt. Cluster analysis was applied to 2-meter air temperature data 1977-2010 at meteorological stations to construct climate divisions for Alaska. Stations were grouped together objectively based on similar homogeneous seasonal and annual climate variability and were refined using local expert knowledge to ultimately identify 13 divisions. Correlation analysis using gridded downscaled temperature and precipitation data validated the final division lines and documented that each division has similar a similar annual cycle in temperature and precipitation. Overall, this work documented substantial links and identified mechanisms joining the large-scale climate to that of Alaska. A better understanding of the role of large-scale climate variability in river ice breakup or tundra greening holds promise for developing seasonal and longer-term forecasts.
  • Analysis Of Model And Observation Data For The Development Of A Public Pm2.5 Air-Quality Advisories Tool (Aquat)

    Tran, Huy Nguyen Quang; Molders, Nicole; Bhatt, Uma; Cahill, Catherine F.; Grell, Georg A.; Kramm, Gerhard (2012)
    An air-quality advisory tool (AQuAT) that combines mobile measurements of particulate matter less than or equal to 2.5mum in diameter (PM2.5) with air-quality simulations performed with the Alaska adapted version of the Community Multiscale Air Quality (CMAQ) model was developed to interpolate PM2.5-measurements into unmonitored neighborhoods in Fairbanks, Alaska. AQuAT was developed as traditional interpolation methods of interpolating the mobile measurements were unsuccessful. Such a spatially differentiated air-quality advisory is highly desired in Fairbanks due to health concerns of PM2.5, and the need to improve the quality of life. The accuracy of AQuAT depends on the accuracy of the air-quality simulations used for its database. Evaluation of these simulations showed that they captured the observed relationships between PM2.5-concentrations and major meteorological fields (e.g., wind-speed, temperature, and surface-inversions) well. Skill scores for simulated PM2.5-concentrations fell in the range of modern models. The AQuAT database can include information on the nonlinear impacts of various emission sources on PM2.5-concentrations. This benefit was illustrated by investigating the impacts of emissions from point sources, uncertified wood-burning devices, and traffic on the distribution of PM 2.5-concentrations in the neighborhoods. Sensitivity studies on the effects of wood-burning device changeouts on the PM2.5-concentrations suggested that the emission inventory should be updated as soon as possible to capture recent changes in the emission situation in response to the changeout program. The performance of AQuAT was evaluated with PM2.5-measurements from mobile and stationary sites, and with simulated PM2.5-concentrations of winter 2010/2011 which were assumed to be "grand-truth" data. These evaluations showed that AQuAT captured the magnitudes and temporal evolutions of the PM 2.5-measurements and the "grand-truth" data well. The inclusion of wind-speed, wind-direction, and temperature in AQuAT did not improve its accuracy. This result may be explained by the fact that the relationships between meteorology and PM2.5-concentrations were already captured by the database. AQuAT allows quick spatial interpolation after the mobile measurements were made and provides error bars. It also allows for any route within the area for which a database of simulated concentrations exists. It was shown that AQuAT can be easily transferred for applications in other regions.
  • Atmospheric Forcing Of Wave States In The Southeast Chukchi Sea

    Francis, Oceana P.; Bhatt, Uma (2012)
    The objective of this study was to assess the impact that the ocean state, particularly ocean waves, have on coastal communities and operations in the Western Alaska region. In situ measurements and one-dimensional spectra models, were used to link observed wave activity -- wind-sea and swells -- to their synoptic drivers. Bottom-mounted Recording Doppler Current Profilers (RDCPs) were placed at offshore and nearshore locations in the southeast Chukchi Sea, Alaska, during 2007 and 2009-2010. The highest significant wave height (SWH) "events" were defined as wave heights above 2m and 3m for a duration of 6h or more. Results show that SWH events appeared to be driven by three types of systems, 1) cyclonic systems that moved into the eastern Bering Sea and then stalled there, 2) cyclonic systems that moved into the eastern Chukchi Sea and then loitered there, and 3) a cyclonic system over the Brooks Range, a less common occurrence. Results also show the offshore region having highest SWHs with an east wind and wave direction, and classified as a wind-sea state. For the nearshore region, highest SWHs with south and west wind and wave directions, generally showed a swell state. Agreement between one-dimensional spectral models and in situ measurements was greatest for the higher wind-sea state in the offshore region, while discrepancies arose for the lower swell state in the nearshore region. Cross-validation of in situ measurements with satellite altimeter radar measurements were also conducted. Good correlation was found for the offshore regions but not for the nearshore regions. Satellite observations were also used to assess wave conditions in the Arctic during the years 1993-2011. A 0.020m/year increase of SWH for the SE Chukchi Sea and a 0.025m/year increase for the Pacific-Arctic, was found which correlates well with diminishing sea ice and the heighted wind speed, also shown in this study.
  • 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.
  • Role Of Waves On The Circulation Of The Arctic Middle Atmosphere: Rayleigh Lidar Measurements And Analysis

    Thurairajah, Brentha (2009)
    Rayleigh lidar measurements of the upper stratosphere and mesosphere are made on a routine basis over Poker Flat Research Range (PFRR), Chatanika, Alaska, (65�N, 147�W). Rayleigh lidar measurements have yielded high resolution temperature and density profiles in the 40-80 km altitude. These measurements are used to calculate gravity wave activity in the 40-50 km altitude. The thermal structure of the stratosphere and mesosphere is documented using an eight year data set, and the role of small scale gravity waves on the large scale meridional circulation is analyzed in terms of the synoptic structure of the Arctic stratospheric vortex, Aleutian anticyclone, and planetary wave activity. The monthly mean temperature indicates colder January temperatures that appear to be due to the increase in frequency of occurrence of stratospheric warming events from 1997-2004. The gravity wave potential energy density is analyzed during stratospheric warming events in two experimental time periods. From the first study consisting of three winters, 2002-2003, 2003-2004, and 2004-2005, the first direct measurement of suppression of gravity wave activity during the formation of an elevated stratopause following the 2003-2004 stratospheric warming event is presented. The gravity wave potential energy density at Chatanika is positively correlated with horizontal wind speeds in the stratosphere, and indicates that the wave activity in the 4050 km altitude is partially modulated by the background flow. In the second study with more recent winters of 2007-2008 and 2008-2009, no systematic difference in the magnitude of potential energy density between the vortex displacement warming event during the 2007-2008 winter and vortex split warming event during the 2008-2009 winter is found. However, the low correlation between gravity wave potential energy and horizontal wind speed after the first warming in January 2008, and a higher correlation after the January 2009 warming suggests that while the gravity wave activity after the 2009 warming is modulated by the background flow, other wave sources modulate the gravity wave activity after the 2008 warming.
  • A Concept To Assess The Performance Of A Permafrost Model Run Fully Coupled With A Climate Model

    Paimazumder, Debasish (2009)
    Soil-temperatures simulated by the fully coupled Community Climate System Model LCM version 3.0 (CCSM3) are evaluated using three gridded Russian soil-temperature climatologies (1951-1980, 1961-1990, and 1971-2000) to assess the performance of permafrost and/or soil simulations. CCSM3 captures the annual phase of the soil-temperature cycle well, but not the amplitude. It provides slightly too high (low) soil-temperatures in winter (summer) with a better performance in summer than winter. In winter, soil-temperature biases reach up to 6 K. Simulated near-surface air temperatures agree well with the near-surface air temperatures from reanalysis data. Discrepancies in CCSM3-simulated near-surface air temperatures significantly correlate with discrepancies in CCSM3-simulated soil-temperatures, i.e. contribute to discrepancy in soil-temperature simulation. Evaluation of cloud-fraction by means of the International Satellite Cloud Climatology project data reveals that errors in simulated cloud fraction explain some of the soil-temperature discrepancies in summer. Evaluation by means of the Global Precipitation Climatology Centre data identifies inaccurately-simulated precipitation as a contributor to underestimating summer soil-temperatures. Comparison to snow-depth observations shows that overestimating snow-depth leads to winter soil-temperature overestimation. Sensitivity studies reveal that uncertainty in mineral-soil composition notably contributes to discrepancies between CCSM3-simulated and observed soil-temperature climatology while differences between the assumed vegetation in CCSM3 and the actual vegetation in nature marginally contribute to the discrepancies in soil-temperature. Out of the 6 K bias in CCSM3 soil-temperature simulation, about 2.5 K of the bias may result from the incorrect simulation of the observed forcing and about 2 K of the bias may be explained by uncertainties due network density in winter. This means that about 1.5 K winter-bias may result from measurement errors and/or model deficiencies. Overall, the performance of a permafrost/soil model fully coupled with a climate model depends partly on the permafrost/soil model itself, the accuracy of the forcing data and design of observational network.
  • 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.
  • Investigations On The Impacts Of Land-Cover Changes And/Or Increased Carbon Dioxide Concentrations On Four Regional Water Cycles And Their Interactions With The Global Water Cycle

    Li, Zhao; Molders, Nicole (2007)
    A suite of simulations that combine reference (355ppmv), doubled and tripled CO2 concentrations alternatively without and with land-cover changes in four similar-sized study regions, the Yukon, Ob, St. Lawrence and Colorado basin and adjacent land, are performed with the fully coupled Community Climate System Model to investigate the impact on global and regional water cycles and the interaction of these regional water cycles with the global water cycle. The relative changes in water-cycle quantities caused by increased CO 2 enhance with latitude and CO2 concentrations. Regional and global water cycles interactions are more pronounced in a warmer climate, but regional precipitation and evapotranspiration is affected differently in high-latitudes (Yukon, Ob) than mid-latitudes (Colorado, St. Lawrence). Land-cover changes can have comparable impacts on regional water cycles than increased CO2 concentrations do. Land-cover changes substantially alter the high-latitude water cycles through enhanced snow-albedo feedback and mid-latitude water cycles through vegetation activity in the warm season. The land-cover changes in different regions interact with each other through heat and moisture advections and secondary effects. This interaction enhances with increasing CO2 concentrations. Interactions between land-cover changes and increasing CO2 concentrations enhance with increasing CO2 due to the high sensitivity of regional water cycles to temperature changes.
  • Autonomous Full-Time Lidar Measurements Of Polar Stratospheric Clouds At The South Pole

    Campbell, James R.; Sassen, Kenneth (2006)
    Polar stratospheric clouds (PSC) are an artifact of extremely low temperatures in the lower-stratosphere caused by a lack of sunlight during winter. Their presence induces increased concentrations of chlorine and bromine radicals that drive catalytic ozone destruction upon the return of sunlight in spring. An eye-safe micropulse lidar (MPL; 0.23 mum) was installed at the Scott-Amundsen South Pole Station, Antarctica in December 1999 to collect continuous long-term measurements of polar clouds. A four-year data subset for analyzing PSC is derived from measurements for austral winters 2000 and 2003--2005. A statistical algorithm based on MPL signal uncertainties is designed to retrieve PSC boundary heights, attenuated scattering ratios and demonstrate instrument performance for low signal-to-noise measurements. The MPL measurements consist mostly of Type II PSC (i.e., ice). The likelihood for Type I measurements are described for specific conditions. Seasonal PSC macrophysical properties are examined relative to thermodynamic and chemical characteristics. The potential for dehumidification and denitrification of the lower Antarctic stratosphere is examined by comparing PSC observations to theoretical predictions for cloud based on common scenarios for water vapor and nitric acid concentrations. Conceptual models for seasonal PSC occurrence, denitrification and dehumidification and ozone loss are described. A linear relationship is established between total integrated PSC scattering and ozone loss, with high correlation. Polar vortex dynamics are investigated in relation to PSC occurrence, including synoptic-scale geopotential height anomalies, isentropic airmass trajectories and local-scale gravity waves. Moisture overrunning, from quasi-adiabatic cooling and transport along isentropic boundaries, is considered a primary mechanism for PSC occurrence. Middle and late-season PSC are found to be the result of mixing of moist air from the outer edges of the vortex that coots upon reaching South Pole. Gravity waves are considered to be only a secondary influence on PSC nucleation and growth.
  • Midlatitude Cirrus Cloud Structural Properties Analyzed From The Extended Facility For Atmospheric Remote Sensing Dataset

    Wang, Likun; Sassen, Kenneth; Cahill, Cathy; Mölders, Nicole; Shaw, Glen; Starr, David O'C (2004)
    The knowledge on cirrus inhomogeneous structural properties is important not only in radiation calculations, but also in deeply understanding the dynamics mechanism including the formation, development, and dissipation of cirrus clouds. The midlatitude cirrus inhomogeneous structural properties have been evaluated by analyzing the 10-year high cloud datasets obtained at the University of Utah, Facility for Atmospheric Remote Sensing in Salt Lake City, UT. Three goals have been reached in this research. First, the means to analyze lidar data using wavelet analysis, an advanced approach to obtain information on the structure of cirrus clouds, has been successfully developed. And then, typical cirrus structures including Kelvin-Helmholtz instabilities, cirrus mammata, and the uncinus cells have been analyzed by case studies and statistical survey. Their dynamical mechanisms, environmental characteristics, and vertical and horizontal length scale have been studied. Thirdly, using the method based on the wavelet transform and other methods, a climatology of midlatitude cirrus horizontal inhomogeneous properties is developed from the FARS lidar backscattered power data, the proxies of real cirrus clouds.
  • 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.
  • 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.
  • Rayleigh lidar studies of mesospheric inversion layers at Poker Flat Research Range, Chatanika, Alaska

    Irving, Brita K. (2012-08)
    Rayleigh lidar observations at Poker Flat Research Range, Chatanika, Alaska (65°N, 213°E), have yielded density and temperature measurements from 40-80 km. These measurements have been made under clear nighttime skies since November 1997. This thesis presents a study of Mesospheric Inversion Layers (MILs) and lidar performance at Chatanika. MILs are identified and characterized in the 40-70 km altitude region on 55 of the 149 wintertime observations over two periods, November 1997-April 2005 and November 2007-March 2009, using a new detection algorithm. Investigation of the MILs compared with planetary wave activity as observed by satellite finds a strong correlation between the presence of MILs and the structure of the planetary waves. These two periods are marked by strong planetary wave activity and sudden stratospheric warming events. MILs are found to occur more frequently than previously reported at Arctic sites, but less frequently than at lower latitudes. In spring 2012 the existing lidar system was extended by incorporating a larger aperture telescope and higher power laser and field trials were conducted. The results from these field trails are presented and the ability of the new lidar system to extend the scope of future studies at Chatanika is assessed.

View more