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Linda M. Keller and Donald R. Johnson

Abstract

Time series of hemispheric available potential (A) and kinetic (K) energies were used to examine the results of a series of observing system simulation experiments that were performed with the Goddard Laboratory for Atmospheres model to determine the impact of the proposed space-based wind profiler on forecast accuracy. The simulated data for the series of 5-day forecasts were produced from a 20-day integration using the ECMWF model, which was also used to produce the verification forecast for the 5-day period. The three simulated observational sets of data that represented conventional observations, satellite-retrieval temperatures, and wind profiles were produced by NMC.

The results in the Northern Hemisphere show that the magnitudes of A and K from the simulation forecasts are quite similar to each other and are uniformly higher than the verification forecast, reflecting systematic differences in the energy levels of the two models. In the Southern Hemisphere, differences in magnitude of A between simulation and verification forecasts are larger than in the Northern Hemisphere. The time series for K shows greater diversity in magnitude among the simulation forecasts, with all the simulation forecasts for K being higher than the verification forecast. The S 1 skill scores and root-mean-square (rms) differences reveal little variation in the accuracy of the forecasts among the three simulation datasets in the Northern Hemisphere. In the Southern Hemisphere, however, forecasts using satellite temperature and wind-profiler data have much smaller rms differences and S 1 scores, indicating an improvement in forecast accuracy over conventional observations. The addition of wind-profiler data provides the greatest improvement in forecast accuracy.

Geographical distributions of vertically integrated eddy A (Ae and K in the Northern Hemisphere reveal that these quantities in the three simulation forecasts are more similar to each other than with the verification forecast. In the Southern Hemisphere, the geographical distributions of Ae and K are more varied with the wind-profiler dataset producing a forecast closest to the verification forecast. In general, the impact of the addition of wind-profiler data on forecast accuracy of energy parameters is negligible in the Northern Hemisphere but distinctly positive in the Southern Hemisphere.

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David D. Houghton, Robert G. Gallimore, and Linda M. Keller

Abstract

Two 100-year seasonal simulators, one performed with a low resolution atmospheric general circulation model (GCM) coupled to a mixed-layer ocean formulation and the other made with the GCM forced by prescribed ocean conditions, are compared to assess the effects of an interactive ocean and sea-ice component on the stability and interannual variability of a climate system. Characteristics of the time variation of surface temperature, 700 mb temperature and sea-ice coverage are analyzed for selected land and ocean areas. Both simulations showed stable seasonal cycles of basic variables, although small trends were found. These trends were roughly linear in nature and quite distinct from all other components of variability. Detrended time series were used to describe the other aspects of variability.

There was pronounced interannual variability in the simulations from both models as seen in the time series for temperature and sea ice over the entire 100-year time period. Consistent with observations, variations tended to be larger in polar areas and over land. The inclusion of the interactive ocean and sea-ice component produced a red spectrum for surface temperature but not for 700-mb temperature. Using a linearized air-sea model patterned after the coupled models, this result is shown to be linked to the combined effects of the model longwave cooling and ocean-atmosphere energy exchange. The shift towards lower frequency in surface temperature was most evident in polar regions and occurred in conjunction with very low frequency (even decadal-scale) variability in the computed sea-ice coverage. The simulated mean and variability characteristics of sea ice corresponded fairly well with observations. This suggests that the low resolution model is able to represent some relevant aspects of the physics of climate fluctuations and thus provide useful simulations for studies of interannual variability.

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Matthew A. Lazzara, George A. Weidner, Linda M. Keller, Jonathan E. Thom, and John J. Cassano

Antarctica boasts one of the world's harshest environments. Since the earliest expeditions, a major challenge has been to characterize the surface meteorology around the continent. In 1980, the University of Wisconsin—Madison (UW-Madison) took over the U.S. Antarctic Program (USAP) Automatic Weather Station (AWS) program. Since then, the UW-Madison AWS network has aided in the understanding of unique Antarctic weather and climate. This paper summarizes the development of the UW-Madison AWS network, issues related to instrumentation and data quality, and some of the ways these observations have and continue to benefit scientific investigations and operational meteorology.

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Lee J. Welhouse, Matthew A. Lazzara, Linda M. Keller, Gregory J. Tripoli, and Matthew H. Hitchman

Abstract

Previous investigations of the relationship between El Niño–Southern Oscillation (ENSO) and the Antarctic climate have focused on regions that are impacted by both El Niño and La Niña, which favors analysis over the Amundsen and Bellingshausen Seas (ABS). Here, 35 yr (1979–2013) of European Centre for Medium-Range Weather Forecasts interim reanalysis (ERA-Interim) data are analyzed to investigate the relationship between ENSO and Antarctica for each season using a compositing method that includes nine El Niño and nine La Niña periods. Composites of 2-m temperature (T 2m), sea level pressure (SLP), 500-hPa geopotential height, sea surface temperatures (SST), and 300-hPa geopotential height anomalies were calculated separately for El Niño minus neutral and La Niña minus neutral conditions, to provide an analysis of features associated with each phase of ENSO. These anomaly patterns can differ in important ways from El Niño minus La Niña composites, which may be expected from the geographical shift in tropical deep convection and associated pattern of planetary wave propagation into the Southern Hemisphere. The primary new result is the robust signal, during La Niña, of cooling over East Antarctica. This cooling is found from December to August. The link between the southern annular mode (SAM) and this cooling is explored. Both El Niño and La Niña experience the weakest signal during austral autumn. The peak signal for La Niña occurs during austral summer, while El Niño is found to peak during austral spring.

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Linda M. Keller, Michael C. Morgan, David D. Houghton, and Ross A. Lazear

Abstract

A climatology of large-scale, persistent cyclonic flow anomalies over the North Pacific was constructed using the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) global reanalysis data for the cold season (November–March) for 1977–2003. These large-scale cyclone (LSC) events were identified as those periods for which the filtered geopotential height anomaly at a given analysis point was at least 100 m below its average for the date for at least 10 days. This study identifies a region of maximum frequency of LSC events at 45°N, 160°W [key point 1 (KP1)] for the entire period. This point is somewhat to the east of regions of maximum height variability noted in previous studies. A second key point (37.5°N, 162.5°W) was defined as the maximum in LSC frequency for the period after November 1988. The authors show that the difference in location of maximum LSC frequency is linked to a climate regime shift at about that time. LSC events occur with a maximum frequency in the period from November through January.

A composite 500-hPa synoptic evolution, constructed relative to the event onset, suggests that the upper-tropospheric precursor for LSC events emerges from a quasi-stationary long-wave trough positioned off the east coast of Asia. In the middle and lower troposphere, the events are accompanied by cold thickness advection from a thermal trough over northeastern Asia. The composite mean sea level evolution reveals a cyclone that deepens while moving from the coast of Asia into the central Pacific. As the cyclone amplifies, it slows down in the central Pacific and becomes nearly stationary within a day of onset. Following onset, at 500 hPa, a stationary wave pattern, resembling the Pacific–North American teleconnection pattern, emerges with a ridge immediately downstream (over western North America) and a trough farther downstream (from the southeast coast of the United States into the western North Atlantic). The implications for the resulting sensible weather and predictability of the flow are discussed. An adjoint-derived sensitivity study was conducted for one of the KP1 cases identified in the climatology. The results provide dynamical confirmation of the LSC precursor identification for the events. The upper-tropospheric precursor is seen to play a key role not only in the onset of the lower-tropospheric height falls and concomitant circulation increases, but also in the eastward extension of the polar jet across the Pacific. The evolution of the forecast sensitivities suggest that LSC events are not a manifestation of a modal instability of the time mean flow, but rather the growth of a favorably configured perturbation on the flow.

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Melissa A. Nigro, John J. Cassano, Matthew A. Lazzara, and Linda M. Keller

Abstract

The Ross Ice Shelf airstream (RAS) is a barrier parallel flow along the base of the Transantarctic Mountains. Previous research has hypothesized that a combination of katabatic flow, barrier winds, and mesoscale and synoptic-scale cyclones drive the RAS. Within the RAS, an area of maximum wind speed is located to the northwest of the protruding Prince Olav Mountains. In this region, the Sabrina automatic weather station (AWS) observed a September 2009 high wind event with wind speeds in excess of 20 m s−1 for nearly 35 h. The following case study uses in situ AWS observations and output from the Antarctic Mesoscale Prediction System to demonstrate that the strong wind speeds during this event were caused by a combination of various forcing mechanisms, including katabatic winds, barrier winds, a surface mesocyclone over the Ross Ice Shelf, an upper-level ridge over the southern tip of the Ross Ice Shelf, and topographic influences from the Prince Olav Mountains. These forcing mechanisms induced a barrier wind corner jet to the northwest of the Prince Olav Mountains, explaining the maximum wind speeds observed in this region. The RAS wind speeds were strong enough to induce two additional barrier wind corner jets to the northwest of the Prince Olav Mountains, resulting in a triple barrier wind corner jet along the base of the Transantarctic Mountains.

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Matthew A. Lazzara, Linda M. Keller, Charles R. Stearns, Jonathan E. Thom, and George A. Weidner

Abstract

For over 30 years, weather forecasting for the Antarctic continent and adjacent Southern Ocean has relied on weather satellites. Significant advancements in forecasting skill have come via the weather satellite. The advent of the high-resolution picture transmission (HRPT) system in the 1980s and 1990s allowed real-time weather forecasting to become a reality. Small-scale features such as mesocyclones and polar lows could be tracked and larger-scale features such as katabatic winds could be detected using the infrared channel. Currently, HRPT is received at most of the manned Antarctic stations. In the late 1990s the University of Wisconsin composites, which combined all available polar and geostationary satellite imagery, allowed a near-real-time hemispheric view of the Southern Ocean and Antarctic continent. The newest generation of satellites carries improved vertical sounders, special sensors for microwave imaging, and the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor.

In spite of the advances in sensors, shortcomings still impede the forecaster. Gaps in satellite data coverage hinder operations at certain times of the day. The development and implementation of software to derive products and visualize information quickly has lagged. The lack of high-performance communications links at many of the manned stations reduces the amount of information that is available to the forecasters.

Future applications of weather satellite data for Antarctic forecasting include better retrievals of temperature and moisture and more derived products for fog, cloud detection, and cloud drift winds. Upgrades in technology at Antarctic stations would allow regional numerical prediction models to be run on station and use all the current and future satellite data that may be available.

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Uma S. Bhatt, Michael A. Alexander, David S. Battisti, David D. Houghton, and Linda M. Keller

Abstract

The impact of an interactive ocean on the midlatitude atmosphere is examined using a 31-yr integration of a variable depth mixed layer ocean model of the North Atlantic (between 20° and 60°N) coupled to the NCAR Community Climate model (CCM1). Coupled model results are compared with a 31-yr control simulation where the annual cycle of sea surface temperatures is prescribed. The analysis focuses on the northern fall and winter months.

Coupling does not change the mean wintertime model climatology (December–February); however, it does have a significant impact on model variance. Air temperature and mixing ratio variance increase while total surface heat flux variance decreases. In addition, it is found that air–sea interaction has a greater impact on seasonally averaged variance than monthly variance.

There is an enhancement in the persistence of air temperature anomalies on interannual timescales as a result of coupling. In the North Atlantic sector, surface air and ocean temperature anomalies during late winter are uncorrelated with the following summer but are significantly correlated (0.4–0.6) with anomalies during the following winter. These autocorrelations are consistent with the “re-emergence” mechanism, where late winter ocean temperature anomalies are sequestered beneath the shallow summer mixed layer and are reincorporated into the deepening fall mixed layer. The elimination of temperature anomalies from below the mixed layer in a series of uncoupled sensitivity experiments notably reduces the persistence of year-to-year anomalies.

The persistence of air temperature anomalies on monthly timescales also increases with coupling and is likely associated with “decreased thermal damping.” When coupled to the atmosphere, the ocean is able to adjust to the overlying atmosphere so that the negative feedback associated with anomalous heat fluxes decreases, and air temperature anomalies decay more slowly.

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Marian E. Mateling, Matthew A. Lazzara, Linda M. Keller, George A. Weidner, and John J. Cassano

Abstract

Because of the harsh weather conditions on the Antarctic continent, year-round observations of the low-level boundary layer must be obtained via automated data acquisition systems. Alexander Tall Tower! is an automatic weather station on the Ross Ice Shelf in Antarctica and has been operational since February 2011. At 30 m tall, this station has six levels of instruments to collect environmental data, including temperature, wind speed and direction, relative humidity, and pressure. Data are collected at 30-, 15-, 7.5-, 4-, 2-, and 1-m levels above the snow surface. This study identifies short-term trends and provides an improved description of the lowest portion of the boundary layer over this portion of the Ross Ice Shelf for the February 2011–January 2014 period. Observations indicate two separate initiations of the winter season occur annually, caused by synoptic-scale anomalies. Sensible and latent heat flux estimates are computed using Monin–Obukhov similarity theory and vertical profiles of potential air temperature and wind speed. Over the three years, the monthly mean sensible heat flux ranges between 1 and 39 W m−2 (toward the surface) and the monthly mean latent heat flux ranges between −8 and 0 W m−2. Net heat fluxes directed toward the surface occur most of the year, indicating an atmospheric sink of energy.

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Nicholas J. Weber, Matthew A. Lazzara, Linda M. Keller, and John J. Cassano

Abstract

Numerous incidents of structural damage at the U.S. Antarctic Program’s (USAP) McMurdo Station due to extreme wind events (EWEs) have been reported over the past decade. Utilizing nearly 20 yr (~1992–2013) of University of Wisconsin automatic weather station (AWS) data from three different stations in the Ross Island region (Pegasus North, Pegasus South, and Willie Field), statistical analysis shows no significant trends in EWE frequency, intensity, or duration. EWEs more frequently occur during the transition seasons. To assess the dynamical environment of these EWEs, Antarctic Mesoscale Prediction System (AMPS) forecast back trajectories are computed and analyzed in conjunction with several other AMPS fields for the strongest events at McMurdo Station. The synoptic analysis reveals that McMurdo Station EWEs are nearly always associated with strong southerly flow due to an approaching Ross Sea cyclone and an upper-level trough around Cape Adare. A Ross Ice Shelf air stream (RAS) environment is created with enhanced barrier winds along the Transantarctic Mountains, downslope winds in the lee of the glaciers and local topography, and a tip jet effect around Ross Island. The position and intensity of these Ross Sea cyclones are most influenced by the occurrence of a central Pacific ENSO event, which causes the upper-level trough to move westward. An approaching surface cyclone would then be in position to trigger an event, depending on how the wind direction and speed impinges on the complex topography around McMurdo Station.

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