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Julien P. Nicolas
and
David H. Bromwich

Abstract

High-resolution numerical weather forecasts from the Antarctic Mesoscale Prediction System (AMPS) archive are used to investigate the climate of West Antarctica (WA) during 2006–07. A comparison with observations from West Antarctic automatic weather stations confirms the skill of the model at simulating near-surface variables. AMPS cloud cover is also compared with estimates of monthly cloud fractions over Antarctica derived from spaceborne lidar measurements, revealing close agreement between both datasets. Comparison with 20-yr averages from the Interim ECMWF Re-Analysis (ERA-Interim) dataset demonstrates that the 2006–07 time period as a whole is reflective of the West Antarctic climate from the last two decades. On the 2006–07 annual means computed from AMPS forecasts, the most salient feature is a tongue-shaped pattern of higher cloudiness, accumulation, and 2-m potential temperature stretching over central WA. This feature is caused by repeated intrusions of marine air inland linked to the sustained cyclonic activity in the Ross and western Amundsen Seas. It is further enhanced by the ice sheet’s topography and by the mid–low-tropospheric wind flow on either side of the central ice divide. Low pressures centered over the Ross Sea (as opposed to the Bellingshausen Sea) are found to be most effective in conveying heat and moisture into WA. This study offers a perspective on how recent and projected changes in cyclonic activity in the South Pacific sector of the Southern Ocean may affect the climate and surface mass balance of WA.

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Bruce P. Briegleb
and
David H. Bromwich

Abstract

Present-day Arctic and Antarctic radiation budgets of the National Center for Atmospheric Research Community Climate Model version 3 (CCM3) are presented. The CCM3 simulation is from a prescribed and interannually varying sea surface temperature integration from January 1979 through August 1993. Earth Radiation Budget Experiment (ERBE) data from 1985 through 1989 are used for validation of top-of-atmosphere (TOA) absorbed shortwave radiation (ASR) and outgoing longwave radiation (OLR). Summer ASR in both polar regions is less than the observations by about 20 W m−2. While the annual mean OLR in both polar regions is only 2–3 W m−2 less than the ERBE data, the seasonal amplitude in OLR of 40 W m−2 is smaller than the observed of 55–60 W m−2. The annual polar TOA radiation balance is smaller than observations by 5–10 W m−2. Compared to selected model and observational surface data, downward shortwave (SW) is too small by 50–70 W m−2 and downward longwave (LW) too large by 10–30 W m−2. Surface downward LW in clear atmospheres is too small by 10–20 W m−2. The absence of sea-ice melt ponds results in 10–20 W m−2 too much SW absorption during early summer and from 20 to 40 W m−2 too little during late summer. Summer cloud covers are reasonably well simulated, but winter low cloud cover is too high by 0.5–0.7 compared to surface cloud observations. Comparison with limited satellite and in situ observations indicates cloud water path (CWP) is too high by about a factor of 2. While cloud particle sizes are approximately in the range of observed values, regional variation between maritime and continental droplet sizes is too strong over coastlines. Despite several improvements in CCM3 radiation physics, the accuracy of polar TOA annual radiation balance is degraded against the ERBE data compared to CCM2. Improvement in CCM3 polar radiation budgets will require improved simulation of CWP, clear sky LW, and sea ice albedo.

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Bruce P. Briegleb
and
David H. Bromwich

Abstract

Present-day Arctic and Antarctic climate of the National Center for Atmospheric Research (NCAR) Community Climate Model version 3 (CCM3) is presented. The CCM3 simulation is from a prescribed and interannually varying sea surface temperature integration from January 1979 through August 1993. Observations from a variety of sources, including the European Centre for Medium-Range Weather Forecasts analyses, rawinsonde, and surface station data, are used for validation of CCM3’s polar climate during this period. Overall, CCM3 can simulate many important polar climatic features and in general is an incremental improvement over CCM2.

The 500-hPa polar vortex minima are too deep by 50–100 m and too zonally symmetric. The Arctic sea level pressure maximum is displaced poleward, while the Icelandic region minimum is extended toward Europe, and the Aleutian region minimum is extended toward Asia. The Antarctic circumpolar trough of low sea level pressure is slightly north of the observed position and is 2–3 hPa too low. Antarctic katabatic winds are similar to observations in magnitude and regional variation. The Antarctic surface wind stress is estimated to be 30%–50% too strong in some regions. Polar tropospheric temperatures are 2°–4°C colder than observations, mostly in the summer season. Low-level winter inversions over the Arctic Ocean are only 3°–4°C, rather than the observed 10°C. In the Antarctic midcontinent they are around 25°–30°C (about 5° stronger than observed) and continue to be stronger than observed along the coast. Although water vapor column is uniformly low by 10%–20% compared to analyses in both polar regions, the regional patterns of minima over Greenland and the East Antarctic plateau are well represented. Annual 70° to pole CCM3 values are 5.8 kg m−2 for the Arctic and 1.7 kg m−2 for the Antarctic. The regional distribution of precipitation minus evaporation compares reasonably with analyses. The annual 70° to pole values are 18.1 cm yr−1, which are close to the most recent observational estimates of 16 to 18 cm yr−1 in the Arctic and 18.4 ± 3.7 cm yr−1 in the Antarctic. In both polar regions, summer surface energy budgets are estimated to be low by roughly 20 W m−2.

Suggestions as to causes of simulation deficiencies are 1) polar heat sinks that are too strong; 2) inadequate representation of sea-ice–atmosphere heat exchange, due to lack of fractional coverage of sea ice of variable thickness; 3) effects of low horizontal resolution; and 4) biased extrapolar influence.

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David H. Bromwich
and
Ryan L. Fogt

Abstract

The European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) and the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis (NCEP1) data are compared with Antarctic and other mid- to high-latitude station observations for the complete years of overlap, 1958–2001. Overall, it appears that ERA-40 more closely follows the observations; however, a more detailed look at the presatellite era reveals many shortcomings in ERA-40, particularly in the austral winter.

By calculating statistics in 5-yr moving windows for June–July–August (JJA), it is shown that ERA-40 correlations with observed MSLP and surface (2 m) temperatures are low and even negative during the mid-1960s. A significant trend in skill in ERA-40 is observed in conjunction with the assimilation of satellite data during winter, eventually reaching a high level of skill after 1978 that is superior to NCEP1. NCEP1 shows consistency in its correlation with observations throughout time in this season; however, the biases in the NCEP1 MSLP fields decrease significantly with time. Similar problems are also found in the 500-hPa geopotential height fields above the direct influences of the mountainous topography. The height differences between ERA-40 and NCEP1 over the South Pacific are substantial before the modern satellite era throughout the depth of the troposphere. The ability for ERA-40 to be more strongly constrained by the satellite data compared to NCEP1, which is largely constrained by the station observational network, suggests that the differing assimilation schemes between ERA-40 and NCEP1 lead to the large discrepancies seen here. Thus, both reanalyses must be used with caution over high southern latitudes during the nonsummer months prior to the assimilation of satellite sounding data.

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Andrew J. Monaghan
and
David H. Bromwich

Antarctica is a challenging region for conducting meteorological research because of its geographic isolation, climate extremes, vastness, and lack of permanent human inhabitants. About 15 observing stations have been in continuous operation since the onset of the modern scientific era in Antarctica during the International Geophysical Year in 1957/58. Identifying and attributing natural- and human-caused climate change signals from the comparatively short Antarctic dataset is confounded by large year-to-year fluctuations of temperature, atmospheric pressure, and snowfall. Yet there is increasing urgency to understand Antarctica's role in the global climate system for a number of reasons, most importantly the potential consequences of ice-mass loss on global sea level rise. Here, we describe recently-created records that allow Antarctic near-surface temperature and snowfall changes to be assessed in all of Antarctica's 24 glacial drainage systems during the past five decades. The new near-surface temperature and snowfall records roughly double the length of previous such datasets, which have complete spatial coverage over the continent. They indicate complex patterns of regional and seasonal climate variability. Of particular note is the occurrence of widespread positive temperature trends during summer since the 1990s, the season when melt occurs. In forthcoming years, careful monitoring of the summer trends will be required to determine whether they are associated with a natural cycle or the start of an anthropogenic warming trend. Key questions are raised during the International Polar Year.

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Keith M. Hines
and
David H. Bromwich

Abstract

Low-level clouds are extensive in the Arctic and contribute to inadequately understood feedbacks within the changing regional climate. The simulation of low-level clouds, including mixed-phase clouds, over the Arctic Ocean during summer and autumn remains a challenge for both real-time weather forecasts and climate models. Here, improved cloud representations are sought with high-resolution mesoscale simulations of the August–September 2008 Arctic Summer Cloud Ocean Study (ASCOS) with the latest polar-optimized version (3.7.1) of the Weather Research and Forecasting (Polar WRF) Model with the advanced two-moment Morrison microphysics scheme. Simulations across several synoptic regimes for 10 August–3 September 2008 are performed with three domains including an outer domain at 27-km grid spacing and nested domains at 9- and 3-km spacing. These are realistic horizontal grid spacings for common mesoscale applications. The control simulation produces excessive cloud liquid water in low clouds resulting in a large deficit in modeled incident shortwave radiation at the surface. Incident longwave radiation is less sensitive. A change in the sea ice albedo toward the larger observed values during ASCOS resulted in somewhat more realistic simulations. More importantly, sensitivity tests show that a reduction in specified liquid cloud droplet number to very pristine conditions increases liquid precipitation, greatly reduces the excess in simulated low-level cloud liquid water, and improves the simulated incident shortwave and longwave radiation at the surface.

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Ryan L. Fogt
and
David H. Bromwich

Abstract

Antarctic Mesoscale Prediction System (AMPS) forecasts of atmospheric moisture and cloud fraction (CF) are compared with observations at McMurdo and Amundsen–Scott South Pole station (hereafter, South Pole station) in Antarctica. Overall, it is found that the model produces excessive moisture at both sites in the mid- to upper troposphere because of a weaker vertical decrease of moisture in AMPS than observed. Correlations with observations suggest AMPS does a reasonable job of capturing the low-level moisture variability at McMurdo and the upper-level moisture variability at South Pole station. The model underpredicts the cloud cover at both locations, but changes to the AMPS empirical CF algorithm remove this negative bias by more than doubling the weight given to the cloud ice path.

A “pseudosatellite” product based on the microphysical quantities of cloud ice and cloud liquid water within AMPS is preliminarily evaluated against Defense Meteorological Satellite Program (DMSP) imagery during summer to examine the broader performance of cloud variability in AMPS. These comparisons reveal that the model predicts high-level cloud cover and movement with fidelity, which explains the good agreement between the modified CF algorithm and the observed CF. However, this product also demonstrates deficiencies in capturing low-level cloudiness over cold ice surfaces primarily related to insufficient supercooled liquid water produced by the microphysics scheme, which also reduces the CF correlation with observations.

The results suggest that AMPS predicts the overall CF amount and high cloud variability notably well, making it a reliable tool for longer-term climate studies of these fields in Antarctica.

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David L. Williamson
,
David H. Bromwich
, and
Ren-Yow Tzeng

Abstract

No abstract available

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Mark C. Serreze
,
Martyn P. Clark
, and
David H. Bromwich

Abstract

An effort is under way aimed at historical analysis and monitoring of the pan-Arctic terrestrial drainage system. A key element is the provision of gridded precipitation time series that can be readily updated. This has proven to be a daunting task. Except for a few areas, the station network is sparse, with large measurement biases due to poor catch efficiency of solid precipitation. The variety of gauges used by different countries along with different reporting practices introduces further uncertainty. Since about 1990, there has been serious degradation of the monitoring network due to station closure and a trend toward automation in Canada.

Station data are used to compile monthly gridded time series for the 30-yr period 1960–89 at a cell resolution of 175 km. The station network is generally sufficient to estimate the mean and standard deviation of precipitation at this scale (hence the statistical distributions). However, as the interpolation procedures must typically draw from stations well outside of the grid box bounds, grid box time series are poorly represented. Accurately capturing time series requires typically four stations per 175-km cell, but only 38% of cells contain even a single station.

Precipitation updates at about a 1-month time lag can be obtained by using the observed precipitation distributions to rescale precipitation forecasts from the NCEP-1 reanalysis via a nonparametric probability transform. While recognizing inaccuracies in the observed time series, cross-validated correlation analyses indicate that the rescaled NCEP-1 forecasts have considerable skill in some parts of the Arctic drainage, but perform poorly over large regions. Treating climatology as a first guess with replacement by rescaled NCEP-1 values in areas of demonstrated skill yields a marginally useful monitoring product on the scale of large watersheds. Further improvements are realized by assimilating data from a limited array of station updates via a simple replacement strategy, and by including aerological estimates of precipitation less evapotranspiration (P − ET) within the initial rescaling procedure. Doing a better job requires better observations and an improved atmospheric model. The new ERA-40 reanalysis may fill the latter need.

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Keith M. Hines
,
David H. Bromwich
, and
Thomas R. Parish

Abstract

The meteorology of high Southern latitudes during winter is simulated using a cloud-free version of The Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model version 4 (MM4) with a 100-km horizontal resolution. Comparisons between idealized simulations of Antarctica with MM4 and with the mesoscale model of Parish and Waight reveal that both models produce similarly realistic velocity fields in the boundary layer. The latter model tends to produce slightly faster drainage winds over East Antarctica. The intensity of the katabatic winds produced by MM4 is sensitive to parameterizations of boundary layer fluxes. Two simulation are performed with MM4 using analyses from the European Centre for Medium-Range Weather Forecasts for June 1988 as initial and boundary conditions. A simulation of the period from 0000 UTC 2 June to 0000 UTC 8 June produces realistic synoptic phenomena including ridge development over East Antarctica, frontogenesis over the Amundsen Sea, and a katabatic surge over the Ross Ice Shelf. The simulated two-averaged fields for June 1988, particularly that of a 500-hPa height, are in good agreement with time-averaged fields analyzed by the European Centre for Medium-Range Weather Forecasts. The results of the simulations provide detailed features of the Antarctic winter boundary layer along the steeply sloping terrain. Highest boundary layer wind speeds averaged over the month-long simulation are approximately 20 m s−1. The lack of latent heating in the simulations apparently results in some bias in the results. In particular, the cloud-free version of MM4 underpredicts the intensity of lows in the sea level pressure field.

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