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Richard I. Cullather
,
David H. Bromwich
, and
Michael L. Van Woert

Abstract

The spatial and temporal variability of net precipitation (precipitation minus evaporation/sublimation) for Antarctica derived from the European Centre for Medium-Range Weather Forecasts operational analyses via the atmospheric moisture budget is assessed in comparison to a variety of glaciological and meteorological observations and datasets. For the 11-yr period 1985–95, the average continental value is 151 mm yr−1 water equivalent. Large regional differences with other datasets are identified, and the sources of error are considered. Interannual variability in the Southern Ocean storm tracks is found to be an important mechanism for enhanced precipitation minus evaporation (PE) in both east and west Antarctica. In relation to the present findings, an evaluation of the rawinsonde method for estimating net precipitation in east Antarctica is conducted. Estimates of PE using synthetic rawinsondes derived from the analyses are found to compare favorably to glaciological estimates. A significant upward trend of 2.4 mm yr−1 is found for the Antarctic continent that is consistent with findings from the National Centers for Environmental Prediction, formerly the National Meteorological Center, and the National Center for Atmospheric Research Reanalysis precipitation dataset. Despite large regional discrepancies, the general agreement on the main features of Antarctic precipitation between studies suggests that a threshold has been reached, where the assessment of the smaller terms including evaporation/sublimation and drift snow loss is required to explain the differences.

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David H. Bromwich
,
Zhichang Guo
,
Lesheng Bai
, and
Qiu-shi Chen

Abstract

Surface snow accumulation is the primary mass input to the Antarctic ice sheets. As the dominant term among various components of surface snow accumulation (precipitation, sublimation/deposition, and snow drift), precipitation is of particular importance in helping to assess the mass balance of the Antarctic ice sheets and their contribution to global sea level change.

The Polar MM5, a mesoscale atmospheric model based on the fifth-generation Pennsylvania State University–NCAR Mesoscale Model, has been run for the period of July 1996 through June 1999 to evaluate the spatial and temporal variability of Antarctic precipitation. Drift snow effects on the redistribution of surface snow over Antarctica are also assessed with surface wind fields from Polar MM5 in this study. It is found that areas with large drift snow transport convergence and divergence are located around escarpment areas where there is considerable katabatic wind acceleration. It is also found that the drift snow transport generally diverges over most areas of East and West Antarctica with relatively small values.

The use of the dynamic retrieval method (DRM) to calculate precipitation has been developed and verified for the Greenland ice sheet. The DRM is also applied to retrieve the precipitation over Antarctica from 1979 to 1999 in this study. Most major features in the spatial distribution of Antarctic accumulation are well captured by the DRM results. In comparison with predicted precipitation amounts from atmospheric analyses and reanalyses, DRM calculations capture more mesoscale features of the precipitation distribution over Antarctica. A significant upward trend of +1.3 to +1.7 mm yr−2 for 1979–99 is found from DRM and forecast precipitation amounts for Antarctica that is consistent with results reported by other investigators and indicates that an additional 0.05 mm yr−1 is being extracted from the global ocean and locked up in the Antarctic ice sheets. While there is good agreement in this trend among all of the datasets, the interannual variability about the trend on the continental scale is not well captured. However, on the subcontinental scale, the interannual variability about the trend is well resolved for sectors in West Antarctica and the South Atlantic. It is also noted that the precipitation trend is weakly downward over much of the continent.

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Zhichang Guo
,
David H. Bromwich
, and
Keith M. Hines

Abstract

The impacts of the El Niño–Southern Oscillation (ENSO) on the Antarctic region are of special importance in evaluating the variability and change of the climate system in high southern latitudes. In this study, the ENSO signal in modeled precipitation over West Antarctica since 1979 is evaluated using forecast precipitation from several meteorological analyses and reanalyses. Additionally, a dynamical retrieval method (DRM) for precipitation is applied. Over the last two decades, the Southern Oscillation index (SOI) has an overall anticorrelation with precipitation over the West Antarctic sector bounded by 75°–90°S, 120°W–180° while it is positively correlated with precipitation over the South Atlantic sector bounded by 65°–75°S, 30°–60°W.

Decadal variations are found as the relationship between the SOI and West Antarctic precipitation is stronger in the 1990s than that in the 1980s. The polar front jet stream, West Antarctic precipitation, and the SOI show a well-ordered correspondence during the 1990s as the jet zonal speed is negatively correlated to the SOI and positively correlated to West Antarctic precipitation. These relationships are weaker during the 1980s, consistent with the change in sign of the correlation between the SOI and West Antarctic precipitation. The decadal variations are apparently related to changes in the quasi-stationary eddies that determine the local onshore and offshore flow over West Antarctica.

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E. Richard Toracinta
,
Robert J. Oglesby
, and
David H. Bromwich

Abstract

Global climate simulations are conducted to examine the sensitivity of the Last Glacial Maximum (LGM) climate to prescribed sea surface temperatures (SSTs) that are modified from the Climate: Long-range Investigation, Mapping, and Prediction (CLIMAP) study. Based on the consensus from various LGM proxy data, the SSTs are cooled by 4°C uniformly in the Tropics (30°N–30°S) relative to CLIMAP, and the high-latitude sea ice extent is reduced. Compared to results from a simulation with CLIMAP SSTs, the modified LGM SSTs cause significant opposing changes in the hemispheric and regional-scale atmospheric circulation, which are most pronounced in the winter hemisphere. For instance, there is significant weakening of the midlatitude circulation and reduction of 500-hPa eddy kinetic energy and midlatitude precipitation resulting from the decreased meridional temperature gradient in the modified SST simulation. In contrast, reduced sea ice extent during the boreal winter causes increased regional baroclinicity and intensified atmospheric circulation in the western North Pacific and the North Atlantic. Cooled tropical SSTs also increase the land–ocean temperature contrast, which strengthens the Asian summer monsoon circulation. Both LGM simulations produce enhanced low-level convergence and increased precipitation along the South Pacific convergence zone (SPCZ) relative to present day, despite the cooler LGM climate. The SPCZ orientation and intensity are closely linked to the distribution of South Pacific SSTs. Comparison of surface temperature estimates from land- and ocean-based proxy data with model output suggests that uniform cooling of the tropical SSTs and modification of the high-latitude sea ice extent may be sufficient to accurately simulate the first-order characteristics of the LGM climate.

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Qiu-shi Chen
,
David H. Bromwich
, and
Lesheng Bai

Abstract

In order to calculate the vertical motion over some high mountain regions, such as Greenland, an ω-equation without the quasigeostrophic approximation in σ-coordinates has been developed. A dynamic method for retrieving precipitation over Greenland is based on this ω-equation. The retrieved annual mean precipitation distribution for 1987 and 1988 is in very good agreement with the observed annual accumulation pattern over the Greenland Ice Sheet.

The major weather system producing precipitation over Greenland is the frontal cyclone. Based on the precipitation characteristics, Greenland can be divided into five subregions. Precipitation over the north coastal and central interior regions primarily occurs in summer. For the three other subregions, if the composite monthly mean sea level pressure charts for high and low monthly precipitation amounts are constructed, a clear relationship between precipitation and cyclonic activity emerges. If a mean cyclone exists in the Labrador Sea, heavy precipitation will fall over Greenland during that month. By contrast, if a mean cyclone exists near Iceland, precipitation over Greenland will be reduced. This is an important relationship between Greenland precipitation and cyclonic activity.

The cyclonic tracks near Greenland are established. A synoptic example is used to show the relation between precipitation and a cyclone moving up the west side of Greenland (track B) combined with movement across the southern tip of the island (track C). In this example, lee cyclogenesis is caused by the southern part of the Greenland Ice Sheet. The lee cyclone develops on the east coast along track C. During lee cyclogenesis, heavy precipitation falls over the southern region. The “parent” cyclone moves along track B, and precipitation falls along the west coast of Greenland.

A possible feedback between cyclonic activity and the mass balance of the Greenland Ice Sheet is proposed. On the one hand, cyclonic activity has a significant influence on snow accumulation over the ice sheet. The development of Icelandic cyclones is not favorable for precipitation over Greenland. On the other hand, the Greenland Ice Sheet has an important dynamic effect in producing lee cyclogenesis and affecting the frequency of Icelandic cyclones. This possible feedback may be important for understanding how the mass balance of the Greenland Ice Sheet and the Icelandic low are maintained in the present climate state.

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John Turner
,
David H. Bromwich
, and
Gary M. Carter
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David H. Bromwich
,
Richard I. Cullather
, and
Robert W. Grumbine

Abstract

Analyses and medium-range numerical weather forecasts produced by the National Centers for Environmental Prediction are evaluated poleward of 50°S during the July 1994 special observing period of the Antarctic First Regional Observing Study of the Troposphere project. Over the Antarctic plateau, the poor representation of the continent’s terrain creates ambiguity in assessing the quality of surface variables. An examination of the vertical temperature profile, however, finds the near-surface temperature inversion strength to be substantially smaller than the observed climatology at the zero forecast hour. This arises from surface temperatures that are warmer than expected. Significant adjustment occurs in a variety of fields over the first few days of the medium-range forecast, which likely results from the initial hour’s suspect temperature profile. A spatially oscillating series of forecast anomalies in the zonally averaged temperature cross section stretches to middle latitudes by day 3. Near-surface and upper-troposphere values are found actually to improve at the South Pole with forecast time, although some fields continue to adjust through day 7. Although the examination presented here does not give a complete diagnosis, differences between observations and analyses suggest deficiencies with the model initial fields have a major role in producing the substantial model drift found. Atmospheric moisture over the continental interior does not change significantly with forecast hour, although the distinct contrast between nearshore and interior conditions lessens with forecast time. A spurious high-latitude wave pattern is found for a variety of variables. The pattern of this distortion remains constant with forecast hour. Over the ocean, large forecast pressure and height differences with analyses are associated with blocking conditions. However, it is unclear whether this results from deficiencies in the forecast model or the meager observational network over the Southern Ocean.

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Svenja H. E. Kohnemann
,
Günther Heinemann
,
David H. Bromwich
, and
Oliver Gutjahr

Abstract

The regional climate model COSMO in Climate Limited-Area Mode (COSMO-CLM or CCLM) is used with a high resolution of 15 km for the entire Arctic for all winters 2002/03–2014/15. The simulations show a high spatial and temporal variability of the recent 2-m air temperature increase in the Arctic. The maximum warming occurs north of Novaya Zemlya in the Kara Sea and Barents Sea between March 2003 and 2012 and is responsible for up to a 20°C increase. Land-based observations confirm the increase but do not cover the maximum regions that are located over the ocean and sea ice. Also, the 30-km version of the Arctic System Reanalysis (ASR) is used to verify the CCLM for the overlapping time period 2002/03–2011/12. The differences between CCLM and ASR 2-m air temperatures vary slightly within 1°C for the ocean and sea ice area. Thus, ASR captures the extreme warming as well. The monthly 2-m air temperatures of observations and ERA-Interim data show a large variability for the winters 1979–2016. Nevertheless, the air temperature rise since the beginning of the twenty-first century is up to 8 times higher than in the decades before. The sea ice decrease is identified as the likely reason for the warming. The vertical temperature profiles show that the warming has a maximum near the surface, but a 0.5°C yr−1 increase is found up to 2 km. CCLM, ASR, and also the coarser resolved ERA-Interim data show that February and March are the months with the highest 2-m air temperature increases, averaged over the ocean and sea ice area north of 70°N; for CCLM the warming amounts to an average of almost 5°C for 2002/03–2011/12.

Open access
David H. Bromwich
,
Frank M. Robasky
,
Richard I. Cullather
, and
Michael L. Van Woert

Abstract

Moisture budget calculations for Antarctica and the Southern Ocean (40°–72deg;S) are performed using operational numerical analyses from the European Centre for Medium-Range Weather Forecasts (ECMWF), the National Meteorological Center (NMC), and the Australian Bureau of Meteorology (ABM). The analyses are intetcompared for an 8-yr period from 1985 to 1992 and are evaluated against representative rawinsonde sites, which are considered accurate depictions of moisture transport at these sites.

The comparisons to East Antarctic rawinsondes and those from Macquarie Island show the ECMWF analyses to be superior in reproducing sounding values at each level. While results are highly variable depending on the station location, agreement of the ECMWF analyses to zonally averaged sounding moisture flux values along the East Antarctic coast is very close. The zonally averaged annual meridional moisture flux, for example, is within as little as 0.03 g kg−1 m s−1, or 2% at the surface. This is particularly good considering the highly variable inflow and outflow patterns along the Antarctic perimeter. The NMC and ABM analyses generally underestimate transport at each level; error cancellation occurs during vertical integration however. A comparison of moisture convergence for East Antarctica with values calculated from rawinsonde data indicates the ECMWF analysis is within 5 mm yr−1 of the observed value, while the NMC result is severely deficient. Overall these results are not surprising given the coarse resolution and spectral nature of the analyses. The ability of the ECMWF analyses to reproduce the observed moisture transport at each level is reassuring.

Comparison of the moisture transport convergence derived from the numerical analyses with previous moisture flux studies over the Southern Ocean reveals general agreement in the location of the boundary between the moisture source and sink. The ECMWF and NMC analyses place the convergence maximum slightly farther south than has been previously found. It is inferred that this results from the blocking effect of the Antarctic coastal topography. At full resolution this point is at approximately 64°S.

Long-term net precipitation (precipitation minus sublimation/evaporation) derived from the numerical analyses is somewhat smaller than values determined by glaciological methods. Net precipitation varies interannually by 25%, with most of the variation concentrated in the South Pacific sector, the region of greatest poleward moisture transport.

The results presented here offer a substantially more positive outlook on the prospects of determining continental precipitation trends in Antarctica through atmospheric methods than has been previously found and demonstrate that the ECMWF analyses provide generally good estimates.

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Tae-Kwon Wee
,
Ying-Hwa Kuo
,
David H. Bromwich
, and
Andrew J. Monaghan

Abstract

In this study, the GPS radio occultation (RO) data from the Challenging Minisatellite Payload (CHAMP) and Satellite de Aplicaciones Cientificas-C (SAC-C) missions are assimilated. An updated version of the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) four-dimensional variational data assimilation system (4DVAR) is used to assess the impact of the GPS RO data on analyses and short-range forecasts over the Antarctic. The study was performed during the period of intense cyclonic activity in the Ross Sea, 9–19 December 2001. On average 66 GPS RO soundings were assimilated daily. For the assimilation over a single 12-h period, the impact of GPS RO data was only marginally positive or near neutral, and it varied markedly from one 12-h period to another. The large case-to-case variation was attributed to the low number of GPS RO soundings and a strong dependency of forecast impact on the location of the soundings relative to the rapidly developing cyclone. Despite the moderate general impact, noticeable reduction of temperature error in the upper troposphere and lower stratosphere was found, which demonstrates the value of GPS RO data in better characterizing the tropopause. Significant error reduction was also noted in geopotential height and wind fields in the stratosphere. Those improvements indicate that early detection of the upper-level precursors for storm development is a potential benefit of GPS RO data. When the assimilation period was extended to 48 h, a considerable positive impact of GPS RO data was found. All parameters that were investigated (i.e., temperature, pressure, and specific humidity) showed the positive impact throughout the entire model atmosphere for forecasts extending up to 5 days. The impact increased in proportion to the length of the assimilation period. Although the differences in the analyses as a result of GPS RO assimilation were relatively small initially, the subtle change and subsequent nonlinear growth led to noticeable forecast improvements at longer ranges. Consequently, the positive impact of GPS RO data was more evident in longer-range (e.g., greater than 2 days) forecasts. A correlation coefficient is introduced to quantify the linear relationship between the analysis errors without GPS RO assimilation and the analysis increments induced by GPS RO assimilation. This measure shows that the growth of GPS RO–induced modifications over time is related to the prominent error reduction observed in GPS RO experiments. The measure may also be useful for understanding how cycling analysis accumulates the positive impact of GPS RO data for an extended period of assimilation.

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